Ba, A. and Sénéchal, N. Extreme winter storm versus summer storm: morphological impact on a sandy beach

This contribution presents the morphologic response of a double-barred sandy beach facing the Atlantic swells in the south west of France under two storm events: the first one occurring during summer period while the beach is experiencing seasonal accretive conditions and the second one during winter while the beach is experiencing seasonal erosion conditions. Shoreline positions, inner bar positions and beach states were extracted from rectified merged video data. Despite a significant difference in usually considered storms thresholds (Hs, duration), the winter storm (Joaquim) being associated to Hs up to 8.5 m during 120 hours and the summer storm being associated to Hs up to 4.5 m during 17 hours, the shoreline and the inner bar positions one week after the apex of each storm were very similar to the one observed previous the apex. Analysis of wave conditions preceding the two events indicates that, relative to them, the two storms were similar in energy but also that conditions observed during falling conditions were favourable to rapid recovery. The beach morphology was probably also close to its equilibrium shape prior to each storm, and this may have vanished severe beach morphology readjustment. Indeed, computed averaged Gourlay parameters are consistent with observed beach states while sync-storm Gourlay parameters are not necessarily. Data also suggest that tide together with the storm duration may explain why no up-state transitions was observed during the summer storm while an slight up-state transition was observed during the winter storm. Thus our data suggest that recovery period can be very rapid even under extreme storms.