BED DRAG COEFFICIENT VARIABILITY IN A TIDAL ESTUARY: FIELD MEASUREMENTS AND NUMERICAL MODELING

Abstract

In this paper, we report the results of a study of the variation of shear stress and the bottom drag coefficient C_D with sea state and currents at a shallow site in San Francisco Bay. Via field experiments, we find that themodel of Styles & Glenn, though formulated to predict C_D and shear stress under ocean swell on the continental shelf, accurately predicts enhanced drag and mean stress under wind waves in an estuary, albeit only very close tothe bed. Higher up in the water column, the steady wind-driven boundary layer at the free surface overlaps with the steady bottom boundary layer, and this overlap needs to be accounted for to accurately model shear stress.
Applying the enhanced C_D of Styles & Glenn to the estuarine circulation model TRIM-3D of Gross et al and Inagaki et al, we find that enhanced drag under wind waves rctards the flushing of contaminants and sediment in South San Francisco Bay, and it enhances channel-shoal asymmetry.