Sediment-Induced Stratification in an Estuarine Bottom Boundary Layer

We took field observations on the shallow shoals of South San Francisco Bay to examine how sediment-induced stratification affects the mean flow and mixing of momentum and sediment throughout the water column. A Vectrino Profiler measured near-bed velocity and suspended sediment concentration profiles, which we used to calculate profiles of turbulent sediment and momentum fluxes. Additional turbulence statistics were calculated using data from acoustic Doppler velocimeters placed throughout the water column. Results showed that sediment-induced stratification, which was set up by strong near-bed wave shear, can reduce the frictional bottom drag felt by the mean flow. Measured turbulence statistics suggest that this drag reduction is caused by stratification suppressing near-bed turbulent fluxes and reducing turbulent kinetic energy dissipation. Turbulent sediment fluxes, however, were not shown to be limited by sediment-induced stratification. Finally, we compared our results to a common model parameterization which characterizes stratification through a stability parameter modification to the turbulent eddy viscosity and suggest a new nondimensional parameter that may be better suited to represent stratification when modeling oscillatory boundary layer flows. Plain Language Summary In estuaries, mud is eroded through the combined effect of waves and currents exerting stress on the bed. Because the mud particles are denser than water, they tend to settle downward and form thin layers of high-mud-concentration fluid near the bed. This is called sediment-induced stratification. We measured sediment concentrations and flow properties near the bed in a shallow area of South San Francisco Bay to determine how sediment-induced stratification affected the interactions between waves, currents, and mud. We found that sediment-induced stratification can make the bed feel smoother by damping turbulence. These results are important for understanding and modeling how sediment moves around in estuaries.