ARS | Publication request: Measurements of Coupled Fluid and Sediment Motion Over Mobile Sand Dunes in a Laboratory Flume

Submitted to: International Journal of Sediment Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 9, 2008
Publication Date: N/A

Interpretive Summary: The flow of water over the bed of a stream or river is very complex, and it is difficult to determine what fluid motions are important for moving sand particles. Examining these processes over a dune covered bed requires creative experimental design and data processing. In order to address this question, the time between high magnitude changes in fluid motion and in particle concentration were calculated from measurements of turbulence and acoustic backscatter from particles. The resulting findings show that over most of the dune, the highest magnitude changes in sediment concentration are on different time scale from changes in fluid velocity, but their times are much more similar for lower magnitude fluctuations. This shows that larger, less frequent, turbulent events may be responsible for maintaining more sediment in suspension than smaller, more frequent events.

Technical Abstract: The relation between turbulent fluid motions and sediment particles over mobile sand dunes may be better understood by examining the time scales over which the quantities fluctuate. In laboratory experiments performed at the USDA-ARS-National Sedimentation Laboratory, profiles of acoustic backscatter and at-a-point turbulence were collected while translating downstream with mobile dune bedforms. The resulting data set has been used to examine the frequency at which fluctuating backscatter and fluid velocity signals exceeded magnitude thresholds based on the standard deviation (sigma) of the signal. It was found that the recurrence frequency for backscatter, downstream, and vertical velocities was strongly dependent on elevation, threshold magnitude, and position relative to bedform length. Vertical velocities showed the largest decrease in frequency with increasing depth. The recurrence frequency for acoustic backscatter data at a 1 (sigma) threshold was best matched by the vertical velocity component in the trough and low stoss region. The downstream velocity component displayed a better match higher on the stoss and near the crest.