Mobility of dry granular flows over contrasting erodible bed materials in laboratory experiments

Erosion and entrainment exchange mass and momentum across the flow–bed interface and can strongly modify mass-flow mobility. However, the material properties that control whether erosive flows accelerate, decelerate, or maintain their mobility remain insufficiently understood. Here, we conducted dry granular flow experiments to investigate erosion, flow evolution, and deposition across systematically imposed flow–bed contrasts, using surface measurements of the evolving flow and final deposit. A quartz slide of constant solid particle density was released over beds with lower, equal, and higher solid particle densities and compared with a reference case without erosion. Each scenario was repeated using finer sand-sized particles and coarser sand-to-gravel-sized particles. The low-density beds produced deeper erosion, higher average flow-front velocities, and longer runout. By contrast, the equal- and high-density beds produced nearly identical results, with shallower erosion depths, lower average flow-front velocities, and more compact deposits, despite their substantially different solid particle densities. The same relative patterns were observed for both particle-size fractions, although coarse-particle flows generally formed longer and flatter deposits. This asymmetric response shows that mobility cannot be explained by solid particle density alone. Instead, the enhanced mobility over the low-density beds is more consistently explained by coupled effects of lower internal friction, rounded particle shape, greater bed deformation, and more efficient entrainment. The similar response of the equal- and high-density beds reflects their comparable and higher frictional resistance and angular particle shape, which appear to outweigh the contrast in solid particle density under the tested conditions. Yet, surface-based measurements alone cannot resolve whether more subtle differences persist in basal slip, shear localization, and the incorporation of mobilized bed material. Depth-resolved observations at the flow–bed interface are therefore needed to directly relate erosion and entrainment to mass and momentum exchange.