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Érosion, enjeux et aménagements

Bathymetric impacts of a seawall on a micro-tidal beach, Gulf of Lions, France

Impacts d’une digue sur la bathymétrie d’une plage micro-tidale, golfe du Lion
Olivier Samat, François Sabatier and Adrien Lambert
p. 119-124

Abstracts

The analysis of bathymetric profiles opposite and on either side of a seawall built at the shoreline on a barred sandy coast indicates an increase in sediment losses and a deepening of the trough at the foot of the structure. A longitudinal pattern is observed in relation to the dominant littoral drift, suggesting that the impact of a seawall should be analysed both transversely and longitudinally.

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Index terms

Geographical index :

France, Golfe du Lion
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Author's notes

This work is a contribution to ORE RESYST (CNRS) and GIZCAM (MEDD) French programmes. The authors thank the PNRC (Parc Naturel Régional de Camargue) and SALINS for their financial support.

Full text

1 - Introduction

1In coastal engineering, it is common practice to use seawalls to protect the coast and limit storm floods (Basco, 2004). When these seawalls are constructed at the shoreline or in the breaker zone, their impact on bathymetric evolution remains poorly known. Indeed, some authors (Dean, 1986; Basco et al., 1992; Wiegel, 2002) consider that a seawall does not notably increase the erosion of the submerged part, except for local erosion at the foot of the structure. On the contrary, other authors highlight the negative role of seawalls in relation to a modification of bathymetry and/or an increase in longshore sediment transport that amplify the processes of erosion (Komar and McDougal 1988; Fletcher et al., 1997; Miles et al., 2001). In any case, there are very few field surveys around seawalls, which makes it difficult to draw any generalizations about their effectiveness or their impact on beaches. The more detailed types of study are based solely on topographic profiles that do not cover the breaker zone, and are essentially concerned with mesotidal beaches without bars (Tait and Griggs, 1990; Griggs et al., 1996; Basco et al., 1997).

2Following the major problems of coastal erosion and management of the littoral zone of the Camargue (Mediterranean Sea, France), this study presents the results of a bathymetric monitoring programme carried out in front of a seawall on a barred sandy coast in a micro-tidal setting. The objective is to provide new information on the erosional phenomena related to the presence of a coastal defence structure. We study the recent evolution (2000-2004) of this sector compared with the evolution of the bottom before the construction of the seawall in its current state in 1998.

2 - Presentation of the site

Fig. 1 - Site location, bathymetric survey and photograph of the seawall.
Localisation du site, des profils bathymétriques, et photographie de la digue.

Fig. 1 - Site location, bathymetric survey and photograph of the seawall.Localisation du site, des profils bathymétriques, et photographie de la digue.

3The studied site is located on the shoreline of the Rhone Delta (Mediterranean Sea, France). The beach is composed of fine sand (D50 = 0.2 mm) and forms a double or triple barred coast of the “Dissipative and Longshore-Bar-Trough” type according to the Wright and Short (1984) classification, with a net longitudinal transport predominantly towards the west. In this micro-tidal context (TR<0.3 m), the storms come from the SE quadrant (Hsig=3 m, T=7 s annual, Hsig=6 m extreme) and produce a net longshore transport from east to west (Fig. 1).

4The seawall now extends along a distance of 2.7 km, and before its construction in 1998, the beach was retreating at rates ranging between 3 and 8 m. yr-1 (Sabatier and Suanez, 2003) mainly due to an increase in the gradient of the longshore transport (Sabatier, 2001). However, a reduction in gradient farther west resulted in intense sedimentation (Beauduc spit). In reality, we could consider the seawall as a dyke before 1998 because it was then separated from the shoreline by a beach undergoing natural retreat (Fig. 2). Built in the 1970s, this dyke was originally intended to contain storm-related floods. Indeed, these floods posed a threat to the salt industry which uses evaporation basins (salt pans) situated behind the beach. Following the continuous retreat of the beach and a 50-year storm in 1997, the dyke was completely destroyed and then rebuilt the following year at the same site, in the form of a seawall (Fig. 2).

