BEDROCK EROSION BY DEBRIS FLOWS: EXPERIMENTS IN A LARGE ROTATING DRUM

Field and topographic evidence indicate that bedrock channel incision by debris flow is a fundamental process in the evolution of many steep, mountainous landscapes. However, the mechanisms by which debris flows cut bedrock channels are not fully understood. We conducted experiments in a 4-meter diameter, 80-cm wide vertically rotating drum to explore the bedrock wear processes. Specifically, we tested the hypotheses that bedrock erosion scales with the bulk inertial stresses, the bulk average stresses, or the fluctuation in stresses at the bed of the debris flow. The advantages of this large-scale experiment are the minimization of sediment down-scaling and the ability to use natural sediment with wide grain size distributions. We varied the grain size distribution from simple homogeneous gravel with a mean diameter of 7, 10, 13 and 21 mm to natural grain size distributions including clay to 15 cm diameter rocks. All flows were water-saturated. The drum velocity varied from 0.4 to 1.25 m/s tangentially and the erodible bedrock tensile strength varied from 250 to 1700 kPa. We measured the topographic wear of the bedrock samples, the bed normal force, and the longitudinal height profile during the flow.

For a wide range of flow conditions, our observations of grain kinematics, erosion, and normal force indicate that both sliding and impact wear occur concurrently. From our experimental data, we identified the key variables that determine the relative importance of sliding and impact wear to be the grain size distribution, fluid content, flow velocity, bedrock strength, and boundary roughness. A combination of these variables can be used to predict the force signal at the bed of the flow, the kinetic energy dissipated at the bed, and the resulting bedrock erosion. We propose a debris flow incision rule that includes both sliding and impact wear, scaled by these influential variables. This work provides experimental evidence for the primary factors in debris flow erosion of bedrock channels.