http://www.vliz.be/v/api.php?action=feedcontributions&user=Sergent&feedformat=atomKust Wiki - Gebruikersbijdragen [nl]2020-08-13T00:25:20ZGebruikersbijdragenMediaWiki 1.31.7http://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53116Wave energy converters in coastal structures2012-09-03T10:37:28Z<p>Sergent, Philippe: /* Technologies */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math>. Moreover the wave period is derived as follows <ref name ="ref2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained <ref name="ref2"/> :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (6), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (6) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name=ref4/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine.<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53115Wave energy converters in coastal structures2012-09-03T10:25:04Z<p>Sergent, Philippe: /* Technologies */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math>. Moreover the wave period is derived as follows <ref name ="ref2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained <ref name="ref2"/> :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (6), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (6) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name=ref4/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53114Wave energy converters in coastal structures2012-09-03T10:22:15Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math>. Moreover the wave period is derived as follows <ref name ="ref2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained <ref name="ref2"/> :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (6), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (6) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name=ref4/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53113Wave energy converters in coastal structures2012-09-03T10:18:38Z<p>Sergent, Philippe: /* Technologies */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math> Moreover the wave period is derived as follows <ref name ="ref2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained <ref name="ref2"/> :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (6), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (6) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name=ref4/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53112Wave energy converters in coastal structures2012-09-03T10:13:40Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math> Moreover the wave period is derived as follows <ref name ="ref2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained <ref name="ref2"/> :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (6), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (6) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53111Wave energy converters in coastal structures2012-09-03T10:11:45Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math> Moreover the wave period is derived as follows <ref name ="ref2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained <ref name="ref2"/> :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53110Wave energy converters in coastal structures2012-09-03T10:11:14Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math> Moreover the wave period is derived as follows <ref name ="ref2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained <ref name="ref3"/> :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53109Wave energy converters in coastal structures2012-09-03T10:07:46Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math> Moreover the wave period is derived as follows <ref name ="ref 2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53108Wave energy converters in coastal structures2012-09-03T10:04:07Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math> Moreover the wave period is derived as follows <ref name ="ref 2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained <ref name = "ref2"/>:<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53107Wave energy converters in coastal structures2012-09-03T10:01:45Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math> Moreover the wave period is derived as follows <ref name ="ref 2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53106Wave energy converters in coastal structures2012-09-03T10:00:17Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used. <math>H_{m0} </math> the spectral estimate of significant wave height is based on zero-order moment of the spectral function as <math>H_{m0} = 4 \sqrt{m_0} </math> Moreover the wave period is derived as follows <ref name ="ref 2"> Vicinanza D., Cappietti L., Ferrante V. and Contestabile P. (2011) : Estimation of the wave energy along the Italian offshore, journal of coastal research, special issue 64, pp 613 - 617. </ref>. <br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>T_e = \frac{m_{-1}}{m_0}<br />
</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
where <math>m_n</math><br />
<br />
represents the spectral moment of order n. An equation similar to that describing the power of regular waves is then obtained :<br />
<p><br />
<br><br />
<p><br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(5)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2, T_e </math>) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(6)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equations (5 and 6) can be reduced to (4) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), pp 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, pp. 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(7)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(8)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53085Wave energy converters in coastal structures2012-09-03T08:23:56Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For real seas, whose waves are random in height, period (and direction), the spectral parameters have to be used like the spectral estimate of significant wave height <math>H_{m0} </math> based on zero-order moment of the spectral function <math>H_{m0} = 4 \sqrt{m_0} </math><br />
<br />
<br />
<br />
<br><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(5)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53084Wave energy converters in coastal structures2012-09-03T08:16:58Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For irregular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(5)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53083Wave energy converters in coastal structures2012-09-03T08:16:29Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For irregular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>P_{w1} = \frac{\rho g^2}{32 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(5)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53082Wave energy converters in coastal structures2012-09-03T08:16:04Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For irregular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(5)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53080Wave energy converters in coastal structures2012-09-03T08:15:24Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For irregular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(4)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(5)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53079Wave energy converters in coastal structures2012-09-03T08:13:57Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<p><br />
<br><br />
<p><br />
<br />
For irregular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(4)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53077Wave energy converters in coastal structures2012-09-03T08:12:56Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
<br />
<br><br />
For irregular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(4)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53074Wave energy converters in coastal structures2012-09-03T08:10:13Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. <br />
For irregular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>P_{w1} = \frac{\rho g^2}{64 \pi} H_{m0}^2 T_e</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
<br />
