Modeling of net sediment transport rate due to wave-driven oscillatory flow over vortex ripples

Abstract

A one-dimensional-vertical (1-DV) model is developed for predicting net ripple-averaged sediment transport rate over a rippled bed under wave-driven oscillatory boundary layer flows. This model is extended from a sheet-flow model developed by Yuan and Tan [52], which is able to account for the effect of wave nonlinearity (velocity skewness and asymmetry) together with a mild bottom slope. The rippled bed is conceptualized as a flat bed with a large bottom roughness accounting for the presence of ripples in modeling the boundary layer flow and sediment concentration. The vortex-shedding effect is incorporated into the model by adding a pick-up rate on top of the one induced by skin-friction turbulence. The model performance is good for 2-D ripples under periodic oscillatory flows, which are used for calibrating the model. We also showed that it is applicable for 3-D ripples and irregular oscillatory flows. In addition, this 1-DV model can capture some intermediate ripple- and period-averaged physical processes, e.g., cycle averages of velocity, sediment concentration and sediment flux, suggesting that the model correctly captures the underlying physics. The relative importance of vortex-shedding effect is shown to be controlled by ripple dimension, sediment grain size and the driving mechanisms for net sediment transport rates.