Morphodynamic of a sandy-muddy macrotidal estuarine beach under contrasted energy conditions (Vilaine estuary, France)

International audienceEstuarine and bay beaches are important areas for human activities. These beaches are variably affected by tides, waves, and winds that can commonly generate marked topographic and sedimentological contrasts. Betahon beach (South Brittany, France) is an intermediate-type beach exhibiting a low-tide terrace linked to a steeply sloping gravelly-sandy upper foreshore, and separated by a shore-parallel bluff from a mudflat on the lower foreshore. The beach exhibits linear ridge and runnel (R-R) bedforms perpendicular to the shoreline. Seasonal monitoring of the beach shows mudflat accretion by fluid mud deposition and erosion of R-R bedforms. A core obtained from the mudflat shows alternations of mud and sand. In order to understand the cross-shore dynamics of the beach, topographic surveys and wave and current monitoring were carried out during two contrasting energy conditions. Bed return flows occurred during high-energy events, inducing an infill of runnels by non-cohesive fine sediments and coarser sediments from the reflective upper beach. During low-energy conditions, a longshore flow channel was identified between the shore-parallel low-tide terrace bluff and the mudflat. Throughout the tide, onshore currents prevailed over the mudflat, inducing the filling of runnels and the base of the bluff with fluid mud