Friction Factors for Wave Uprush

Abstract

The non-linear shallow water theory contains a set of solutions for the problem of swash following bore collapse on a hydraulically smooth and impermeable beach. These have recently been compared with field data from a number of natural sandy beaches (HUGHES, 1992). The comparison between the inviscid equations and the data was generally favourable; however, the parameters measured were consistently over predicted by the theory. It is assumed here that this discrepancy is due to energy dissipation effects not originally represented in the theory. The inviscid equation of motion for the shoreline following bore collapse is expanded  to include a shear stress term to account for bed friction. This equation is then solved for the time-history of the shoreline position and the maximum swash height. Field measurements of the maximum swash height, initial shoreline velocity, swash depth, beach slope and grain size are used  with these equations to determine the inferred friction factor for the uprush. The magnitude of the friction factor is found to be of the order of 0.1 for the sandy beaches considered here. A recent model for the bottom boundary layer in the presence of sheet flow is capable of predicting the magnitude of the observed friction factor.