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Sediment Fluxes through Carquinez Strait
 Sediment supply to a subembayment of an estuary is determined by watershed sediment input and the sediment exchange with adjacent embayments. Sediment supply is a critical variable for investigations of habitat stability, restoration potential, and contaminant fate/transport. Suspended sediment is needed to create and sustain valuable estuarine habitats such as tidal wetlands (Pont et al., 2002; Reed, 2002; Temmerman et al., 2003; Zedler and Callaway, 2001), though sediment-associated contaminants can also accumulate wherever sediment preferentially deposits (Arzayus et al., 2002; Hornberger et al., 1999; Taylor et al., 2004). In addition, nutrients and biota accumulate near estuarine turbidity maxima (ETM), where high suspended-sediment and contaminant concentrations are found (Peterson et al., 1975; Jassby and Powell, 1994). Enhanced biological activity in these areas may increase contaminant uptake by the food web (Kimmerer et al., 1998).
These issues converge in Suisun Bay, California. Over 90% of marsh area has been lost in San Francisco Bay since the 19th century, and current management goals in Suisun Bay include marsh restoration. Deposited sediment in Suisun Bay is thought to be high in mercury concentrations due to 19th century gold mining in the watershed (Hornberger et al., 1999), leading to concerns about resuspension of these deposits over long timescales (>10 y). Jaffe et al. (1998) show net erosion in Suisun Bay since the first bathymetric surveys in the 19th century. The net sediment budget of Suisun Bay in the current era may shed light on the viability of habitat restoration as well as the magnitude of mercury introduction to the water column. In order to quantify this sediment budget, sediment flux at the upstream and downstream boundaries of the subembayment must be determined.
Proper estimation of suspended-sediment flux through Carquinez Strait must account for the entire cross-section. While monitoring a large cross-section is both physically and financially impossible over the necessary timeframes, it may be possible to identify surrogate data that can be continuously monitored. In this study we occupied portions of the channel with autonomous equipment for three months, and calibrated those data to tidal-cycle cross-sectional measurements. The results from the three-month deployment were extrapolated to quantify fluxes over water year 2004, and other years when surrogate data were available.
The surrogates are upstream watershed discharge, suspended-sediment concentration at one location in the Strait, and the longitudinal salinity gradient. The first two surrogates substitute for tidally averaged discharge and velocity-weighted suspended-sediment concentration in the Strait, thereby providing advective flux estimates, while Stokes drift is estimated with suspended-sediment concentration alone. Dispersive flux is estimated using the product of longitudinal salinity gradient and the root-mean-square value of velocity-weighted suspended-sediment concentration as an added surrogate variable. Cross-sectional measurements validated the use of surrogates during the monitoring period. During high freshwater flow advective and dispersive flux were in the downstream direction, while upstream dispersive flux dominated and advective flux approached zero during low freshwater flow. Stokes drift flux was consistently in the upstream direction. Wetter than average years led to net export from Suisun Bay, while dry years led to net sediment import. Relatively low watershed sediment fluxes to Suisun Bay contribute to net export during the wet season, while gravitational circulation in Carquinez Strait and higher suspended-sediment concentrations in San Pablo Bay (downstream end of Carquinez Strait) are responsible for the net import of sediment during the dry season. Annual predictions of suspended-sediment fluxes, using these methods, will allow for a sediment budget for Suisun Bay, which has implications for marsh restoration and nutrient/contaminant transport. These methods also provide a general framework for estimating sediment fluxes in estuarine environments, where temporal and spatial variability of transport are large.
Publication
Ganju, N.K. and Schoellhamer, D.H., 2006, Annual sediment flux estimates in a tidal strait using surrogate measurements. Estuarine, Coastal and Shelf Science, 69, 165-178.
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