Double dam-break-generated swash flows on a rough planar beach

Swash–swash interactions are investigated using a numerical model. The numerical model is based on open field operation and manipulation (OpenFOAM), which solves the Reynolds-averaged Navier–Stokes equations for two immiscible fluids with a k-ω shear stress transport turbulence closure. The numerical results are validated with laboratory data for swash flows on a rough planar beach driven by a double dam-break swash apparatus. Good agreement is obtained for free surface elevation, flow velocity, and horizontal pressure gradient. Based on the numerical results, swash–swash interactions are classified into wave merging (WM), wave capturing (WC), and backwash–uprush interaction (BUI), with distinct flow characteristics identified for each swash type. The analysis shows the potential for bed instability due to shear stress over a wider region for an extended duration during the WM backwash stage. Bed instability induced by pressure gradients is observed during the WC and BUI interaction stages. The BUI type generates the strongest turbulent kinetic energy, featuring intense rolling motion and enhanced flow near the bed, contributing to elevated potential for bed failure in the swash zone. These findings contribute to understanding the hydrodynamics and bed instability characteristics in the swash zone, providing insights for predicting coastal erosion processes.