Exploring the effects of bed inertia on debris-flow mobility

Erosion and entrainment are key processes that modulate debris-flow mobility. However, the conditions under which erosive debris flows accelerate and attain longer runout or decelerate and come to rest earlier remain insufficiently understood. The Pudasaini and Krautblatter (2021) landslide mobility model attributes these divergent behaviors to inertial contrasts between the moving mass and the erodible bed, suggesting that the incorporation of inertially weaker, neutral, or stronger material can enhance, maintain, or reduce the flow mobility, respectively. Here, we use flume experiments and surface-based measurements to investigate how bed inertia influences the velocity, erosion, and runout of dry, single-phase debris flows by systematically varying solid densities. A quartz slide of constant solid density is released over erodible beds with lower, equal, and higher densities representing inertially weak, neutral, and strong scenarios. Our results reveal consistent and repeatable patterns. Debris flows over low-density beds exhibit higher apparent mean erosion rates, increased flow-front velocities before deposition, and longer runout than in the inertially neutral scenario. In contrast, debris-flow evolution over equal- and high-density beds is nearly identical, characterized by lower frontal velocities, reduced erosion, and shorter, thicker deposits. These findings indicate that the entrainment of the low-density material enhances debris-flow mobility relative to the inertially neutral scenario, whereas the incorporation of high-density material does not lead to the expected mobility reduction. This asymmetric response suggests that solid density alone does not fully explain the observed mobility behavior under the experimental conditions considered here. Additional influences related to particle shape and internal friction are likely involved, too. The low-density bed combines more rounded particles with a low internal friction angle facilitating entrainment, whereas the equal- and high-density beds comprise more angular grains with similar and higher internal friction angles, which may lead to comparable resistance to erosion despite their contrasting densities. Ongoing work focuses on resolving processes at the flow-bed interface to capture grain-scale dynamics at depth and resolve temporal variations in erosion intensity, which may help to identify subtle differences between the inertial scenarios that are not detectable using surface-based measurements alone.