Numerical modeling of pressure gradients in swash-swash interactions driven by a double dam-break

Swash zone hydrodynamics control shoreline change and beach erosion/accretion. Understanding of swash hydrodynamics on a wave-by-wave basis is needed for the parameterization of numerical models to predict these morphodynamics. Detailed hydrodynamics have been investigated in the laboratory by analyzing single swash events. However, swash-swash interactions, occurring on natural beaches, play an important role on the spatial and temporal distributions of hydrodynamic parameters controlling sediment transport (e.g., pressure gradients). In this work, swash interactions are investigated using a phase- and depth-resolving numerical model (VOF-RANS). The numerical model is validated with free-surface elevation and nearbed swash velocity measurements, from double dam-break laboratory experiments, for different swash interaction types (capture, weak, and strong interactions). A satisfactory agreement of the complex swash dynamics was found between the numerical model and the laboratory data (typical correlation values > 0.92 and root-mean-square-error <0.35 m/s for nearbed velocity). The numerical model was employed to investigate the spatial and temporal evolution of horizontal and vertical pressure gradients. Simulations indicate the bed-parallel total force associated with swash interactions can be approximated by the total acceleration in the weak and strong interactions cases mainly at the maximum peaks associated with the pressure gradient. In the capture case, there is a poor fit in the outer and middle swash zone, with model skill improvement in the inner swash zone. However, large differences were predicted between the total force and the total acceleration in the bed-orthogonal direction near the bed, implying turbulence stresses cannot be neglected. The location of the maximum pressure gradient is strongly correlated with the normalized swash separation time and the excursion ratio of interacting bores. The numerical results suggest the potential parameterization of hydrodynamic conditions based on runup time series.