Topographic steering of dense overflows: Laboratory experiments with V-shaped ridges and canyons

Abstract

Topographic corrugations such as canyons and ridges cross-cutting the path of a dense plume may effectively steer all or part of the plume downslope. Here, topographically steered flows are investigated experimentally, as laminar, dense gravity currents are observed to impinge on and flow along sloping, V-shaped canyons and ridges. Ridges, as well as canyons, were observed to steer the dense water downslope. A dynamical regime, in which the along-slope transport is balanced by a return flow in the Ekman layer to maintain a geostrophically balanced downslope flow along the corrugation, has been proposed. Results from a previously published analytical model describing such flows are compared with the laboratory experiments. The response of the flow to variations in four governing parameters (slope, rotation, volume flux and reduced gravity) is generally described well by the model and results agree qualitatively, although theory slightly underestimates the dense layer thickness. Vertical velocity profiles resolving the Ekman spiral were obtained using a laser Doppler velocimeter and they showed the secondary, transverse circulation superimposed on the primary, downslope flow. A particle flowing down the canyon/along the ridge can be expected to follow a helix-like path, and dye released within the dense layer showed this. The experiments support the analytical model and the dynamical regime proposed for topographically steered flows. The gravity current split in two when the transport capacity of the corrugation was exceeded; one part continued along the slope and the other flowed downslope along the corrugation.