Wave-impact driven dune face erosion processes

Abstract

For socio-economic and safety reasons it is desirable to be able to model shoreline retreat. To that end, it is essential that models predict dune and beach profile change under storm conditions as accurately as possible. Several approaches to dune erosion modelling have been proposed. The wave-impact driven dune erosion model decouples inner surf and swash zone sediment transports from dune face erosion, enabling a combination of this model with a process-based morphodynamic model for the nearshore. Such a combined model allows for a physical feedback between nearshore hydrodynamics, dune face erosion and the evolution of the nearshore and foreshore from that erosion. The present report describes a study on wave-impact driven sediment flux. Previous research by Fisher et al. (1986) resulted in a linear relation between wave impact and dune face erosion volumes. In their approach the total erosion of a dune during a certain period is considered as the summation of the specific erosion volumes from single impacts on the dune face. From observations during large-scale dune erosion experiments in the Delta flume, it is concluded that dune face erosion under wave attack takes place in periodical slump events, preceded by a period of wave attack. In this research, the relation between wave impact and dune face erosion volumes is studied taking into account the periodicity of slump events. To this end, wave impact, period of wave attack and corresponding dune erosion volumes are defined and derived from data of the large-scale dune erosion experiments. Wave impact for a predefined period is derived from in-situ pressure and flow velocity measurements, while erosion volumes for entire tests and for individual slump events are derived from respectively in-situ profile measurements and video data. Stereo video calculations are used to process video data into 3D-profiles. The period of wave attack is derived from video measurements. The resulting values show a clear coherence between wave impact and sediment flux. Several relations can be fitted through the data points for the situation where the dune face is entirely above still water level (SWL) and the dune face has a near-critical slope. Fitted relations are not applicable for the situation where the toe of the dune is below SWL, as this implies a fundamentally different dune erosion process.