Morphologic response of four pocket beaches to high energy conditions: including the Xynthia storm (South Brittany, France)
Résumé
This work highlights the morphological behavior of four reflective pocket beaches (Fogeo, Kerver, Govelins and Suscinio) during an episod of high-energy events cumulating in the Xynthia storm. A total of 23 topographic profiles were surveyed monthly at spring high tide using a DGPS. Located in the atlantic French rocky coastline, the studied beaches present a "low-tide terrace" morphology. They are sheltered from the Altantic wave climate by the Quiberon peninsula and islands. During high-energy conditions, the break in slope moves landward with an increasing upper beachface gradient (tanβ). For example, tanβ increased from 0.108 to 0.126 in the center of Kerver beach and the break in slope moved 4.8 m landward. This does not follow the morphodynamic behavior expected for the morphological parameters of these beaches. This trend is reversed during low-energy conditions. High-energy conditions, and especially the Xynthia storm, caused a massive erosion (about 10% of the initial volume) in the Suscinio embayed beach whereas the three other beaches underwent an accretion (around 2%). In spite of this erosion, the western profile of the Suscinio beach accreted which suggests a longshore drift oriented westward assimilated as a beach rotation. On other beaches, The accretion is not uniform on all beaches but it suggests a cross-shore sediment transport from the lower shoreface to the upper shoreface also associated with a beach rotation. These results highlight the importance of geological settings and wave diffraction to the variability of morphological response with high-energy events of closely-spaced reflective beaches and the need to be taken into account for predicting morphological model as much as oceanographic parameters.