A numerical study of intermittent sediment concentration under breaking waves in the surf zone

A three-dimensional Large Eddy Simulation (LES) model is presented together with a sediment pickup/advection module for the study of spatial and temporal variation of sediment concentration due to three-dimensional fluid motion under breaking waves on a sloping beach. The LES flow module calculates the turbulent flow due to wave breaking where a large-scale turbulence can be directly solved by the governing flow equations and a small-scale turbulence can be evaluated using a sub-grid-scale (SGS) model. In the sediment pickup/advection module, the sediment pickup rate is estimated by a sediment pickup function using the Shield's number at the bottom of the fluid layer, and suspended sediment particles are assumed to move with the ambient fluid. The numerical results show that the flow field including the wave breaking is represented well by the LES model and the sediment pickup/advection module is able to simulate spatial and temporal intermittency of the sediment suspension. Three noticeable sediment pickup areas are defined, namely around the breaking point, break impact area and run-up zone. In the inner region of the surf zone, the sediment concentration distribution shows strong three-dimensionality. The calculated results of the time-averaged and the equi-phase-averaged sediment concentration distributions are compared to earlier laboratory experiment results. The sediment concentration distribution and advection calculated by the model show general correspondence to the experimental results, but the averaged sediment concentration is approximately 1.5 times greater than the experimental value.