Coupling Monitoring and Mathematical Modelling of Beaches to ...

Journal of Coastal Research

27

6A

104–115

West Palm Beach, Florida

November 2011

Coupling Monitoring and Mathematical Modelling of Beaches to Analyse a Problem of Harbour Sedimentation: Case Study Filipa S.B.F. Oliveira and Paula M.S. Freire

www.cerf-jcr.org

Laborato´rio Nacional de Engenharia Civil Avenida do Brasil 101 1700-066 Lisboa, Portugal [email protected]

ABSTRACT OLIVEIRA, F.S.B.F. and FREIRE, P.M.S., 2011. Coupling monitoring and mathematical modelling of beaches to analyse a problem of harbour sedimentation: case study. Journal of Coastal Research, 27(6A), 104–115. West Palm Beach (Florida), ISSN 0749-0208.

www.JCRonline.org

This article addresses the problem of sedimentation at the entrance of a harbour by evaluating and understanding the sediment dynamics in the adjacent beaches. The results of the methodology applied to acknowledge the beaches’ sediment dynamics were used to diagnose the problem’s cause and to interpret its evolution. The methodology includes analysis of data from a monitoring programme and process-based mathematical modelling of the alongshore and crossshore beach dynamics. The integration of both allowed the authors to investigate the hydromorphological behaviour of the harbour-adjacent beaches and to conclude that (i) the harbour and adjacent beaches are a single morphological system, and thus require integrated management; (ii) the study area is exposed to a seasonal wave regime, which induces a local sediment transport pattern and consequently the main seasonal morphological characteristics of the study area; and (iii) the process of sand accumulation at the harbour entrance is irreversible without human intervention. Because harbours should be designed and constructed based on two criteria—capacity of depth self-maintenance and integration, with minimum impact on the local morphodynamics—this study highlights the need for monitoring and identifying the total extension of the active beach, particularly in coastal environments with seasonal hydromorphological variations, before deciding on harbour layout relative to the sedimentary littoral transit.

ADDITIONAL INDEX WORDS:

Morphodynamics, coastal seasonality, Portuguese coast.

INTRODUCTION Harbours are indispensable for shipping and fishery activities. These maritime infrastructures have large economic, environmental, and social impacts on the local populations, but they also have increasing worldwide importance due to their role in the globalization process. Presently, 67% of external trading in Portugal is done by sea (Fortes et al., 2006; Oliveira, 2004). Commonly, harbours have associated problems, like contamination of bodies of water by metals and organic compounds and frequent dredging to maintain minimum depth for safe navigation. Dredging operations are presently the most applied solution for maintenance of desirable depth in harbours with sedimentation tendency. However, more self-sustainable solutions have been proposed: (i) solutions that contemplate change of the water exchange conditions within the harbour by implementing a new structure (e.g., a current-deflecting wall) whose layout can influence currents and sediment transport and can lead to a reduction of sedimentation (Kuijper et al., 2005) and (ii) solutions in which harbour layout was designed

DOI: 10.2112/JCOASTRES-D-09-00075.1 received 1 July 2009; accepted in revision 24 February 2010. ’ Coastal Education & Research Foundation 2011.

not only to shelter the harbour from incident wave regime but also to optimize the bypass of the littoral drift (Brøker, 2008). ˆ ncora harbour, located on the north coast The Vila Praia de A of Portugal, was constructed from March 2002 to November 2003 for sheltering small fishery vessels. A few months after its completion, the harbour was not accessible at low tide conditions due to sedimentation at its entrance. To decide on the best solution to guarantee the harbour permanent accessibility, taking into account the possible effects on the morphological evolution of the adjacent beaches, the port authority needed to diagnose the causes of the sedimentation and predict its evolution (permanence, natural reversibility, or aggravation). The relevant research issues addressed in this study are the methodology applied to interpret the sedimentation tendency at the harbour entrance, a topic of international relevance for being ‘‘a critical element in the economic feasibility of a project,’’ as pointed out by Van Rijn (2004), and the particular characteristics of the sediment dynamics of the case study, governed by a strong meteooceanographic seasonality. The methodology applied is focused on the investigation of the hydromorphological behaviour of the adjacent beaches. It is based on high-quality data and application of suitable mathematical models. The models applied were two-dimensional-vertical (2D–V) process-based models. When comparing

Integration of Different Coastal Disciplines for Littoral Management

Figure 1.

