Numerical simulation of sediment suspension and transport under plunging breaking waves

Abstract

We perform a simulation-based study on the suspension and transport of sediment under a plunging wave breaker. Navier–Stokes equations for a multi-phase incompressible flow are solved using an in-house finite-difference code. The air–water interface is captured by using the coupled level-set and volume-of-fluid method. The convection–diffusion equation with a settling term for a passive scalar is solved to describe the transport of the sediment concentration. The pick-up function is used to address the sediment flux at the bottom. Five cases with different water depth and wave steepness have been simulated. Both instantaneous and statistical results on the sediment concentration are analyzed to investigate the processes of sediment suspension and transport. It is observed from the simulation results that during wave breaking, sediment is suspended due to the high shear stress at the water bottom. The wave breaking also induces an upward motion of water between two large scale counter-rotating vortices, which in turn transports the suspended sediment towards the water surface. Based on the analysis of the mass center of suspended sediment, the process of sediment transport under the plunging breaking wave is divided into the erosion stage, suspension stage, and transport stage. The effects of water depth and wave steepness on the bulk concentration of suspended sediment, bottom erosion and deposition fluxes, and the mass center of sediment are discussed. As the water depth decreases and as the wave steepness increases, more sediment is picked up during the wave breaking. The bed deformation is also predicted based on the transport equation of the bed elevation.