(19) Interaction of granular flows and their boundaries: basal forces, particle kinematics, and bed erosion measured in a large vertically rotating drum flume
Leslie Hsu, Lamont-‐Doherty Earth Observatory, Columbia University, NY, USA (firstname.lastname@example.org)
William E. Dietrich, University of California, Berkeley, CA, USA
Leonard S. Sklar, San Francisco State University, CA, USA
Field studies suggest that bedrock incision by granular flows may be the primary process cutting valleys in steep, unglaciated landscapes. The mechanisms of granular flow incision, however, are not well quantified. Here we present a suite of laboratory experiments describing processes and rates of bedrock erosion by granular flows. We used a debris flow flume facility comprised of a 4-‐meter diameter, 80-‐cm wide vertically rotating drum to measure mean and fluctuating normal forces at the base of granular flows. We analyzed the time series of bed forces generated in flows composed of granular material for both narrow (gravel-‐water) and wide (muddy, sand-‐gravel-‐cobble) grain size distributions. The mean bulk force equaled the static weight of the flow, while the force fluctuations, represented by the standard deviation and the top 1% of force, were a near-‐linear function of effective grain diameter and flow velocity, and a ~0.5 power function of an inertial stress scaling term. The fluctuating force component was a function of grain diameter, flow velocity, and matrix fluid properties. These results provide quantitative relationships between a metric for the collisional energy at the boundary and measurable properties of field-‐scaled flows.
As part of our investigations of controls o boundary forces and bedrock wear, we observed grain segregation processes and fluid-‐sediment interactions that were previously undescribed in the literature. These included lateral oscillations of the flow front and the formation of asymmetric coarse-‐particle gyres. We also measured erosion of synthetic bedrock samples in the 4-‐meter diameter drum to test different models for the relationship between bedrock erosion rate and measured basal forces. Based o the experimental observations, we propose a debris flow erosion rule that includes components of both sliding and impact wear, whose relative importance is scaled by experimentally-‐tested variables.