Waves, levees and impact pressures in snow avalanches

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Snow avalanches are a major natural hazard in mountainous regions and pose a significant risk affecting people and infrastructure in many countries throughout the world. Avalanches typically have a dilute powder cloud that obscures the underlying dense flow beneath. This dense flow often causes much of the damage. Snow scientists have therefore spent many years developing GEODAR, Doppler and FMCW radar, as well as instrumented pylons, to visualize the internal dense flow dynamics (Köhler et al. 2016; 2018). These technologies shows that dense avalanches develop surges and internal waves (figs. 3) (Sovilla et al. 2010). In addition, field observations (fig. 4) show that a significant proportion of avalanches develop levees. These waves and levees are important because (i) they strongly enhance the mobility and run-out of the flow (Edwards et al. 2021, Rocha et al. 2019) and (ii) they vastly concentrate the energy within avalanches, dramatically magnifying the impact forces they exert on buildings and obstacles in their path (see figure 4). Despite their importance, the underlying mechanisms that cause waves and levees in snow avalanches are poorly understood, and consequently they are not well predicted by current models.This proposal combines the breakthroughs in visualisation of dense snow avalanches with a new general theory for wave and levee formation in shallow flows, originally developed in Manchester for small-scale dry granular flows (Gray & Edwards 2014, Edwards & Gray 2015, Viroulet et al. 2018, Rocha et al. 2019, Edwards et al. 2021). This theoretical framework will allow us to use the radar observations of wave amplitude, wavelength and coarsening dynamics, to provide important constraints on the rheological properties of snow avalanches, which are strongly temperature dependent. Köhler et al. (2018) identified seven main types of snow avalanche and this proposal focuses on the three main dense-flow types: (i) cold shear, (ii) warm shear and (iii) warm plug. Cold flows are cohesionless granular flows, while warm flows have some liquid water in them, which allows large snowballs to agglomerate by cohesive forces (Steinkogler et al. 2015). The snowballs dramatically increase the mean particle size, and warm-shear flows then have a tendency to form huge levees in the run-out zone (fig. 4a). This suggests that these flows may be closely analogous to the small-scale self-channelizing flows in Manchester (fig. 4b), for which we have developed a quantitative model (Rocha et al. 2019). This proposal therefore aims to develop a new friction law for snow-avalanche models, that will capture the spontaneous formation and growth of waves, as well as self-channelization in the run-out zone. We will also examine how self-channelization and wave formation are able to concentrate the impact forces on structures and enhance run out.