105

Study area.

these with the possible application of two-dimensional-horizontal (2D–H) process-based models, the former had the following advantages: (i) the consideration of the beach phenomena of wave breaking and undertow current (which can be observed in the vicinity of the harbour entrance), (ii) an included hydrodynamic model (instead of having to combine the models), (iii) ease of application, and (iv) the possibility of longer-term simulations. Unfortunately, three-dimensional models are not yet at a stage of development suitable for simulations of the duration required in this analysis.

PHYSICAL SETTING Geomorphology ˆ ncora bay in northwest The study area is the Vila Praia de A Portugal. It is confined between two rocky headlands (Forte da Lagarteira to the north and Forte do Caõ to the south) and includes the harbour, constructed in the extreme north sector of ˆ ncora and Gelfa (Figure 1). The the bay, and two beaches: A ˆ ncora beach, extending 400 m, is limited in the north by the A ˆ ncora. The Gelfa harbour and in the south by the mouth of River A beach, extending 1700 m, is limited in the north by the mouth of

ˆ ncora and in the south by the Forte do Caõ headland. River A Although this coastal stretch is predominantly sandy, sparse rocky outcrops can be seen along the foreshore of the extreme south sector of the Gelfa beach. The stretch has a slightly curved development, particularly at the extreme north sector, which ˆ ncora. Its main alignment is NNE– includes the mouth of River A ˆ ncora is SSW. The two beaches have distinct backshores: A limited by a revetment, followed by village frontage, and Gelfa is limited by a dune field covered by vegetation. The average annual ˆ ncora is 3.2 m3?s21. However, during the discharge of River A months of June, August, and September, natural closure of the river mouth, with sand mobilized from the beach foreshore, happened several times. Due to the existence of a small dam, built for agricultural purposes, 900 m from the river mouth, the volume of sediment that reaches the river mouth is not significant (Impacte, 2000); thus, the river was not considered as a source of sediment in this study. Besides the construction of the harbour, the main geomorphological change in the last two decades was erosion of the foredune in the north sector of the Gelfa beach during an exceptional storm event in January 1990. This episode caused dune slumping, overwashing, and breaching, which resulted in the lowering of 8

Journal of Coastal Research, Vol. 27, No. 6A, (Supplement), 2011

106

Oliveira and Freire

to 9 m of the dune crest within 200 m of dune width. The dune was later protected with an ancient fencing system technique and wood paths suspended in woodpiles (oblique photograph 2 in Figure 1) to avoid human traffic and damage and to protect the dune from wind erosion, since without vegetation the dune surface became more vulnerable to wind action.

Wave Climate The time series of wave climate data (parameters of height, period, and direction) analysed and used within the scope of the present study concerns the period from October 2001 to April 2006, which from now on will be designated as the study period, the same as the monitoring period. These data occurred at a nearshore point, in front of the study area, at 10 m below chart datum (CD), which is 2 m below mean sea level (MSL). The wave regime is characterised by two distinct maritime seasons: the maritime winter, from 1 October to 31 March, and the maritime summer, from 1 April to 30 September. Thus, the study period contemplates five maritime winters and four maritime summers, from now on designated as winters and summers. During the winters, the highest waves occurred predominantly from WNW, the directional sector with a higher frequency of occurrence for most of the winters (Figure 2). In opposition, during the summers, the highest waves were not distinctively associated to any particular directional sector of incidence, and the directional sector NNW had the highest frequency of occurrence in each of the four summers. During the study period, interannual variations of the wave regime occurred, particularly in the winters. In the winters of 2003 and 2004, there was a lower frequency of occurrence of the highest waves (with a root-mean-square wave height above 3 m) from the directional sector WNW than in the other 3 years. During the last three winters analysed, there was a higher frequency of occurrence of the highest waves in the directional sector NNW than in the 2 previous years. In the winter of 2004, atypically, a very high frequency of waves occurred from the farther north directional sectors. Although less significant than in the winters, the interannual variations that occurred in the summers of the study period were (i) the exceptional predominance of waves incoming from the directional sector NNW in 2004 and (ii) the higher frequency of occurrence of the highest waves in the directional sector NNW in 2005 (Figure 2).

