A novel and unified solution to multi-phase mass flows

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Debris flows are gravity driven mixture flows of soil, sand, rock and water. The solid particles and viscous fluid governs the rheological properties, and their coupling significantly influences the dynamics. Debris flows can dramatically increase their volume and destructive potential, and become exceptionally mobile by entraining bed sediment. The mixture composition can evolve to strikingly change the spatial distribution of particles and fluid, and thus frictional and viscous resistance. So, erosion-deposition and phase-separation between solid and fluid, which strongly depend on material composition, play a critical role in debris flow dynamics. Proper understanding of these complex physical processes is very important in accurate description of impact forces, inundation areas, landscape evolution and developing reliable mitigation plans. Predicting the underlying processes of erosion, phase-separation and deposition in debris flow are long-standing challenges. However, due to lack of data and suitable models, there exists no runout prediction method that includes observed processes of erosion, entrainment and diffusion of eroded material, grain sorting, phase-separation, levee formation and deposition patterns.Based on innovative mechanical models for erosion-deposition and phase-separation that explicitly consider changes in local flow compositions (Univ. Bonn), and their validations with unique well controlled pioneering experiments (Univ. Utrecht), we aim to develop a novel, unified, efficient and fully coupled solution to these true multi-phase, three-dimensional mass flow problems. As debris flows are better described by a three-phase mixture that include viscous fluid, and fine and coarse grains as compared to often used single-phase models, proposed model consists of three-phases including yield strength. As wave-dispersion plays a critical role when a landmass impacts a water body, our novel model includes non-hydrostatic effects. Great advantage is that with such a proposed unified three-phase approach, realistic inter-connected processes of erosion, phase-separation, flow transformation from debris flow to turbidity current and their influences on the flow structure, composition and deposition can be achieved. The model will be validated by conducting new experiments with multi-component erosive debris flows combining all complex governing processes. These processes include erosion of dry and saturated beds, entrainment, particle sorting, phase-separation, levee/lobe formation, flow transformation from the subaerial to submarine environment and evolution of debris deposits with variable reservoir fluid level. Thus, this project results in an advanced open source mass flow simulation model aiming to accurately predict debris flow dynamics, phase-separation, erosion, deposition and runout. As such, the model will substantially help debris-flow hazard mitigation and reduce damage and fatalities worldwide.