Estimates of bottom roughness length and bottom shear stress in South San Francisco Bay, California

A field investigation of the hydrodynamics and the resuspension and transpo rt of particulate matter in a bottom boundary layer was carried out in Sout h San Francisco Bay (South Bay), California, during March-April 1995. Using broadband acoustic Doppler current profilers, detailed measurements of tur bulent mean velocity distribution within 1.5 m above bed have been obtained . A global method of data analysis was used for estimating bottom roughness length z(o) and bottom shear stress (or friction velocities u*). Field dat a have been examined by dividing the time series of velocity profiles into 24-hour periods and independently analyzing the velocity profile time serie s by flooding and ebbing periods. The global method of solution gives consi stent properties of bottom roughness length z(o) and bottom shear stress va lues (or friction velocities u*) in South Bay. Estimated mean values of z(o ) and u* for flooding and ebbing cycles are different. The differences in m ean z(o) and u* are shown to be caused by tidal current flood-ebb inequalit y: rather than the flooding or ebbing of tidal currents. The bed shear stre ss correlates well with a reference velocity, the slope of the correlation defines a drag coefficient. Forty-three days of field data in South Bay sho w two regimes of z(o) (and drag coefficient) as a function of a reference v elocity. When the mean velocity is >25-30 cm s(-1), the In z(o) (and thus t he drag coefficient) is inversely proportional to the reference velocity. T he cause for the reduction of roughness length is hypothesized as sediment erosion due to intensifying tidal currents thereby reducing bed roughness. When the mean velocity is <25-30 cm s(-1), the correlation between z(o) and the reference velocity is less clear. A plausible explanation of scattered values of z(o) under this condition mat be sediment deposition. Measured s ediment data were inadequate to support this hypothesis: but the proposed h ypothesis warrants further field investigation.