HR: 0800h
AN: H51C-1153    [Abstracts]
TI: Downslope Asymmetry in Rainsplash Transport of Sand Revealed With High-Speed Imaging
AU: * Furbish, D J
EM: david.j.furbish@vanderbilt.edu
AF: Departments of Earth and Environmental Sciences and Civil and Environmental Engineering, Vanderbilt University, Nashville, TN 37235-1805
AU: Schmeeckle, M W
EM: schmeeckle@asu.edu
AF: Department of Geography, Arizona State University, Tempe, AZ 85287-0104
AU: Borosund, M N
EM: miriam.n.borosund@vanderbilt.edu
AF: Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN 37235-1805
AB: An understanding of rainsplash detachment and transport is required to formulate theories for a host of hillslope processes, including downslope rainsplash transport, rill development and destruction, and sheetwash transport resulting initially from rainsplash. Net downslope transport of soil particles by rainsplash results from a bias in the number and/or length of particle trajectories. The relative contributions of these attributes of particle motion bear on formulations relating rainsplash transport rates to surface slope angle, as the directional asymmetry of trajectories depends on details of motion during drop impact, including drop incidence angle, whereas the bias in trajectory lengths mostly obtains for geometrical reasons. To clarify these attributes of particle motions initiated by rain drop impacts, we are conducting experiments that allow us to directly visualize them. Specifically, in a first set of experiments we have obtained high-speed video imagery of water-drop impacts and associated motions of medium sand particles on varying slopes under dry and moist (drained) conditions. Drops with diameters of 3 mm were released from a height of 5 m onto a sand target. The target consisted of a circular hole (diameter of 2.5 cm, depth of 1.9 cm) drilled into a wooden block. The quartz sand was angular with a nominal diameter 0.35 mm. In each run the sand surface was initially smooth and flush with the surrounding target surface. Moist conditions were obtained by filling the target hole with sand, wetting it, then allowing the sand to fully drain. Target slopes were 0, 10, 20 and 30 degrees. Images of the drop impacts and particle motions were obtained with a high-speed digital video camera running at 500 frames per second. The videos reveal a clear radial symmetry in particle trajectories at 0 degrees, and only a slight asymmetry at 10 degrees. Noticeable asymmetry occurs at 20 degrees and, at 30 degrees, virtually no particles move upslope. During drop impact the mass and momentum of the drop spreads laterally, causing a surface layer of particles (several particle diameters thick) to move radially outward as much as several drop diameters. This motion is strongly asymmetrical at larger slopes and, with dry conditions, may be as important as ballistic motions in producing net transport. Dry conditions involve more ballistic motions than do moist conditions. The effects of surface tension associated with initially moist conditions are to reduce the detachment of surface grains, reduce plowing and, for particles that do detach, increase the likelihood that particles cling together during their ballistic motions. These results suggest the possibility of decreasing rainsplash transport with initial surface wetting following the onset of a storm, followed by increased transport as surface-tension effects are reduced with further wetting toward saturation, before the onset of rainsplash-enhanced particle mobilization with sheetflow. The results also suggest that the drop incidence angle, set by both surface slope and drop trajectory, contributes significantly to asymmetry in particle trajectories. Clarification of these points, however, will require experiments that cover a wider rage of parametric quantities (e.g. drop size, particle size, moisture content, surface roughness) than those considered in these initial runs.
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting