Laboratory Experiments Investigating Entrainment By Debris Flows

Abstract

As debris flows course down a steep hillside they entrain bed materials such as loose sediments, increasing the size and hazard of the debris flow. The mechanics underlying the particle entrainment are not well-understood. Existing models for the entrainment process include those that explicitly consider stress ratios, the angle of inclination, and the particle fluxes relative to those achieved under steady conditions. Other considerations have included particle-scale dynamics, such as the “granular temperature” (kinetic energy associated with particle velocity fluctuations). We investigate how total and instantaneous entrainment and deposition vary with macroscopic stresses and particle-scale interactions using laboratory experiments in an instrumented experimental laboratory debris flow flume. Using a high-speed camera, we monitor volume fractions, flow velocities, and instantaneous entrainment and deposition. Our measurements of total erosion support previous results indicating a relatively linear relationship between total erosion and the slope angle. Measurements of instantaneous entrainment rates reveal a more complex interdependence between the stress associated with the flow, the granular temperature and the entrainment rate. Specifically, for most cases, with the initial experimental debris flow, as stress associated with flow increases, entrainment rate increases as well, but this effect quickly saturates. When the experimental flow is still in relatively early stages, the stress associated with the flow on the bed is maintained at a high level and even can increase, but the entrainment rate appears to saturate and quickly decrease. This decrease in the entrainment rate is correlated with a decrease in granular temperature, a decrease that can be represented approximately with an exponential relationship. We present the methods developed to obtain these results and the details of these results for monosized systems subjected to a variety of boundary conditions.