If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(4)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53073Wave energy converters in coastal structures2012-09-03T08:05:42Z<p>Sergent, Philippe: /* Application for wave energy converters */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(4)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53072Wave energy converters in coastal structures2012-09-03T08:01:17Z<p>Sergent, Philippe: /* Wave energy and wave energy flux */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular water waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H</math> the wave height of regular water waves. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H_{m0}^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(4)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53071Wave energy converters in coastal structures2012-09-03T07:59:31Z<p>Sergent, Philippe: /* Wave energy and wave energy flux */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
For regular waves, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height H squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H_{m0}^2</math> the spectral estimate of significant wave height. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H_{m0}^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(4)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=Wave_energy_converters_in_coastal_structures&diff=53070Wave energy converters in coastal structures2012-09-03T07:57:40Z<p>Sergent, Philippe: /* References */</p>
<hr />
<div>== Introduction ==<br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Construction_of_a_coastal_structure.jpg|300px]]<br />
|-<br />
|'''Fig 1: Construction of a coastal structure.'''<br />
|}<br />
Coastal works along European coasts are composed of very diverse structures. Many coastal structures are ageing and facing problems of stability, sustainability and erosion. Moreover climate change and especially sea level rise represent a new danger for them. [[Sea dikes|Coastal dykes]] in Europe will indeed be exposed to [[waves]] with [[Wave height|heights]] that are greater than the dykes were designed to withstand, in particular all the structures built in shallow water where the depth imposes the maximal amplitude because of wave breaking. <br />
<p><br />
These structures need therefore to be modernized and adapted to [[climate change]] on one hand and to increase of [[Maritime_Traffic|maritime traffic]] and size of container carriers on the other hand.<br />
<p><br />
This necessary adaptation will be costly but will provide an opportunity to integrate converters of sustainable energy in the new maritime structures along the coasts and in particular in harbours. This initiative will contribute to the reduction of the greenhouse effect. Produced energy can be directly used for the energy consumption in harbour area and will reduce the carbon footprint of harbours by feeding the docked ships with green energy. Nowadays these ships use their motors to produce electricity power on board even if they are docked. Integration of [[wave energy converters]] (WEC) in coastal structures will favour the emergence of the new concept of future harbours with zero emissions.<br />
<p><br />
<br><br />
<p><br />
__toc__<br />
<br />
== Wave energy and wave energy flux ==<br />
<p><br />
In a sea state, the time-mean [[waves|wave]] energy density E per unit horizontal area on the water surface (J/m²) is the sum of kinetic and potential energy density per unit horizontal area. The potential energy density is equal to the kinetic energy <ref name="ref1">Mei C.C. (1989) The applied dynamics of ocean surface waves. Advanced series on ocean engineering. World Scientific Publishing Ltd </ref> both contributing half to the time-mean wave energy density E that is proportional to the wave height squared according to linear wave theory <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(1)<br />
</div><br />
<math>E= \frac{1}{8} \rho g H_{m0}^2</math> <br />
<br />
</div><br />
<p><br />
<br><br />
<p><br />
g is the gravity and <math>H_{m0}^2</math> the spectral estimate of significant wave height. As the waves propagate, their energy is transported. The energy transport velocity is the group velocity. As a result, the time-mean wave energy flux per unit crest length (W/m) perpendicular to the wave propagation direction, is equal to <ref name="ref1"/>:<br />
<p><br />
<br><br />
<p><div style="text-align: center;"><br />
<div style="float: right"><br />
(2)<br />
</div> <br />
<math> P= Ec_{g}</math> <br />
</div> <br />
<p><br />
<br><br />
<p><br />
with <math>c_{g}</math> the group velocity (m/s). Due to the dispersion relation for water waves under the action of gravity, the group velocity depends on the wavelength λ (m), or equivalently, on the wave period T (s). Further, the dispersion relation is a function of the water depth h (m). As a result, the group velocity behaves differently in the limits of deep and shallow water, and at intermediate depths:<br />
<div style="text-align: center;"><br />
<math>(\frac{\lambda}{20} < h < \frac{\lambda}{2})</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
<br />
== Application for wave energy converters ==<br />
<br />
For regular waves in deep water:<br><div style="text-align: center;"><br />
<div style="float: right">(3)</div><br />
<math>c_{g} = \frac{gT}{4\pi} </math> and <math>P_{w1} = \frac{\rho g^2}{32 \pi} H_{m0}^2 T</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The time-mean wave energy flux per unit crest length is used as one of the main criteria to choose a site for wave energy converters. If local data are available (<math>H_{m0}^2 </math>, T) for a sea state through in-situ wave buoys for example, satellite data or numerical modelling, the last equation giving wave energy flux <math>P_{w1}</math> gives a first estimation. Averaged over a season or a year, it represents the maximal energetic resource that can be theoretically extracted from wave energy. <br />
If the directional spectrum of sea state variance F (f,<math>\theta</math>) is known with f the wave frequency (Hz) and <math>\theta</math> the wave direction (rad), a more accurate formulation is used:<br />
<p><br />
<br><br />
<div style="text-align: center;"><br />
<p><div style="float: right">(4)</div><br />
<math>P_{w2} = \rho g\int\int c_{g}(f,h)F(f,\theta) dfd \theta</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-right: 1em; text-align:center; font-size:85%"<br />
|[[Image:Time-mean wave energy flux along West European coasts.jpg|280px]]<br />
|-<br />
|'''Fig 2: Time-mean wave energy flux along <br> West European coasts''' <ref name = ref3> Mattarolo G., Benoit M., Lafon F. (2009), Wave energy resource off the French coasts: the ANEMOC database applied to the energy yield evaluation of Wave Energy, 10th European Wave and Tidal Energy Conference Series (EWTEC’2009), Uppsala (Sweden)</ref>'''.'''<br />
|}<br />
It can be shown easily that equation (4) can be reduced to (3) with the hypothesis of regular waves in deep water. The directional spectrum is deduced from directional wave buoys, SAR images or advanced spectral wind-wave models, known as third-generation models, such as WAM, WAVEWATCH III, TOMAWAC or SWAN. These models solve the spectral action balance equation without any a priori restrictions on the spectrum for the evolution of wave growth.<br />
<p><br />
<br />
From TOMAWAC model, the near shore wave atlas ANEMOC along the coasts of Europe and France based on the numerical modelling of wave climate over 25 years has been produced <ref name="Ref 2">Benoit M. and Lafon F. (2004) : A nearshore wave atlas along the coasts of France based on the numerical modeling of wave climate over 25 years, 29th International Conference on Coastal Engineering (ICCE’2004), Lisbonne (Portugal), 714-726.</ref>. Using equation (4), the time-mean wave energy flux along West European coasts is obtained (see Fig. 2). This equation (4) still presents some limits like the definition of the bounds of the integration. Moreover, the objective to get data on the wave energy near coastal structures in shallow or intermediate water requires the use of numerical models that are able to represent the physical processes of wave propagation like the refraction, shoaling, dissipation by bottom friction or by wave breaking, interactions with tides and diffraction by islands. <br />
<p><br />
The wave energy flux is therefore calculated usually for water depth superior to 20 m. This maximal energetic resource calculated in deep water will be limited in the coastal zone:<br />