Sediments ˆ ncora and Gelfa beaches are The sediments of the A predominantly medium and coarse sands. At the extreme south sector of the Gelfa beach, near the rocky outcrops, a gravel deposit is present in the lower part of the beach face. Fine sediment fraction (silt and clay) can be found near the mouth of ˆ ncora, not exceeding 2% of the total sample. the River A

METHODOLOGY The study area was monitored based on a programme started in October 2001 and initially planned to end in April 2004. This monitoring programme included topohydrographic and sea

Figure 2. Wave regime by maritime season at 12 m below mean sea level in front of the study area.



Figure 3.

107

Location of cross-shore beach profiles.

conditions (wave regime) surveillance. However, because the sedimentation of the harbour entrance started to be noticed immediately after the harbour’s completion, the topohydrographic surveillance was continued until April 2006. Surface sediment samples were collected along the beach face, at MSL, in different cross-shore beach profiles (Figure 3). Sampling was performed on two dates in October 2001 and June 2004. For each sample, gravel and fine fraction content were obtained and sand grain size analysis was performed. The grain size distribution parameters—median diameter, D50, and pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi geometrical spreading, D84 =D16 —were evaluated. The methodology applied in this study integrates results from two complementary analyses, which were executed simultaneously. The first analysis was of the morphological evolution of the beach, based strictly on the processing, manipulation, and interpretation of the results of the monitoring programme. The second analysis was process-based mathematical modelling of the alongshore and cross-shore beach dynamics, which, using results from the first analysis as input and control conditions, allowed the authors to simulate with continuity the evolution of the beach hydrodynamics and sediment transport conditions and thus to complete the interpretation and understanding of the hydromorphological behaviour of the study area obtained from the first analysis.

The first analysis included (i) analysis of the time and space variation of the sand grain size characteristics (mean diameter and geometrical spreading) to understand the eventual changes of sediment sources and sediment dynamics; (ii) analysis of the geometrical variation of the waterline at MSL to help understanding of the alongshore mass oscillations within the bay; (iii) analysis of several beach profiles, distributed uniformly in the alongshore extension of the study area, and their evolution to conclude the cross-shore sand volume oscillations; (iv) comparison of digital terrain models generated from topohydrographic surveys to identify areas of erosion and deposition and their respective rates; and (v) investigation of relationships between extreme wave events and morphologic changes. The second analysis included (i) calculation of the surf zone hydrodynamics induced by the time series of wave climate data imposed at the offshore boundary of the study area; (ii) identification of seasonal tendencies and interannual variations of the longshore transport induced by the time series of wave climate data; (iii) cross-shore distribution of the longshore transport during the study period; (iv) evaluation of the impact of the longshore transport on beach morphology during the study period; and (v) evaluation of the impact of maritime storms (erosion episodes), when significant morphological changes occurred in a short period, on beach morphology.


108

Figure 4.

Oliveira and Freire

ˆ ncora and Gelfa beaches. Sampling dates: October 2001 (in black) and June 2004 (in grey). Sediment grain size characteristics of the A

The mathematical modelling of the sediment dynamics of the beach was executed through the application of the three processbased numerical models Litdrift, Litline, and Litprof of the wellknown (among the coastal modelling community) software

Figure 5.

package Litpack (DHI, 2005), which had already been successfully applied in other coastal stretches near the study area (Larangeiro and Oliveira, 2005; Larangeiro, Oliveira, and Freire, 2003; Oliveira, Freire, and Larangeiro, 2002; Silva et al., 2004).

Variation of the waterline at mean sea level.



Figure 6.

Variation of the beach width at mean sea level.

RESULTS Monitoring Programme Sedimentary Evolution ˆ ncora and The sediment grain size characteristics of the A Gelfa beaches are relatively constant in space and time (Figure 4): the median diameter varies between 0.4 and 0.6 mm and between 0.3 and 0.7 mm, respectively, at the ˆ ncora and Gelfa beaches, and the geometrical spreading varies A between 1.3 and 1.4 and between 1.3 and 1.7, respectively, at ˆ ncora and Gelfa beaches. Low values of geometrical the A spreading, which correspond to better-sorted sediments, are mainly associated to sediments with lower median diameter.

Morphological Evolution The alongshore geometrical variation of the waterline at MSL during the monitoring period shows no consistent seasonal pattern (Figure 5). However, a general pattern of the alongshore sediment net transferences between the two extremes of the coastal stretch can be deduced: towards SSW during summer and towards NNE during winter.

