Full-2D to quasi-3D sediment transport models in surf zones

The mechanism of accretion beach has been the subject of extensive studies, as the mode of sediment transport is more sophisticated than bed-load transport alone. These studies have suggested a high concentration of suspended sediment near the free surface induced by strong plunging breakers. However, predictions by models have not been successful due to the localized interactions among the plunging jet, turbulent production, and sediment movement. This study proposes a new calculation model for turbulent flow and suspended sediment transport in the surf zone.

A numerical model was developed to simulate the flow and sediment motion in connection with plunging breakers in the surf zone. The Reynolds-Averaged Navier-Stokes (RANS) equations in two spatial dimensions were employed to simulate the flow field, together with a k-ε model for turbulence and the Volume of Fluid (VOF) method for multiple free-surface tracking. In quasi-3D, the Boussinesq equations were utilized to simulate the flow, combined with a k-ε turbulence model and a bottom boundary layer thickness method for estimating bottom shear stress. An advection-diffusion equation was used for the suspended sediment concentration with a bottom boundary condition based on the reference concentration formulation. The performance of the suspended sediment transport model under plunging breaking waves was examined through comparison with experimental data.

Good agreement between the model and experimental data was obtained for surface elevation and velocity, turbulent kinetic energy, eddy viscosity, bottom shear velocity and stresses, as well as suspended sediment concentration. The overturning waves, plunging jet, and transport of high concentrations of suspended sediment near the free surface are reproduced by the present model with a selected fine mesh resolution. The study shows the applicability of the present model in turbulent and suspended sediment-dominated regions induced by strong plunging breakers. The applicability of the new computational model is further tested on broad coasts under extreme and regular wave conditions. By incorporating bed-load, suspended load, and seabed evolution formulations into the present model, the performance of the new model in predicting erosion and accretion areas around coastal structures is presented and discussed.