Three-Dimensional Modeling of Sediment Trapping and Dispersal on River- Influenced Continental Shelves

Many of the processes that transport sediment in coastal seas (wave/current resuspension, wind-, tidal-, and buoyancy-forced currents) are included in a three-dimensional, numerical model that has been applied to the Eel River shelf, northern California (Harris, Geyer and Signell, 2000). This accurately estimates the volume of sediment deposited on the shelf by floods of the Eel River, but predicts deposition on the inner shelf (Figure 1), which is contrary to observations that flood deposition is concentrated off-shore of the 50m isobath (Wheatcroft, et al., 1997). Evidence from the Eel River Shelf shows that sediment concentrations can become high enough (> 10’s g/L) in the near-bed region to induce down-slope transport driven by the weight of the suspension (Ogston, et al., 2000; Traykovski, et al., 2000). To account for sediment dispersal on energetic, depositional margins, it therefore appears necessary to consider gravity-driven transport of sediment, wave-current resuspension, and plume processes. Dense near-bed layers were not represented in the calculations shown in Figure 1, and models that do include them neglect either the three-dimensionality of the system, or transport processes above the waveboundary layer (see, e.g. Scully and Friedrichs, in revision). We have incorporated gravitationally-forced transport of a dense near-bed layer into our calculations so that we can compare the relative contributions of this mechanism with wave/current resuspension and plume processes and evaluate whether these three processes can explain observed shelf deposition.