Bore-driven swash on beaches: Numerical modeling and large-scale laboratory experiments

Novel laboratory experiments have been carried out to measure swash hydrodynamics on smooth and rough impermeable beach slopes. The experiments use a swash rig which produces highly repeatable, full-scale, single swash events, enabling detailed measurements of velocities and depths across the swash zone. Example results are presented, showing comparisons between smooth and rough beach hydrodynamics. The effects of roughness are most evident in the maximum run-up, in depths at the top end of the beach and in the backwash velocities. Experimental results have been compared with predictions of depth and depth-averaged velocity from a 1D NLSWE-based numerical model with a simple quadratic stress law for the bed shear stress. Comparison of experimental and model results shows reasonably good agreement for depth and velocities for much of the swash cycle. However, differences in the backwash velocities point to poor bed shear stress modeling. The results imply that good agreement between measured and predicted depths and velocities for swash uprush is not an indication that the bed shear stress is well predicted by the model.