Particle scale studies of erosion and segregation dynamics in debris flows

Debris flows are massive multiphase flows consisting of mixtures of particles of a wide variety of sizes and interstitial fluids. As debris flows course down a steep hillside they entrain loose bed materials, a process that not only increases the size of the debris flows but the mobility as well along with their associated hazards. There is no agreement in the dominant entrainment dynamics; models include those that explicitly consider macroscopic stresses to those that include an explicit consideration of the physics of the “granular state”. The situation appears to be complicated by the particle-size dependence of the phenomenology, particularly the spontaneous sorting that occurs in these systems that affects the details of the interaction of the flow with the bed. In this talk we describe experimental and computational efforts to understand how certain particle-scale dynamics contribute toward large scale debris flow dynamics. In an instrumented laboratory debris flow flume, we monitor flow velocities, local structure, and instantaneous entrainment and deposition rates to understand the controls on erosion in these flows. We use computational simulations to determine the details of particle size evolution via segregation dynamics. We present the details of these results and comment how they contribute to larger-scale entrainment dynamics in these flows.