reset of a subtidal bar during an energetic event : application to Le Truc Vert beach case (France)

A large number of beach morphologies across the world exhibits nearshore sandbars with complex 3D patterns. During large storms, these shapes disappear and the bars get to a certain extent alongshore uniform. This phenomenon is called a reset. Numerous studies have been conducted on the development of the bar shapes (3D) or on the cross-shore migration of sandbars (2D). The present work attends to determine the relevant processes involved in the reset of the three dimensional bars and to describe the relative influence of each of these processes. To perform this study, data collected during the ECORS campaign at Le Truc Vert (France) in 2008 are analysed. In addition, a numerical approach is performed using a new research Delft3D model forced by the Xbeach wave generator to investigate the processes involved in a reset-event. The effects of the hydrodynamic external conditions on the currents in the surfzone are investigated. Then the reset is studied in details in order to understand the role of the different processes taken into account by the numerical model. The significant role of the longshore currents induced by oblique wave incidence is clearly assessed. In addition, the long wave orbital velocities as well as the mean flow related to the short wave forcing control the stirring of sediments and influence the time scale of the reset-event.