Unravelling the relative importance of impact forces in debris flows of different composition

Debris flows are mixtures of water and sediment that flow rapidly down a slope due to gravity force (Iverson, 1997; Takahashi, 2014). They are destructive to property and often fatal, a single event potentially killing thousands of people (Iverson, 1997 and 2011; Haas and Woerkom, 2016). The catastrophe is worsened when the volume of debris flow increases by bed entrainment caused by the interaction between flowing particles and bed particles (Takahashi, 1981; Pierson et al., 1990; Haas and Woerkom, 2016). Bed entrainment may result from impact forces and basal-shear forces and are mainly driven by the debris flow composition. However, it has long been unclear whether impact or basal-shear forces bear more responsibility in the erosion process, or it is a combination of both forces. This study shows that impact forces play a critical role in the occurrence of debris flows, and thereby in the erosion of bed material. Geophone capable of quantifying impact forces, and load cell capable of measuring flow weight in dry granular flows (representing debris flows) were used on laboratory-scale simulations. Impact forces were strongly controlled by debris flow composition (i.e. grain-size distribution) and moderately by flow properties (i.e. flow depth, flow weight, flow velocity). Thus, impact forces significantly affect debris-flow erosion, mainly by progressive scour rather than mass failure. Impact forces possibly result in bed entrainment, which is related to increasing debris-flow volume. A better understanding of how the debris-flow volume increases will help minimise the negative impacts of debris flows as well as strengthen hazard mitigation strategies.