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

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.Pour déterminer l’impact et l’efficacité d’une digue frontale, nous avons réalisé une campagne de mesures bathymétriques durant 4 années. Nous montrons une augmentation de la profondeur des fonds au pied de la digue et du déficit sédimentaire du secteur. La digue joue un role sur les dynamiques perpendiculaires, mais aussi longitudinales, dans la direction de la dérive littorale dominante

Connecting large-scale coastal behaviour with coastal management of the Rhône delta

International audienceThe aim of this paper is to connect the Large Scale Coastal Behaviour (LSCB) of the Rhône delta (shoreface sediment budget, river sediment input to the beaches, climatic change) with the impact and efficiency of hard engineering coastal structures. The analysis of the 1895 to 1974 bathymetric maps as well as 2D modelling of the effect of wave blocking on longshore transport allows us to draw up a conceptual model of the LSCB of the Rhône delta. The river sand input, settled in the mouth area (prodeltaic lobe), favours the advance of adjacent beaches. There is however a very weak alongshore sand feeding of the non-adjacent beaches farther off the mouth. After a mouth shift, the prodelta is eroded by aggressive waves and the sand is moved alongshore to build spits. This conceptual model suggests that there is a “timeshift” between the input of river sediments to the sea and the build up of a beach (nonadjacent to the mouth). Nowadays, as the river channels are controlled by dykes and human interventions, a river shift is not possible. It thus appears unlikely that the river sediments can supply the beaches of the Rhône delta coast. Under these conditions, we must expect that the problems of erosion will continue at Saintes-Maries-de-la-Mer and on the Faraman shore, in areas with chronic erosion where the shoreline retreat has been partially stopped by hard engineering practices in the 1980s. Therefore, these arti.cially stabilised sectors remain potentially under threat because of pro.le steepening and downdrift erosion evidenced in this paper by bathymetric profile measurements. In the long-term (1905 to 2003), the temporal analysis of the storm surges and the sea level show very weak but reliable increasing trends. Thus, these climatic agents will be more aggressive on the beaches and on the coastal structures calling their ef.ciency into question. We also evidence that the hard engineering structures were built in a favourable climatic context during the 1980s meanwhile the storm surges and the sea-level rise are stronger since the 1990s. Regarding to the LSCB of the Rhône delta, and the impact of hard engineering coastal structures, we suggest that classical hard coastal protections are not the best option to protect the coast

Lutte contre l’érosion littorale : efficacité des méthodes de stabilisation par drainage de plage, le cas de la baie d’Agay, Var

Cet article s’intéresse à l’efficacité des méthodes de stabilisation du trait de côte par drainage de plage en Méditerranée française, sur le site de la baie d’Agay (Var). L’étude des tendances sédimentaires à long terme met en évidence l’existence d’une érosion naturelle du site entre 1950 et 1976, puis l’influence anthropique (épis et rechargements) sur la stabilisation constatée de cette plage entre 1976 et 1998. Le suivi topo-bathymétrique réalisé durant deux ans, avant et après l’installation du système de drainage, démontre la stabilité du trait de côte à partir de l’implantation des drains, sans rechargement et après destruction des aménagements en enrochements existants. L’étude des relations physiques entre niveaux de la nappe de plage et morphologie de l’estran sous et hors contrainte de drainage met en évidence l’impact instantané de ce dernier sur la position de la nappe, ainsi que l’influence du rabattement de la nappe sur la progradation de l’estran.The subject of this article is the effectiveness of beach dewatering stabilization methods on a micro-tidal, reflective beach of the French Mediterranean (Bay of Agay, Var). Study of the long-term sedimentary tendencies highlights the existence of a natural erosion of the site between 1950 and 1976, before anthropogenic influence (groins and beach nourishment) on the observed beach stabilization between 1976 and 1998. The two-year topo-bathymetric survey carried out before and after the establishment of the beach dewatering system shows a stabilization of the shoreline starting from the drain installation, without beach nourishment and after destruction of the existing installations. The study of the physical relationship between levels of the beach watertable and beachface morphology with and without the drainage system shows the instantaneous impact of this parameter on the watertable level, and the influence of watertable lowering on beachface progradation