For this “Avalunsj” Callum Treagaskis from the NGI will give us a presentation of his PhD and couple it with some of his later work at the NGI. The title of his talk is Dynamic modelling of snow avalanches.

Abstract: Flows of granular material may exhibit both solid-like and liquid-like behaviour simultaneously, for example, in large amplitude surge waves that are a destructive feature of debris flows or in snow avalanches where the erosion of snow at the front is fundamental to the dynamics of the avalanche. When modelling such a flow, the frictional properties are essential to accurately model the behaviour and mobility of the avalanche and also how deposits of static material influence the flow. Often models adopt a simple frictional term, however, this choice can underpredict the velocity field of the avalanche and can require unphysical values for the friction to match runout distances. The depth-averaged $\mu(I)$-rheology extended to include hysteresis effects (where $\mu$ is the friction and $I$ the inertial number), is a recent advancement in the continuum modelling of granular flow that can describe the interaction of static and flowing regions in the flow ,and thus, the influence of erosion and deposition. The depth-averaged model using this rheology is used to study the erosion and deposition properties of granular flow impacting an obstacle with results compared to small-scale experiments. The results are of direct relevance to geophysical mass flows and snow avalanches, which flow over rough terrain and may impact barriers, forests or other infrastructure.

Such a model can be further developed to describe fluidised powder snow avalanches. Using a regularised form of the rheology has extended the applicable range into regions of rapid highly-collisional flow. With this adaption such rheologies may now be applicable to dry fluidised snow avalanches which have regions of rapid collisional flow alongside a dense core. Theories for particle segregation, where the material species that make up the granular mixture separate during the flow, have also been advanced with work linking the rheology to segregation theory. Combining these theoretical frameworks it may be possible to derive a model for a fluidised snow avalanches that captures the behaviour of fluidised flow while maintaining solving efficiency. Including a fluidisation process is important in order to describe the density variation observed in powder snow avalanches which affects the velocities and mobility of the differing flow regimes, and thus, the runout of the flow. Once combined with the additional considerations for varying topography and erosion, entrainment and deposition of material, such a model is of direct relevance for hazard mapping of snow avalanches in mountainous areas.

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