:* at low tide by wave breaking;<br />
:* at high tide in storm event when the wave height exceeds the maximal operating conditions;<br />
:* by screen effect due to the presence of capes, spits, reefs, islands,...<br />
<p><br />
<br><br />
<p><br />
<br />
== Technologies ==<br />
<br />
According to the [http://www.iea.org/ International Energy Agency (IEA)], more than hundred systems of [[Wave energy converters|wave energy conversion]] are in development in the world. Among them, many can be integrated in coastal structures. Evaluations based on objective criteria are necessary in order to sort theses systems and to determine the most promising solutions.<br />
<p><br />
Criteria are in particular:<br />
# the converter efficiency : the aim is to estimate the energy produced by the converter. The efficiency gives an estimate of the number of kWh that is produced by the machine but not the cost. <br />
# the converter survivability : the capacity of the converter to survive in extreme conditions. The survivability gives an estimate of the cost considering that the weaker are the extreme efforts in comparison with the mean effort, the smaller is the cost.<br />
<p><br />
Unfortunately, few data are available in literature. In order to determine the characteristics of the different wave energy technologies, it is necessary to class them first in four main families <ref name="ref3"/>.<br />
<br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|Valign="top"| [[Image:Overtopping_with_low-head_hydraulic_turbine.jpg|250px]]<br />
|Valign="top" Colspan="2"|[[Image:Submerged_oscillating_bodies_with_hydraulic_motor%2C_hydraulic_turbine%2C_linear_electrical_generator.jpg|350px]]<br />
|-<br />
|Valign="top"| Overtopping with low-head hydraulic turbine<br />
|Valign="top" Colspan="2"|Submerged oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|-<br />
|Valign="top"|[[Image:Floating oscillating bodies with hydraulic motor, hydraulic turbine.jpg|250px]]<br />
|Valign="top"|[[Image:Oscillating water column with water turbine 1.jpg|180px]]<br />
|[[Image:Oscillating water column with water turbine 2.jpg|180px]]<br />
|-<br />
|Valign="top"|Floating oscillating bodies with hydraulic motor, <br> hydraulic turbine, linear electrical generator.<br />
|Valign="top" colspan="2"|Oscillating water column with water turbine.<br />
|-<br />
|colspan ="3"| '''Fig 3: The various wave energy technologies'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
An interesting result is that the maximum average wave power that a point absorber can absorb <math>P_{abs} </math>(W) from the waves does not depend on its dimensions <ref name ="ref4">De O. Falcão A. F. (2010) Wave energy utilization: A review of the technologies. Renewable and Sustainable Energy Reviews, Volume 14, Issue 3, April 2010, Pages 899–918. </ref>. It is theoretically possible to absorb a lot of energy with only a small buoy. It can be shown that for a body with a vertical axis of symmetry (but otherwise arbitrary geometry) oscillating in heave the capture (or absorption) width <math>L_{max}</math>(m) is as follows <ref name =ref4/>:<br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(5)</div><br />
<math>L_{max} = \frac{P_{abs}}{P_{w}} = \frac{\lambda}{2\pi}</math> or <math>1 = \frac{P_{abs}}{P_{w}} \frac{2\pi}{\lambda}</math><br />
</div><br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Upper limit of mean wave power absorption for a heaving point absorber.jpg|300px]]<br />
|-<br />
|'''Fig 4: Upper limit of mean wave power <br>absorption for a heaving point absorber.'''<br />
|}<br />
where <math>{P_{w}}</math> is the wave energy flux per unit crest length (W/m). An optimally damped buoy responds however efficiently to a relatively narrow band of wave periods.<br />
<p><br />
Babarit et Hals propose <ref name="ref5">Babarit A. and Hals J. (2011) On the maximum and actual capture width ratio of wave energy converters – 11th European Wave and Tidal Energy Conference Series (EWTEC’2011) – Southampton (U-K).</ref> to derive that upper limit for the mean annual power in irregular waves at some typical locations where one could be interested in putting some wave energy devices. The mean annual power absorption tends to increase linearly with the wave power resource. Overall, one can say that for a typical site whose resource is between 20-30 kW/m, the upper limit of mean wave power absorption is about 1 MW for a heaving WEC with a capture width between 30-50 m.<br />
<p><br />
In order to complete these theoretical results and to describe the efficiency of the WEC in practical situations, the capture width ratio <math>\eta</math> is also usually introduced. It is defined as the ratio between the absorbed power and the available wave power resource per meter of wave front times a relevant dimension B [m]. <br />
<p><br />
<br><br />
<p><br />
<div style="text-align: center;"><div style="float: right">(6)</div><br />
<math>\eta = \frac{P_{abs}}{P_{w}B} </math><br />
</div><br />
<p><br />
<br><br />
<p><br />
The choice of the dimension B will depend on the working principle of the WEC. Most of the time, it should be chosen as the width of the device, but in some cases another dimension is more relevant. Estimations of this ratio <math>\eta</math> are given <ref name = "ref5"/>: 33 % for OWC, 13 % for overtopping devices, 9-29 % for heaving buoys, 20-41 % for pitching devices. For energy converted to electricity, one must take into account moreover the energy losses in other components of the system.<br />
<p><br />
<br><br />
<p><br />
<br />
== Civil engineering ==<br />
<p><br />
Never forget that the energy conversion is only a secondary function for the coastal structure. The primary function of the coastal structure is still [[Shore protection, coast protection and sea defence methods|protection]]. It is necessary to verify whether integration of WEC modifies performance criteria of overtopping and stability and to assess the consequences for the construction cost.<br />
<p><br />
Integration of WEC in coastal structures will always be easier for a new structure than for an existing one. In the latter case, it requires some knowledge on the existing coastal structures. Solutions differ according to sea state but also to type of structures (rubble mound breakwater, caisson breakwaters with typically vertical sides). Some types of WEC are more appropriate with some types of coastal structures.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Oscillating water column configuration 1.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 2.jpg|325px]]<br />
|-<br />
|[[Image:Oscillating water column configuration 3.jpg|325px]]<br />
|[[Image:Oscillating water column configuration 4.jpg|325px]]<br />
|-<br />
|colspan ="2"| '''Fig 5: Several OWC (Oscillating water column) configurations (by Wavegen – Voith Hydro).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Environmental impact ==<br />
<p><br />
Wave absorption if it is significant will change hydrodynamics along the structure. If there is mobile bottom in front of the structure, a sand deposit can occur. Ecosystems can also be altered by change of hydrodynamics and but acoustic noise generated by the machines.<br />
<p><br />
<br><br />
<p><br />
{|align="right" style="margin-left: 1em; text-align:center; font-size:85%"<br />
|[[Image:Finistere area and locations of the six sites.jpg|250px]]<br />
|-<br />
|'''Fig 6: Finistere area and locations of<br> the six sites (google map).'''<br />
|}<br />
<br />
=== Study case: Finistere area ===<br />
<p><br />
Finistere area is an interesting study case because it is located in the far west of Brittany peninsula and receives in consequence the largest wave energy flux along the French coasts (see Fig.2). This area with a very ragged coast gathers moreover many commercial ports, fishing ports, yachting ports. The area produces a weak part of its consumption and is located far from electricity power plants. There are therefore needs for renewable energies that are produced locally. This issue is important in particular in islands. The production of electricity by wave energy will have seasonal variations. Wave energy flux is indeed larger in winter than in summer. The consumption has peaks in winter due to heating of buildings but the consumption in summer is also strong due to the arrival of tourists. <br />
<p><br />
