What is it about?

In some cases, snow avalanches have gone much farther than conventional models (both dynamical and statistics-based ones) predict. Both observations and experiments have indicated for some time that dry-snow avalanches consist not only of a dense core and a dilute powder-snow cloud, but that there is a third, intermediate-density flow regime, which shows high mobility combined with significant impact pressure. We call this flow regime fluidized because snow particle collisions produce enough dispersive pressure to drive particles away from each other and thus to reduce friction significantly. We extend the Norem–Irgens–Schieldrop avalanche model so that it can describe both the dense and fluidized flow regimes. For the density-dependent rheological parameters, results of theoretical analyses and numerical simulations of granular materials are used. Applying the model to different observed avalanche events, we find that it successfully simulates long-runout avalanches without much tuning of the friction parameters in the model.

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Why is it important?

The proposed model provides a physics-based description of important aspects of avalanche flow – in particular the density reduction that occurs when the avalanche speed increases. In the fluidized flow regime, the flow resistance is much lower than in the dense regime so that the run-out can be much longer. Importantly, the model achieves this more realistic modeling without the need to invoke low values of the basic friction parameters, as is necessary e.g. in Voellmy-type models. This is an important step towards a more objective way of producing avalanche hazard maps.

Perspectives

In the decade since the publication of this exploratory study, full-scale avalanche experiments at Vallée de la Sionne, Switzerland have produced further confirmation for the importance of the intermediate-density, fluidized flow regime. Bartelt and co-workers have developed a numerical model with variable density as an extension of the Voellmy-type model RAMMS. (However, there is disagreement in the scientific community about several aspects of that model, see (Issler et al., J. Glaciol. 64(243), 148–164) and (Bartelt and Buser, J. Glaciol. 64(243), 165–170).) The simple block model presented in the paper is too simplistic for use in hazard mapping. It should be developed into a full-featured quasi-three-dimensional code. Moreover, as already mentioned in the paper, the dispersive pressure from particle collisions is by itself insufficient to reduce the density as much as the experiments suggest. Additional mechanisms that would reduce the density, like aerodynamic under-pressure on the surface of the front, need to be studied in more detail and incorporated into the model.

Dieter Issler
Norwegian Geotechnical Institute

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This page is a summary of: Exploring the significance of the fluidized flow regime for avalanche hazard mapping, Annals of Glaciology, January 2008, Cambridge University Press, DOI: 10.3189/172756408787814997.
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