Mobility and erosion of granular flows travelling on an erodible substrate: Insights from the small-scale flume experiments

Rock avalanches are extremely rapid, flow-like movements of dry debris mass with extreme destructive potential. Among the dynamical aspects of rock avalanches still inadequately understood, the flow along an erodible substrate stands out as a particularly significant one. We carried out a series of flume experiments to investigate the mobility and the erosion of granular flows on an erodible substrate. It was found that granular flows with a large total mass could evolve into a stable multi-layer flow when suffering the enhanced basal resistance (e.g., loose and dry erodible substrate). Without the variation of the properties of erodible material, the increase of total mass (from 5 kg to 15 kg) as well as the reduction of grain size (from 4 to 8 mm to 1-2 mm) has a positive influence on the mobility of the granular flows (max. 12% and max. 3.4% growths in runout distance, respectively), emphasizing the important roles of both the event magnitude and the internal friction. Total mass and the grain size have prominent influences also on the erosion effect of granular flow. An increase in flow mass causes an enhancement of the eroded depth and of the expansion in both backward and forward directions, while grain size enhanced only forward expansion in addition to the eroded depth. Altogether, the experiments indicate that an erodible substrate may have dramatic effects on the dynamics of an experimental granular flow. When applied to the field, they point to the importance of the nature of the basal material onto which a natural landslide collapses.