Rheology of debris flow materials is controlled by the distance from jamming

Debris flows are dense and fast-moving complex suspensions of soil and water that threaten lives and infrastructure. Assessing the hazard potential of debris flows requires predicting yield and flow behavior. Reported measurements of rheology for debris flow slurries are highly variable and sometimes contradictory due to heterogeneity in particle composition and volume fraction (phi) and also inconsistentmeasurement methods. Here we examine the composition and flow behavior of source materials that formed the post-wildfire debris flows inMontecito, CA, in 2018, for a wide range of phi that encapsulates debris flow formation by overland flow. We find that shear viscosity and yield stress are controlled by the distance from jamming, Delta phi = phi(m) - phi, where the jamming fraction phi(m) is a material parameter that depends on grain size polydispersity and friction. By rescaling shear and viscous stresses to account for these effects, the data collapse onto a simple nondimensional flow curve indicative of a Bingham plastic (viscoplastic) fluid. Given the highly nonlinear dependence of rheology on Delta phi, our findings suggest that determining the jamming fraction for natural materials will significantly improve flow models for geophysical suspensions such as hyperconcentrated flows and debris flows.

Automated summary

Debris flows, which are dense and fast-moving suspensions of soil and water, pose a threat to lives and infrastructure. This study examines the composition and flow behavior of source materials that formed post-wildfire debris flows in Montecito, CA in 2018. The results show that shear viscosity and yield stress are influenced by the distance from jamming, and by rescaling the shear and viscous stresses, the data collapses onto a flow curve indicative of a Bingham plastic fluid. These findings suggest that determining the jamming fraction can significantly improve flow models for geophysical suspensions such as debris flows.