Shahrzad Davarpanah Jazi, Mathew Wells

The length-scale of deposition beneath a buoyant sediment-laden river plume can be strongly influenced by enhanced settling-driven convection, and is directly related to the horizontal velocity of the plume and a sedimentation time-scale. In our experiments, a buoyant plume of fresh water and sediment spreads over a denser saline layer. The speed of the plume increases with the net density difference between the layers, while in contrast, the time-scale of the settling-driven convection is related inversely to the density anomaly due to the sediment concentration difference. These competing effects result in the length-scale of propagation increasing as L ~ sqrt(1-1/R_rho)*R_rho^(2/3), where R_rho= delta rho_S / delta rho_C is the ratio of density differences due to salt and sediment. When R_rho is close to one, settling-driven convection is vigorous and the propagation length-scale is very small. Beneath the surface plume, the descending sediment laden fluid can also form an interflow or underflow, which is able to propagate greater distances than the surface plume. The implication of these results for sediment-laden river plumes in lakes and the coastal ocean are discussed.