Modeling the Effects of Bed Drag Coefficient Variability under Wind Waves in South San Francisco Bay

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 found that the model of Styles & Glenn, though formulated to predict C_D under ocean swell on the continental shelf, accurately predicted enhanced drag under wind waves in an estuary. Knowing this, we apply the enhanced drag coefficient determined by Styles & Glenn to the estuarine circulation model TRIM-3D of Casulli & Cattani and Gross et al, and use it to examine the effects of variable roughness on contaminant and sediment transport in South San Francisco Bay. We also investigate the importance of the wave model used in TRIM-3D, by comparing the results of Inagaki et al's fetch and wave model with those of SWAN. The different wave models we use generate noticeably different trends in sedimen transport and roughness variation. Given a wave model, we find that tidal stage in South San Francisco Bay is quite insensitive to variability in roughness, but tidally-averaged RMS and residual currents are quite sensitive (modified by about 10%), and channel-shoal asymmetry is enhanced. Along with this, the deep channel experiences more erosion of sediment, and the shoals more deposition. Furthermore, variable roughness extends the hydraulic residence time (inhibits flushing) south of the Dumbarton Bridge from 18 days to 19 days.