Six sites are selected (see figure 7) for a preliminary study of wave energy flux and capacity of integration of wave energy converters. The wave energy flux is expected to be in the range of 1 – 10 kW/m. The length of each [[Application of breakwaters|breakwater]] exceeds 200 meters. The wave power along each structure is therefore estimated between 200 kW and 2 MW. Note that there exist much longer coastal structures like for example Cherbourg (France) with a length of 6 kilometres.<br />
<p><br />
<br style="clear:both;"/> <br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Satelite_picture_Roskof.jpg|219px]]<br />
|[[Image:Satelite_picture_Molene.jpg|225px]]<br />
|[[Image:Satelite_picture_Le_conquet.jpg|225px]]<br />
|-<br />
|(1) Roscoff (300 meters)<br />
|(2) Molène (200 meters)<br />
|(3) Le Conquet (200 meters)<br />
|-<br />
|[[Image:Satelite_picture_Esquibien.jpg|220px]]<br />
|[[Image:Satelite picture Saint-Guenole.jpg|227px]]<br />
|[[Image:Satelite_picture_Lesconil.jpg|225px]]<br />
|-<br />
|(4) Esquibien (300 meters)<br />
|(5) Saint-Guénolé (200 meters)<br />
|(6) Lesconil (200 meters)<br />
|-<br />
|colspan ="3"| '''Fig.7: Finistere area, the six coastal structures and their length (google map).'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
Wave power flux along the structure depends on local parameters: bottom depth that fronts the structure toe, the presence of caps, the direction of waves and the orientation of the coastal structure. See figure 8 for the statistics of wave directions measured by a wave buoy located at the Pierres Noires Lighthouse. These measurements show that structures well-oriented to West waves should be chosen in priority. Peaks of consumption occur often with low temperatures in winter coming with winds from East- North-East directions. Structures well-oriented to East waves could therefore be also interesting even if the mean production is weak.<br />
<p><br />
<br><br />
<p><br />
{| style ="margin-left: 1em" align="center" style="text-align:center; font-size:85%"<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 1.jpg|225px]]<br />
|[[Image:Waves measures at the Pierres Noires Lighthouse 2.jpg|435px]]<br />
|-<br />
|colspan ="2"| '''Fig 8: Wave measurements at the Pierres Noires Lighthouse.'''<br />
|}<br />
<p><br />
<br><br />
<p><br />
<br />
== Conclusion ==<br />
<p><br />
Wave energy converters (WEC) in coastal structures can be considered as a land renewable energy. The expected energy can be compared with the energy of land wind farms but not with offshore wind farms whose number and power are much larger. As a land system, the maintenance will be easy. Except the energy production, the advantages of such systems are :<br />
* a “zero emission” port<br />
* industrial tourism <br />
* test of WEC for future offshore installations.<br />
<p><br />
<br><br />
<p><br />
<br />
== Acknowledgement ==<br />
<p><br />
This work is in progress in the frame of the national project EMACOP funded by the French Ministry of Ecology, Sustainable Development and Energy.<br />
<p><br />
<br><br />
<p><br />
== See also ==<br />
<br />
* [[Waves]]<br />
* [[Wave transformation]]<br />
* [[Groynes]]<br />
* [[Seawall]]<br />
* [[Seawalls and revetments]]<br />
* [[Coastal defense techniques]]<br />
* [[Wave energy converters]]<br />
* [[Shore protection, coast protection and sea defence methods]]<br />
* [[Overtopping resistant dikes]]<br />
<p><br />
<br><br />
<p><br />
<br />
==References==<br />
<references/><br />
<p><br />
<br><br />
<p><br />
<div align="center"><br />
{| style="border:1px solid #abd5f5; background:#f1f5fc; margin:0em 0em 0em 0em;"<br />
|<center>The main author of this article is [http://www.coastalwiki.org/index.php?option=com_imis&module=person&Itemid=17&persid=11176 Sergent, Philippe]<br><small>With contributions by: François Bouttes, Bertrand Michard, Emmanuel Cosquer, Alain Clément, Aurélien Babarit, Virginie Baudry, Michel Benoit and Giovanni Mattarolo</small></center><br />
----<br />
<br />
* For other articles by this author see [[:Category:Articles by Sergent, Philippe]]<br />
|}</div><br />
<br />
[[Category:Protection of coastal and marine zones]]<br />
[[Category:Coastal_defence]]<br />
[[Category: coastal wiki event Delft 2012 ]]<br />
[[Category: Coastal defense technique]]<br />
[[Category:Coastal_management]]<br />
[[Category:Techniques and methods in coastal management]]</div>Sergent, Philippehttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6278CNEXO experimental SandPit in Seine Estuary2007-04-06T12:50:56Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO sandpit]]<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
<br />
===Currents modelling ===<br />
<br />
<br />
<br />
Tide data come from SOGREAH's hydrodynamic model for the Seine Estuary where the Cnexo pit lies. These results are given from April 1st 2002 to April 30th 2002. This period comprises 56 tides. Data for a medium tide is then extracted. Hydrodynamic velocity components (u,v) and water heights h are given at the 4 corners of a rectangular domain around the Cnexo pit. The rectangular domain is 10 km x 10 km centred on the Cnexo pit which is 3 km long. The maximum water height is 8.25 meters and the maximum current velocity is 0.77 m/s.<br />
<br />
[[Image:Current.JPG|thumb|Figure 8: Current velocity for a medium tide]] <br />
<br />
<br />
A short hydrodynamic modelling is run by CETMEF using the REFLUX® model that solves shallow water equations by the Finite Elements Method. The boundary conditions are the water levels. Velocity results are obtained inside the computation area and compared with SOGREAH’s data on the same point. Rough results without calibration are shown in Figure 8. These results are correct and should be improved with a little calibration. However as the current itself has a very poor influence on the sediment transport it was decided to take a homogeneous current field among the whole domain but just changing with time.<br />
<br />
== Morphological response modelling ==<br />
<br />
Preliminary results of sediment transport model show that waves have more effect on transport than currents do. That shows that the simplification to homogeneous currents among the domain is a correct hypothesis in a first approach. We also found that storm waves (classes H3 + H4) which represent only 4.5% of total wave events are responsible of 90% of sediment transport. It was decided to model first only storm waves for calibration and then to carry out a final complete modelling in order to get the definitive results.<br />
<br />
[[Image:Section8.JPG|thumb|Figure 9: Results for section S8 with Soulsby –Van Rijn sediment transport law]] <br />
<br />
<br />
After a rough calibration we ran out our model for 21 years with every wave conditions and a homogeneous current field. The newer part of the pit is more filled (around 6 m of deposit) than the older part (around 2 m of deposit) and southern slopes of the pit are much eroded.<br />
<br />
With cross-sections shown in Figure 9 we can see that the model does not reproduce perfectly the measurement in cross-sections with a too strong erosion of the southern part. Figure 10 shows on the contrary quite good results for the longitudinal section except a slightly too important filling of the older part. <br />
<br />
[[Image:Sectionlong.JPG|thumb|Figure 9: Results for section Slong with Soulsby –Van Rijn sediment transport law]] <br />
<br />
<br />
We also see in the latter figure that results with the total wave events are very close to results keeping only storm events. Modelling of the total wave events improves however slightly the results.<br />
<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6277CNEXO experimental SandPit in Seine Estuary2007-04-06T12:49:23Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO sandpit]]<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
<br />
===Currents modelling ===<br />
<br />
<br />
<br />
Tide data come from SOGREAH's hydrodynamic model for the Seine Estuary where the Cnexo pit lies. These results are given from April 1st 2002 to April 30th 2002. This period comprises 56 tides. Data for a medium tide is then extracted. Hydrodynamic velocity components (u,v) and water heights h are given at the 4 corners of a rectangular domain around the Cnexo pit. The rectangular domain is 10 km x 10 km centred on the Cnexo pit which is 3 km long. The maximum water height is 8.25 meters and the maximum current velocity is 0.77 m/s.<br />
