Modelling Solid-Fluid Phase Separation and Dewatering in Debris Flows

The runout behaviour of debris flows is strongly governed by their solid-fluid composition.  In mitigation projects it is often necessary to predict when the solid phase deposits and if there exists the possibility of fluid washes.  The solid-fluid composition in the runout zone often controls the size and type of mitigation measures, as well as how land is zoned around a specific torrent.  This problem is extremely difficult to solve in general terrain because of the difficulty to establish initial conditions for both sediment and fluid, the inability to accurately account for torrent geometry and erosion, or the complexity of the muddy-granular flow rheology.  Here we present a dilatant, two-phase debris flow model that predicts the deposition of the solid phase with eventual dewatering.  Theoretically, the model exhibits a specific solid-fluid composition ratio for a debris flow in steady-state conditions.  In the runout zone, when the flow decelerates, the shear-work is no longer capable of sustaining this steady-state, leading to the deposition of solid material with decoupling of the fluid phase.  We apply the model to simulate several debris flow events where the stopping/dewatering behaviour of flow was captured using high-resolution drone scans.   Finally, we show that the wide range of empirical friction coefficients used in single phase debris flow models can be constrained by application of two-phase models, with varying solid-fluid compositions.