Predictions of vertical sediment flux in oscillatory flows using a two-phase, sheet-flow model

A two-phase sheet flow model is utilized to investigate the wave-induced vertical sediment flux in the benthic boundary layer. The near-bed vertical sediment flux is a critical parameterization for suspended load models. In this paper, we show that the vertical sediment fluxes predicted by the two-phase model agree with the U-tube data for oscillatory flows. Both the measured data and model results indicated that the sediment upward flux (pick-up) increases with Shields parameter (or bottom bed stress) during the flow acceleration phase, but quickly reaches a saturated value with a " plateau" near the peak flow. This feature is also captured by the reference concentration formulation, but not the pick-up function formulation, developed empirically for steady flows. With further consideration of wave-current interaction and wave-induced streaming inside the boundary layer, we conclude that to accurately parameterize the upward sediment flux, the feedback of the sediment phase to the carrier flows must be included. The feedback mechanism is introduced through sediment stresses and drag forces.