Effect of sediment supply and flow rate on the initiation and topographic evolution of sandbars in laboratory and numerical channels

Abstract

The evolution of barforms from a bed of uniform sediment and changes in sediment storage were measured in a laboratory flume and simulated numerically. Flume experiments were conducted with several upstream sediment supplies and flow conditions. For the sediment supply rates (no upstream supply, equilibrium supply, and 133, 166, and 200 percent of the equilibrium supply) and flow rates examined, the plane bed tended to evolve into mid-channel bars early in the runs ~15 minutes. As the flume experiments progressed, the bed transitioned to a lower mode configuration of alternate bars or a single-thread meandering thalweg. Increasing the upstream sediment supply to 133 percent or more of the equilibrium rate, increased the height and volume of deposited sediment relative to experiments conducted at the equilibrium rate and those experiments without sediment supply. Experiments conducted at flow rates of 0.5 and 1.0 L/s without sediment supply demonstrated that an increase in flow corresponded to a greater volume of erosion. A coupled two-dimensional flow and sediment transport model, Nays2DH, was used to simulate the evolution of bed topography for three sediment supply rates. We compared the morphodynamics and sediment storage predicted by Nays2DH for two initial bed conditions: one set of calculations used a plane bed with a small upstream perturbation as the initial bed condition, and the other set used the bed topography measured 15 minutes after the start of the flume run. Whereas initializing the model with measured flume topography provided a somewhat better analog to the final evolved morphology, predictions of sedimentation were not substantially improved over simulations using the plane bed as the initial condition.