Robert P Kostynick , Hadis Matinpour, Shravan Pradeep, Sarah Haber, Alban Sauret, Eckart Meiburg, Thomas Dunne, Paulo Arratia, Douglas J Jerolmack 

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 grain size, shape, chemical composition, and solid-volume fraction (ϕ). Here we examine the composition and flow behavior of source materials that formed the post-wildfire debris flows in Montecito, CA in 2018, for a wide range of ϕ that encapsulates debris-flow formation by overland flow. We find that shear viscosity and yield stress are controlled by the distance from jamming, ∆ϕ = ϕm − ϕ, and that the jamming fraction ϕm depends on grain-size polydispersity and friction. By re-scaling shear and viscous stresses to account for these effects, the data collapse onto a simple non-dimensional flow curve indicative of a Bingham plastic (viscoplastic) fluid. Given the highly nonlinear dependence of rheology on ∆ϕ, 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.