INTEGRATED ASSESSMENT AND ANALYSIS OF PHYSICAL LANDSCAPE PROCESSES THAT IMPACT THE QUALITY AND MANAGEMENT OF AGRICULTURAL WATERSHEDS
Location: Watershed Physical Processes Research Unit
Title: Effects of sand addition on turbulent flow over an immobile gravel bed
Submitted to: Journal of Geophysical Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 23, 2010
Publication Date: March 4, 2011
Citation: Wren, D.G., Langendoen, E.J., Kuhnle, R.A. 2011. Effects of sand addition on turbulent flow over an immobile gravel bed. Journal of Geophysical Research. 116 doi:10.1029/2010JF001859.
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 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.
The factors controlling the complex interaction of a coarse stream bed with flow and sediment are difficult to measure. However, planning for reservoir flushing or dam removal requires knowledge of these interactions. In both cases, impounded sediments are introduced to channel beds that have had fine sediment particles removed without replacement. The channel bed pore space interacts with the flow and provides storage for particles. In order to address the need for information on such systems, an adjustable-slope, recirculating laboratory flume was used to study the changes in flow and turbulence caused by sand added to an immobile gravel bed. Detailed measurements were made using an acoustic Doppler velocimeter that collected three velocity components at a rate of 200 Hz. Because of the rough nature of the bed, individual velocity profiles varied significantly; therefore, in order to determine general trends, 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. The increasing elevation of sand relative to the gravel layer resulted in a decreased water surface slope to maintain uniform flow, decreased bed shear stress, decreased Reynolds stress, increased relative turbulence intensity, and a near-bed shift towards sweep dominated turbulence.