Portuguese case studies: Caparica: verschil tussen versies
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[[Category:Coastal risk management]]
[[Category:Coastal risk management]]
[[Category:Coastal flooding management]]
[[Category:Coastal flooding management]]
Huidige versie van 20 feb 2019 om 13:57
A case study has been carried out on the Costa da Caparica, Portugal. The information gathered in this section features in Veloso Gomes et al. (2005) .
- 1 Area discription
- 1.1 Geology and coast classification
- 1.2 Morphology, topography and bathymetry of the coast
- 1.3 Physical processes
- 1.3.1 Transport agents
- 1.3.2 Sediment transport
- 1.3.3 Sediment budget
- 1.4 Assessment of capital at risk
- 2 Problem description
- 3 Solutions/measures
- 4 Effects and lessons learned
- 5 References
The area is located south of the Tagus river mouth and north of Setúbal Peninsula. This coastal zone is characterised by a large fluvial morpho-dynamic influence in the river inlet area. The study area extends from Cova do Vapor to south of Costa da Caparica village.
Millions of people use Costa da Caparica beach during the summer and urban seafront improvement is included in a major national programme for urban area re-qualification, known as the POLIS programme.
Geology and coast classification
Geologically, the area is characterised by an alluvium deposition related with the interactive effects of waves/tides and river flow, with refraction/diffraction patterns that direct the local alluvium transport from south to north - in recent years creating a small sand spit near the NATO harbour. In the past this sand spit had more significant dimensions, extending towards the Bugio lighthouse. It was created by extensive alluvium deposits on the south side of the Tagus river (Cova do Vapor – Espichel Cape and Bugio bar). South of Costa da Caparica village a fossil cliff stretches in the direction of Albufeira lagoon. This cliff has a maximum height of 70m and has undergone some changes over geological time (Quaternary period). Due to the erosive processes that have given it particular characteristics, this cliff is unique in Portugal. In 1984 this area was defined as a protected area (DL 168/84, 22th May).
This coastal zone is a crescent-shaped plain, which includes the most important fossil cliff in Portugal. The cliff is protected from the sea by a considerable extension of sand. This is constituted of sub-horizontal layers of sedimentary rock, with ages that vary from the medium Miocenic period to modern times.
Morphology, topography and bathymetry of the coast
The topography of the study area coast has a simple characterisation; it has a crescent shape and lies in a SSE-NNW orientation. The coastal plain extends from the coastline to the 70m-high fossil cliff.
The sea bathymetry is very smooth, with bathymetric lines almost parallel to the coastline and a very gentle slope. Near this area there are two sand banks, parallel to the navigation channel of the Tagus estuary; Cabeça do Pato (or north Cachopo) to the north, and Bugio bar (or south Cachopo) to the south. The latter has suffered very significant morphologic changes in recent decades. According to Mota Oliveira (1973) , between 1939 and 1985 the Bugio bar moved 700m to the north and underwent an accentuated erosion process.
The monthly average wave regime in the Bugio lighthouse neighbourhood is characterised by average periods between 5 and 9 sec, with a spectral direction between WSW and WNW (Oliveira et al., 1999) . The significant average wave height and the maximum wave height vary between 0.7m and 2.6m, and 1.2m and 4.1m respectively.
The general regime observed in this area, according to Oliveira et al. (1999), is characterised “by periods of up-crossing zeros, Tz, between 3 and 16 sec, that are associated to local average spectral directions between S-20-W and WNW, respectively. Significant wave heights near Bugio are limited by breaking, and are smaller than 8.4m. Finally, the most frequent local wave directions are between 260º and 290º, usually with wave heights of between 0.5 and 2.5m. The most frequent period varies between 5 and 8 sec”.
The astronomic tides are semidiurnal type, with tidal cycles of approximately 12h25m and propagating from south to north.
The maximum spring tide registered at Cascais tide gauge was 3.83m and the neap tide was 0.27m. In meteorological tides and when low pressures occur, wind persistent waves can reach 5m.
Near shore currents
According to the results of a bi-dimensional finite elements model, the velocities of tidal currents in the Tagus estuary are strong, although they have low heights. Spring tides exceed 2.0m/s during the flood and 1.8 m/s during the ebb. The correspondent average values are 1.5m/s and 1.4m/s respectively.