<br />
[[Image:Current.JPG|thumb|Figure 8: Current velocity for a medium tide]] <br />
<br />
<br />
A short hydrodynamic modelling is run by CETMEF using the REFLUX® model that solves shallow water equations by the Finite Elements Method. The boundary conditions are the water levels. Velocity results are obtained inside the computation area and compared with SOGREAH’s data on the same point. Rough results without calibration are shown in Figure 8. These results are correct and should be improved with a little calibration. However as the current itself has a very poor influence on the sediment transport it was decided to take a homogeneous current field among the whole domain but just changing with time.<br />
<br />
== Morphological response modelling ==<br />
<br />
Preliminary results of sediment transport model show that waves have more effect on transport than currents do. That shows that the simplification to homogeneous currents among the domain is a correct hypothesis in a first approach. We also found that storm waves (classes H3 + H4) which represent only 4.5% of total wave events are responsible of 90% of sediment transport. It was decided to model first only storm waves for calibration and then to carry out a final complete modelling in order to get the definitive results.<br />
<br />
[[Image:Section8.JPG|thumb|Figure 9: Results for section 8 with Soulsby –Van Rijn sediment transport law]] <br />
<br />
<br />
After a rough calibration we ran out our model for 21 years with every wave conditions and a homogeneous current field. The newer part of the pit is more filled (around 6 m of deposit) than the older part (around 2 m of deposit) and southern slopes of the pit are much eroded.<br />
<br />
With cross-sections shown in Figure 9 we can see that the model does not reproduce perfectly the measurement in cross-sections with a too strong erosion of the southern part. Figure 10 shows on the contrary quite good results for the longitudinal section except a slightly too important filling of the older part. <br />
<br />
[[Image:Sectionlong.JPG|thumb|Figure 9: Results for sectionlong with Soulsby –Van Rijn sediment transport law]] <br />
<br />
<br />
We also see in the latter figure that results with the total wave events are very close to results keeping only storm events. Modelling of the total wave events improves however slightly the results.<br />
<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=Bestand:Sectionlong.JPG&diff=6276Bestand:Sectionlong.JPG2007-04-06T12:46:07Z<p>Sergent: </p>
<hr />
<div></div>Sergenthttp://www.vliz.be/v/index.php?title=Bestand:Section8.JPG&diff=6275Bestand:Section8.JPG2007-04-06T12:43:09Z<p>Sergent: </p>
<hr />
<div></div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6274CNEXO experimental SandPit in Seine Estuary2007-04-06T10:31:46Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO sandpit]]<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
<br />
===Currents modelling ===<br />
<br />
<br />
<br />
Tide data come from SOGREAH's hydrodynamic model for the Seine Estuary where the Cnexo pit lies. These results are given from April 1st 2002 to April 30th 2002. This period comprises 56 tides. Data for a medium tide is then extracted. Hydrodynamic velocity components (u,v) and water heights h are given at the 4 corners of a rectangular domain around the Cnexo pit. The rectangular domain is 10 km x 10 km centred on the Cnexo pit which is 3 km long. The maximum water height is 8.25 meters and the maximum current velocity is 0.77 m/s.<br />
<br />
[[Image:Current.JPG|thumb|Figure 8: Current velocity for a medium tide]] <br />
<br />
<br />
A short hydrodynamic modelling is run by CETMEF using the REFLUX® model that solves shallow water equations by the Finite Elements Method. The boundary conditions are the water levels. Velocity results are obtained inside the computation area and compared with SOGREAH’s data on the same point. Rough results without calibration are shown in Figure 8. These results are correct and should be improved with a little calibration. However as the current itself has a very poor influence on the sediment transport it was decided to take a homogeneous current field among the whole domain but just changing with time.<br />
<br />
== Morphological response modelling ==<br />
<br />
Preliminary results of sediment transport model show that waves have more effect on transport than currents do. That shows that the simplification to homogeneous currents among the domain is a correct hypothesis in a first approach. We also found that storm waves (classes H3 + H4) which represent only 4.5% of total wave events are responsible of 90% of sediment transport. It was decided to model first only storm waves for calibration and then to carry out a final complete modelling in order to get the definitive results.<br />
<br />
After a rough calibration we ran out our model for 21 years with every wave conditions and a homogeneous current field. The newer part of the pit is more filled (around 6 m of deposit) than the older part (around 2 m of deposit) and southern slopes of the pit are much eroded.<br />
<br />
With cross-sections shown in FIgure 9 we can see that the model does not reproduce perfectly the measurement in cross-sections with a too strong erosion of the southern part. Figure 10 shows on the contrary quite good results for the longitudinal section except a slightly too important filling of the older part. <br />
<br />
We also see in the latter figure that results with the total wave events are very close to results keeping only storm events. Modelling of the total wave events improves however slightly the results.<br />
<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6273CNEXO experimental SandPit in Seine Estuary2007-04-06T10:26:37Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO sandpit]]<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
<br />
===Currents modelling ===<br />
<br />
<br />
<br />
Tide data come from SOGREAH's hydrodynamic model for the Seine Estuary where the Cnexo pit lies. These results are given from April 1st 2002 to April 30th 2002. This period comprises 56 tides. Data for a medium tide is then extracted. Hydrodynamic velocity components (u,v) and water heights h are given at the 4 corners of a rectangular domain around the Cnexo pit. The rectangular domain is 10 km x 10 km centred on the Cnexo pit which is 3 km long. The maximum water height is 8.25 meters and the maximum current velocity is 0.77 m/s.<br />
<br />
[[Image:Current.JPG|thumb|Figure 8: Current velocity for a medium tide]] <br />
<br />
<br />
A short hydrodynamic modelling is run by CETMEF using the REFLUX® model that solves shallow water equations by the Finite Elements Method. The boundary conditions are the water levels. Velocity results are obtained inside the computation area and compared with SOGREAH’s data on the same point. Rough results without calibration are shown in Figure 8. These results are correct and should be improved with a little calibration. However as the current itself has a very poor influence on the sediment transport it was decided to take a homogeneous current field among the whole domain but just changing with time.<br />
<br />
== Morphological response modelling <br />
<br />
Preliminary results of sediment transport model show that waves have more effect on transport than currents do. That shows that the simplification to homogeneous currents among the domain is a correct hypothesis in a first approach. We also found that storm waves (class H4) which represent only 4.5% of total wave events are responsible of 90% of sediment transport. It is decided to model first only storm waves for our model calibration and then to carry out a final complete modelling in order to get the definitive results.<br />
<br />
After a rough calibration we ran out our model for 21 years with every wave conditions and a homogeneous current field. We found results that are presented in figure 20. In this figure we see that the newer part of the pit is more filled (around 6 m of deposit) than the older part (around 2 m of deposit) and that the southern slopes of the pit are much eroded.<br />
<br />
