How does three-dimensional canopy geometry affect the front propagation of a gravity current?

Large-eddy simulations are performed to investigate how three-dimensional canopy geometry affects the front propagation of an incoming gravity current under a given initial forcing. A regular array of rigid square cylinders are used to represent the distributed canopy elements. It is shown that the conventional geometrical parameter of submerged canopies in constant-density flows (ah, where a is the frontal area per canopy volume and h is the canopy height) is misleading when applied to buoyancy-driven flows due to the additional complexity arising from the internal density gradients. Instead, the present study suggests a new geometrical framework consisting of a canopy density (ϕ) and a canopy-to-current height ratio (h), which can jointly provide an unambiguous description of the state of the current–canopy interaction. Two propagation regimes of the gravity current are identified, either along the channel bed (through-flow) or above the canopy’s top boundary (over-flow). Our analysis reveals that ϕ and h counteract each other’s effect on the transition of flow regimes. Large ϕ implies a strong suppression of horizontal advection within the canopy and thus promotes the through-to-over flow transition; in contrast, large h tends to promote the over-to-through flow transition due to the lack of sufficient potential energy to overcome the height jump. The end product is a complex variation pattern of a propagation regime and front velocity in the two-dimensional ϕ–h parameter space.