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

A simple model for the bottom boundary layer on the continental shelfis presented. The governing equations are developed for a stratified,turbulent Ekman layer in a combined wave and current flow over a moveablesediment bed. An eddy diffusivity closure scheme that includes theeffect of suspended sediment, temperature, and salinity inducedstratification on the vertical turbulent diffusion of mass and momentumcouples the resulting unsteady conservation equations for fluid momentum,fluid mass, and suspended sediment mass. The wave velocity, currentvelocity, and suspended sediment concentration profiles predicted by thesimultaneous solution of the conservation equations require the physicalbottom roughness and a sediment reference concentrati on to be specifiedas boundary conditions. The physical bottom roughness associated withbiologically generated bedforms, wave generated ripples, and near bedsediment transport are calculated as functions of the flow and sedimentconditions. Using expressions for the height of sediment transportinglayer and the sediment velocity, an expression for the sediment referenceconcentration is developed by matching laboratory measurements ofsediment transport rates in oscillatory flow. The model predicts thatthe bottom flow field is highly dependent on (1) the nonlinear wave andcurrent interaction, which increases the boundary shear stress andenhances vertical turbulent diffusion, (2) the effect of the boundaryshear stress on a moveable sediment bed, which determines the physicalbottom roughness and the amount of sediment in suspension, and (3) theeffect of stable stratification, which inhibits vertical turbulenttransport and couples the flow to the suspended sediment and fluiddensity profiles. The validity of the theoretical approach is supportedby model predictions that are in excellent agreement with high qualitydata collected during two continental shelf bottom boundary layerexperiments for a wide range of flow and bottom conditions.

Funding for the work resulting in this Thesis has been provided bythe American Gas Association (Project No. PR-153-126), the NationalScience Foundation (Grant No. OCE~8014930), and NOAA-Sea Grant(NA-79AA-D-0010l; NA 79AA-D-00102).

Submitted in partial fulfillment of the requirements for the degree of Doctor of Science at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution January 1983