Mechanisms of Beach Ground Water and Swash Interaction

Previous studies of swash-ground water interactions have given prominance to infiltration of swash and backwash into unsaturated sand as the principal mechanism explaining sedimentation patterns in the inter-tidal zone. Here we consider the role of fluidisation in explaining such sedimentation patterns. It appears that fluidisation due to general ground water outflow from a beach is insufficient to induce fluidisation of surface sand. We propose a different mechanism for explaining fluidisation whereby very small amounts of swash infiltration into the seepage face cause rapid increases in pore water pressure below the beach surface. During the backwash, pressure unloading on the surface causes rapid ground water outflow from the surface sediment. The rate of this outflow is shown, using a simple model, to be sufficient to induce fluidisation. We also consider a model of general beach ground water behaviour and conclude that it can be satisfactorily used to provide boundary conditions for smaller scale models of fluidisation under swash.