Control of Subaqueous Delta Morphology by Wave-Supported Sediment Gravity Flows

Wave-supported gravity flows are a class of turbidity currents which rely on the velocity shear produced by waves near the seabed to keep sediment in suspension during the gravity-induced downslope movement of the sediment flow. Offshore of many high load rivers, wave-supported sediment gravity flows dominate the seaward transport of sediment across the subaqueous portion of the river delta. As a result, waves play a fundamental role in controlling the bathymetric profile off of these rivers. The characteristic shape of the landward portion of these subaqueous deltas is convex upwards. This is because the seaward increase in bed slope associated with a convex profile allows the attenuation of wave agitation with depth to be compensated for by a downslope increase in the contribution of gravity. This minimizes the across-shore convergence of sediment transport within the gravity flow, allowing a stable delta profile to result. Observations indicate that off of rivers exposed to high wave energy, the characteristically convex profile is less steep and extends further from shore relative to rivers exposed to low wave energy. An analytical solution for equilibrium delta profiles associated with wave-supported gravity flows suggests that delta steepness should increase in proportion to Q/H3, where Q is riverine sediment discharge and H is wave height. Larger waves decay less rapidly with depth, so at equilibrium, a less rapid increase in slope with depth is required to prevent sediment convergence. Predictions by the analytical solution are consistent with observations of wave height, sediment supply and bathymetric profiles from a wide variety of real deltas, from high wave energy rivers along the Pacific coast, to low wave energy rivers in the Mediterranean.