Figure 7.

109

During the study period, a generalized enlargement of the ˆ ncora and Gelfa beaches occurred. It was well expressed by A the increasing of the beach width at MSL, with a maximum ˆ ncora beach (profile C) and 124 m at the value of 168 m at the A Gelfa beach (profile 17 in Figure 6). The cross-shore morphological evolution of several beach profiles, distributed uniformly in the alongshore extension of the study area, was consistent with the seasonal variations of the wave regime: sediment transference from the beach face to the surf zone occurs during high-energy conditions, and the opposite takes place during low-energy conditions. In the ˆ ncora beach, the natural migration of the River A ˆ ncora mouth A originates high variability of the cross-shore morphology. During the monitoring period, in February 2003, a storm event had consequences in the north sector of the Gelfa beach, particularly at profile 14, with retreat of the beach face at MSL of 26 m and lowering of the beach backshore of 4 m. Alongshore variability of the seasonal morphological evolution is patent in the Gelfa beach, where a contrasting evolution can be seen between the northern and the southern extremes: the south sector is relatively stable, probably due to the presence of the rocky outcrops (profile 5); in the north sector,

Examples of the morphological evolution of the Gelfa beach cross-shore profiles.


110

Oliveira and Freire

Figure 8. Comparison of digital terrain models generated from topohydrographic surveys: (a) April 2004 with end of summer 2004, (b) November to December 2005 with April to May 2006, and (c) April 2004 with April to May 2006.

significant cross-shore variation is observed (profile 17 in Figure 7). The comparison of the beach cross-shore morphology at two similar energetic conditions, in April 2004 and in May 2006, shows that a general deposition occurred between the CD level and 4 m below CD at the north sector of the Gelfa beach, adjacent to the harbour mouth (profile 17 in Figure 7).

The comparison of digital terrain models, generated from topohydrographic surveys, shows significant morphological changes of the subaerial beach and at depths between CD and 5 m below CD. A relationship between the seasonality of the wave regime and the distribution pattern of the erosion and deposition areas is unequivocal:



Figure 9.

111

Seasonal longshore transport at a central cross-shore transect.

(1) During the summer of 2004, a shoal, with longitudinal development from the mouth of the harbour towards south, was formed at depths near CD. Erosion of the ˆ ncora beach and accretion of the subaerial part of the A subaerial part of the Gelfa beach, particularly in the south sector, also occurred (Figure 8a). (2) During the winter of 2005, accretion in the surroundings of the harbour mouth and along depths near CD increased, and the shoal migrated to west and south, probably due to the displacement of material from the subaerial part of the Gelfa beach to higher depths (Figure 8b). General accretion near the harbour entrance, with the formation of the shoal along depths near CD, is well illustrated by the comparison between the morphology at the end of the winter of 2004 and that at the end of the winter of 2006 (Figure 8c). The rate of deposition in the entrance of the harbour, between April and November 2004, was 0.7 m?y21.

Mathematical Modelling Longshore Transport The cross-shore distribution of the longshore sediment transport (in both modes, bed load and suspension) during the nine consecutive maritime seasons of the study period was calculated through the simulation of the action of each of the 6month time series of wave climate data over the beach profile (representative of the respective season) measured in a crossshore transect in the central part of the beach. The results show a clear seasonal pattern that is in agreement with the morphological monitoring data: predominance of sediment transport towards NNE during winter and towards SSW during summer (Figure 9). The estimated average seasonal longshore transport was 460 3 103 m3?y21 in the NNE direction and 370 3 103 m3?y21 in the SSW direction for the five winters and 100 3 103 m3?y21 in the NNE direction and 420 3 103 m3?y21 in the SSW direction for the four summers. Taking into account the main alignment of the coastline, these results agree with the seasonal characteristics of the wave regime described earlier in the Wave Climate section. The interannual variations of the wave regime that occurred during the study period, mainly during winter, had repercussions in the longshore transport. The net transport in the

