Internal Shear and Velocity Fluctuations Promote Granular Flow Mobility: Insights from Flume Experiments

High mobility of granular flows is commonly attributed to basal lubrication and fluid–solid interactions, yet the role of internal shear and velocity fluctuations in promoting flow runout remains insufficiently quantified. Here we present a series of controlled flume experiments designed to isolate the effects of internal deformation on granular‐flow mobility. Using synchronized measurements of surface velocity fields, basal forces, and high‐frequency velocity fluctuations, we quantify the spatial and temporal evolution of shear localization, fluctuation intensity, and basal stress transmission.
Results show that intense internal shear zones generate pronounced velocity fluctuations, which propagate downward through the flow depth and modulate basal stresses. The amplitude of basal stress fluctuations increases systematically with both shear rate and fluctuation intensity, indicating an efficient transfer of internal agitation toward the base. This process weakens effective basal resistance and enhances slip, leading to significantly increased runout and mean flow velocity under otherwise identical conditions.
By integrating kinematic measurements with stress analysis, we identify a scaling relationship that links basal friction, flow thickness, inertial number, and normalized fluctuation stress through a power‐law form. This law provides a quantitative bridge between internal dynamics and macroscopic mobility. Our findings demonstrate that internal shear and velocity fluctuations are not merely byproducts of granular motion, but key drivers of enhanced mobility, offering new insights into the mechanics of long‐runout granular flows such as landslides, debris avalanches, and dry granular surges.