Modeling of Critically-Stratified Gravity Flows: Application to the Eel River Continental Shelf, Northern California

An analytical and numerical model are presented and applied to predict gravitydriven transport and deposition o f fluid mud layers that form within the wave boundary layer on the continental shelf off the Eel River in northern California. Observations indicate that following floods of the Eel River down-slope transport of fluid mud trapped within the wave boundary layer is the dominant across-shelf transport mechanism. The models are based upon the assumption that following significant floods, an abundant supply o f easily suspended fine sediment is delivered to the coastal ocean, allowing a negative feedback mechanism to maintain the near-bed Richardson number at its critical value. Thus, sediment-induced stratification effectively limits the amount of fine sediment that can be maintained in suspension, allowing the calculation of down-slope transport and deposition knowing only the appropriate near-bed velocity scale. Analytic predictions o f mid-shelf mud transport and deposition are spatially and temporally consistent with field observations and provide strong evidence that gravitydriven processes control the emplacement and location of the Eel margin flood deposit. Analytic predictions of deposition suggest that the magnitude of wave energy is more important than the magnitude o f the flood event in controlling the thickness of mid-shelf gravity-driven deposition following floods. Higher wave energy increases the capacity for critically-stratified gravity flows to transport sediment to the mid-shelf and results in greater deposition. The bathymetry o f the Eel margin plays a critical role in gravitydriven transport and deposition. Analytic predictions indicate that gravity-driven deposition on the mid-she If begins roughly 7-8 km north of the river mouth. Closer to the river mouth, the seaward increasing mid-shelf slope associated with the concave downward subaqueous delta causes gravity-driven flux divergence, preventing significant mid-shelf gravity-driven deposition and favoring sediment bypassing. Seaward decreases in shelf slope in the vicinity o f the observed flood depo-center leads to greater flux convergence by gravity-driven flows, and hence greater deposition. The numerical model predicts gravity-driven deposition on the continental shelf for four consecutive flood seasons of the Eel River using realistic bathymetry, waves and river forcing. Results from the numerical model are consistent with observations of deposition on the mid-self and support the results of the analytical model that suggest wave intensity and bathymetry are the dominant factors controlling the location and magnitude o f observed deposition. Despite significantly greater sediment input near the river mouth, little mid-shelf deposition is predicted in this region due to the increasing off-shelf slope. The numeric results suggest that gradients in the along-shelf components o f bed-slope also favor gravity-driven deposition 10-30 km north o f the river mouth. Including the influence of along-shelf currents had little impact on the location o f mid­ shelf deposition, providing further support for bathymetric control of flood sedimentation on the Eel margin. A significant fraction of sediment from the Eel River was predicted to leave the shelf as a gravity-driven flow during floods with large wave energy. However, in extremely large floods, gravity-driven processes were not capable o f removing riverderived fine sediment from the inner-shelf. MODELING OF CRITICALLY-STRATIFIED GRAVITY FLOWS: APPLICATION TO THE EEL RIVER CONTINENTAL SHELF, NORTHERN CALIFORNIA