On the Costa da Caparica waterfront, near the river inlet, the residual tidal currents have a smaller intensity due to the velocities that occur in the inlet, represented in the model results with values smaller than 0.2 m/s, and in a south to north direction in front of the Costa da Caparica beach. This south-north current is the result of a closed circuit.
According to Abecasis (1997) , a great part of the sea regime reaching the Cova do Vapor – south Caparica stretch is rotated to the south-west due the diffraction effect (around Cape Raso for sea conditions coming from the north and west quadrants) and refraction (due to the complex area bathymetry), which induces a net alluvium transport from south to north. The ebb tide currents, combined with the influence of the bar in the Golada zone, reinforce the capacity of sand movement in the same direction, finally depositing on the north face of the bar slope and contributing to its progression in this direction. During the ebb, strong currents in the natural channel promote sand transport to the external side of the bar, depositing it as the intensity decreases and thereby contributing to the reduced depth. It is incidental sea action, in particular from the north quadrant, which moves the transport process onto the coast along the slope to the south, thereby closing the alluvium movement circuit.
Sea level rise
In terms of sea level changes, there is not much information concerning future trends on the Portuguese coast. However, if the sea level rises it will likely have negative effects, mostly on the wave climate, the level and propagation of tides, coastal erosion, flooding and sediment balance. In the study area of Cova do Vapor – Costa da Caparica those effects could be aggravated by the fact that this coast is constituted of alluviums and fossil cliffs that have a limited resistance to sea action.
Data of inputs and outputs of sediments
This area depends exclusively on the Tagus River and its estuary inputs. According to the National Water Plan of 2001, the amount of sediment received by the estuary is mainly dependent of storms and flooding, and it was estimated at 1 to 77*106 tons per year. In dry years, the total estuary discharges to the sea are estimated at 0.4 to 1*106 tons of sediment. The average rates of sedimentation calculated between 1983 and 1984, indicate values of between 1.1 to 1.5cm/year. Between 1928 and 1986, 65% of the sediments could stay in the system. In recent times, intense silting in the upstream estuary area has been observed, due to the loss of capacity of the fluvial system and due to the fact that is a natural sediment retention area. According to the National Service of Parks, Reserves and Nature Conservation in 1989 the bank stabilisation works in the Tagus river basin, done from the 20’s to the 50’s, had a great impact on the amount of sediment carried out from the river. During this period the coastline retreat was enormous.
In littoral areas, huge amounts of sediment were dredged for works in other places, including artificial sand nourishment of Costa do Sol beaches.
Direction and rates of transport
The main sediment transport direction in this area is south-north due to the littoral drift currents. The sand spit on the south side of the Tagus river inlet can easily demonstrate this fact.
Sedimentary transport rates are not available or do not exist. In order to give an idea of the sea capacity transport, some references indicate that it is around 1,000,000m3 per year.
In this area, reports of erosion on south of Cova do Vapor date from 1947, particularly reaching the village of Costa da Caparica in 1958.
It has been verified that since 1870 an important physiographic transformation and retreat in the coastline of the area has occurred. The disappearance of the sand spit and the retreat of the coastline are evident. The sand spit moved by wave action up to the NATO harbour. This process was reduced by the construction of defence works (groyne field).
Figure XI.7 shows a cross section of the coastline evolution in Costa da Caparica between 1957 and 1964, before the construction of defence works. Over this period the disappearance of the sand spit facilitated sand transport to the north; as a result, Caparica beach experienced a process of accentuated erosion. The evolution of this process can be observed in the decreasing dune crest heights (from +14.0m to +8.0m HZ), and the retreat of submerged areas.
The +4.0m HZ curve retreated 150m, and the +2.0m retreated about 100m in seven years, which is significant. South of Caparica the waves are almost perpendicular to the beach and, therefore, the coast remains dynamically stable.
Erosion rates and trends
To establish trends in this area, several documents were analysed. After the analysis, it was possible to identify the degree of vulnerability and some evolution scenarios.
According to the National Water Plan of 2001, in the Tagus river inlet in the years between 1939 and 1985 the Bugio sand bank, or south Cachopo, advanced about 700m to the north, suffering generalised erosion and the north Cachopo, or Bico de Pato, moved about 800m to the south-east.
The coastline retreat is evident, massive and of immense concern. Between 1870 and 1966 the sand spit almost disappeared during a period of great erosion.
