The influence of intertidal bar-trough morphology on sediment transport on macrotidal beaches, northern France

International audienceSediment transport measurements, coastal hydrodynamics and beach morphological surveys were undertaken on three sandy barred macrotidal beaches of Northern France. Sand trapping took place at several locations across the intertidal zone during rising and falling tides in order to obtain estimates of both longshore and onshore sediment flux from the lower to the upper beach. Results show that intertidal bars are responsible for significant wave energy dissipation leading to a decrease of sand transport towards the upper beach. Sediment transport appeared to be substantially higher on the bars than within the troughs due to wave breaking processes that are responsible for intense sand remobilization. The comparison of onshore sediment transport with longshore transport showed that onshore-directed transport is generally higher than the longshore sediment flux, suggesting that significant amounts of sand are directed shoreward across the bars, contributing to the onshore migration of intertidal bars. Sediment transport rates within the troughs proved to be significant only during runnel drainage when flow channelization was responsible for a local increase in sediment transport, this seaward-directed transport representing a sand recycling mechanism that contributes to the stability of the intertidal bars. Our measurements also showed a low alongshore variability in longshore sediment transport rates, which is an additional factor that plays a role in maintaining the linear bar and trough morphology on these macrotidal beaches