Mixing at the front of a gravity current

Summary

The mixing at the front of a gravity current such as a saline flow or a turbidity current depends on two main processes.

The main process is the formation of Kelvin Helmholtz billows. These form at the frontal interface where both velocity and density gradients exist between the two fluids.

The Kelvin-Helmholtz billows may be distorted and broken up by an second instability caused by the over-running of less dense fluid beneath the nose. In a shifting pattern of lobes and clefts, much of the flow is diverted into the clefts and can cause lineations in deposits. The form of the head is also modified by ambient head- or tail-flows, which modify the velocity gradient and hence the form of the billows.

The initial effect of external turbulence is to increase the mixing rate. In a strong turbulent field a sharp front can be dissipated; the advance is then only by turbulent diffusion, a much slower rate of distribution of the dense fluid.

One important frontal feature occurs only in three dimensional gravity currents. In the initial stages of a radial divergent flow the rapid stretching of the leading edge results in the formation of a large roll which can extend down to the ground.