The Front Condition for Gravity Currents Propagating over Rough Boundaries

Gravity currents – fluid flows generated by horizontal density gradients – are ubiquitous in the environment. An area of increasing interest in this general field is the interaction of gravity currents with boundary roughness, where that roughness may be of the scale of the current itself. This paper describes an experimental study investigating the impact of varying boundary roughness densities on the flow structure and propagation speed of a gravity current. The gravity currents exhibit two primary flow regimes, a “flow through” regime where the current is enmeshed in the roughness elements, and an “overriding” regime where the current is forced to flow almost entirely over the roughness elements. As the flow transitions from the first to the second of these regimes through increasing roughness density there is the possibility that the Froude number of the current will actually increase.