A 3D SEDIMENT TRANSPORT MODEL FOR COMBINED WAVE-CURRENT FLOWS

Abstract

Accurate prediction of current velocity and bottom shear stress, which both can be significantly influenced by wind waves, is essential for sediment transport predictions in the coastal environment. Consequently wind-wave effects must be taken into account in a numerical sediment transport model for application in coastal waters. In the present study, elements of a large-scale 3D numerical coastal circulation and sediment transport model are developed to predict net, i.e. the wave-period-averaged, sediment transport rates. The sediment transport components considered are (i) bed-load transport; (ii) mean suspended load sediment transport within the wave boundary layer, which is obtained from an analytical solution; and (iii) suspended load sediment transport above the wave bottom boundary layer, which is obtained from a numerical model. In all model components wind wave effects are accounted for through simple analytical models. Thus, the roughness prescribed for the hydrodynamic part of the numerical coastal circulation model is the apparent roughness, i.e. the roughness experienced by a slowly varying current in the presence of waves. Similarly, the reference concentration specified for the sediment transport part of the numerical model is obtained from analytical solutions for suspended sediment concentrations within the combined wave-current bottom boundary layer. Stratification effects caused by suspended sediment are included in the large-scale numerical sediment transport model. Results of idealized tests suggest that wind wave effects can be pronounced, e.g. in some typical coastal scenarios sediment can only be mobilized when wind waves are present and accounted for. It is also shown that stratification can significantly affect suspended sediment transport rates of fine sediments.