Erosion by experimental debris flows: Particle size effects

The mobilization of surface material by particle-laden flows involves phenomenology that cross multiple scales: particle-scale interactions and mesoscopic stresses have significant implications for landscape evolution and associated hazard mitigation issues. Here, we consider the problem of erosion of bed materials by debris flows – flows of boulders, gravel, sand, fine particles, and fluids – as they entrain soils and rocks from steep hillsides. In this paper we report results from laboratory experiments investigating the effect of changing coarse particle concentration in a dry “debris flow” on the erosion of a bed over which it flows. We find that increasing the fraction of coarse particles in the bed often increases the bed erosion. However, for some systems, the details are noisier and harder to discern. We associate the variable erosion and noisiness in part with the competing dynamics of small scale interactions, such as the coarse grain impacts, and larger scale details, such as those related to angles of repose. We also present preliminary results measuring instantaneous erosion rates and demonstrate that size dependence of the erosion rates can vary considerably from that of the net erosion. We conclude by summarizing some limitations of our experiments and ongoing next steps to address these limitations.