Sediment suspension under sheet flow conditions beneath random waves plus current

The suspended sediment concentration under large random waves alone as well as under large random waves plus current has been examined by a classical sediment diffusion model in conjunction with a dynamic turbulent boundary layer model based on the linearized boundary layer equations with horizontally uniform forcing. The turbulence closure is provided by a high Reynolds number k – e model. Under such conditions wave or current-induced sea bed ripples are washed out, such that a sheet flow and a suspension layer of high sediment concentration exist. This sediment diffusion model has been verified against a range of existing measurements, for regular waves alone, as well as regular waves plus current. Overall, these comparisons show that the present sediment diffusion model does a fair job except in the outer suspension layer, where the existence of a `convective' peak near flow reversal becomes important. These peaks cannot be captured by standard sediment diffusion models. Correlations between suspended sediment concentration and boundary layer quantities due to the grouping of the largest waves in a realistic sea state have been examined. Sediment pump-up under a wave group is visualized. Estimates of probability density functions for individual sediment concentration maxima are given. Superposition of a mean current on the waves at the outer boundary induces a mean sediment flux as well as an enhancement of the suspended sediment concentration; these effects have been demonstrated and quantified. A non-zero angle between the waves and the current leads to a decrease in both the mean sediment concentration and the mean sediment flux. The resulting mean sediment concentration and mean sediment flux for random waves plus current has been shown to agree reasonably well with those obtained by an equivalent sinusoidal wave plus current. Overall, the results obtained in this paper support the application of an equivalent wave to predict mean sediment concentrations, mean sediment fluxes, as well as the total sediment transport under sheet flow conditions.