Experiences with beach nourishments in Portugal

Case study Vale do Lobo

This article provides an example of beach nourishment at Vale do Lobo beach, which is located in the Algarve in Portugal. The information gathered in this section features in Veloso Gomes et al. (2005) . The articles describes the area, the erosion problem and discusses possible solutions. For definitions of coastal terms, see the article definitions of coastal terms. For information on different types of nourishment, see shore nourishment.

General description of the area

Figure 1: Red sand cliffs (source: DRAGAPOR - Dragagens de Portugal, S.A.).

Vale do Lobo beach is located on the south Portuguese Coast at Quarteira, between Albufeira and Faro in the district of Loulé, about 5km east of Vilamoura.

Geomorphology

The cliffs of Vale do Lobo beach are almost vertical formations of red Pleistocene sands (see also Figure 1). This shoreline stretch is characterised by a set of high cliffs, composed of highly fractured formations with weak cohesion, and a narrow subjacent beach. The base of the cliffs is often hit by breaking waves, especially in winter. The width of the subjacent beach is the main factor on which the evolution of the cliffs depends, as it is the main protection against waves breaking directly on the cliffs.

Physical processes

Along the south Portuguese coast the sea currents generated by the wind or the tides are negligible in relation to the drift currents generated by the waves. The breaking wave currents are comparatively low in the intervention area, in the range of 0.25 to 0.5m/s. On the downdrift area wave breaking currents can reach values of around 2m/s.

The typical values for tides are the following (INAG, 1997) :

• Average tide level: +2.00m (HZ);
• Spring tides:
  • Maximum high tide: +3.75m (HZ);
  • Minimum low tide: +0.40m (HZ);
  • High tide: +3.35m (HZ);
  • Low tide: +0.70m (HZ);
• Neap tides:
  • High tide: +2.65m (HZ);
  • Low tide: +1.40m (HZ);

The dominant wave climate directions are in the W–SW quadrant (~68%), with a low probability around the E–SE quadrant. Wave heights are lower than 2m in approximately 90% of the cases, of which the majority are lower than 1m. Even so, 5 to 10% of the wave heights are between 2.5 to 3.5m. The typical wave period is around 5 sec, although there are some cases of periods reaching 13 sec and longer. The tide and wave values indicate that, in general, this is a mesotidal, wave-dominated coast.

Longshore transport is dominant along the shoreline, and is mainly wave induced. The dominant direction is from west to east. Aeolian transport is negligible.

The erosion problem

Type of coastal erosion

Beyond the decrease in beach widths, erosion on this stretch can also be noticed on the cliffs, which are also suffering a substantial retreat. The following three different processes dominate the cliff evolution of the Quarteira Region (Marques, 1997 ):

• Erosion due to run off;
• Ravining erosion;
• Landslide movements.

See also Rocky shore morphology and Cliff stabilisation..

Causes of erosion

Since 1974, when the construction of the Vilamoura Marina and the Quarteira coastal defences interrupted the longshore littoral drift, coastal erosion has significantly increased downdrift (Marques, 1997). This erosion reached Vale do Lobo beach between 1980 and 1983 after a retreat of approximately 30m between 1976 and 1980 at the New Fort cliff, updrift of Vale do Lobo. The erosion threatened the Vale do Lobo Resort and motivated construction of coastal defences in 1983/84.

After the intense erosion period of the seventies the retreat rates diminished to values of around 0.6-0.7 m/year (between 1990 and 1996). These values were slightly higher than the ones registered in the fifties and sixties before the construction of the sea works, suggesting that the littoral system has reached a situation of near equilibrium after two decades of changing depths (Marques, 1997).

Also pointed out as being responsible for increasing the vulnerability to erosion of these cliffs is golf course watering. Water load due to watering increases the probability of landslides.

Problem description

The erosion in Vale do Lobo is mainly a direct consequence of human activities. The cliff has been retreating further and further, becoming a danger to people and buildings. The cliff retreat can be easily observed all along the shoreline, and is mainly a result of:

• beach narrowing due to the updrift interception of littoral sand transport at Quarteira, Vilamoura Marina and other defence structures;
• the rill and gully erosion after heavy storm episodes;
• golf course and garden watering.

Solutions/measures

Policy Options

This is a tourism area of major importance, with a significant built-up structure that has to be protected despite its high vulnerability. Because of this, the adopted policy option is clearly to Hold the Line and there has been important public and private investment to attain this goal. However, there are some effects that cannot be satisfactorily controlled and it is therefore essential to adequately plan the use of this area, and to considerer managed realignment to a more inland location.

