Modeling the effect of entrainment and air pore pressure on the mobility of snow avalanches: new insights from DEM and CFD-DEM simulations

Snow avalanches pose significant threats to both populations and infrastructure in mountainous regions. A critical factor influencing the dynamics and hazards associated with these events is the entrainment of bed material, which can substantially increase flow volume and mobility. However, the detailed assessment of entrainment mechanisms and rates, particularly in relation to various flow and bed material properties, has rarely been conducted. In this study, we developed a three-dimensional model based on the Discrete Element Method (DEM) to simulate the interaction between a snow avalanche and an erodible, porous snow cover generated through cohesive ballistic deposition. Initially, we analyse the effects of bed porosity and depth on entrainment and flow mobility in the absence of interstitial fluid. By holding macroscopic strength parameters constant, we isolate the influence of bed porosity on entrainment dynamics. The results reveal a transition in entrainment mechanisms: frontal ploughing and increased mobility dominate in highly porous beds, while basal abrasion and decreased mobility prevail in less porous ones. Subsequently, we incorporate interstitial air effects using coupled Computational Fluid Dynamics (CFD) and DEM simulations. For realistic snow conditions, our simplified snow impact model configuration suggests that pressurized pore air can weaken and fluidize the snow thus enhancing avalanche mobility. In the future, laboratory and field experiments will be carried out to validate and complement these modeling efforts. Additionally, the model will be extended to simulate liquefaction-induced entrainment in debris flows over saturated sediments.