Quantification of sediment bed - water column exchange processes in the South San Francisco Bay estuary
The salinity gradient formed by the mixing of saline ocean waters and fresh river waters within estuaries forms diverse habitats and creates the conditions for highly dynamic sediment behavior. Sediments which enter the estuarine system may pass multiple times through the water column and sediment bed before finally being carried out to sea or deposited as a more permanent part of the sediment bed. The exchange of sediments between the water column and sediment bed is affected not only by physical forcing such as tides and winds but also be properties inherent to estuarine sediments including flocculation and consolidation. Like many others, the San Francisco Bay estuary has been highly impacted by human activities over the last 150 years. Faced with an ever changing environment, understanding how the estuary will be impacted in the future depends in part upon understanding how sediments move between the bed and water column.
Two instrument deployments were carried out in the spring and fall of 2009 which measured suspended sediment concentrations, sediment bed properties and water column physical properties in the South San Francisco Bay. Analysis of the in situ measurements from the instrument deployments, sediment cores collected at the study site and results from a coupled sediment bed - water column numerical model of the instrument deployment period were used to estimate several important properties of sediments in the estuary. The depth of the layer of sediments readily resuspended from the bed was found to be less than 1 cm, an order of magnitude smaller than had been previously assumed. Acoustic Doppler velocimeters positioned near the bed were used to make collocated measurements of water velocity and suspended sediment concentrations (SSC). Turbulence data extracted from the velocity time series was used to develop sediment erosion relationships as a function of bed shear stress. Settling velocities were also estimated from the suspended sediment time series data. Upward looking acoustic Doppler current profilers were also deployed to measure water column velocities and collocated SSC. An error analysis showed that, with the proper information, the SSC calibrations produced reliable values. Results from the coupled sediment bed - water column model corroborated many of the results estimated from the experimental data. The results from this study provide several important estuary specific values for sediment exchange processes and demonstrate the need to use in situ measurements to test our mechanistic understanding of these processes.