Determining granular and cohesion controls on debris flow rheology

Abstract:

While the dynamics of initiation and cessation of debris flows are extraordinarily complex and transient, one may gain insight by simplifying the boundary conditions to probe the flow resistance of these suspensions to prescribed shear forces; this is the essence of rheology. For decades, researchers have measured and debated the meaning of rheology from suspensions of debris-flow materials. Here we draw on progress in the physics of dense suspensions to examine the rheology of source materials that formed the post-wildfire debris flows in Montecito, CA in 2018. Our goal is determining how the physical and chemical properties of these soils, including burning, affect flow behavior using a rheometer. Clay content and organic matter contribute to cohesion through aggregation; limited variation in these quantities is observed across our samples. Sand content modulates granular-frictional resistance; we find that the volumetric sand fraction varies widely across our samples. All materials examined exhibit shear thinning viscosity behavior, and yield stress increases with the volume fraction of solids in the suspensions. While qualitatively similar, the flow curves (viscosity vs. shear rate) vary significantly among samples. These curves can be collapsed onto a single master curve, using two parameters: yield stress, and the critical volumetric jamming fraction at which viscosity becomes infinite. Empirically, these quantities correlate with the sand volume fraction in our samples. To better understand the contributions of cohesion and granular-friction to rheology, we examine flow curves of idealized mixtures of kaolinite clay and sand at various proportions. We find a non-monotonic relation between sand content and viscosity, consistent with a sudden onset of granular friction at a critical sand fraction. These results suggest that two empirical parameters, yield stress and jamming fraction, are physically meaningful quantities that largely determine debris-flow rheology. We suggest that these quantities are controlled by clay/organic or sand content, which determine cohesion and granular friction, respectively.