Particle size ratios and ice content effects on rock-ice avalanche propagation and deposition: Flume experiments and DEM simulations

Rock-ice avalanches in cold high-mountain regions pose severe hazards due to their high mobility, yet the quantitative controls of particle-size ratio and ice content remain insufficiently constrained. This study investigates their coupled effects using inclined-flume experiments and Discrete Element Method (DEM) simulations, covering three gravel sizes (2-5 mm, 5-7 mm, 7-10 mm) and four ice-content levels (0%, 20%, 40%, 60%). Run-out distance, velocity, energy components, flow regime (Savage number), and segregation index α were quantified. Increasing ice content significantly enhances mobility, but with diminishing marginal effectiveness. From 0% to 40% ice content, run-out distance increases by 41%-86%, whereas the additional increase from 40% to 60% contributes only 12%-23%. Particle-size ratio strongly governs segregation intensity. Fine-gravel groups reach segregation indices of α=0.92-0.98, indicating nearly complete upward migration of ice, whereas medium-gravel and coarse-gravel groups exhibit much weaker segregation, stabilizing at α=0.68-0.74 and 0.60-0.69. Savage number analyses reveal marked flow-regime transitions. At 0% ice content, Savage numbers reach 1.0-1.5, indicating a collisional regime. Increasing ice content suppresses collisionality, with Savage numbers decreasing to 0.03-0.07 at 60% ice content, consistent with dense-regime flow. DEM energy analyses confirm this regime shift: for fine-gravel mixtures, collision energy decreases by 14%, while sliding-friction energy increases by 33% as ice content increases from 0% to 60%, reflecting enhanced overburden effects imposed by upward-segregated ice layers. Medium and coarse mixtures exhibit weaker or opposite energy-shift patterns, demonstrating strong size dependence. Mechanistically, large particle-size contrasts promote strong segregation and form dense basal rock layers that increase basal friction and reduce mobility. When particle sizes are similar or ice content is high, segregation remains limited, allowing ice to mix into the basal layer, thereby reducing basal friction and enhancing mobility. This research quantitatively demonstrates how composition controls particle spatial distribution, flow regime, and energy dissipation, offering new mechanistic insights into the propagation and deposition behaviors of rock–ice avalanches and improving hazard assessment in vulnerable high-mountain regions.