Basal Force Fluctuations and Granular Rheology: Linking Macroscopic Descriptions of Granular Flows to Bed Forces With Implications for Monitoring Signals

Abstract

Granular flows are ubiquitous in nature with single flows traversing a wide range of dynamic conditions from initiation to deposition. Many of these flows are responsible for significant hazards and can generate remotely detectable seismic signals. These signals provide a potential for real-time flow measurements from a safe distance. To fully realize the benefit of seismic measurements, basal-granular forces must be linked to macroscopic internal flow dynamics across a wide range of flow conditions. We utilize discrete element simulations to observe dry and submerged granular flows under plane-shear and inclined-flow configurations, relating bulk kinematics to basal-force distributions. We find that the power and frequency of force fluctuations scale with non-dimensional shear rate (I). This scaling tracks three pre-established regimes that are described by μ(I) rheology: (a) an intermittent particle rearrangement regime, where basal forces are dominated by low frequencies; (b) an intermediate regime where basal forces start to increase in frequency while showing correlations in space and (c) a fully collisional regime where the signal is nearly flat up to a cutoff frequency. We further identify a newly defined fourth regime that marks a “phase change” from the intermediate to collisional regime where increases in basal force fluctuations with increasing shear rates stalls as the granular bed dilates, partially destroying the contact network. This effort suggests that basal forces can be used to interpret complex granular processes in geophysical flows.