Groundwater effects on sediment transport: A modelling study of the mechanisms underlying beach dewatering for erosion control

Field and laboratory observations have shown that a relatively low beach groundwater table enhances beach accretion while a high water table promotes beach erosion. These observations have led to the beach dewatering technique (artificially lowering the beach water table) for combating beach erosion. The aim of this study is to quantify the interactions between the ocean and coastal aquifer. Such interactions affect swash sediment transport and beach profile changes. A process-based numerical model is developed to simulate the interacting wave motion on the beach, coastal groundwater flow, swash sediment transport and beach profile changes. The non-linear shallow water equation is modified to simulate swash/backwash motion interacting with the beach groundwater. Saturated flow in the coastal aquifer is governed by the Laplace equation. An additional term is added into the free surface boundary conditions for the water table to incorporate capillary effects. The instantaneous cross-shore sediment transport rate is calculated according to Bagnold's sediment transport model. The net sediment transport rate is obtained for every swash/backwash cycle and is used to calculate the resulting beach profile changes. Results of model testing demonstrate that the model replicates (1) bar/berm formation at beaches under different wave conditions, (2) the equilibrium state of a beach exposed to constant wave conditions, and (3) accretionary effects of a low beach water table on beach profile changes