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Abstract We conducted field work in South San Francisco Bay to examine cohesive sediment flocculation dynamics in a shallow, wave‐ and current‐driven estuarine environment. Drawing on data collected using a suite of acoustic and optical instrumentation over three distinct seasons, we found that the factors driving floc size variability differed substantially when comparing locally sourced sediment (i.e., through wave‐driven resuspension) to suspended sediment advected from upstream. Statistical analysis of our extensive field data revealed additional seasonal variability in these trends, with wave stress promoting floc breakup during the summer and winter months, and biological processes encouraging floc growth during the spring productive period. Combining these data with fractal dimension estimates, we found that seasonally varying floc composition can lead to differences in floc settling velocity by a factor of approximately two to five for a given floc size. Finally, by analyzing co‐located turbulence and sediment flux measurements from the bottom boundary layer, we present evidence that the relationship between floc size and the inverse turbulent Schmidt number varies with floc structure. These results can be used to inform sediment transport modeling parameterizations in estuarine environments. 

Abstract Over the course of a year, we conducted three field deployments in South San Francisco Bay to examine seasonal variability in bottom drag. Our data consisted of turbulence measurements both within and outside the bottom boundary layer and benthic characterization surveys adjacent to our study site. Our results suggest that canopies of benthic worm and amphipod feeding tubes, which were denser during summer, can increase the drag coefficient by up to a factor of three relative to the smoother beds found in winter and spring. The extent of the drag increase varied depending on the measurement device, with the greatest increase inferred by measurements taken further from the bed. The small scale and temporally varying population densities of these living roughness elements pose significant challenges for hydrodynamic models, and future work is needed to begin incorporating benthic biology statistics into drag coefficient parameterizations. 

Abstract 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. 

Abstract We report direct measurement of drag forces due to tidal flow over a submerged seagrass bed in Ngeseksau Reef, Koror State, Republic of Palau. In our study, drag is computed using an array of high‐resolution pressure measurements, from which values of the drag coefficients,C_D, referenced to measured depth‐averaged velocities,V, were inferred. Reflecting the fact that seagrass blades deflect in the presence of flow, we find thatC_Dis O(1) when flows are weak and tends toward a value of 0.03 at the highest velocities, behavior that is consistent with existing theory for canopy flows with flexible canopy elements. A limited subset of velocity profiles obey the law of the wall, producing values of shear velocity that, while noisy, broadly agree with values inferred from the pressure measurements.