University of Cambridge > > Institute for Energy and Environmental Flows (IEEF) > Density stratification in gravity currents; topographic interactions, shear layer stability, and impact on the global sediment budget

Density stratification in gravity currents; topographic interactions, shear layer stability, and impact on the global sediment budget

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Gravity currents are major agents of transport of particulate materials on the Earth’s surface, where they commonly interact with surface relief. The vertical mean velocity profile of particulate gravity currents can be treated as essentially a combination of a logarithmic boundary layer resulting from its no-slip lower boundary, and an error function velocity profile across the shear layer in the upper part of the current. The density stratification results from a balance between particle settling and an upward flux due to turbulent diffusion.

Where the height of the topography is on a smaller scale than the thickness of the boundary layer, approximate scaling laws can be derived by applying models for interactions between topography and stratified flow borrowed from atmospheric science. This is illustrated across a range of scales by examples from the 1980 Mt St Helens lateral blast, and by inferred interactions between turbidity currents and small-scale sand dunes on the bed.

The overwhelming majority of material removed from the continents by erosion and subsequent transport by rivers ultimately forms large submarine fans (O 10^6 km3) in the deep ocean off the continental margins. The subaqueous part of the transport is effected mainly by turbidity currents that flow through sinuous submarine channels that may be hundreds to thousands of kilometres long. In many laboratory or numerical experiments on gravity currents, shear at the boundary with the overlying ambient fluid is sufficient to destabilise the density stratification. The consequent Kelvin-Helmholtz instabilities result in entrainment of ambient fluid, and in turbulence whose viscous dissipation constitutes an effective drag on the current. This drag and the vertical expansion of the flow due to entrainment would render long range sediment transport by such flows impossible on the extremely low gradients of submarine fan channels. A model is presented here for currents in which the upper flow boundaries are stable (high gradient Richardson numbers), a condition favoured by the low gradients of submarine fans, and by the presence of fine-grained suspended sediment in the upper part of the flow.

This talk is part of the Institute for Energy and Environmental Flows (IEEF) series.

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