Morphodynamics of bedforms in a supercritical-flow regime: a depth-resolved numerical modelling approach

Both open-channel flows and density currents are able to create supercritical-flow bedforms. The \nmorphodynamics of these supercritical-flow bedforms are, however, still poorly understood. This is \nmainly due to a lack of measurements of flow processes occurring within these types of flows. Cyclic \nsteps have successfully been simulated in open-channel flow using a depth-resolved numerical \nmodel. The equilibrium conditions at which certain supercritical-flow bedforms are stable are \ninvestigated. The temporal variation in Froude number is indicative of at which conditions cyclic \nsteps are in a macroscopic equilibrium at a variability of grain sizes, discharges and sediment \nconcentrations. The depth-resolved model provides insight into the dynamic interaction between \nvelocity structure, shear stresses, and sediment concentrations within the flows and resulting erosion \nand deposition patterns, which, in their turn affect the flow-properties again. The velocity structure \ndownstream of a hydraulic jump displays highest flow velocities near the bed, whilst lowest or even \nnegative velocities are located at the top of the flow, causing the flow to remain exerting shear \nstresses on the bed even after the hydraulic jump. The sediment concentrations within the flow only \ndecrease after a 30 second, or half a meter lag, causing most of the deposition to take place at the \nlast two-thirds of subcritical region of the flow. The resulting depositional pattern consists of \nupstream-dipping backset laminations deposited on the stoss-side of the bedform, cross-cut by the \nerosive surface of the lee-side of the cyclic step, this interplay between erosion and deposition also \ncauses an upstream migration of the cyclic steps.