A continental shelf bottom boundary layer model : the effects of waves, currents, and a movable bed

Abstract

A simple model for the bottom boundary layer on the continental shelf is presented. The governing equations are developed for a stratified, turbulent Ekman layer in a combined wave and current flow over a moveable sediment bed. An eddy diffusivity closure scheme that includes the effect of suspended sediment, temperature, and salinity induced stratification on the vertical turbulent diffusion of mass and momentum couples the resulting unsteady conservation equations for fluid momentum, fluid mass, and suspended sediment mass. The wave velocity, current velocity, and suspended sediment concentration profiles predicted by the simultaneous solution of the conservation equations require the physical bottom roughness and a sediment reference concentrati on to be specified as boundary conditions. The physical bottom roughness associated with biologically generated bedforms, wave generated ripples, and near bed sediment transport are calculated as functions of the flow and sediment conditions. Using expressions for the height of sediment transporting layer and the sediment velocity, an expression for the sediment reference concentration is developed by matching laboratory measurements of sediment transport rates in oscillatory flow. The model predicts that the bottom flow field is highly dependent on (1) the nonlinear wave and current interaction, which increases the boundary shear stress and enhances vertical turbulent diffusion, (2) the effect of the boundary shear stress on a moveable sediment bed, which determines the physical bottom roughness and the amount of sediment in suspension, and (3) the effect of stable stratification, which inhibits vertical turbulent transport and couples the flow to the suspended sediment and fluid density profiles. The validity of the theoretical approach is supported by model predictions that are in excellent agreement with high quality data collected during two continental shelf bottom boundary layer experiments for a wide range of flow and bottom conditions.