A solid physical understanding of debris-flow erosion is needed for both hazard prediction and understanding long-term 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 the water and clay content of an unconsolidated bed significantly control erosion magnitude by affecting the transfer of pore pressure, loading conditions, and cohesion of the bed. Bed-water content increases erosion rapidly when the bed comes close to saturation, 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.

Debris flows are an active geomorphological agent that, on the short term, pose a threat to human life, property and infrastructure (e.g.,