Segregation-Induced Flow Transitions in Rock-Ice Mixtures: Implications for Rock-Ice Avalanche Dynamics

Global climate change has been intensifying the scale and frequency of rock-ice avalanches and similar catastrophic mass movements in high-mountain regions. The difference in the physical characteristics of rock and ice particles leads to mixing and segregation during flow. Although, both particle segregation and the presence of ice fundamentally alter flow behavior, the joint influence and feedback of these two aspects are overlooked in state-of-the-art rock-ice avalanche models. Using discrete element simulations, we show that by controlling the distribution of inter-particle frictional interactions within the mixture, segregation patterns resulting from the size, density, concentration, and surface friction differences of rock and ice phases can induce sharp velocity gradients along the flowing thickness. Flowing layers where low friction contacts with ice are abundant tend to flow faster and can induce slow creeping motion in an otherwise static basal layer dominated by more frictional rocks. Based on these observations, we find that the effective friction of rock-ice flows for various mixture concentrations and size ratios can be obtained as a sum of the single-phase rheologies of rocks and ice weighted according to their microscopic contact probabilities. This effective friction for rock-ice mixtures allows us to extend a recent non-local granular fluidity framework that captures the complex segregation-flow feedback mechanism in rock-ice flows. The findings provide a deeper micromechanical understanding of how particle interactions influence rock-ice avalanche mobility, which ultimately improves flow models needed for hazard assessment and mitigation.