Turbulent Flow and Sand Transport over a Cobble Bed in a Laboratory Flume

Improving the prediction of sand transport downstream of dams requires characterization of the interaction between turbulent flow and near-surface interstitial sands. The advanced age and impending decommissioning of many dams have brought increased attention to the fate of sediments stored in reservoirs. Sands can be reintroduced to coarse substrates that have available pore space resulting from periods of sediment-starved flow. The roughness and porosity of the coarse substrate are both affected by sand elevation relative to the coarse substrate; therefore, the turbulence characteristics and sediment transport over and through these beds are significantly altered after sediment is reintroduced. Past work by the writers on flow over sand-filled gravel beds revealed that the fine-sediment level controls the volume of material available for transport and the area of sediment exposed to the flow. The present work expands on the gravel-bed experiments by conducting similar measurements of turbulent flow and sand transport over a bed of cobbles with a median diameter of approximately 150mm. This change in scale expands the generality of the previous experiments and broadens the range of sand transport and turbulence measurements. It was found that the same relationship between bed shear stress and sand elevation was valid for both gravel and cobble systems. Reductions in bed shear stress, relative to the clear-water case, were observed as the sand elevation was increased, although the highest sand elevation did not yield the lowest shear stress. Quadrant analysis showed that, for stronger turbulent events, there was an increase of sweeps and a decrease in bursts as the sand level was raised. This effect was observed for a region with a height of approximately 1.4 times the thickness of the roughness layer.