Numerical study of swash dynamics: Development of a porous layer boundary treatment for rough surfaces modelling

A key element for the simulation of swash events over rough beds using depth- and phase-resolving numerical models is the treatment of the bottom boundary condition. In this contribution, a new boundary treatment for rough surfaces, inspired by canopy models, is proposed. It is based on the use of a porous layer to model the effects of the roughness elements on the flow. The parameters of the porous layer are derived from the physical and geometrical properties of the roughness elements. A classical boundary condition model for rough surfaces is used as reference. The results of the models are compared to two sets of experiments. First, to a fully developed boundary layer flow over a rough surface, which shows that the proposed model is able to correctly capture such flows. Then, to bore-driven swash experiments on impermeable rough slopes, showing that the porous layer model significantly improves the prediction of the position and shape of the swash lens tip, which leads to a better prediction of the shoreline and bed shear stress. The swash lens tip shape appears to be an essential parameter to accurately describe the swash dynamics, and it is shown to be controlled, at least in part, by the beach surface boundary condition. A 2D approximation is compared to the 3D results, which indicates that it is a viable option to simulate such swash experiments in narrow flumes at low cost, while capturing most of the underlying physics. • A rough surface boundary treatment inspired by canopy models is relevant for swash. • The new boundary treatment improves the prediction of the swash lens tip shape. • Improved bed shear stress predictions are achieved during early run-up. • Bed shear stress is essential for correctly predicting morphodynamic evolution.