Settling-driven convection limits the spatial scale of deposition beneath buoyant flows

Session: 47. - Physical Processes in Lakes

Shahrzad Davarpanah Jazi, University of Toronto, [email protected]
Mathew Wells, University of Toronto Scarborough, [email protected]

Abstract

The length-scale of deposition beneath a buoyant river plume can be strongly influenced by enhanced particle settling such as settling-driven convection. The length of deposition should be directly related to a propagation length-scale due to the horizontal velocity and the sedimentation time-scale. In our experiments a plume of fresh water, laden with sediment, spreads over a denser saline layer. The speed of the plume increases with the net density difference between the layers. In contrast, the time-scale of the settling-driven convection is related inversely to the density anomaly due to only the sediment. These competing effects result in the length-scale of propagation increasing as Lprop ~ (1 - 1 / R?)^1/2 R?^2/3, where  R_? = ??_S / ??_C is the ratio of density differences due to salt and sediment. When R_? is small and close to one, settling-driven convection is vigorous and hence the propagation length-scale is very small. In contrast, the Stokes settling velocity would imply much greater length-scales and no dependence upon R_?. Beneath the surface plume, sediment-laden fluid can form an interflow or underflow, which is able to propagate greater distances than the surface plume. We discuss the implication of these results for sediment-laden river plumes in lakes and the coastal ocean.

1. Keyword
sediment transport

2. Keyword
hydrodynamics

3. Keyword
estuaries

4. Additional Keyword
Settling-driven convection

5. Additional Keyword
Particle-laden gravity current

6. Additional Keyword
Stokes settling velocity