How Volume Increases the Mobility of Geophysical Granular Flow: A Unified Rheological Perspective

Geophysical granular flows, involving rapidly flowing granular materials, can exhibit volume-enhanced mobility. Lacking a mechanistic understanding of such size effects limits the applications of lab-scale findings to natural events. Using discrete element method simulations, we find that increasing granular system size suppresses energy-dissipating velocity fluctuations while promoting sustained creeping motion. This nonlocal phenomenon of granular materials enhances the mobility in various granular column collapse scenarios. This mechanism is reflected in the rheological data, which deviate from traditional $\mu (I)$ rheology but are collapsed under a recent power-law rheology that incorporates velocity fluctuations. Moreover, this size-dependent power-law rheology exhibits universality in transient simulations with varied flow geometries, slope angles, and base roughness. This rheologically consistent framework, spanning inertial to quasi-static states, bridges small-scale investigations and continuum models for large-scale simulations, enabling improved predictive capability of the entire flow processes, from initiation to deposition, in natural geophysical flows.