HR: 1340h
AN: H53C-1282    [Abstracts]
TI: Experimental investigation of the turbulent structures that initiate bedload motion
AU: * Schmeeckle, M W
EM: schmeeckle@asu.edu
AF: Arizona State University, Department of Geography P.O. Box 870104, Tempe, AZ 85287-0104 United States
AU: Nelson, J M
EM: jmn@usgs.gov
AF: USGS, Box 25046 Denver Federal Center, Lakewood, CO 80225 United States
AU: Shreve, R L
EM: shreve@u.washington.edu
AF: University of Washington, School of Ocenaography 685 Spring Street #123, Friday Harbor, WA 98250-8058 United States
AB: Knowledge of the coupling between turbulent structures and the subsequent motion of bed grains is important to formulating sediment transport relations in complex turbulent flows. Previous experiments involving measurements of forces on fixed particles have shown that bed grains experience fluctuations in drag and lift that are several times the mean, and these high drag and lift events are well correlated with positive fluctuations in downstream velocity near the particle. In order to extend these experiments to the initial motion of grains that are not fixed, laboratory flume measurements of the initial motion of glass spheres were conducted using a high-resolution (1280X1024 pixels) and high-speed digital video camera at a rate of 400 frames per second. Six experimental runs were conducted on each of four spheres having diameters of 1.1, 1.6, 2.5, and 3.6 cm. Each of these spheres was placed on a bed pocket formed by three other 2.5cm glass spheres that were glued together to form a known bed-pocket angle. All of the spheres were entrained at approximately the same discharge. A 1.3 mm thick laser light sheet aligned parallel to the flume walls illuminated flow seed particles in a two-dimensional plane that included the center of the test sphere. Two-dimensional velocity fields were calculated for all successive pairs of images in the video sequence using particle imaging velocimetry (PIV) algorithms. Results showed that the downstream velocity immediately upstream of the test sphere at the initiation of motion was generally 2-3 standard deviations greater than the local mean velocity. The vertical extent of the volume of high-speed fluid that caused the sphere to begin motion generally extended less than two median bed-grain diameters above the bed. A volume of relatively slow moving fluid was generally found above this volume of near-bed high-speed fluid. Thus, to observe the turbulent structures that cause particle entrainment in a gravel bed requires measurements within a couple grain diameters of the bed.
DE: 4558 Sediment transport
DE: 4568 Turbulence, diffusion, and mixing processes
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting