Settling-driven convection: A mechanism of sedimentation from stratified fluids

Convection driven by sediment particles may play an important role in sedimentation from the base of buoyant (hypopycnal) plumes, for example, fluvial plumes in stratified estuaries and lakes, black smokers on the ocean floor, volcanic clouds, and coastal currents. In addition to the well-known double-diffusive convection mechanism, another mode of convective instability development is by settling across the density interface. We performed laboratory experiments to investigate this fingering/convective instability mechanism and its effect on particle distribution in the water column and deposition at the bed. A simple theoretical model of finger formation at a fluid density interface is developed based on an analogy with thermal/plume formation at a flat heated plate. This model, which involves a thickening interface layer that becomes gravitationally unstable relative to the ambient fluid, is in good agreement with measurements of finger size and instability wavelength from visualization experiments. Since fingering at the density interface drives larger-scale convection in the fluid below, a mass balance model of the lower layer, assuming strong mixing (i.e., uniform sediment concentration) is successfully applied to predict sediment concentration in the water column and deposition at the bed. Strong mixing can be assumed since convective velocities are usually much greater than the particle fall velocities. As convection proceeds, the sediment concentrations in the two layers approach each other and convection will die out. Using the model equations, we develop analytical expressions for the time when convection ceases and the portion of sediment remaining in the water column.