Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997

San Francisco Bay is an urbanized estuary that has been affected by anthropogenic activities (Nichols and others, 1986). The U.S. Geological Survey began a broad program of scientific study in San Francisco Bay in 1968 (Cloern and others, 1994), and sediment chemistry and sediment transport have been and continue to be elements of this program. This abstract discusses present USGS sediment-transport research in the Bay, which includes analysis of time series of suspended-solids concentrations, remote sensing, particle-size analysis, bottom boundary layer processes, bedform migration, wetlands morphology, and changes in sedimentation rates. Another abstract in this volume discusses sediment chemistry research (Luoma, this volume).

Sediments are an important component of the San Francisco Bay estuarine system. Potentially toxic substances, such as metals and pesticides, adsorb to sediment particles (Kuwabara and others, 1989; Domagalski and Kuivila, 1993). The sediments on the bay bottom provide the habitat for benthic communities that can ingest these substances and introduce them into the food web (Luoma and others, 1985). Bottom sediments also are a reservoir of nutrients that contribute to the maintenance of estuarine productivity (Hammond and others, 1985). The transport and fate of suspended sediments are important factors in determining the transport and fate of constituents adsorbed on the sediments. In Suisun Bay, the maximum concentration of suspended solids usually marks the position of the turbidity maximum, which is a crucial ecological region in which suspended sediments, nutrients, phytoplankton, zooplankton, larvae, and juvenile fish accumulate (Peterson and others, 1975; Arthur and Ball, 1979, Jassby and Powell, 1994). Suspended sediments also limit light availability in the bay, which, in turn, limits photosynthesis and primary production (Cloern, 1987; Cole and Cloern, 1987), and deposit in ports and shipping channels, which must be dredged to maintain navigation (U.S. Environmental Protection Agency, 1992).

To determine the effects of wind waves, tidal currents, spring/neap cycle, watershed runoff, and other factors that affect suspended-solids concentration (SSC) in San Francisco Bay (Schoellhamer, 1996), a network of eight sites at which SSC is monitored has been established (Buchanan and Schoellhamer, 1996). Optical backscatterance sensors are deployed at two depths at each site, and measurements are automatically collected every 15 minutes. Water samples are collected periodically and analyzed for SSC, and the results of these analyses are used to calibrate the sensors. In addition to the continuous monitoring sites, which are located in relatively deep water (greater than 10 meters), instrument packages that measure current velocity, water depth, SSC, and wave properties have been deployed in relatively shallow waters (about 3 meters) of the Bay for periods of several weeks (Buchanan and others, 1996; Warner and others, in press).

The monitoring sites provide good temporal resolution but poor spatial resolution, so remote sensing techniques are being developed to map SSC distribution in the Bay. Satellite images for San Francisco Bay during 1994-96 were selected on the basis of hydrologic and meteorologic conditions and processed. The images show where SSC in the Bay and adjacent coastal waters was increased by floods in 1995. SSC data from the monitoring stations in the Bay and a semi-analytic method (Stumpf and Pennock, 1989) were used to relate satellite-observed reflectance to SSC.

Instrumentation is being developed and tested to measure the in-situ size distribution of suspended particles in the Bay, which are typically fine grained and flocculated (Krank and Milligan, 1992). A Laser In-situ Sediment Scattering Transmissometer (LISST, Agrawal and Pottsmith, 1994) from the Navy that is designed to record time series of particle-size distribution has been deployed once in San Francisco Bay and twice in San Diego Bay. During these deployments, independent measurements of SSC were made. Ralph Cheng has worked very closely with the manufacturer to process the LISST data. Thus far, only qualitative agreement between the LISST and independent measurements of SSC has been achieved. The LISST has potential for in-situ determination of sediment particle-size distribution, but it requires additional development. A LISST is being purchased to continue developing this capability and it will be tested initially in the lab.

Hydrodynamics is an important factor in determining the resuspension and transport of sediments in the Bay, so concurrent measurement of water velocity, water depth, and wave properties are often collected with SSC time-series data. Acoustic Doppler Current Profilers (ADCP) are used to collect velocity profiles either in the bottom boundary layer (Gartner and Cheng, 1996; Cheng and others, in press; Gartner and others, in press) or throughout the water column (Burau and others, 1993; Cheng and others, 1996). Point velocity meters are deployed (Cheng and Gartner, 1984; Buchanan and others, 1996) and also can be used to measure properties of wind waves in shallow water (Schoellhamer, 1995). The Geoprobe instrument tripod (Cacchione and Drake, 1979) contains several point velocity meters that are used to measure velocity profiles in the bottom boundary layer (Sternberg and others, 1986, Cacchione and others, 1996). Velocity and water-depth measurements are used to calculate horizontal, residual (tidally averaged) fluxes of suspended solids (Tobin and others, 1995; Lacy and others, 1996; Warner and others, in press), and bottom boundary layer measurements provide a better understanding of sediment resuspension and deposition (Cacchione and others, 1996; Gartner and others, in press). In addition, a numerical model of sediment transport in the Bay has been developed (McDonald and Cheng, in press).

Bedform migration is being studied in Central San Francisco Bay, initially with field surveys using side-scan sonar and sub-bottom profiler mapping. On the basis of side-scan sonar records (Rubin and McCulloch, 1979), the Geoprobe tripod (Cacchione and Drake 1979) with rotating side-scan sonar (Rubin and others, 1983) and an upward-looking ADCP will be deployed in a well-defined field of large sand waves off Tiburon in Central San Francisco Bay during summer 1997. The repeated side-scan sonar images will reveal the movement of sand ripples and sand waves, while the Geoprobe system will provide simultaneous measurements of water levels, currents, and suspended sediments. Bedform migration models of Rubin and Hunter (1987) will be tested with these data, and transport estimates for the region will be made.

In an attempt to recover some of the 95 percent of the San Francisco Bay tidal wetlands that have been converted to other land types since 1850 (Dingler, 1994), various Federal and State agencies have banded together to restore 350 acres of farm land to tidal wetland. Morphodynamic processes will be monitored to improve the design of future restoration sites. This information is provided by measuring sedimentation patterns and rates of change, geotechnical characteristics, and tidal-channel development within the restoration site and in the existing tidal wetland bayward of the site.

To study sedimentation processes at the decadal time scale, bathymetric surveys have been analyzed to determine spatial and temporal sedimentation patterns (Jaffe and others, in press). Bathymetric surveys of San Pablo Bay, a subembayment of San Francisco Bay, indicate that a reduction in riverine sediment supply (Oltmann, 1996) is reflected by a decrease in sediment volume (increasing water depth) during the latter half of the 20th century. In addition, the surface area of mudflats in San Pablo Bay is decreasing. Mudflats are shallow-water areas adjacent to wetlands that probably contribute sediments to wetlands.

Although the studies mentioned in this extended abstract pertain to San Francisco Bay, the techniques and findings of the studies are applicable to other sediment studies conducted by the USGS. Sediment erosion, transport, and deposition are of critical importance to a broad range of environmental and economic issues, many of which are addressed by the USGS: coastal erosion, reservoir sedimentation, pollutant transport, and the origin of sedimentary deposits, such as petroleum reservoirs. In addition to diverse projects in the USGS, industry, academia, and other federal and state government agencies are confronted with problems associated with sediment transport. Despite the importance of such projects, their success is often severely handicapped by a lack of ability to accurately measure and predict the rate and direction of sediment transport as a function of flow conditions, particularly where flow varies through time or spatially. Studies of sediment chemistry and of the effect of sediments and sediment chemistry on biological resources require knowledge of how sediments move in the environment.

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