Mobility of dry granular debris flows over erodible beds: Experimental insights into the influence of flow–bed inertia

Abstract. Debris-flow mobility responds sensitively to erosion and entrainment that exchange mass and momentum across the flow-bed interface. Yet, the mechanical controls that cause some debris flows to accelerate during erosion while others decelerate are insufficiently understood. Recent theory attributes this divergent behavior to inertial contrasts between the moving mass and the erodible bed, suggesting that incorporating inertially weaker, neutral, or stronger substrate into the flow enhances, maintains, or reduces flow mobility, respectively. Here, we conducted flume experiments and surface-based measurements to assess how the inertia of the erodible bed affects the flow kinematics, erosion, entrainment, and runout of dry granular single-phase debris flows. We systematically imposed inertial contrasts by releasing a quartz slide of constant solid density over erodible beds with lower, equal, and higher solid densities representing inertially weak, neutral, and strong scenarios, and compare these alongside a reference case without erosion. Each scenario was repeated for fine sand and a sand-gravel mixture. Our results reveal consistent behavior across both particle-size distributions. Debris flows over low-density beds exhibit higher apparent mean erosion rates, faster flow fronts before deposition, and longer runout lengths, whereas flows over equal- and high-density beds evolve similarly, with shallower erosion, slower flow fronts, and shorter, more compact deposits. Relative to the neutral scenario, the entrainment of low-density material thus appears to enhance debris-flow mobility, while incorporating high-density material does not lead to the anticipated mobility loss. This asymmetric response suggests that solid-density contrasts alone are insufficient to explain the observed trends under the experimental conditions considered here. Differences in particle shape and internal friction likely also contribute. Whereas the low-density bed comprises more spherical particles with a lower friction angle facilitating entrainment, the equal- and high-density beds consist of angular particles with similar and higher internal friction angles, leading apparently to similar resistance to erosion despite their divergent densities. However, resolving whether more subtle differences persist between the inertial scenarios will require direct observations at the flow-bed interface to capture grain-scale dynamics and temporal variability in erosion intensity.