Microscopic Description of Basal Stress Generated by Granular Free-Surface Flows

Geophysical flows grow in volume and impact potential by eroding and entraining bed material during movement, posing potential risks to human lives and facilities downstream. This granular process is controlled by the basal stress and its fluctuations at the flow-bed interface. Therefore, it is essential to provide physical insights into the basal stresses generated by granular flows for hazard risk management. In this study, we conduct a series of DEM simulations of channelized granular free-surface flows to gain a micromechanical understanding of particle-bed interactions. Basal normal and shear stresses are recorded and analyzed quantitatively through three statistical metrics: time-averaged mean value, maximum value, and standard deviation. The relationship of these quantities with the bulk and basal flow properties and the micromechanical quantities that control stress fluctuations are discussed. Results show that the basal stress exhibits a distinct transition from a dense to dilute flow regime at a critical Savage number of 0.1, coinciding with the transition of stress-generating and transmission mechanisms from being friction-dominated to collision-dominated. Maximum basal normal and shear stresses scale with the particle-bed impact velocities in the form of a power law relation. Stress fluctuations scale with the granular temperature, revealing its dependence on random particle motions. A unified scaling law is proposed to link the standard deviation of basal stresses with the free particle volume, covering a wide range of flow regimes from dense to dilute flow.