Strategy

The engineering option for the coastal defence of Vale do Lobo beach is essentially a soft one, complemented with hard measures over short extensions. The strategy is to maintain the natural beauty of the cliffs and wide beaches as much as possible since they are among the most valuable aspects of this region, even though the need for coastal protection is inevitable. The maintenance of beach widths will avoid more direct wave action on the cliffs.

Measures

Figure 2: Offshore dredging

The aim of the “Artificial Sand Nourishment of Vale do Lobo Beach – Algarve 97” project was to stabilize the coastal stretch of Vale do Lobo which had been showing pronounced, and in some cases irreversible, coastal erosion (INAG, 1997).

According to INAG (1997), the main goal of the programmed intervention was to create a “new” beach on the Vale do Lobo base, 2 or 3m above the existing level, extending for 1,400m and with a beach width growth of approximately 80m. The sand needed (~700,000 m³) for this operation was obtained by offshore dredging in a predefined zone.

This operation consisted of dredging 700,000 m³ of sand on the shoreline band frontal to the beach, and its propulsion to the beach through a floating pipe so that the designed beach platform could be created. The dredging area was located approximately 4km from the beach at about 20m depth. The dredging equipment was a trailing suction hopper dredger.

This operation started in October 1998 and finished in the first week of January 1999, when the desired volumes were reached.

In complement to the artificial sand nourishment operations the existing support works were reinforced, namely that protecting the swimming pool. In addition, rocks were placed at the foot of cliffs that clearly showed signs of instability. The drainage system was also improved.

The estimated cost of this operation was evaluated in 1997 at € 3.2 million, which would be partially supported by public and private investment. There is still an unresolved judicial litigation process between the Vale do Lobo beach resort management society and the Water Institute.

Effects and lessons learned

Figure 3: State of the beach before beach nourishment

Figure 4: State of the beach during the beach nourishment

Figure 5: State of the beach after beach nourishment

There is no information regarding the current situation of Vale do Lobo beach since no monitoring has been done since the artificial sand nourishment finished in January 1999.

As previously mentioned, there is a judicial litigation process between the Vale do Lobo beach resort management society and the Water Institute about the volume of sand placed during the artificial sand nourishment operations. A short time after completing the Vale do Lobo beach nourishment work, sea actions removed a great amount of the sand from the beach, causing the resort society to conclude that the work was not finished – contrary to the Water Institute's point of view.

Taking the position that the work was not finished because the volume of sand remaining on the beach after the first storm event was less than predicted for the project, the managing society of Vale do Lobo beach resort decided to interrupt their part of the payment. The judicial litigation on this matter continues. Because of this dispute the planned nourishment reloads and execution of artificial dunes over the nourished beach were never carried out. The Vale do Lobo beach resort society had agreed to bear the total responsibility for the work, and this would have permitted the maintenance of a good beach area.

The long term effects of artificial sand nourishment of Vale do Lobo beach, in terms of stopping the existing erosion, can only be achieved if new reloads are made periodically. Indeed, the effect of a nourishment operation is temporary and even though it has proven to be effective in the sense of slowing down the process of cliff erosion it is necessary to be aware that this is an exposed area – on an open coast – thus the frequency in which new interventions will be needed is always going to be an essential aspect to consider.

In effect, as was proven a short time after the nourishment operation in 1999, the high erosive strength of the sea can easily remove a large part of any sand volume placed artificially on the beach. Therefore, it is expected that due to the severe wave climate the need for new reloads will be frequent, thus increasing the costs.

The situation of Vale do Lobo beach is similar to others happening along the entire Portuguese coast: there is significant capital at risk due to coastal erosion, and there is a demand on the government for coastal defence measures from owners of the threatened properties. However, the motivating interests are mostly private, and it is, therefore, unfair to expect that the taxpayers should pay for this work without any benefit for society as a whole. On the other hand, this work is too expensive to be supported by private funds alone, due to the required frequency for sand re-nourishment.

Case study Costa de Caparica

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) .

Area discription

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, 22^th 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.

Physical processes

Transport agents

Wave climate

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”.

Tidal regime

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.

Sediment transport

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.

Sediment budget

Coastline variation

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.

Problem description

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 beach 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.

Solutions/measures

Policy options

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.

Related articles

• Shore nourishment
• Beach nourishment

References

The main author of this article is Luciana, Das Neves Please note that others may also have edited the contents of this article. Citation: Luciana, Das Neves (2022): Experiences with beach nourishments in Portugal. Available from http://www.coastalwiki.org/wiki/Experiences_with_beach_nourishments_in_Portugal [accessed on 24-11-2024] For other articles by this author see Category:Articles by Luciana, Das Neves For an overview of contributions by this author see Special:Contributions/Lpneves