Turbulent flow and sand transport over a cobble bed in a laboratory flume

Authors: Wren, Daniel; Kuhnle, Roger; Langendoen, Eddy; Rigby Jr, James

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/30/2013
Publication Date: 4/10/2014
Publication URL: https://handle.nal.usda.gov/10113/5678118
Citation: Wren, D.G., Kuhnle, R.A., Langendoen, E.J., Rigby Jr, J.R. 2014. Turbulent flow and sand transport over a cobble bed in a laboratory flume. Journal of Hydraulic Engineering. 140(4). doi:04014001. 2014.

Interpretive Summary: The interaction of a coarse stream bed with flow and sediment is complex, and the controlling factors, such as bed roughness, slope, and availability of fine sediments, are difficult to measure. However, planning for reservoir flushing or dam removal requires knowledge of these interactions. In both cases, sediment may be reintroduced to downstream channel beds that have had fine particles removed without replacement from upstream, leaving pore space which interacts with the flow and represents available storage capacity. The proportion of a cobble bed stream that is covered by sand strongly affects the amount of sediment transported; however, the relationship between bed coverage, transport rate, and bed shear stress is poorly understood. The goal of this research is to measure changes in turbulence and sand transport caused by adding sand to an immobile cobble bed. The data will be used to better understand the changes caused by the sand addition. The U.S. Bureau of Reclamation plans to use this research to help in making decisions related to the removal of dams. The data will also be used to aid in developing models of flow, turbulence, and sediment transport over rough beds. It was found that a similar relationship for predicting sand transport over a gravel bed also works for the cobble bed. This is an important finding that will help to develope a general formula sediment transport over rough beds.

Technical Abstract: The turbulence structure of flow over rough beds and its interaction with fine sediments in the bed are important for efforts to predict sediment transport downstream of dams. The advanced age and impending decommissioning of many dams have brought increased attention to the fate of sediments stored in reservoirs. Fine sediments 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 the fine sediment 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 authors 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 turbulence and sand transport over a bed made of 150 mm cobbles. This change in scale helps to define how general the results of the previous experiments are and to broaden 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 ejections 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.