Fig. 2 - Historical evolution of the seawall.
Évolution historique de la digue.

Fig. 2 - Historical evolution of the seawall.Évolution historique de la digue.

3 - Methods

5We have monitored six lines of profiles since September 2000 (performing four surveys per year) in order to determine the impact of the seawall on the evolution of the bathymetric profiles (Fig. 1). Four lines are located in front of the seawall and the two others on either side. These two latter lines are assumed to represent « natural » profiles, and are used as a comparison with lines in front of the seawall. The profiles were carried out using a sounder (error in Z +/- 0.3 m) and a differential GPS embarked on board a rubber dinghy, and extend offshore to a distance of approximately 1 500 m to reach a water depth of 10 m. All the profiles were readjusted compared to the NGF (NGF = French sea-level datum) based on the tide-gauge data of the day, which were recorded at less than 3 km from the site. Between 1988 and 1999, annual bathymetric profiles were measured on lines G14, G15 and G16, but these surveys extend only 500 m offshore to a depth of approximately 4 m. In fact, the second bar was not systematically measured as a whole. These three lines of profiles are used here to compare the evolution of the bottom before and after construction of the seawall. The bathymetric profiles are used to calculate sediment budgets, the average migration of the shoreface bars, the depth of closure, and to evaluate scour at the foot of the seawall.

4 - Results

4.1 - Sediment budget of the profiles

6In order to determine if the seawall enhances the processes of erosion, we compare the sediment budgets of the profiles before (1988-1998) and after (1998-2004) the construction of the seawall. Since the profiles between 1988 and 1999 extend less far offshore than those measured after 2000, we compare the profiles on the part that is common to both periods (Fig. 3).

7The sector is currently in deficit in terms of the average annual sediment budget, but up until 1998, there were no significant differences between the profiles. Between 1998 and 2004, the sediment budget was always deficient, but the profiles in the centre and west of the seawall exhibit more intense erosion and indicate a pattern in longshore erosion along the direction of the dominant littoral drift (Fig. 3). The easternmost profile (GI14) records a deficit of approximately 0.3 m3.m-1 corresponding to 40 m3.yr-1. The deficit then grows from east to west along the seawall to reach a maximum of almost 1.2 m3.m-1 (250 m3.yr‑1) at GI18. Although this deficit begins to fall again towards the west, it remains high with an erosion of about 0.8 m3.m-1 (200 m3.yr-1) at GI17. Finally, the total volume of sand loss extrapolated for the whole of the sector between GI14 and GI17 amounts to 545 000 m3.yr-1. Finally, these results not only show an increase in the erosion of the profiles after the construction of the seawall but also increasing erosion in the direction of the longshore drift.

Fig. 3 - Sediment budget (in grey, the location of the seawall).
Bilan sédimentaire (en gris, localisation de la digue).

Fig. 3 - Sediment budget (in grey, the location of the seawall).Bilan sédimentaire (en gris, localisation de la digue).

4.2 - Scour at the foot of the structure

8All the bathymetric profiles indicate the presence of a deepening at the foot of the seawall (Fig. 4). However, this deepening corresponds to the inner trough situated between the shoreline and the inner bar on the natural profiles. Thus, it is appropriate to ask whether this deepening is influenced by the structure or whether it corresponds to the natural morphology of the profile.

9We thus monitored the depth of the inner trough since 1988 to determine if the impact of the seawall increases the depth of the trough by scouring (Fig. 5). The profiles on both sides of the structure display relatively regular and homogeneous evolution of their trough since 1988, with a slight tendency to deepening. On the other hand, the profiles in front of the seawall (GI15’, GI16, GI17) were hollowed out markedly after 1998, with values ranging between - 1 and - 2 m between 1988 and 1997, and values from - 4 to - 5 m in 2004. In detail, even though the innermost trough of profile GI15 is in front of the seawall, it does not show any notable evolution. Here again, we assume a longshore trend in the influence of the seawall because this profile - situated in the east - is upstream of the dominant longshore transport and seems undisturbed by the seawall. Finally, the increased depth of the inner trough in front of the seawall is interpreted as scour directly related to the presence of the structure.

