Gravity-Driven Sediment Transport on the Continental Shelf: Implications for Equilibrium Profiles Near River Mouths

An analytical model is developed for equilibrium bathymetric profiles off river mouths associated with the shoreward, convex upward portion of subaqueous deltas and clinoforms. The model builds on recent field results demonstrating that gravity-driven flux of suspended mud is important on shelves provided that wave-induced suspension of sediment supports the requisite turbid hyperpycnal layer. Because the maximum sediment load is determined by the critical Richardson number, the results are independent of the properties of the suspended mud or the bed. The model assumes the equilibrium state to represent a balance between the supply of sediment by a river at the coast and the down-slope bypassing of sediment to deep water within wave-supported turbid near-bed layers. Progressive seaward increases in bed slope across the convex shelf profile allow the attenuation of wave agitation with depth to be compensated for by a down-slope increase in the contribution of gravity. The model is consistent with shelf profiles off the mouths of the Po (Italy), Rhone (France), Eel (California), Ganges-Brahmaputra (Bangladesh), and Waiapu (New Zealand) Rivers. The equilibrium profile is predicted to be a function of wave climate and riverine sediment supply only, with deeper and broader profiles associated with decreasing sediment supply, increasing wave height and/or increasing wave period.