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Monitoring coastal morphodynamics using high-precision multibeam technology

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Versie door Wouter Kreiken (Overleg | bijdragen) op 15 dec 2008 om 12:11

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Monitoring coastal morphodynamics is of importance when for example coastal erosion of accretion is present or protective measure are taken and the effectiveness of these measures need to be known. Monitoring this process in time is of importance to manage the developments in the right way. A technique to do this is by high-precision multibeam technology. This technology is explained in this article and an example of application is given. This shows how high-precision multibeam technology can be used in practice.


Introduction

Coastal erosion, bathymetric changes of the near coast seabed or the stability and dispersal of dredged and disposed material are among the key questions of coastal management: What are the effects of coastal protection measures? How effective are they? Do they generate new burdens on the coastal system? Which processes control the movement of the seabed? What are the typical scales for the rearrangement of bed material? What types of seabed structures contribute most to the transport of the bed material? To what extent are morphological changes predictable? Answers to these questions are still limited by a lack of observational evidence. Most of all, area-wide data are required to allow for comprehensive views of the relevant bed structures ranging over horizontal scales from decimetres to kilometres. The detection of seabed changes over these wide ranges of scales requires observational systems that combine high spatial resolution down to decimetres with high precision and accuracy in the horizontal and vertical positioning of the seabed structures in the range of centimetres.

Techniques and Methods

Table 1 Characteristics of the EM 3002 multibeam system.

In the last decade multibeam echosounder systems became a widely used tool for seabed mapping and have replaced the traditional single beam surveys. Seabed mapping based on single beam transects relies substantially on spatial interpolation methods, often guided by manual expert interference. In contrast, multibeam systems provide dense observational two-dimensional networks of depth soundings, from which a continuous digital terrain model of the seabed can be constructed.

Figure 1 The multibeam system as implemented onboard the RV “Ludwig Prandtl”. The light underwater cone symbols the beam swath, devices inside the circles show (from left to right) gyro compass, motion senor, ship-borne GPS-RTK, sound profiler, and RTK land base.

The high spatial resolution of the multibeam method is achieved by high frequency transmission of an echosounder swath that is received in highly directional re- solution by means of phase discrimination technique. The characteristic numbers of the system used by GKSS are listed in Table 1 and the system’s components depicted in Fig. 1. Beam arrival times are transformed into distances by sampled vertical sound profiles combined with modelling of ray propagation. Any ship movement is compensated for by rapidly sampling motion sensors. Dynamical vertical and horizontal positioning of the multibeam-transducer with cm-precision is achieved using real-time kinematic global positioning (GPS-RTK) and a high quality gyro compass.

Limitations and Problems

Figure 2 Left: Digital terrain model (DTM) around a fishing drag board trace as obtained at the Baltic Coast in August 2001 during the DYNAS project. This drag board trace stayed nearly unchanged over more than three years. The centre coordinate is given in UTM32 projection. “y”-axis is in the north direction. Depth is with respect to the normal chart datum. Right: Difference DTM plot October 2003 - August 2001. The high precision (i. e. reproducibility) of the observations can be derived from the complete disappearance of the trace in the difference plot. Note the expanded vertical scale at the right hand side of the panels, which for the right panel ranges from -1 to +1 mm.

The multibeam system requires a water depth of at least 3 m, a value that is also needed for shipping safety. The quality of the observations depends strongly on the effective compensation of the ship motion and a high quality of the GPS signals. This constrains the range for high-quality mappings to distances of 10 to 20 km away from the land-based GPS- reference stations and survey periods to moderate wave conditions. Repeated sampling of stable immobile seabed structures (fishing drag-board traces, ship wrecks) indicates that the reconstructed overall vertical position with respect to a fixed chart datum may vary by more than 10 cm from survey to survey. The causes of this variation have not been resolved yet. Hence, without a vertical reference present within the surveyed area overall variations in the seabed of less than 20 cm cannot be derived unambiguously over periods longer than several months. In contrast, using a fixed seabed structure as vertical reference the precision of multibeam data can be less then half a meter in the horizontal and one centimetre in the vertical, as demonstrated at the German Baltic coast (Fig. 2).

Figure 3 Map of the study site located at the German Baltic coast. The lower panel represents the DTM of the August 2001 survey. The centre coordinate is given in UTM32 projection. The corresponding WGS 84 co-ordinates are N 54° 12.08’, E 11° 54,14’. The depth is given with respect to the normal chart datum. The mounds of the dumped glacial till (gt) and the mixed soil (ms) are clearly visible. Other bed features are a fishing drag board trace (db) and the scour holes around the anchor stones of the boundary buoys (bb).

