Reduced gravity ($g^{\prime }$)

For two Boussinesq fluids with density difference $\Delta \rho$ and reference density ${\rho }_0$, reduced gravity is $g^{\prime }=g\Delta \rho /{\rho }_0$.

An entraining shallow-water layer changes depth, density, and momentum as ambient fluid crosses its interface. Entrainment dilutes its reduced gravity and accelerates or decelerates it through the momentum needed to bring new fluid into the layer.

A gravity current is a predominantly horizontal flow driven by a density difference. Its front speed scales as the square root of reduced gravity times current depth.

Removing a barrier between fluids of different densities creates oppositely directed gravity currents. In an energy-conserving Boussinesq exchange through a full-depth opening, each layer occupies half the depth.