winters of 2003 and 2004 was directed towards SSW, in opposition to the predominant tendency. In the winter of 2005, the net longshore transport recovered its direction, towards NNW, but the total transport was well distributed in both directions (Figure 9). During winter, the extension of the active zone of the submerged beach and the closure depth (both relative to the longshore sediment transport) are considerably higher than during summer. The values of these two parameters, which characterise the submerged zone of the beach where significant sediment transport occurs, can double from summer to winter, as occurred in the winter of 2002, when the extension of the active zone reached about 850 m and the closure depth 29 m CD (11 m below MSL) (Figure 10). The beach profile varies within the maritime season, but in the calculations of the longshore transport it was considered constant for each season. As already mentioned, it was used an observed profile representative of the respective prevailing characteristics in summer and winter. The effect of the profile geometry in the estimation of the longshore transport was investigated. The results obtained from the simulation of the action of the 6-month time series of wave climate data of the winter of 2003 over the beach profiles measured consecutively in the summer of 2003 and winter of 2003 show that the differences in the profile geometry cause significant variations in the cross-shore distribution of the longshore transport.

Figure 10. Beach morphological characterisation parameters at a central cross-shore transect.


112

Figure 11.

Oliveira and Freire

Cross-shore distribution of the longshore transport: (a) net and total and towards (b) NNE and SSW.

Comparing the transport curves generated with both profiles, it can be observed that the typical morphology of the winter profile induces the following (Figure 11): (1) The starting point of the submerged active zone of the beach farther offshore than in the summer profile. The

extension of the submerged active zone and the closure depth are 710 and 27.2 m CD, respectively, for the winter profile and 698 and 26.9 m CD, respectively, for the summer profile. This is due to the submerged bar in winter (formed from erosion of the beach face), which induces breaking of the higher waves farther from the shore;



Figure 12.

113

Example of verification of the evolution of the numerical waterline at mean sea level.

(2) Higher gradient of the longshore transport than in the summer profile. (3) Higher peaks of transport than in the summer profile. (4) Location of the transport peak closer to the shore farther offshore than in the summer profile Concerning the integral of the transport in the submerged active zone of the profile, the results show that the typical geometry of the winter profile induces higher transport than the summer profile: about 20% more in the NNE direction and 24% more in the SSW direction.

Coastline Evolution The evolution of the waterline at MSL during the study period was numerically simulated to verify the impact of the time series of wave climate data on the beach planform. Geomorphological characteristics (initial waterline, beach profile geometry, and beach sediment size), the time series of wave climate data, and boundary conditions at the northern and southern extremes of the beach were imposed as initial, forcing, and calibration conditions in the numerical setup. The evolution of the waterline at MSL was calculated relative to a baseline with a N–S orientation and a resolution spacing of 20 m. The numerical simulations show that the longshore sediment dynamics had an impact, although small, on the variation of the emerged beach width. At the southern extreme of the Gelfa beach, the results show clearly that the beach width was maximum and minimum by the end of summer and end of winter, respectively. The numerical waterlines were compared

with the waterlines extracted from the digital terrain models produced based on the topohydrographic surveys. They show good agreement in the south zone but not in the north zone (Figure 12) at the river mouth, where the disagreement is particularly large in the last two winters of the study period (2004 and 2005). The reasoning for this disagreement is, in the authors’ opinion, the limitation of the numerical model. As a one-line model that uses a profile-type approach, it approximates high longshore gradients of the variables that characterise the nearshore processes; i.e., the model does not realistically simulate the sediment dynamics in areas where the hydrodynamic processes are predominantly bidimensional (e.g., due to irregularities of the bathymetry, not parallel to the coastline). Thus, because the coastal stretch under analysis is slightly curved, particularly at the northern extreme (at the river mouth), the results ought to be evaluated cautiously.

Short-Term Cross-Shore Dynamics The cross-shore morphodynamic evolution during the most severe (the highest incident wave height) short-term storm event of the study period was simulated to estimate data that, generally, are never registered due to the lack of safety conditions for field workers and technical equipment. The objective was to estimate the sediment transport rates across the beach profile, the retreat of the beach face, the volume of sediment eroded from the foreshore and transferred to the nearshore, and the location of deposition of this sediment. The results revealed the following:


114

Figure 13.

