Bottom Stress in Wind-Driven Depth-Averaged Coastal Flows

Abstract The relationship between depth-averaged velocity and bottom stress for purely wind-driven flows in unstratified coastal waters is examined using a one-dimensional (vertically resolving) current model. Results indicate that conventional drag laws employed in depth-averaged coastal circulation models may produce large directional errors in bottom stress estimated. A drag tensor is suggested as an alternative to conventional drag coefficient formulations. The drag tensor allows for variation in direction between depth-averaged velocity and bottom stress. Two physically relevant cases are studied in order to quantify drag tensor variation as a function of water depth, wind stress and bottom roughness. The drag tensor allows for phenomena excluded by drag coefficient formulations. Results indicate that the drag tensor is a simple and physically pleasing alternative to dray coefficients.