HR: 16:15h
AN: H44A-02    [Abstracts]
TI: Experimental Study of Surface Erosion by Granular Flows
AU: * Hsu, L
EM: lhsu@eps.berkeley.edu
AF: University of California, Berkeley, 307 McCone Hall Department of Earth and Planetary Science, Berkeley, CA 94720 United States
AU: Dietrich, W E
EM: bill@eps.berkeley.edu
AF: University of California, Berkeley, 307 McCone Hall Department of Earth and Planetary Science, Berkeley, CA 94720 United States
AB: Field studies suggest that in steep landscapes mass flows of coarse grained material may be the primary agents responsible for cutting canyons. No process-based theory for bedrock incision by such flows exists, and the infrequency of such events makes them impractical to study in the field. Stock (2003, Ph.D. UC Berkeley) has suggested that in the case of debris flows, the wear rate arises primarily from particle collisions with the bed. Therefore, the stresses on the bed should be correlated with the inertial stresses in the flow. Here, we explore the relationship between wear rate of synthetic bedrock and inertial stresses in granular flows by making measurements in a 60 cm diameter, 15 cm wide vertically rotating (horizontal axis) drum. The debris composition consists of varying amounts of gravel, water, and fines, from dry granular flows to muddy slurries. The shear rate is estimated from the difference between the surface and bottom velocities of the flow divided by the flow depth. We estimate Bagnold and Savage numbers for each experimental material and conditions range from inertially dominated to more viscous states. We measure erosion by differencing the initial and final mass of the imbedded erodible rock sample. For no-slip conditions, we observe that erosion rate increases with higher shear rates and larger grain diameters. Experiments with observable slip at the bed have a lower shear rate, but high erosion rates. To explore this further, we vary boundary roughness, affecting the amount of sliding that occurs at the bed of the flow. For no-slip conditions, the wear appears to occur at the front of the flow, when the faster-moving surface particles overtake the flow front and impact the bed. These results suggest that inertial stresses, which scale with shear rate and grain diameter, are correlated with erosion, but any sliding will alter the amount of wear. Consequently, bed roughness and its effect on bottom slip significantly affects bedrock wear. Results from these experiments are being used to guide the design of future experiments in a larger 4-meter diameter, 0.75 meter wide vertically rotating drum. These experiments will more accurately simulate the scale and behavior of natural debris flows.
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting