Numerical modeling of gravity-driven sediment transport and deposition on an energetic continental shelf: Eel River, northern California

A two-dimensional numerical model was applied to predict large-scale deposition by wave-supported sediment gravity flows on the Eel River continental shelf for four consecutive flood seasons using measured bathymetry, waves and river forcing. The model assumes that sediment-induced stratification maintains the near-bed Richardson number at its critical value, which determines the sediment carrying capacity of the wave boundary layer. Deposition is predicted when the gravity-driven flux of sediment exceeds the carrying capacity. The model predicted 26% of fine sediment discharged by the Eel River to be deposited on the midshelf with a magnitude and distribution largely consistent with field observations. Greatest deposition on the midshelf was predicted well north of the river mouth despite greater sediment input nearest the river mouth. Model results indicate that when the river delivers sufficient sediment to critically stratify the wave boundary layer, wave intensity and the bathymetry of the Eel shelf are the dominant factors controlling the observed pattern of deposition. Large wave energy caused the majority of fine sediment (65%) to escape the shelf as gravity-driven flows. The greatest amount of sediment was predicted to leave the shelf from the region off-shelf of the river mouth (including 11% into the Eel Canyon) where inshore sediment input was high and the concave downward bathymetry associated with the Eel River subaqueous delta prevents significant midshelf gravity-driven deposition.