Effects of ambient currents and waves on gravity-driven sediment transport on continental shelves

Observations from several shelf environments show that down-slope gravity-driven transport may constitute an important mode of suspended sediment dispersal across shelves and highlight the influence of ambient waves and currents on gravity-induced sediment flux. The phenomena discussed here involve high concentrations of suspended sediment mixed with seawater and thus differ in genesis from hyperpycnal plumes released directly from sediment-laden rivers. The field sites examined are the Gulf of Bohai off the mouth of the Yellow River (Huanghe), the northern California shelf off the mouth of the Eel River, and the Louisiana shelf west of the mouths of the Mississippi River. Off the Yellow River, rapid down-dope transport over distances of a few km occurred when frictional resistance, induced by strong along shelf currents, was temporarily relaxed. More prolonged down-slope motion over longer distances occurred following floods of the Eel River, when wave and current agitation provided turbulence to sustain gravity-driven transport of fluid mud. On the Louisiana inner shelf, the down-slope gravity force was much weaker, but observations suggest that thin gravity flows may still have occurred in the presence of waves. A simple analytical theory is developed that incorporates the influence of ambient shelf currents on gravity-driven transport of suspended sediment. This theory is quantitatively consistent with the observations from the three sites. If the supply of easily suspended sediment is less than the capacity of ambient currents (including waves) to carry sediment, then intense turbulence limits gravity-induced sediment transport by increasing the drag at the base of the flow. When ambient currents abruptly cease, rapid down-slope transport can then occur over short distances until the sediment settles. Such flows do not remain intensely turbulent because the slope of the continental shelf is too gentle to induce shear instability within the gravity flow. The maximum sustained rate of gravity-induced sediment transport occurs when ambient currents are strong, but the supply of easily suspended sediment exceeds the resuspension capacity of the ambient currents. Feedback then leads to values of the gradient Richardson number (Ri) within the flow that are near the critical value of 1/4. This partially damps bottom drag, but still allows the generation of sufficient turbulence to maintain sediment in suspension. Observations also indicate systematic relationships among Ri, the supply of easily suspended sediment and the bottom drag coefficient acting on the gravity flow. (C) 2001 Elsevier Science B.V. All rights reserved.