Oliveira and Freire

ˆ ncora beach under a maritime storm event. Cross-shore morphological evolution at the A

(1) Under a highly energetic event (i.e., an erosion episode), ˆ ncora beach is more vulnerable than the Gelfa the A beach. The volume of sand mobilized (i.e., extracted from its initial position) in the Gelfa beach is 84% and 63% of ˆ ncora beach for the winter the volume mobilized in the A and the summer profiles, respectively. (2) The impact of a storm event is higher over the winter profile (i.e., if the storm event occurs over an already eroded profile) than over the summer profile in the entire stretch. The harm concerns not only the volume of sediment transferred from the beach face towards an ˆ ncora beaches offshore area, which was in the Gelfa and A 30% and 7%, respectively, higher in the winter profile than in the summer profile, but also the modifications of the beach morphology. The action of a storm episode over the winter morphology causes not only the retreat of the beach face but also the displacement of the submerged bar to higher depths. This process is particularly ˆ ncora beach, where the volume of significant at the A sediment removed from the submerged bar was higher than the volume removed from beach face (Figure 13). (3) The sediment, which is eroded from the beach face and the submerged bar and transported in the offshore direction until it reaches the lower part of the beach profile, accumulates until a depth 27 and 25 m CD in the ˆ ncora and Gelfa beaches, respectively. The closure A depth regarding the cross-shore transport is higher for the winter profiles (to which the values mentioned are related) than for the summer profiles.

DISCUSSION The temporal stability of the grain size characteristics is in agreement with the embayed nature of this coastal stretch, and it suggests the absence of drastic changes in sediment sources and sediment dynamics. The most efficient factor in the generation of longshore transport is distinct in both seasons. In winter, the longshore

transport is mainly due to the amplitude of the waves. They reach the coastline more parallel to the shore than during summer but with higher height. In summer, the longshore transport is mainly due to the wave obliquity. The test of the effect of the beach profile geometry in the longshore transport described earlier in the Mathematical Modelling section gives information on the error introduced in seasonal longshore transport due to considering the calculations over a static profile, which, although representative of the prevailing seasonal morphological characteristics, does not take into account the variability of the profile during the season. Because some of the winter profiles used do not show a preponderant offshore bar, it is likely that they are a transition between a dissipative and a reflective profile. Thus, it is legitimate to derive that (i) the extension of the submerged active zone of the beach is higher than the maximum value obtained, 850 m; (ii) the closure depth, concerning the longshore transport, is higher than 29 m CD; and (iii) more sediment transport occurs with higher frequency between distances 300 and 450 m from the waterline at MSL. These findings, derived from the analysis of the cross-shore distribution of the longshore sediment transport, agree with the observed seabed geometry changes and reveal that the harbour entrance is clearly in the beaches’ submerged active zone. During the study period, there was no consistent pattern of seasonal evolution of the beach planform, i.e., a cyclic retreat and advance of the coastal stretch extremes corresponding to a longshore displacement of sand volume. Despite this result (obtained from the comparison of the topohydrographic surveys), a general tendency for net transport was observed towards the south during the maritime summer (from April to September) and towards the north during the maritime winter (from October to March). This apparent inconsistency is in agreement with the interannual variation of the wave regime observed in the study period, during which two atypical maritime winters were identified. There is a significant amount of seasonal cross-shore transport (i.e., sand transference between the beach face and



the surf zone, and vice versa) based on comparison of the profiles surveyed. Mathematical simulations of the short-term profile evolution under storm events (i.e., erosion episodes) ˆ ncora beach is more vulnerable (i.e., suffers reveal that the A more changes) than the Gelfa beach under highly energetic wave action. This can be explained by the relation between the direction of the incident storm wave and the shore orientation (during storm episodes, the wave’s incoming direction is ˆ ncora beach) and the geometry normal to the shoreline at the A of the beach profile (independently of the maritime season, the ˆ ncora beach than at the Gelfa beach face slope is milder at the A beach). Moreover, the rocky outcrops at the SSW extreme of the Gelfa beach should play an important role in the reduction of the cross-shore transport because they provide a platform of energy dissipation where no sediment is mobilized. Based on these findings, it can be deduced that during the maritime summer, under milder wave conditions that cause predominant onshore transport, part of the sand accumulated at the entrance of the harbour and surrounding area (until 29 m CD, as the results show) during the previous season is pushed and maintained into the interior of the harbour.