Assessment of capital at risk
Proper attention has not yet been given to this important subject in Portugal, with the possible exception of the first version of the Risk Maps – which did not give an assessment of capital at risk.
The uncertainties and the limited scientific capacity to forecast extreme coastal forcing events (storms, sea level rise, tsunamis), non-tangible and cumulative environmental changes and impacts, and the lack of quantification of the needs, values and aspirations of coastal communities are some of the current limitations to assessing capital at risk.
Management plans should be based on an adequate understanding of coastal dynamics. It is necessary to continue research on many aspects of coastal dynamics in order to better assess and understand erosion and sedimentation problems, predictions of shoreline positions for various scenarios and timescales of climate variability and direct human influence, the vulnerability of beaches, dunes and coastal structures to storms and other extreme events, and the impacts of coastal structures and ecological changes.
Intervention was called for, since an extremely dynamic morphological evolution on this coastal zone was evident. After hundreds of years of submarine accumulation and (sand spit) progress seaward (Bugio bar and Bico de Pato bar) wave and sediment dynamics changed. There are records of erosion in Costa do Vapor dating from 1947, reaching Costa da Caparica village in 1958.
Erosion events, increasing urbanisation near the danger area (coastline) and the insufficient coastal management plans increase the risk vulnerability and the need for protection against coastline retreat, overtopping, flooding and infrastructure destruction.
The Coastal Zone Management Plan (POOC – Plano de Ordenamento da Orla Costeira) for Sintra – Sado (which includes the Cova do Vapor/Costa da Caparica stretch) recognises some dangerous situations and gives some guidelines such as:
- The waterfront is in a possible risk situation;
- The defence, in relation to the urban nucleus, is performed by the seawall;
- The relative unimportance of the groyne fields for reconstruction of the sand beach;
- Groyne inefficiency attributed to the fact that the groynes are short and too close together;
- The need for studies of the sedimentary dynamics in the coastal zone;
- A large artificial nourishment intervention on north extremity of the stretch is proposed, to configure the coast approximately to previous configurations.
- It indicates that interventions have to be made in the Tagus River inlet sediments;
- It is possible to expand the beach area and contribute to the enhancement of Caparica Beach.
This area is more or less in equilibrium after the groyne field construction in Costa da Caparica (1972) and three groyne expansions in Cova do Vapor (1968/1971).
It is important to note the great urban expansion that occurred between 1972 and 1996, despite the fact that the coastal stretch was still vulnerable to storms, even with the coastal defence works. This expansion of the waterfront urban area could reach the adjacent area, provoking worsening and increasing effects of sea actions and risks.
In the persistent winter of 2000/2001, the destruction of the beach users facilities improperly located above the primary dune in S. João beach occurred. This beach lost a great amount of sand and the dunes suffered an intense erosive process.
The beach user facilities must and will be relocated, but the beach and the dunes, according to the historical record and the recent dynamics, are unlikely to recover their profiles through only natural actions.
Coastal erosion is a very serious problem and will probably increase as a response to the continuous weakening of the river sediment sources, the dredging works, the mean sea-level rise, urban waterfront settlements, the changing morphodynamics patterns and other causes.
The current urban waterfront is defended by coastal structures that need maintenance and remodelling in terms of the groyne lengths and seawall crests. In such highly exposed areas "soft" coastal defences are probably not effective.
It is necessary to adopt a “preventive” policy as well as a “curative” one because of the severity of the present problems.
The research into new coastal defence technical solutions and/or the improvement of the current ones should be promoted. However, it is not expected that more environmental and cost effective solutions will appear in the foreseeable future.
The adopted policy in this area was to hold the line. This policy and the new intervention for this area were proposed to protect the urban area and expand the beach area through artificial sand nourishment.
Coastal defence measures/financial aspects
In this area erosion began due to the collapse of the natural protection afforded by the sand spit formation north of the beaches, which ends at the zone of the Bugio lighthouse (Barceló, 1971) .
When this natural protection on the north collapsed, an intense longshore drift began that formed a spit in the root of the existing formation; this spit was fed in part by the sands carried northward, the remainder being deposited in the channel of the Tagus estuary, which avoided serious disturbances. This erosion phenomenon began to affect Caparica in 1960.
