Bed dynamics in a microtidal swash zone under small wave conditions

National audienceThe knowledge of the swash zone is of primary importance to understand the morphological evolution of sedimentary beaches. The swash zone is highly dynamic, with instantaneous sediment fluxes often being several orders of magnitude greater than their surf zone counterparts. Despite of the increased research efforts dedicated to the swash hydro- and morphodynamics during the last decades, several key processes remain to be understood before the swash morphodynamics can be predicted and modeled. A large part of existing studies have focused on meso to macrotidal swash zones exposed to moderate to storm wave conditions. The present communication reports on a field study of small-scale swash bed processes in microtidal and small wave conditions. It is shown that even such calm conditions can result in significantly dynamic swash zone. The selected field site is the Pont de Rousty sandy beach, Camargue, France. A high-resolution survey of the swash zone sand bed has been performed over a cross-shore transect on both hydro and morphodynamic aspects. The instrumentation included a set of both buried and non-buried pressure sensors, a network of ultrasonic altimeters, a 3D high resolution acoustic velocimeter and other pressure and velocity sensors deployed outside the swash zone to measure the incoming forcing. Results show that the swash bed and slope permanently evolve under the action of waves. Each swash event can individually induce bed level variations ranging from 1 to 5mm but larger evolution (up to 4cm) are observed at longer time scales. Ongoing work is focused, on one hand, to the role played by the swash physical parameters (velocity, duration, height, etc) on the event-related transport and, on the other hand, to the identification of a " bottom-up " relationship between the succession of individual actions of swash events and the long term evolution of swash morphology