CONCLUSIONS A methodology that couples monitoring and mathematical modelling was applied to investigate the coastal dynamics in the bay and thus diagnose the cause and predict the future evolution of the sedimentation tendency observed at the ˆ ncora Harbour. The results obtained entrance of Vila Praia de A allowed the authors to conclude the following: (1) The sedimentation process occurs mainly during the maritime winter, when the longshore transport, which occurs to 29 m CD (to deeper water than the harbour entrance), is predominantly towards north. Simulta neously, there is transference of sediment from the beach ˆ ncora beach face to deeper water than 25 m CD for the A (again, deeper water than the harbour entrance). (2) During the maritime summer, when the longshore transport is predominantly towards south (from the ˆ ncora to the Gelfa beach) and sand is transferred to A the beaches’ face (incoming from a greater depth), the volume of sand that was accumulated at the harbour entrance during the previous maritime winter is not mobilized because the harbour breakwater has a shel tering effect at the harbour entrance, where through diffraction the accumulated sand tends to be pushed into the harbour interior. These two conclusions, which summarise the main characteristics of the hydromorphological behaviour of the study area, led to the final conclusion that the process of accumulation of sand at the entrance of the harbour is not reversible without human intervention. Two alternative types of intervention could be recommended to maintain the harbour access: (i) a programme of dredging the harbour entrance and repositioning the sand volume in the bay’s southern extreme (i.e.,

115

temporary removal of the sand from the harbour entrance but guarantee of its maintenance within the beach system) or (ii) modification of the harbour layout (structural intervention), involving extension of the southern jetty to deeper water to keep the harbour entrance away from the beaches’ submerged active zone. The recommendation arising from this case study is that harbour layout should be designed while bearing in mind not only its function of protecting against wave attack and its possible impact on the coastal dynamics of the adjacent beaches but also its positioning relative to the sedimentary littoral transit.

ACKNOWLEDGMENTS The authors thank their colleagues in the NPE division for the transference of the offshore wave regime and Instituto Portua´rio dos Transportes Marı´timos for authorizing the publication of this study.

LITERATURE CITED Brøker, I., 2008. The use of advanced numerical models to support the design of coastal structures. Revue Europeeńne de Geńie Civil, 12(1–2), 67–86. DHI (Danish Hydraulic Institute), 2005. Litpack: Noncohesive Sediment Transport in Currents and Waves—User Guide. Denmark: DHI. Fortes, C.J.; Santos, J.A.; Clı´maco, M., and Oliveira, I.M., 2006. Some aspects of port and coastal engineering in Portugal. 2nd Seminar and Workshop in Ocean Engineering (Rio Grande, Brazil), CDROM [in Portuguese]. Impacte, 2000. Maritime Infrastructures of the Fishery Harbour of ˆ ncora: Study of Environmental Impact. Lisbon, Vila Praia de A Portugal: Instituto Maritimo-Portua´rio, 220p [in Portuguese]. Kuijper, C.; Christiansen, H.; Cornelisse, J.M., and Winterwerp, J.C., 2005. Reducing harbor siltation. II. Case study of Parkhafen in Hamburg. Journal of Waterway, Port, Coastal & Ocean Engineering, 131, 267–276. Larangeiro, S.H.C.D. and Oliveira, F.S.B.F., 2005. Application of formulations and models to estimate the longshore sediment transport between Praia da Vieira and Praia Velha, west coast of Portugal. In: Proceedings of the First International Conference on Coastal Conservation and Management in the Atlantic and Mediterranean (Tavira, Portugal), pp. 433–441. Larangeiro, S.H.C.D.; Oliveira, F.S.B.F., and Freire, P.M.S., 2003. Longshore sediment transport along a sandy coast with hard rock outcrops. Shore and Beach, 71, 20–24. Oliveira, F.S.B.F.; Freire, P.M.S., and Larangeiro, S.H.C.D., 2002. Characterisation of the dynamics of Figueira da Foz beach, Portugal. Journal of Coastal Research, Special Issue No. 36, pp. 552–563. Oliveira, I.B.M., 2004. Forty years of coastal engineering in Portugal. In: Proceedings of the 29th International Conference on Coastal Engineering (Lisbon, Portugal, ASCE), pp. 3–39. Silva, R.; Silva, A.J.; Rusu, E.; Oliveira, F.S.B.F.; Larangeiro, S.H.C.D., and Taborda, R., 2004. Evaluation of the longshore current for a sector of the Portuguese west coast: application of different methodologies. In: Proceedings of the 29th International Conference on Coastal Engineering (Lisbon, Portugal, ASCE), pp. 1455–1467. Van Rijn, L.C., 2004. Estuarine and coastal sedimentation problems. In: Proceedings of the 9th International Symposium on River Sedimentation (Yichang, China), pp. 105–120.
