Constraint-based approach towards debris flow rheology

Debris flows are dense, fast-flowing suspensions that form when intense rainfall fluidizes soil on steep mountain hillslopes. Such flows are associated with complex fluid-particle mixtures with particulates ranging from clay to gravel to ash, resulting in a diverse range of interparticle interactions (friction, cohesion, lubrication etc.) giving rise to correspondingly rich rheological behaviors under external natural disturbances. Here we use a constraint-based approach to interpret how interparticle interactions give rise to complex flow curves, and apply this framework to synthetic and natural debris-flow suspensions. Suspensions of sand and kaolinite powder - generated over a range of volume fractions (φ) are used as an idealized model for debris flows, where interparticle interactions can be tuned with water chemistry. We also examine soils that served as the source of deadly post-wildfire debris flows in Montecito, California in 2018. This allows us to test whether results obtained for idealized suspensions may be extended to highly heterogeneous natural materials. Experimental flow curves can be fully explained by a loosening of cohesive contraints with increasing stress (σ), where the yielding and shear-jamming states are separated by distinct stress scales. The flowing and jammed regions of each suspension are demarcated by a line in σ-φ phase space. We use this finding to propose a rheological state diagram for debris flows, and map our data and additional debris-flow data from the literature onto this diagram.