Modeling stratified wave and current bottom boundary layers on the continental shelf

The Glenn and Grant [1987] continental shelf bottom boundary layer model for the flow and suspended sediment concentration profiles in the constant stress layer above a noncohesive movable sediment bed has been updated. The Reynolds fluxes for sediment mass and fluid momentum are closed using a continuous, time-invariant linear eddy viscosity modified by a continuous stability parameter to represent the influence of suspended sediment-induced stratification throughout the constant stress region. Glenn and Grant [1987] use a less realistic discontinuous eddy viscosity and neglect the stratification correction in the wave boundary layer. For typical model parameters the two models produce currents above the wave boundary layer that are in better agreement than the suspended sediment concentrations. Within the wave boundary layer the differences are much greater for both the current and the sediment concentration. This leads to significant differences in the sediment transport throughout the constant stress layer. Sensitivities of the updated model were examined on the basis of observed wave and current data acquired during storms on the inner continental shelf. Comparisons between the stratified and neutral versions of the updated model indicate that the stratified version produces a total depth-integrated sediment transport that can be 2 orders of magnitude less than, and time-averaged shear velocities that can be nearly half of, that predicted by the neutral version. Sensitivities to grain size distributions indicate that even a small amount of finer sediment can stratify the storm-driven flows. Sensitivities to closure constants within the range of reported values also produce up to an order of magnitude variation in sediment transport, illustrating the need for dedicated field experiments to refine further estimates of these parameters.