Observations of Near-Bed Shear Stress in a Shallow, Wave- and Current-Driven Flow

We present in situ observations of mean and turbulent bottom stresses in a shallow, wave- and current-driven flow over a cohesive sediment bed on the eastern shoals of South San Francisco Bay. Data from a Nortek Vectrino Profiler deployed with its measurement volume overlapping the bed allowed us to calculate mean velocity profiles and turbulent Reynolds stresses over a 1.5-cm profile with 1-mm vertical resolution. Additional acoustic instrumentation and pressure sensors provided mean current and wave data. From these observations we found that biological roughness elements protruding from the sediment bed result in a mean velocity profile qualitatively similar to that found in canopy shear mixing layers. Despite fundamental differences between this measured velocity structure and that assumed by wave-current boundary layer models, we also found that the addition of waves to mean currents increases the net drag felt by the flow. The near-bed momentum flux was often dominated by a wave-induced component, which was generated by interactions between the wavy flow and the rough bed. Finally, we estimated the friction velocity using several different calculation methods and compared results to the measured bottom stress. This analysis revealed that traditional methods (e.g., log law fitting and point turbulence measurements) are consistent with one another when measuring the stress outside the wave boundary layer but were all poor approximations of the total stress at the bed. Plain Language Summary We conducted field work in South San Francisco Bay to investigate how waves and tidal currents exert stress on a muddy sediment bed. Our data show that the average velocity near the bed is qualitatively similar to flow over a submerged canopy, for example, flow over seagrass, or atmospheric flow over a city. This velocity structure was caused by dense colonies of bottom-dwelling worms at the bed. We also found that the addition of waves to the tidal currents caused the flow to feel additional drag. Finally, we compared various methods of estimating the shear stress near the bed and found that none of the estimation methods could adequately represent the measured bed stress. These results are important for sediment transport modeling in estuaries.