Direct Bed Shear Stress Measurements in Bore-Driven Swash and Swash Interactions

Bore-driven swash and swash interactions are investigated experimentally. Direct bed shear stress measurements suggest bore/swash backwash interactions act to interrupt backwash (seaward) sediment transport. Catch up and absorption causes landward directed bed shear force (enhanced by impact forces onto the bed) of a short duration that does not follow the measured flow velocity trend. A single swash interaction is likely to restrict seaward direct transport and/or suspend sediment in large volumes but not directly contribute to the landward advection of sediment. Bore-driven swash hydrodynamics for a single swash event were predicted well using a non-linear shallow water wave equation model. In the mid-swash region measured uprush bed shear stress was predicted well using the drag law estimate with constant friction factor C_f approx.0.02. A smaller or time-varying C_f is required to accurately predict measured backwash bed shear stress.