Froude number ($\mathrm{Fr}$)

The Froude number compares inertial speed with a gravity-wave speed. For a shallow gravity current of depth $h$ and reduced gravity $g^{\prime }$, $\mathrm{Fr}=U/\sqrt{g^{\prime }h}$.

A common high-Reynolds number closure for a deep-ambient gravity current is $\dot{L}=\mathrm{Fr}\sqrt{g^{\prime }h}$, where $L$ is front position and the order-one Froude number depends on the chosen front model.