Example: The fate of dumped material at the German Baltic Coast

Problem

Harbours, rivers and estuaries all over the world have to be dredged to remain navigable for modern sea traffic. Dredged material is often highly polluted and there is much concern about the dispersal of the material during and after dumping. For the German Baltic coast, this was studied in more detail close to Rostock-Warnemünde harbour (Fig. 3) within the framework of the DYNAS-project (Dynamics of natural and anthropogenic sedimentation) (Harff et al., 2003[1], 2005[2]). The fate of dredged material dumped into a site closed for shipping activity was followed over more than three years from June 2001 to August 2004 by means of multibeam technology (Stockmann et al., in print[3]).

Figure 4 a) Bed elevation profiles across the glacial till transect shown in the right panel as obtained in August 2001, October 2003, and October 2004. b) DTM of the glacial till dumping site as obtained in August 2001 and October 2004. The centre coordinate is given in UTM32 projection. “y”-axis is in North direction. Depth is with respect to the normal chart datum. The inset in the left panel shows the not exaggerated bed elevation profile along the transect across one of the ring structures as indicated by the black line.

Results

The settled disposals consisted mainly of glacial till and formed crater-like structures reflecting the transfer of downwards into radial momentum after impact (Fig. 4). The seabed structures of the disposals showed only little change over time and their quantification by the multibeam echosounder surveys required a high measuring precision in the order of a few cm in the vertical. Volume estimations of the bed disposals revealed that material was transported away only during the dumping phase. Over the following years only reworking of the material took place: the surface smoothed out (see Fig. 4) and this process was faster at smaller horizontal scales (see Fig. 5). From the rate of the observed bathymetric changes, the material properties and model based wave and bed shear stress statistics it was estimated that these structures as a whole would persist for about a century (see Table 2).

Table 2 Estimated surface flattening times for increasing horizontal structure size. The temporal smoothing of the surface structures was extrapolated into the future using two different approaches: (1) a linear function, i.e. assuming a constant smoothing rate, and (2) by an exponential function, i.e. assuming a smoothing rate proportional to the surface variance.
Figure 5 Mean structure height of the glacial till mounds versus the horizontal scales of the mounds’ surface structure elements for August 2001, October 2003 and October 2004. Flattening of the smaller structures takes place first at the smallest scales and becomes very slow at horizontal scales above eight meters.

Management implications

The observations have some consequences for the dumping management. As long as glacial till is dumped at comparably sheltered sites the loss and further dispersal of the dredged material is negligible over decades. Dumped material consisting of silt and sand may be transported away to a substantial degree in the form of plumes and be widely dispersed due to low settling velocities. As silt is the material with the lowest settling velocities in dredged sediment mixtures and also the most polluted material, coastal zone management must focus on the handling of this fraction of the dredged material. At the same time, glacial till chunks have a limited use in covering highly polluted seabed. Although they possess a high stability when deposited on the bed it will be most demanding to achieve a sufficient area-wide coverage as the radial dispersion of the settled material is very limited and further spreading very slow.

References

  1. Harff, J. et al.(2003).Projekt: DYNAS - Dynamik natürlicher und anthropogener Sedimentation. Vorhaben: Sedimentationsprozesse in der Deutschen Bucht (Final Report). Rostock: University of Rostock, Warnemünde: Leibniz Institute for Baltic Sea Research.
  2. Harff, J. et al. (2005). Projekt: DYNAS - Dynamik natürlicher und anthropogener Sedimentation. Vorhaben: Sedimentationsprozesse in der Deutschen Bucht - Phase II (Final Report). Rostock: University of Rostock, Warnemünde: Leibniz Institute for Baltic Sea Research.
  3. Stockmann, K., Riethmüller, R., Heineke, M. & Gayer, G. (in print, doi:10.1016/j.jmarsys.2007.04.010). On the morphological long-term development of dumped material in a low-energetic environment close to the German Baltic coast. Accepted by Journal of Marine Systems.

See also

The main author of this article is Riethmüller, Rolf
Please note that others may also have edited the contents of this article.


The main author of this article is Stockmann, Karina
Please note that others may also have edited the contents of this article.


The main author of this article is Heineke, Martina
Please note that others may also have edited the contents of this article.