Title: Turbulence structure and sand transport over gravel and cobble beds in laboratory flumes Authors
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: May 14, 2013
Publication Date: November 4, 2013
Citation: Wren, D.G., Kuhnle, R.A., Langendoen, E.J., Pellachini, C. 2013. Turbulence structure and sand transport over gravel and cobble beds in laboratory flumes. Book Chapter. 341-357. 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 that interacts with the flow and represents available storage capacity. The proportion of a gravel 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 caused by adding sand to an immobile gravel 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.
Technical Abstract: Characterizing the turbulence generated by flow over rough beds has become increasingly important in support of 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. In many cases, fine sediments are reintroduced to coarse substrates that have large volumes of pore space available for storage after having sediments removed by years 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. Recent experiments at the USDA-ARS-National Sedimentation Laboratory have focused on detailed measurements of turbulence and sand transport over and through coarse substrates. For six sand bed elevations relative the gravel substrate, turbulence measurements were made using an acoustic Doppler velocimeter that collected three velocity components at a rate of 200 Hz. The rough nature of the bed caused individual velocity profiles to vary significantly; therefore, the data were spatially averaged over six 10 X 20 cm planes parallel to the bed with the lowest plane about 2 cm below the maximum gravel elevation. Detailed measurements of sand transport rate, bed texture, and bed topography were collected for four different discharges for 11 different elevations of sand in the gravel bed. Sand transport was measured using both physical samples and a density cell. A collapse of the transport data was accomplished by relating the sand transport rate to the bed shear stress scaled by the square root of the value of the cumulative probability distribution function of the gravel surface evaluated at the height of the mean sand bed. The increasing elevation of sand relative to the gravel layer resulted in decreased Reynolds stress and turbulence intensity. It is hypothesized that these changes are due to a reduction in the size of coherent structures caused by limiting the depth of flow-penetration into the rough bed as the sand elevation increased.