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

Abstract

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