Rheology of debris flows: Insights from experiments with coarse-grained matrix

Debris flow is characterized by a heterogeneous mixture of water and sediment with varying rheology. The granular effects on rheology are usually attributed to the bulk concentration of solids without considering the variability of granular configuration, as signified in the grain size distribution (GSD). In this work, the GSD effects on debris flow rheological properties were explored using the parameters μ and D_c derived from a unified GSD function, P(D) ~ D^-μ exp(-D/D_c), that are widely applicable for debris flow materials. Compared with other experiments using artificial fine-grained slurry (with grain size <2 mm) at a given solid volume concentration (C_v), the realistic coarse-grained matrix (up to 10 mm) of fresh debris flows was used for the experimentation, under shear rate up to 40 (s⁻¹) as in natural conditions. The results show that the flow can be categorized as Herschel-Bulkley (HB) fluid, with an average consistency index of 0.45, signifying the shear thinning effect. The yield stress and effective viscosity exhibit a power-law with μ and an exponential relationship with D_c, revealing the interlock between fine and coarse grains. Then, a modified HB model was proposed using the GSD parameters to specify the granular effects and explain the velocity fluctuation of debris flow surges. This work represents the first attempt to express rheological properties as a function of the unified GSD parameters and is potentially instrumental in formulating debris flow dynamics incorporating granular effects.