THE FLUCTUATING COMPONENT OF BASAL NORMAL FORCE GENERATED BY DEBRIS FLOWS

Debris flows exhibit highly variable basal force fluctuations which are important for understanding near-bed grain dynamics, bed surface erosion, energy dissipation, and flow runout. However, few quantitative analyses of the controls on the force fluctuations exist. In a 4-meter diameter, 80 centimeter wide vertically rotating debris flow flume, we used a 15x15 cm load plate to measure the bed normal force in a range of granular flows. We analyzed the bed forces generated in flows composed of clay, silt, sand, gravel, cobbles, and water for both narrow and wide grain size distributions. We show that the square of the impulse, which is related to kinetic energy transferred to the bed from the granular collisions, can be quantified as the variance of the force signal. The variance of the force was a function of grain diameter, flow velocity, and matrix viscosity. The tail of the distribution of bed normal forces had distinct shapes for the narrow and wide grain size distributions. The force variance generated by narrow and wide grain size distributions had a power law dependence on an inertial stress scaling term where D₈₄ was used as the effective diameter. These results provide the first quantitative relationships in field-scaled materials between a metric for the collisional energy at the boundary and the properties of the flow.