Flow structure and resistance over subaquaeous high- and low-angle dunes

A prominent control on the flow over subaqueous dunes is the slope of the downstream leeside. While previous work has focused on steep (similar to 30 degrees), asymmetric dunes with permanent flow separation, little is known about dunes with lower lee slope angles for which flow separation is absent or intermittent. Here we present a laboratory investigation where we systematically varied the dune lee slope, holding other geometric parameters and flow hydraulics constant, to explore effects on the turbulent flow field and flow resistance. Three sets of fixed dunes (lee slopes of 10 degrees, 20 degrees, and 30 degrees) were separately installed in a 15m long and 1m wide flume and subjected to 0.20m deep flow. Measurements consisted of high-frequency, vertical profiles collected with a Laser Doppler Velocimeter. We show that the temporal and spatial occurrence of flow separation decreases with dune lee slope. Velocity gradients in the dune leeside depict a free shear layer downstream of the 30 degrees dunes and a weaker shear layer closer to the bed for the 20 degrees and 10 degrees dunes. The decrease in velocity gradients leads to lower magnitude of turbulence production for gentle lee slopes. Aperiodic, strong ejection events dominate the shear layer but decrease in strength and frequency for low-angle dunes. Flow resistance of dunes decreases with lee slope; the transition being nonlinear. Over the 10 degrees, 20 degrees, and 30 degrees dunes, shear stress is 8%, 33%, and 90% greater than a flat bed, respectively. Our results demonstrate that dune lee slope plays an important but often ignored role in flow resistance.