The transport, transformation and dispersal of sediment by buoyant coastal flows

Rivers provide the dominant pathway of terrigenous sediment to the ocean. The density difference between riverine and salt water as well as the density anomaly contributed by sediment have important consequences on the delivery, transport and ultimate fate of the sediment issued from the land. Most fresh water outflows produce positively buoyant plumes; however, the aggregation and settling of sediment cause the route of sediment transport to diverge from the route of fresh water flow. Sediment is trapped in frontal zones, both in estuaries and on the inner shelf, often resulting in large increases in the concentration of sediment relative to the riverine source. At high concentrations, the density anomaly due to the sediment itself contributes to the vertical stability of the flow, often increasing the trapping efficiency of the frontal zone. Sediment trapping may also occur within the wave boundary layer on the continental shelf, leading to high concentrations within a layer only on the order of 10 cm thick, but which may represent an important cross-shelf conduit of sediment. The high concentration layers formed both by frontal trapping and by wave boundary layer trapping can attain excess densities large enough to initiate down-slope motion on the continental shelf, resulting in an important cross-shelf transport process. Because of these trapping processes, the formation of high-concentration layers, and the occurrence of hyperpycnal plumes, the transport of sediment in river-influenced environments is often dominated by near-bottom fluxes rather than fluxes in the surface plume.