Impact of beach states on net longshore sediment transport

Abstract

The response of longshore sediment transport to the morphodynamic state of the beach was assessed with the use of the process based numerical model, Delft 3D. For this purpose, idealized simulation bathymetries that reflect morphological characteristics of two different beach states (namely longshore bar and trough and transverse bar and rip) were created on the basis of observations of the subaqueous topography at the Sand Motor, a mega-nourishment located in the coast of South Holland. Sediment transport simulations were performed for a range of hydrodynamic conditions that reflect typical sea conditions at the Dutch coast. It was found that two dominant morphological aspects determine the response in net longshore transport: 1) the alongshore variability of morphology and 2) the slope and general configuration of the beach profile; and the relative importance of these two are dependent on the hydrodynamic forcing exerted on the system. Relevance of the profile shape on determining sediment transport behaviours increases as the angle of wave approach increases: Steeper slopes in the swash zone promote higher transport rates for medium and especially for high wave angles. Generation of topographically controlled rip currents that increase or decrease net longshore transport rates depends on the angle of the incoming waves with respect to the coast. Situations with nearly normal wave incidence result in larger potential for rip current development than situations with obliquely incident waves.