The first defence work was constructed in 1959, namely the E1 groyne. In 1962 the E2 groyne was built and in 1963 the E3 groyne as well as the seawall between the E2 and E3 groynes were built. The main objective of these works was to construct a hard nucleus to hold the coastline. In 1959 a seawall (dike) was built on the south side (between Cova do Vapor and Costa da Caparica). This seawall was to prevent the sea flooding during the storms that overtopped the dunes.
The situation became worse in 1964, when destruction in the central area of Costa da Caparica occurred. The dike was reinforced and a small groyne was constructed. It was evident that the influence of groynes built in Cova do Vapor was too small to originate sand accumulation on the southern beach, and to reduce/stop the erosion process. Between 1968 and 1971 the three groynes of Cova do Vapor were expanded. The bigger one was extended to 600m in length, while recognising the necessity to expand it even more.
With erosion progressing to the south from Cova do Vapor to Costa da Caparica, the situation was progressively worsening. Two defence objectives were decided on: protection of the urban area against the sea actions and the re-establishment of the beaches. The plan included the construction of a 2,500m seawall and the construction of a groyne field with seven structures with foundations on the seawall. The purpose of the groyne field was to catch and fix the small amount of sand that moves onshore.
In 1973 this groyne field had acceptable performance, while there was still a recognized need to review the groyne lengths. The groyne lengths were around 180m and the distance between them around 330m.
Between 1972 and 2000 the coastline was more or less stable. However, the severe winter weather of 2000/2001 demonstrated that this area is still very unstable and vulnerable.
In Costa da Caparica after this winter it was verified that;
- There was little sand on the beaches, and at high tide the beaches were almost completely covered by water;
- A great part of the sea defences in Costa da Caparica were damaged: generalized groyne shortening, damage to the groyne heads and trunks, reduction and destruction of structures in important zones.
Costa da Caparica beach is used by thousands people during the summer and the re-valuation of its urban seafront and beach is very important. The re-valuation plan is included in a major national programme for the re-qualification of urban areas – the POLIS programme.
The proposed solution consists of a commitment to intervene and improve the existing solutions, and the difficulties of forecasting the performance of interventions. Experience demonstrates that forecasting ability is very limited, in spite of significant scientific progress and the availability of several forecasting models on the market. Prudence and good sense are necessary.
On the one hand, current defences, although seriously damaged in their structure can be recovered and improved. This solution worked well in terms of protection for over 30 years but it was not able to provide a sufficiently developed permanent beach for bathing purposes.
On the other hand, it should be highlighted that extreme hydro-morphological phenomena, as well as its evolution on the medium to long term, are difficult to forecast, in face of both the actual state of the scientific knowledge and the inadequacy and unavailability of field data.
The medium-term (five-year) solution, in conjunction with artificial sand nourishment, will eliminate some intermediate groynes and will increase the length of those that will become the defence structure groynes.
The destruction of some groynes will only be done if monitoring surveys demonstrate their inefficiency. The need to increase the length of the defence structure groynes will be better assessed after monitoring, as well as their configuration (rectilinear, oblique, in T). In addition, the need for constructing a breakwater should also be assessed after monitoring data within the next few years.
A very different concept from the existing one would be, for example, a 700m length for at least two of the groynes in Costa da Caparica to intercept the longitudinal sedimentary transport to the level -5.0 HZ. This could be less efficient than foreseen (if the transport cost is reduced) and/or can have impacts which are difficult to foresee and correct in the waterfront and downdrift areas. With the current state of knowledge, an intervention with subsequent approaches in this very complex and relatively unpredictable dynamic system guided by the results of monitoring tasks would be much more prudent.
The interventions in the defence works with artificial sand nourishment basically consist of:
- 1. Reshaping of the existing groynes:
- increasing the length of groynes that will act as defence structure groynes;
- reducing the length of groynes that could be eliminated in the medium-term;
- these operations should preferably be done downdrift of the structures (north) or updrift (south), in such a way as to improve the capacity of sand retention.
- 2. Reshaping of the support works (seawall) in the urban waterfront:
- recognizing the vital importance of this structure in terms of defence and for alternatives other than an accentuated retreat of the built-up waterfront (streets and buildings);
- increasing the crest level of the seawall so as to improve the seaside road and the future access for equipment in maintenance work.
