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 Local Immobile Gravel Bed Elevation on Turbulence
Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 5, 2010
Publication Date: June 27, 2010
Citation: Wren, D.G., Langendoen, E.J., Kuhnle, R.A. 2010. Effects of Local Immobile Gravel Bed Elevation on Turbulence. Federal Interagency Sedimentation Conference Proceedings. CD Conference Proceedings.
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 such interactions. In both cases, sediment may be reintroduced to 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 generation of turbulence and, hence, the amount of sediment transported. In the present work, the portion of the gravel bed that affects turbulence properties measured at a given point was assessed. This information is useful for those who need to understand the flow in and through gravel beds and will be used to help in efforts to create accurate computer models of the interaction between water flow and rough beds.
Predicting sand transport over irregular substrates is difficult due to complex interactions of flow, substrate, and sediments. A better understanding of these processes is required to understand the impacts of reservoir flushing and dam removal, which both result in the introduction of finer sediments into relatively clean, coarse substrates. Modeling flow and sediment transport in these complex conditions requires an understanding of the connections between flow patterns and bed topography. An adjustable-slope, recirculating laboratory flume was used in measurements of turbulence and bed topography for a partially sand-filled immobile gravel bed with a D50 of 35 mm. Detailed flow measurements were made using an acoustic Doppler velocimeter that measured three velocity components at a rate of 200 Hz. The response of turbulence to local gravel bed roughness was investigated using visual and statistical measures of the connection between local bed elevations and Reynolds stresses. Gravel elevations were averaged over wall-parallel strips 1-20 cm in length and 3.5 cm (about one D50) in width. Mean elevations over shorter lengths were negatively correlated with near-bed Reynolds stress, and longer regions were positively correlated for z/h>0.2. The relationship between the standard deviation of gravel elevation and Reynolds stress was much weaker than for mean elevation.