Non linear shallow water modelling of bore-driven swash: Description of the bottom boundary layer

The paper presents a simple approach to estimate the bottom shear stress in the swash zone by coupling the Non Linear Shallow Water Equations with the momentum integral equation for the bottom boundary layer. The approach allows not only the computation of the frictional dissipation term in the equations but also to have an insight into the flow structure in the water column during a swash event. The numerical results have been compared with a new set of experiments involving a single dam-break generated swash event. Three different grain sizes, ranging from coarse sand to gravel, have been tested in the laboratory.A sensitivity analysis on the only calibration parameter of the model, the bed roughness, is presented and its optimal value is chosen. The numerically predicted free surface, depth averaged velocity and velocity profiles have been compared with the measured ones. Results are very good in the run-up stage, where also the bottom shear stress is well modelled. In the backwash the frictional energy dissipation is better modelled for the gravel beach than for the coarse sand one. Results show that the simple model accurately predicts the flow parameters during the swash event. Also the velocity profiles are reasonably well predicted, especially during the run-up.