Variation in longshore sediment transport under low to moderate energy conditions on barred macrotidal beaches.

The aim of this study was to estimate longshore sediment transport on three sandy barred macrotidal beaches of Northern France using sediment traps. Measurements of longshore sediment transport were carried out at several locations across the intertidal zone during rising and falling tides in order to obtain estimates of longshore sediment flux from the lower to the upper beach and coupled with high-frequency (2 Hz) hydrodynamic data using a series of hydrodynamic instruments. Results showed that longshore sediment transport increased with both wave height and mean flow, but no relation was found with wave angle which is probably due to the influence of tidal currents that interact with wave-induced longshore currents. Although, limited variation of longshore sediment transport was observed, cross-shore variation showed that sediment fluxes were higher on the middle to lower beach, which can be explained by a decrease in tidal flow velocity towards the upper beach as well as wave-energy dissipation over the beach and intertidal bars. Longshore sediment transport direction appeared to be controlled by both incoming wave and tidal current direction, depending on the hydrodynamic zones, sediment transport being tidally-dominated in the shoaling zone and mostly wave-induced in the inner surf zon