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

The Glenn and Grant [1987] continental shelf bottom boundary layer model fo r the flow and suspended sediment concentration profiles in the constant st ress layer above a noncohesive movable sediment bed has been updated. The R eynolds fluxes for sediment mass and fluid momentum are closed using a cont inuous, time-invariant linear eddy viscosity modified by a continuous stabi lity parameter to represent the influence of suspended sediment-induced str atification throughout the constant stress region. Glenn and Grant [1987] u se a less realistic discontinuous eddy viscosity and neglect the stratifica tion correction in the wave boundary layer. For typical model parameters th e two models produce currents above the wave boundary layer that are in bet ter agreement than the suspended sediment concentrations. Within the wave b oundary layer the differences are much greater for both the current and the sediment concentration. This leads to significant differences in the sedim ent transport throughout the constant stress layer. Sensitivities of the up dated model were examined on the basis of observed wave and current data ac quired during storms on the inner continental shelf. Comparisons between th e stratified and neutral versions of the updated model indicate that the st ratified 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. Sensitiv ities to grain size distributions indicate that even a small amount of fine r sediment can stratify the storm-driven flows. Sensitivities to closure co nstants within the range of reported values also produce up to an order of magnitude variation in sediment transport, illustrating the need for dedica ted field experiments to refine further estimates of these parameters.