Cross-sections presented in figure 21 and 22 enables a better representation of the pit evolution. With these cross-sections we can see that the model does not reproduce perfectly the measurement in cross-sections with a too strong erosion of the southern part. Figure 23 shows on the contrary quite good results for the longitudinal section except a slightly too important filling of the older part. <br />
<br />
We also see with these figures that results with the total wave events are very close to results with only storm events. Modelling of the total wave events improves however the results (see figure 23).<br />
<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6272CNEXO experimental SandPit in Seine Estuary2007-04-06T10:14:48Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO sandpit]]<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
<br />
===Currents modelling ===<br />
<br />
<br />
<br />
Tide data come from SOGREAH's hydrodynamic model for the Seine Estuary where the Cnexo pit lies. These results are given from April 1st 2002 to April 30th 2002. This period comprises 56 tides. Data for a medium tide is then extracted. Hydrodynamic velocity components (u,v) and water heights h are given at the 4 corners of a rectangular domain around the Cnexo pit. The rectangular domain is 10 km x 10 km centred on the Cnexo pit which is 3 km long. The maximum water height is 8.25 meters and the maximum current velocity is 0.77 m/s.<br />
<br />
[[Image:Current.JPG|thumb|Figure 8: Current velocity for a medium tide]] <br />
<br />
<br />
A short hydrodynamic modelling is run by CETMEF using the REFLUX® model that solves shallow water equations by the Finite Elements Method. The boundary conditions are the water levels. Velocity results are obtained inside the computation area and compared with SOGREAH’s data on the same point. Rough results without calibration are shown in Figure 8. These results are correct and should be improved with a little calibration. However as the current itself has a very poor influence on the sediment transport it was decided to take a homogeneous current field among the whole domain but just changing with time.<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6271CNEXO experimental SandPit in Seine Estuary2007-04-06T10:14:09Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO sandpit]]<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
<br />
===Currents modelling ===<br />
<br />
[[Image:Current.JPG|thumb|Figure 8: Current velocity for a medium tide]] <br />
<br />
<br />
Tide data come from SOGREAH's hydrodynamic model for the Seine Estuary where the Cnexo pit lies. These results are given from April 1st 2002 to April 30th 2002. This period comprises 56 tides. Data for a medium tide is then extracted. Hydrodynamic velocity components (u,v) and water heights h are given at the 4 corners of a rectangular domain around the Cnexo pit. The rectangular domain is 10 km x 10 km centred on the Cnexo pit which is 3 km long. The maximum water height is 8.25 meters and the maximum current velocity is 0.77 m/s. <br />
<br />
A short hydrodynamic modelling is run by CETMEF using the REFLUX® model that solves shallow water equations by the Finite Elements Method. The boundary conditions are the water levels. Velocity results are obtained inside the computation area and compared with SOGREAH’s data on the same point. Rough results without calibration are shown in Figure 8. These results are correct and should be improved with a little calibration. However as the current itself has a very poor influence on the sediment transport it was decided to take a homogeneous current field among the whole domain but just changing with time.<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=Bestand:Current.JPG&diff=6270Bestand:Current.JPG2007-04-06T10:12:26Z<p>Sergent: </p>
<hr />
<div></div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6269CNEXO experimental SandPit in Seine Estuary2007-04-06T10:06:10Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO sandpit]]<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
<br />
===Currents modelling ===<br />
<br />
Tide data come from SOGREAH's hydrodynamic model for the Seine Estuary where the Cnexo pit lies. These results are given from April 1st 2002 to April 30th 2002. This period comprises 56 tides. Data for a medium tide is then extracted. Hydrodynamic velocity components (u,v) and water heights h are given at the 4 corners of a rectangular domain around the Cnexo pit. The rectangular domain is 10 km x 10 km centred on the Cnexo pit which is 3 km long. The maximum water height is 8.25 meters and the maximum current velocity is 0.77 m/s. <br />
<br />
A short hydrodynamic modelling is run by CETMEF using the REFLUX® model that solves shallow water equations by the Finite Elements Method. The boundary conditions are the water levels. Velocity results are obtained inside the computation area and compared with SOGREAH’s data on the same point. Rough results without calibration are shown in Figure 8. These results are correct and should be improved with a little calibration. However as the current itself has a very poor influence on the sediment transport it was decided to take a homogeneous current field among the whole domain but just changing with time.<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6268CNEXO experimental SandPit in Seine Estuary2007-04-06T09:41:48Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO pit]]<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6267CNEXO experimental SandPit in Seine Estuary2007-04-06T09:40:40Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO pit]]<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6266CNEXO experimental SandPit in Seine Estuary2007-04-06T09:39:05Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
<br />
<br />
===Waves modelling ===<br />
<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results for wave class H4 (Hp=3.29 m; T=7.5s; direction=298°)and in particular the slightly changes of wave heights behind the pit.<br />
<br />
[[Image:wave.JPG|thumb|Figure 7: Wave height behind CNEXO pit]]<br />
<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=Bestand:Wave.JPG&diff=6265Bestand:Wave.JPG2007-04-06T09:35:30Z<p>Sergent: </p>
<hr />
<div></div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6264CNEXO experimental SandPit in Seine Estuary2007-04-06T09:32:38Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is around 298°.<br />
<br />
===Waves modelling ===<br />
<br />
<br />
Because of a large computation area, just wave periods larger than 6.5 s can be computed with REFONDE® model that solves mild slope equation by the Finite Element Methods. Mesh exceeds 800.000 finite elements.<br />
Figure 7 shows results of wave modelling results and in particular the slightly changes of wave heights behind the pit.<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6263CNEXO experimental SandPit in Seine Estuary2007-04-06T09:22:56Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are used for a simplified wave climate shown in Figure 6. The dominant direction is about around 298°.<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6262CNEXO experimental SandPit in Seine Estuary2007-04-06T09:18:16Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
===Differential bathymetric map===<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
===Volume analysis of the Cnexo pit===<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
==Waves and currents data ==<br />
===Waves data ===<br />
<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves. The four dominant angular domains centred around dominant directions are shown in Figure 6. We can clearly see that there is one dominant direction which is about a = 298°.<br />
<br />
== References ==<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6261CNEXO experimental SandPit in Seine Estuary2007-04-06T09:14:44Z<p>Sergent: </p>
<hr />
<div>==Introduction==<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
==Initial state of bathymetry==<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
==Bathymetric data==<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
==Morphodynamic evolution of Cnexo Sandpit since 1981==<br />
<br />
===Cross-sections analysis===<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
==Differential bathymetric map==<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
==Volume analysis of the Cnexo pit==<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
