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Bedrock Erosion by Granular Flow

  • Author(s): Hsu, Leslie;
  • Advisor(s): Dietrich, William E.;
  • et al.

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 I present a suite of laboratory experiments describing processes and rates of bedrock erosion by granular flows. In the first study, experiments in a 56 cm diameter, 15 cm wide rotating drum test the hypothesis that bedrock erosion is related to grain inertial stresses which scale with shear rate and particle size. In 67 experimental runs, the eroded depth of the bedrock sample varied with inertial stresses in the granular flow to a power less than 1.0, and inversely with the bedrock strength. The second study used a new 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. The mean bulk force scaled with the static weight of the flow, while the variance of the force was a function of grain diameter, flow velocity, and matrix fluid properties. I show that the square of the impulse, related to kinetic energy transferred to the bed from granular collisions, can be quantified as the variance of the force signal. My results provide the first quantitative relationships between a metric for the collisional energy at the boundary and measurable properties of field-scaled flows. In the third study, I measured erosion of synthetic bedrock samples in the 4 meter diameter drum to test three models for the relationship between bedrock erosion rate and measured basal forces: (1) erosion by impact wear resulting from forces due to bulk inertial solid stress (2) erosion by sliding wear scaled by bulk normal force (3) erosion by impact wear scaled by the square of the impulse exerted on the bed. Based on my experimental observations, I propose a debris flow erosion rule that includes components of both sliding and impact wear, whose relative importance is scaled by experimentally-tested variables. Finally, as part of my investigations of controls on boundary forces and bedrock wear, I 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. I documented the first order controls on segregation by particle size, boundary conditions, fluid content, and fluid viscosity.

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