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

• DOI: 10.1073/pnas.2209109119
• Publication Date: 2022-10-24T19:13:40Z
• Authors: Robert Kostynick; Hadis Matinpour; Shravan Pradeep; Sarah Haber; Alban Sauret; Eckart Meiburg; Thomas Dunne; Paulo Arratia; Douglas Jerolmack
• Keywords: 0303 health sciences; soft matter; Water; geomorphology; 01 natural sciences; 6. Clean water; 620; Soil; 03 medical and health sciences; Engineering; Geophysics; Suspensions; 13. Climate action; debris flow; Physical Sciences; 0103 physical sciences; Earth Sciences; Rheology; jamming; Plastics

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 (ϕ) and also inconsistent measurement methods. Here we examine the composition and flow behavior of source materials that formed the postwildfire debris flows in Montecito, CA, in 2018, for a wide range ofϕthat encapsulates debris flow formation by overland flow. We find that shear viscosity and yield stress are controlled by the distance from jamming,Δϕ=ϕm−ϕ, where the jamming fractionϕmis 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 onΔϕ, 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.