impact of beach states on alongshore transport

Impact of spatial variability in the nearshore bathymetry on net sediment transport rates has been investigated for a selection of observed beach states at the Dutch coast. The beach states comprise a longshore bar trough and two transverse bar rip situations, which were present at the large scale Sand Motor nourishment at the Holland coast. These observed bathymetric features were then applied multiple times next to each other along a longer stretch of coast to obtain a repeating pattern of the considered beach state. The wave transformation towards the shore, alongshore wave-driven and water-level setup driven currents and sediment transport were computed with the Delft3D model, which has been applied successfully for many other studies at the Sand Motor. It was found that net sediment transport is considerably influenced for the most pronounced transverse bar rip configuration, which was most prominent for conditions with small wave angles (i.e. less than 10° from shore-normal). Furthermore, a decrease in transport rates is found for conditions from larger angles of wave incidence (i.e. 30 to 45° from shore-normal). Impacts of the bathymetries of longshore bar trough and the less pronounced transverse bar rip system on net sediment transport rates were much smaller. The actual cause for the enhancement (or decrease) of the net transport for the transverse bar rip configuration is expected to be related to 1) the oblique orientation of the rip-channel for the considered configuration as well as to 2) a more diffusive pattern of the wave breaking as a result of the refraction on the spatially variable bathymetry.