In fluid dynamics, a gravity current is a primarily horizontal flow in a gravitational field that is driven by a density difference, hence gravity currents also sometimes being referred to as "density currents". Typically, the density difference is small enough for the Boussinesq approximation to be valid.
Gravity currents are typically of very low aspect ratio (that is, height over typical horizontal lengthscale). The pressure distribution is thus approximately hydrostatic, apart from near the leading edge (this may be seen using dimensional analysis). Thus gravity currents may be simulated by the shallow water equations, with special dispensation for the leading edge which behaves as a discontinuity.
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“Grown beyond nature now, soft food for worms,
They lift frail heads in gravity and good faith.”
—Derek Mahon (b. 1941)
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And the profit and loss.
A current under sea
Picked his bones in whispers. As he rose and fell
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—T.S. (Thomas Stearns)