Fig. 4 - Exemple of a bathymetric profile (GI 16, January 2001).
Exemple d’un profil bathymétrique (GI 16, janvier 2001).

Fig. 4 - Exemple of a bathymetric profile (GI 16, January 2001).Exemple d’un profil bathymétrique (GI 16, janvier 2001).

Fig. 5 - Depth variation of the inner trough since 1988 (in grey, the location of the seawall).
Variations de la profondeur de la fosse interne depuis 1988 (en gris, localisation de la digue).

Fig. 5 - Depth variation of the inner trough since 1988 (in grey, the location of the seawall).Variations de la profondeur de la fosse interne depuis 1988 (en gris, localisation de la digue).

4.3 - Modification of bar morphodynamics

10On this point, some authors evoke the presence of strong reflective phenomena to explain the origin of sub-marine morphologies of an undulatory type (Barnetet al., 1988; Kraus, 1988). Other authors show the migration of shoreface bars sometimes towards the coast (Morton, 1988) and sometimes in the offshore direction (Barnetet al., 1988). We thus evaluated the average sediment budget of the bars and their migration between 2000 and 2004. A comparison with the former profiles was not possible because of the ill-adapted surveys between 1988 and 1998.

11The morphology of the bathymetric profiles varies longitudinally. There is a transition from two relatively distinct bars in the east (GI14, GI15) to three bars in front of GI18 and GI16 and to the west of the seawall (GI17). The averaged sediment budget of the first two bars (bar 1 and bar 2) evolves in a similar way (Fig. 6). We observe a volume loss mainly on the western part of the seawall, as well as in the median part, and a gain on the eastern part (GI15 and GI14).

12On all the profiles, bar 2 shows the fastest rates of erosion. The sediment budget of the first two bars also exhibits a tendency to erosion in the direction of the dominant littoral drift. On GI17 and GI18, bar 3 also shows major losses exceeding those of bar 2 on GI17. The averaged cross-shore displacement of the bars indicates opposite movements between the profiles. The bars updrift and in front of the seawall are moving offshore ; meanwhile the bars on the “natural” profile GI17, downdrift of the seawall, are moving onshore (Fig. 7). The offshore displacement of bar 2 is around 20 m.yr-1 and there are no significant longshore changes in front of the seawall. Nevertheless, the inner bar (bar 1) displays slower offshore movement in front of the seawall (around 4 m.yr-1) than on the natural profile GI14 and on profile GI15 (around 25 m.yr-1). This fact is probably caused by the presence of the seawall which acts as a fixed boundary. The onshore migration of the bars 1 and 2 on profile GI17 is caused by a general shift of the profile shape related to the shoreline retreat.

Fig. 6 - Evolution of the sediment budget of the bars (in grey, the location of the seawall).
Évolution du bilan sédimentaire des barres (en gris, localisation de la digue).

Fig. 6 - Evolution of the sediment budget of the bars (in grey, the location of the seawall).Évolution du bilan sédimentaire des barres (en gris, localisation de la digue).

Fig. 7 - Migration of the bars (in grey, the location of the seawall).
Déplacement des barres (en gris, localisation de la digue).

Fig. 7 - Migration of the bars (in grey, the location of the seawall).Déplacement des barres (en gris, localisation de la digue).

4.4 - Variations of the depth of closure

13A major criticism of seawalls concerns their eventual offshore influence, linked directly to an increased reflection against the structure which might carry away sediments towards the offshore zone. To analyse this phenomenon, we determined the closure depth and the width of the “active zone” of the profile defined as the distance between the shore and the closure depth. Indeed, we can consider these two parameters as being representative of the active zone of the profile where we seek to evaluate the possible disturbances caused by the seawall.

14The “active zone” of the profile extends over a width of 400 m in GI14, then increases significantly from east to west on the profiles in front of the structure to reach 1000 m in GI18 (Fig. 8). The active zone then decreases towards the west in GI17. The depth of closure, which varies from - 6.3 to - 9.5 m, shows logically the same tendency. Finally, the depth of closure is much further offshore on the profiles in front of the seawall than on the so-called “natural” profiles. This provides indirect evidence of an offshore influence of the seawall on the beach profile, with a longshore trend from east to west.