- 3. Proceed with artificial sand nourishment:
- off-shore sand will be transported from loan sites already defined by the Instituto Hidrográfico for this purpose (and/or originating from harbour administration (APL) dredging work for navigational proposes);
- feeding the beaches updrift (south) the groynes field of Costa da Caparica, between the groynes of Costa da Caparica and updrift (south) of S. João beach;
- placement sites should always be located updrift (south) of the areas to be nourished in order to allow natural modelling by sea action, moving the sand and reconfiguring the beach. The S. João beach dunes will also benefit from this operation.
Effects and lessons learned
Coastal erosion is a very serious problem and probably will increase as a response to the continuous weakening of the fluvial sediment sources, dynamic changes, the mean sea-level rise, increasing human waterfront settlements and other problems.
In this area, as was described, erosion problems are very serious and have great repercussions in terms of property losses and socio-economic impacts. For this reason, it is important to defend the area with soft or hard interventions that can reduce the erosion rate.
This coastline is evolving and if nothing is done or interventions are not carefully planed, it is possible that major repercussions will occur in the future. The urban area must be restricted to where overwash and flooding risks are nil. It is not rational to expand the construction of buildings or beach supports to areas where there is a high risk of destruction, after building sea defences to protect these infrastructures.
Urban expansion along vulnerable coastlines must be stopped. Local authorities and urban planners cannot continue to ignore the medium/long-term physical dynamics and consequent constraints.
For approximately 30 years, from 1972 until 2000, with the help of the groyne field and the seawall the beach and the coastline were more or less stable. This is enough time for people to forget what happened in the recent past and to build restaurants and beach user facilities in this area.
The sea defences built in 1972 have had an important role in Costa da Caparica defence. The seawall was able to stop Costa da Caparica from flooding, but the now degraded groyne field was not able to recover the beach configuration. This defence work has had an important role in the understanding and knowledge of the zone dynamics. It can be considered as a field experience that demonstrates that the groyne length was too short for sand retention. This experience provided these structures as the base for the recently presented solution for the area.
In 2000/2001 severe and persistent sea actions occurred, where it was shown that the area is vulnerable and that the destruction of the sea defences is a possibility. It is essential to always remember that the area is very unstable, and there is a great probability of change with extreme physical dynamics.
The capacity to forecast the medium and long-term beach evolution continues to be very limited due to scientific constraints. Apart from this limitation, the inadequacy of field data (namely topo-hydrographic studies) is a major obstacle for the quantification, understanding and forecasting of the phenomena. It is important to implement a monitoring plan capable of recording data, and to improve the understanding and comprehension of the dynamic processes in the area.
The quantity of sand needed for artificial sand nourishment is very large (up to two million cubic meters), therefore, using just locally available amounts is out of the question (e.g., using ripping techniques as used in emergency interventions). The sand to be used will be from off-shore or sediments dredged for harbour maintenance, provided that it has the right quality for this purpose. It is important that the artificially introduced sand is similar to what is on the beach, in terms of size, colour and quality.
- Veloso Gomes, F.; Taveira Pinto, F.; das Neves, L. & Pais Barbosa, J. (2006). EUrosion - A European Initiative for Sustainable Coastal Erosion. Pilot Site of River Douro - Cape Mondego and Case Studies of Estela, Aveiro, Caparica, Vale do Lobo and Azores, 317 pp. ISBN 972-752-074-x.
- Mota Oliveira, I. (1973). Inlets. Contribution for the Study of their Equilibrium Conditions, PhD Thesis, IST, Lisboa (in Portuguese).
- Oliveira, E.M.; Fortunato, A.B.; Fortes, J.; Silva, L.G.; Vicente, C. & Pereira, M.C. (1999). Protecção do Farol do Bugio contra a acção das ondas e correntes, Os Estuários de Portugal e os Planos de Bacia Hidrográfica, Ed. Associação Eurocoast - Portugal, pp. 181-201 (in Portuguese). ISBN 972-85558-06-6
- Abecasis, F. (1997). Caracterização Geral Geomorfológica e Aluvionar da Costa Continental Portuguesa, Colectânea de Ideias sobre a Zona Costeira de Portugal, Associação Eurocoast-Portugal, G. Soares de Carvalho, F. Veloso Gomes and F. Taveira Pinto, Porto, Portugal, pp. 9-24.
- Barceló, J.P. (1971). Experimental Study of the Hydraulic Behaviour of Inclined Groyne Systems, LNEC, Ministry of Public Works, Lisbon.
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