How Bed Composition Affects Erosion by Debris Flows-An Experimental Assessment

A solid physical understanding of debris-flow erosion is needed for both hazard prediction and understanding landscape evolution. However, the processes and forces involved in erosion by debris flows and especially how the erodible surface itself influences erosion are poorly understood. Here, we experimentally investigate the effects of bed composition on debris-flow erosion, by systematically varying the composition of an erodible bed in a small-scale debris-flow flume. The experiments show that water and clay content of an unconsolidated bed significantly control erosion magnitude by affecting the transfer of pore pressure, loading conditions, and contraction-dilation behavior of the bed. As the water content increases and the bed comes close to saturation, erosion increases rapidly, whereas for clay content an optimum for erosion exists around a clay content of 3%-4%. Our results show that small variations in bed composition can have large effects on debris-flow erosion, and thus volume growth and hazard potential.