Fig. 8 - Width of the active zone and closure depth of the profile (in grey, the location of the seawall).
Largeur active et profondeur de fermeture du profil (en gris, localisation de la digue).

Fig. 8 - Width of the active zone and closure depth of the profile (in grey, the location of the seawall).Largeur active et profondeur de fermeture du profil (en gris, localisation de la digue).

4.5 - Influence of the seawall on the beach recovery processes

15The question of beach recovery is often discussed in the literature. Some authors show that beach recovery is not necessarily slowed down (Dean, 1986; Barnetet al., 1988; Griggs and Tait, 1988; Wiegel, 2002), while others show that seasonal variations can be temporarily accentuated in front of the structure (Jones and Basco, 1997) and that the intensity of the recovery is a function of the width of the beach (Kraus, 1988). However, most studies are concerned with sectors where the seawall occupies a position at the top of the beach, which prevents a direct comparison with our study site. Therefore, our objective here is to determine whether or not the presence of a seawall reduces the processes of beach recovery during periods of calm weather.

16Two periods were selected as being representative of beach recovery: February to April 2002 and January to August 2004 (Fig. 9). Our results show that, on the lines situated on the margin or just within the extremities of the structure, the sediment budgets are mostly positive or exceptionally negative (January-August 2004 in GI17). On the other hand, on the profiles directly in front of the seawall, beach recovery is non-existent and the sediment budgets are always in deficit. To summarize, the seawall actually reduces the recovery of the beach, but to a variable extent, according to the period and profile.

Fig. 9 - Beach recovery according to profile lines (in grey, the location of the seawall).
Processus de reconstructions sur chaque profil (en gris, localisation de la digue).

Fig. 9 - Beach recovery according to profile lines (in grey, the location of the seawall).Processus de reconstructions sur chaque profil (en gris, localisation de la digue).

5 - Discussion

17The results demonstrate that the seawall actually increases beach erosion. This erosion is observed from the increase in sediment losses, a deepening of the innermost trough at the foot of the structure, a deceleration - even absence - of beach recovery processes and a deepening of the closure depth in front of the seawall. All these data confirm the results of previous studies (Barnet, et al., 1988; Morton, 1988) on meso- and macro-tidal beaches subject to higher energy swell. This evolution is most probably caused by an increase in turbulence due to reflective dynamics related to the presence of the seawall (Kraus, 1988). The offshore bar movement cannot be attributed categorically to the presence of the seawall because this phenomenon is also observed on coasts devoid of coastal defence structures (Ruessink and Kroon 1995), in particular on the Mediterranean beaches of the Gulf of Lions (Sabatier and Provansal 2000; Certain 2002). Nevertheless, the influence of the seawall is most likely felt throughout the breaker zone and probably at greater depths, as shown by the closure depth analysis.

18Our results also show an increase in erosion in the direction of the dominant transport, in agreement with Sabatier (2001) who explains erosion in this sector by an increase in longshore sediment transport. Up to now, relatively few studies have focused on the longshore distribution of erosional phenomena, while previous research concentrated especially on the cross-shore evolution of profiles. The present study is compatible with the in situ current measurements of Miles et al., (2001), which also show a longitudinal increase in erosion along a seawall. The “natural” profile GI17, which is situated downdrift, is probably subject to the influence of the seawall since its longshore effects are in any case apparent in terms of bathymetry.

6 - Conclusion

19Our results and interpretations do not agree with those of some studies (Jones and Basco, 1997; Wiegel, 2002) proposing there are no essential differences in bottom morphology in front of the seawall. On the contrary, we show the negative impact of the seawall on the surrounding bathymetry, thus calling into question the long-term stability of the structure. However, the morphological and hydrodynamic responses to the construction of a seawall depend largely on the local conditions: position of the seawall on the profile (Rakha and Kamphuis, 1997), erosive tendency in the long-term and type of coastal defence structures (Plant and Griggs, 1992). In this context, we highlight the difficulties of understanding erosional phenomena in front of a seawall, since we are interested in case studies and the problems of generalizing the impact of such a structure on bathymetry. Moreover, we stress that the impact of a seawall should be analysed not only transversely but also longitudinally. A series of in situ current measurements should allow us to obtain more concrete information on this phenomenon.

