EQUILIBRIUM CLINOFORMS ASSOCIATED WITH GRAVITY DRIVEN SEDIMENT TRANSPORT NEAR RIVER MOUTHS

An analytical model is developed for equilibrium bathymetric profiles off river mouths associated with the shoreward, convex upward portion of clinoforms and subaqueous deltas. 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. The model assumes the equilibrium state to represent a balance between the supply of sediment by a river at the coast and the downslope 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 downslope increase in the contribution of gravity. The model is consistent with profiles of clinoforms and/or subaqueous deltas off the mouths of the Eel (California), Ganges-Brahmaputra (Bangladesh), Waiapu (New Zealand), Po (Italy), and Rhone (France) Rivers. The equilibrium profile is predicted to be deeper and broader with decreasing sediment supply, increasing wave height and/or increasing wave period.