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.

Novel analysis of in-situ acoustic and optical data collected in a shallow, wave- and current-driven environment enabled determination of: (1) particle characteristics that were most affected by near-bed physical forcing, and (2) characteristic shear stress, τchar, at which the rate of changes to particle characteristics were most pronounced. Results indicated that moderate τchar values of 0.125 Pa drove changes in particle composition during summer. In winter, particle concentration effects were most affected at τchar of 0.05 Pa, suggesting dominance of fluff layer resuspension. Changes to particle size were most relevant during a biologically productive springtime period, with initiation of particle disaggregation occurring most commonly at τchar of 0.25 Pa. The values of τchar determined from acoustic and optical data are consistent with laboratory-based measurements of critical shear stress for resuspension of site sediments, and with typical critical shear stress values for resuspension of cohesive sediments as reported in the literature.

We conducted field work on the shoals of South San Francisco Bay to elucidate the mechanisms driving cohesive sediment erosion in a shallow, wave- and current-driven flow. Compiling data from three deployments, including measurements taken within the combined wave-current boundary layer, we found that waves were strongly correlated to turbulent sediment fluxes across all seasons and a range of deployment depths. Tidal turbulence was only correlated to turbulent sediment fluxes for larger relative depths, or when a wave-driven sediment flux into the boundary layer allowed the tidal shear stress to transport sediment into the overlying flow. Despite the dominance of waves in eroding sediment, we found favorable agreement between in situ boundary layer erosion measurements and laboratory erosion measurements conducted in a steady flume. Results were analyzed in the context of two benthic surveys which provided insight into the sediment bed properties.