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Bibliography

Barnet m.-R., Asce A.-M, Wang H., (1988), Effects of a vertical seawall on profile response, 21st International Conference on Coastal Engineering, Malaga, 1-3, p. 1493-1507.

Basco D.-R, Bellomo D.-A, Pollock C., (1992), Statistically significant beach profile change with and without the presence of seawalls, 23rd International Conference on Coastal Engineering, Venice, p. 1924-1937.

Basco D.-R, Bellomo D.-A, Hazelton J.-M, Jones B.-N., (1997), The influence of seawalls on subaerial beach volumes with receding shorelines, Coastal Enginneering, 30, p. 203-233.

Basco D.-R., (2004), Seawall Impact on Adjacent Beaches: separating Fact from fiction, Journal of Coastal Research, Special Issue 39, p. 24-28.

Certain R., (2002), Morphodynamique d’une côte sableuse micro-tidale à barres : le golfe du lion (Languedoc-Roussillon), Thèse, Université de Perpignan, 209 p.

Dean R.-G., (1986), Coastal Armoring : effect, principles and mitigation, 20th International Conference on Coastal Engineering, p. 1843-1857.

Fletcher C.-H., Mullane R.-A; Richmond B.-M., (1997), Beach loss along armored shorelines on Oahu, Hawaiian Islands, Journal of Coastal Research, 13, 1, p. 209-215.

Griggs G.-B., Tait J.-F., (1988), The effects of Coastal protection structures on beaches along northern Monterey bay, California, Journal of Coastal Research, Special Issue 4, p. 93-111.

Griggs G.-B., Moore L.-J., Tait, J.-F, Scott K., Pembrook D., (1996), The effects of storm waves of 1995 on beaches adjacent to a log term seawall monitoring site in northern Monterey bay, California, Shore and Beach, 64, 1, p. 34-39.

Jones B.-N., Basco D.-R, (1997), Seawall effects on historically receding shorelines. 25th International Conference on Coastal Engineering, Orlando, p. 1985-1994.

Komar p.-D., Mc Dougal W.-G, (1988), Coastal Erosion and engineering structures: the Oregon experience, Journal of Coastal Research, Special Issue 4, p. 77-92.

Kraus N.-C., (1988), The effects of seawalls on the beach: an extended literature review, Journal of Coastal Research, Special Issue 4, p. 1-28.

Miles J.-R., Russel p.-E., Huntley D.-A., (2001), Field measurement of sediment dynamics in front of a seawall, Journal of Coastal Research, 17, 1, p. 195-206.

Morton R.-A., (1988), Interactions of storms, Seawalls, and Beaches of the Texas Coast, Journal of Coastal Research, Special Issue 4, p. 113-134.

Plant N.-G., Griggs G.-B., (1992), Interactions between nearshore processes and beach morphology near a Seawall, Journal of Coastal Research, 8, 1, p. 183-200.

Rakha K.-A., Kamphuis J.-W., (1997), Wave-induced currents in the vicinity of a seawall, Coastal Engineering, 30, 23-52.

Ruessink B.-G., Kroon A., (1995), The behaviour of a multiple bar system in a nearshore zone of Terschelling, the Netherlands, 1965-1993, Marine Geology, 121, p. 187-197.

Sabatier F., (2001), Fonctionnement et dynamiques morpho-sédimentaires du littoral du delta du Rhône, Thèse de doctorat, Université Aix Marseille III, 272 p.

Sabatier F., Provansal m., (2000), Sandbars morphology of Espiguette spit, Mediterranean Sea, France, International Workshop Sandwaves Dynamics, Lille, 23-25 march 2000, p. 179-187.

