The morphodynamics of a swash event on an erodible beach

A high accuracy numerical solution, coupling one-dimensional shallow water and bedevolution equations, with, for the first time, a suspended sediment advection equation, thereby including bed- and/or suspended load, is used to examine two swash events on an initially plane, erodible beach: the event of Peregrine & Williams (2001), and that of a solitary wave approaching the beach. Equations are solved by the method of characteristics, and the numerical model is verified. Full coupling of suspended load to beach change for Peregrine & Williams (2001) yields only slightly altered swash flows, depending on beach mobility and sediment response time; a series of similar final beach change patterns results for different beach mobilities. Suspended- and bed-load transport have distinct morphodynamical signatures. For the solitary wave a backwash bore is created (Hibberd & Peregrine 1979). This morphodynamical bore propagates offshore initially, and leads to the creation of a beach bed-step (Larson & Sunamura 1993), primarily due to bed-load transport. Its height is directly related to bed-load mobility, and also depends strongly on bed friction coefficient. The shock dynamics of this bed-step are explained and illustrated. Bed- and suspended-load mobilities are quantified using field data, and an attempt is made to relate predictions to measurements of single swash events on a natural beach. Average predicted bed change magnitudes across the swash are of the order of 2mm, with maximum bed changes up to about 10cm at the bed-step.