===Waves and currents data ===<br />
==Waves data ==<br />
<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves and results are shown in Figure 6. We can clearly see that there is one dominant direction which is about a = 298° and we decomposed these numerical results into four dominant angular domains centred around dominant directions. These four dominant angular domains and their linked properties are summarized in table 1.<br />
<br />
=== References ===<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6260CNEXO experimental SandPit in Seine Estuary2007-04-06T09:12:33Z<p>Sergent: /* Waves data */</p>
<hr />
<div>===Introduction===<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
===Initial state of bathymetry===<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
===Bathymetric data===<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
===Morphodynamic evolution of Cnexo Sandpit since 1981===<br />
<br />
==Cross-sections analysis==<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
==Differential bathymetric map==<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
==Volume analysis of the Cnexo pit==<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
===Waves and currents data ===<br />
==Waves data ==<br />
<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
[[Image:climate.JPG|thumb|Figure 6: Simplified wave climate]]<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves and results are shown in Figure 6. We can clearly see that there is one dominant direction which is about a = 298° and we decomposed these numerical results into four dominant angular domains centred around dominant directions. These four dominant angular domains and their linked properties are summarized in table 1.<br />
<br />
=== References ===<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=Bestand:Climate.JPG&diff=6259Bestand:Climate.JPG2007-04-06T09:11:48Z<p>Sergent: </p>
<hr />
<div></div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6258CNEXO experimental SandPit in Seine Estuary2007-04-06T09:10:24Z<p>Sergent: /* Waves data */</p>
<hr />
<div>===Introduction===<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
===Initial state of bathymetry===<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
===Bathymetric data===<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
===Morphodynamic evolution of Cnexo Sandpit since 1981===<br />
<br />
==Cross-sections analysis==<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
==Differential bathymetric map==<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
==Volume analysis of the Cnexo pit==<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
===Waves and currents data ===<br />
==Waves data ==<br />
<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
[[Image:climate.TIF|thumb|Figure 6: Simplified wave climate]]<br />
<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves and results are shown in Figure 6. We can clearly see that there is one dominant direction which is about a = 298° and we decomposed these numerical results into four dominant angular domains centred around dominant directions. These four dominant angular domains and their linked properties are summarized in table 1.<br />
<br />
=== References ===<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6257CNEXO experimental SandPit in Seine Estuary2007-04-06T08:29:30Z<p>Sergent: /* Waves data */</p>
<hr />
<div>===Introduction===<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
===Initial state of bathymetry===<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
===Bathymetric data===<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
===Morphodynamic evolution of Cnexo Sandpit since 1981===<br />
<br />
==Cross-sections analysis==<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
==Differential bathymetric map==<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
==Volume analysis of the Cnexo pit==<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
<br />
===Waves and currents data ===<br />
==Waves data ==<br />
<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves and results are shown in Figure 6. We can clearly see that there is one dominant direction which is about a = 298° and we decomposed these numerical results into four dominant angular domains centred around dominant directions. These four dominant angular domains and their linked properties are summarized in table 1.<br />
<br />
=== References ===<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6256CNEXO experimental SandPit in Seine Estuary2007-04-06T08:26:27Z<p>Sergent: /* Waves data */</p>
<hr />
<div>===Introduction===<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
===Initial state of bathymetry===<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
===Bathymetric data===<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
===Morphodynamic evolution of Cnexo Sandpit since 1981===<br />
<br />
==Cross-sections analysis==<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
==Differential bathymetric map==<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
==Volume analysis of the Cnexo pit==<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
===Waves data ===<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation model needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves and results are shown in Figure 6. We can clearly see that there is one dominant direction which is about a = 298° and we decomposed these numerical results into four dominant angular domains centred around dominant directions. These four dominant angular domains and their linked properties are summarized in table 1.<br />
<br />
=== References ===<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6255CNEXO experimental SandPit in Seine Estuary2007-04-06T08:25:48Z<p>Sergent: /* Waves data */</p>
<hr />
<div>===Introduction===<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
===Initial state of bathymetry===<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
===Bathymetric data===<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
===Morphodynamic evolution of Cnexo Sandpit since 1981===<br />
<br />
==Cross-sections analysis==<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
==Differential bathymetric map==<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
==Volume analysis of the Cnexo pit==<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
===Waves data ===<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation software needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves and results are shown in Figure 6. We can clearly see that there is one dominant direction which is about a = 298° and we decomposed these numerical results into four dominant angular domains centred around dominant directions. These four dominant angular domains and their linked properties are summarized in table 1.<br />
<br />
=== References ===<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6254CNEXO experimental SandPit in Seine Estuary2007-04-06T08:25:20Z<p>Sergent: /* Waves data */</p>
<hr />
<div>===Introduction===<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
===Initial state of bathymetry===<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
===Bathymetric data===<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
===Morphodynamic evolution of Cnexo Sandpit since 1981===<br />
<br />
==Cross-sections analysis==<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
==Differential bathymetric map==<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
==Volume analysis of the Cnexo pit==<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
===Waves data ===<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very close to the Cnexo pit (several kilometres far only) as it is shown on the location map (Figure 1) which means that no propagation software needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves and results are shown in figure 6. We can clearly see that there is one dominant direction which is about a = 298° and we decomposed these numerical results into four dominant angular domains centred around dominant directions. These four dominant angular domains and their linked properties are summarized in table 1.<br />
<br />
=== References ===<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=6253CNEXO experimental SandPit in Seine Estuary2007-04-06T08:21:26Z<p>Sergent: /* Waves data */</p>