Sabatier F., Suanez S., (2003), Evolution of the Rhône delta coast since the end of the 19th century, Géomorphologie : relief, processus, environnement, 4, p. 283-300.

Tait, J.-F., Griggs, G.-B, (1990), Beach response to the presence of a seawall, Shore and Beach, 58, 2, p. 11-28.

Wiegel R.-L., (2002), Seawalls, seacliffs, beachrock: what beach effects?, Shore and Beach, 70, 1, p. 17-27.

Wright L.-D., Short A.-D., (1984), Morphodynamic variability of surf zones and beaches: a synthesis, Marine Geology, 56, p. 93-118.

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List of illustrations

Title Fig. 1 - Site location, bathymetric survey and photograph of the seawall.Localisation du site, des profils bathymétriques, et photographie de la digue.
URL http://mediterranee.revues.org/docannexe/image/189/img-1.png
File image/png, 64k
Title Fig. 2 - Historical evolution of the seawall.Évolution historique de la digue.
URL http://mediterranee.revues.org/docannexe/image/189/img-2.png
File image/png, 8.6k
Title Fig. 3 - Sediment budget (in grey, the location of the seawall).Bilan sédimentaire (en gris, localisation de la digue).
URL http://mediterranee.revues.org/docannexe/image/189/img-3.png
File image/png, 12k
Title Fig. 4 - Exemple of a bathymetric profile (GI 16, January 2001).Exemple d’un profil bathymétrique (GI 16, janvier 2001).
URL http://mediterranee.revues.org/docannexe/image/189/img-4.png
File image/png, 10k
Title Fig. 5 - Depth variation of the inner trough since 1988 (in grey, the location of the seawall).Variations de la profondeur de la fosse interne depuis 1988 (en gris, localisation de la digue).
URL http://mediterranee.revues.org/docannexe/image/189/img-5.png
File image/png, 14k
Title Fig. 6 - Evolution of the sediment budget of the bars (in grey, the location of the seawall).Évolution du bilan sédimentaire des barres (en gris, localisation de la digue).
URL http://mediterranee.revues.org/docannexe/image/189/img-6.png
File image/png, 14k
Title Fig. 7 - Migration of the bars (in grey, the location of the seawall).Déplacement des barres (en gris, localisation de la digue).
URL http://mediterranee.revues.org/docannexe/image/189/img-7.png
File image/png, 15k
Title Fig. 8 - Width of the active zone and closure depth of the profile (in grey, the location of the seawall).Largeur active et profondeur de fermeture du profil (en gris, localisation de la digue).
URL http://mediterranee.revues.org/docannexe/image/189/img-8.png
File image/png, 17k
Title Fig. 9 - Beach recovery according to profile lines (in grey, the location of the seawall).Processus de reconstructions sur chaque profil (en gris, localisation de la digue).
URL http://mediterranee.revues.org/docannexe/image/189/img-9.png
File image/png, 12k
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References

Electronic reference

Olivier Samat, François Sabatier and Adrien Lambert, « Bathymetric impacts of a seawall on a micro-tidal beach, Gulf of Lions, France », Méditerranée [Online], 108 | 2007, Online since 01 January 2009, connection on 23 July 2017. URL : http://mediterranee.revues.org/189 ; DOI : 10.4000/mediterranee.189

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About the authors

Olivier Samat

UFR des Sciences Géographiques et de l’Aménagement - Université de Provence - CEREGE Europôle de L’Arbois – BP 80 - 13545 Aix-en-Provence cedex 04 - France - samat@cerege.fr

By this author

François Sabatier

UFR des Sciences Géographiques et de l’Aménagement - Université de Provence - CEREGE Europôle de L’Arbois – BP 80 - 13545 Aix-en-Provence cedex 04 - France - sabatier@cerege.fr

By this author

Adrien Lambert

UFR des Sciences Géographiques et de l’Aménagement - Université de Provence - CEREGE Europôle de L’Arbois – BP 80 - 13545 Aix-en-Provence cedex 04 - France - lambert@cerege.fr

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