A field study of the ripple vortex shedding process in the shoaling zone of a macro-tidal sandy beach

International audienceThis paper presents an analysis of the field data collected at the French atlantic coast during the ECORS project on near-shore flow / sediment processes of macro-tidal sandy beaches. An Acoustic Doppler Velocity Profilers has been deployed in the macro tidal region in order to profile the cross-shore and vertical flow velocity components over a distance of 33cm above the bed. For a moderate wave climate, 5cm high ripples were seen to migrate across the acoustic beam in the shoaling zone. We observe that the flow field in the near bed region is strongly affected by the ripples. As previously described in the literature, two flow separation regions exist for each ripple at each wave cycle: a first one on the landward face of the ripple just after the wave crest and a second one on the seaward face during the wave trough. The vertical orbital velocity shows an inversion of sign during each separation and subsequent reattachment phase. Moreover, the orbital stress is calculated as the product of the measured cross-shore and vertical orbital velocities. The characteristic pattern of the orbital stress field differs strongly for both faces of the ripple. Finally, the transverse vorticity field also reveals different behaviors at each face of the ripple. The analysis of the whole set of data reveals the consistency of this orbital stress and vorticity organization, with more intense values encountered near the ripple crest