<hr />
<div>===Introduction===<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
===Initial state of bathymetry===<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
===Bathymetric data===<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
===Morphodynamic evolution of Cnexo Sandpit since 1981===<br />
<br />
==Cross-sections analysis==<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
==Differential bathymetric map==<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
==Volume analysis of the Cnexo pit==<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
===Waves data ===<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very closed to the Cnexo pit (several kilometres far only) as it is shown on the location map (figure 1) which means that no propagation software needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves and results are shown in figure 6. We can clearly see that there is one dominant direction which is about a = 298° and we decomposed these numerical results into four dominant angular domains centred around dominant directions. These four dominant angular domains and their linked properties are summarized in table 1.<br />
<br />
=== References ===<br />
Lemoine M., Clabaut P., Simon S., Augris C., 1999, Étude de la souille expérimentale d’exploitation de granulats marins dite « souille CNEXO » en baie de Seine : évolution morpho-sédimentologique et faunistique entre 1981 et 1996, Rapport Ifremer.<br />
<br />
Desprez M., 1996, Étude des sédiments superficiels et de la macrofaune benthique dans le secteur de l’ancienne fouille expérimentale du CNEXO. État en décembre 1995. Rapport GEMEL Picardie.<br />
{{author<br />
|AuthorID=11176<br />
|AuthorName= Sergent, Philippe<br />
}}</div>Sergenthttp://www.vliz.be/v/index.php?title=CNEXO_experimental_SandPit_in_Seine_Estuary&diff=4591CNEXO experimental SandPit in Seine Estuary2007-03-09T13:06:45Z<p>Sergent: </p>
<hr />
<div>===Introduction===<br />
<br />
Through the sixty’s, needs of sands and gravels quickly increased and several studies were carried out by Cnexo (former name of Ifremer Institute) on substitution materials to prevent problems of material supply.<br />
In order to study responses of exploitation of submarine materials, a full-scale experiment was launched in 1973 on a site located at the mouth of the Seine estuary.<br />
Morphodynamic behaviour of Cnexo pit was studied within SANDPIT European FP5 project.<br />
<br />
===Initial state of bathymetry===<br />
<br />
The Cnexo sandpit is 2.5 km long, 400 m wide and its direction is SW-NE. It was dug in a region where the depths vary between 16 m and 17.5 m with a slight slope toward the North. Figure 1 gives a good idea of the Cnexo pit location inside the Seine Estuary and also shows both the location of the Candhis wave buoy used to collect wave data and the location of two well-known and well-studied sediment deposits (Octeville deposit and Kannick deposit).<br />
<br />
[[Image:Cnexo.JPG|thumb|Figure 1: Cnexo pit location]] <br />
<br />
A sediment study of the East part of the Seine bay was carried out in 1967 and showed that the Cnexo pit was dug in a region where the bottom material was made of fine quartz sands with a median diameter between 0.25 mm and 0.50 mm. These sands at the surface of the bottom contain from 20% to 30% of limestone but less than 2% of silt.<br />
<br />
Before its dredging, the studied site was located on a vast homogeneous sandy zone under which we find old terraces of the Seine river which are mainly composed of coarser materials, more heterogeneous and containing less limestone. Manufacturers of materials were interested in these terraces and especially in their coarser parts.<br />
<br />
The dredging of the Cnexo sandpit was carried out from 1974 to 1980 through 13 campaigns of materials extraction that removed more than 2.800.000 m3 of materials. <br />
<br />
[[Image:Cnexo2.JPG|thumb|Figure 2: Cnexo dredging]]<br />
<br />
The whole granted domain was not exploited at the same time.<br />
From 1974 to 1977, the only Northeast part of the domain was dredged on a length of 1500 m approximately. At the end of this first stage, this “old” dredging is 200 m wide and his depth varies between 3 and 5 m.<br />
From 1977 to 1980, the Southwest part of the domain was also dredged and this dredging was deeper and also thinner. At the end of the “new” dredging in 1980, the whole dredging is about 3 km long and between 130 m to 300 m wide (These dimensions are found using 18 m isobaths). Its bathymetry is deeper in its “new” Southwest part (between 5 and 13 m deep) than in its “old” Northeast part (between 3 and 6 m deep).<br />
<br />
Figure 2 shows these different stages of Cnexo dredging from 1974 to 1980.<br />
<br />
===Bathymetric data===<br />
<br />
[[Image:Cnexo3.JPG|thumb|Figure 3: 1981 bathymetry with locations of different cross-sections]]<br />
<br />
Three different bathymetries were collected for years 1981, 1996 and 2002. The two most recent bathymetries (1996 and 2002) were made by Le Havre Harbour that used exactly the same Global Positioning System for both studied years. Le Havre harbour considers that precision is around 1 meter for the planimetric coordinates and not more than 20 cm for the altimetric coordinate.<br />
1981 bathymetric data are more hazardous and was found by digitalizing an old map.<br />
Even if the field measurements had been made correctly we should not expect better precision than 10-15 m for planimetric coordinates and 30 cm for altimetric coordinate.<br />
<br />
===Morphodynamic evolution of Cnexo Sandpit since 1981===<br />
<br />
==Cross-sections analysis==<br />
[[Image:Cnexo4.JPG|thumb|Figure 4: Cross-section Slong in 1981, 1996 and 2002]]<br />
<br />
A strong filling of the pit is measured in the southern part of Cnexo pit between 1981 and 1996 and continued between 1996 and 2002 but with a smaller range. See figure 4.<br />
<br />
<br />
==Differential bathymetric map==<br />
<br />
A differential bathymetric map is presented in figure 5. A clear filling of the pit between 1981 and 2002 is found while the two slopes of this pit are eroded which seems physically correct. The phenomenon is stronger in the southern part than in the northern part that was less dug.<br />
<br />
[[Image:Cnexo5.JPG|thumb|Figure 5: Bathymetric differential map between 2002 and 1981]]<br />
<br />
==Volume analysis of the Cnexo pit==<br />
<br />
A volume analysis was carried out on a restricted domain around the pit. From 1981 to 1996 an accretion of about +615.000 m3 (41 000 m3 /year) was found and from 1996 to 2002 accretion was about +90.000 m3 (15 000 m3 /year).<br />
<br />
===Waves data ===<br />
<br />
Two wave databases were collected from two wave buoys located near the CNEXO site. <br />
These buoys belong to the CANDHIS French wave observation network.<br />
The first buoy is located near Antifer and gives omni directional wave data from July 1996 to December 2002 which leads to more than 14000 measurements of (Hp; Tp).<br />
This buoy is a little bit far from the Cnexo site and would require a wave propagation study to get the modified wave field above the studied pit.<br />
The second buoy gives omni directional wave data from January 1997 to February 2003 which leads to more than 29000 measurements of (Hp; Tp). This latter buoy is very closed to the Cnexo pit (several kilometres far only) as it is shown on the location map (figure 1) which means that no propagation software needs to be used to get the correct waves field above the Cnexo pit. Unfortunately wave directions are not given. <br />
Hindcast wave-data at this location have been extracted from the numerical wave data-base along the French coasts, under construction within a joint project between Météo-France, CETMEF and EDF LNHE. This extraction was made for the whole years 1999 and 2000 with a result every 3 hours, which leads to more than 5800 measurements of (Hp; Tp; Direction).<br />
<br />
An analysis of these numerical measurements was made in order to study the main directions of waves and results are shown in figure 6. We can clearly see that there is one dominant direction which is about a = 298° and we decomposed these numerical results into four dominant angular domains centred around dominant directions. These four dominant angular domains and their linked properties are summarized in table 1.</div>Sergent