Bed drag coefficient variability under wind waves in a tidal estuary: Field measurements and numerical modeling

In this dissertation, I 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. I compare shear stresses calculated via high-frequency pointwise velocity measurements, time-averaged vertical profiles of velocity, and the model of Styles & Glenn (2000). Although this model was formulated to predict the drag coefficient under ocean swell on the continental shelf, results from my experiments show that it accurately predicts shear stress under wind waves in an estuary, albeit only very close to the bed. Otherwise, the steady wind-driven boundary layer at the free surface overlaps with the steady bottom boundary layer, and the model's assumptions are invalidated. By calculating the wind stress at the surface and assuming a linear variation of shear between the bed and surface, however, the model predicts water column shear stresses well in agreement with data. Comparison is also made between the wave-turbulence decomposition schemes of Shaw and Trowbridge (2001), Benilov & Filyushkin (1970), and a spectral scheme. This comparison reveals a large amount of wave-turbulence interaction, with turbulent shear stresses being enhanced at wave frequencies.; I then apply the variable C_D determined by Styles & Glenn to the estuarine circulation model TRIM-3D of Gross (1997), and use it to examine the effects of variable roughness on contaminant and sediment transport in South San Francisco Bay. I also investigate the importance of the wave model used in TRIM-3D, by comparing the results of Inagaki's (2000) fetch and wave model with those of SWAN (Booj et al., 1999). The different wave models I use generate noticeably different trends in sediment transport and roughness variation.; Given a wave model, I find that tidal stage is quite insensitive to variability in roughness, but tidally-averaged rms and residual currents are quite sensitive (modified by up to 20%). Sediment erosion and deposition trends are also sensitive to roughness to this degree, and variable roughness extends the hydraulic residence time (inhibits flushing) in Lower South San Francisco Bay from 18 days to 19 days.