What is it about?
The most detailed information on the dynamics of snow avalanches is gained from (difficult and expensive) experiments at a handful of avalanche test sites in the entire world. However, astonishingly much can be learnt from simple field observations if they are properly analyzed. We demonstrate this with three powder-snow avalanche events―ranging from medium-sized to extremely large―that occurred in the Swiss Alps in early 1995. In each case, three clearly different types of avalanche deposits were found. This suggests that powder-snow avalanches not only have a dense core and a dilute suspension layer, but also a part of intermediate density, which we call fluidized flow regime. The order of magnitude of the densities and velocities of the three flow regimes could be estimated by combining the available observations and analyzing them in simple ways. For example, the run-up height of the powder-snow cloud can be very large, but only if its density is not too high. The damage pattern (and also the absence of damage) provides bounds on the pressure and―when combined with information on the density―velocity. Interestingly, later full-scale experiments have confirmed these estimates. An interesting and useful result from these observations and a number of other avalanches investigated later is that the speed of the dense core of dry mixed snow avalanches is about half of the speed of the fluidized layer
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Why is it important?
The fact that much useful qualitative and semi-quantitative information can be extracted from simple field observations opens the door for learning about the dynamics of mixed dry-snow avalanches not only through the relatively few measurements at the very few existing test sites, but also from a large number of events in a much wider variety of avalanche paths. This is, in particular, valuable if one wants to understand the statistical properties of these avalanches.
Perspectives
The three avalanche events that provide most of the "raw material" for the analyses in this paper were crucial for the authors in developing a better understanding of the processes at work in mixed dry-snow avalanches. I have never afterwards encountered avalanche events with such clear evidence for three different flow regimes. Despite this, they were described and to some degree analyzed only in an SLF Internal Report in German from 1996, by the same authors. Furthermore, hardly any scientific papers have explored what information on the physical processes within powder-snow avalanches can be extracted from field observations, without recourse to expensive intruments installed at specific test sites. A good understanding of avalanche flow regimes is important for avalanche hazard mapping: The powder-snow cloud may have a much longer run-out than the dense and fluidized parts, but its pressure is often within a range that buildings can be dimensioned for. The fluidized front of dry-snow avalanches is much more mobile than the dense body, easily climbs over side walls of gully bends and thus may follow a different path from the dense part. Experiments at Vallée de la Sionne have shown that the average stagnation pressures in the fluidized layer are moderate, but peak pressures on small areas may exceed 1 MPa for a few milliseconds. To improve hazard maps, numerical models incorporating all three flow regimes identified in this paper need to be developed.
Dieter Issler
Norwegian Geotechnical Institute
Read the Original
This page is a summary of: Inferences on Mixed Snow Avalanches from Field Observations, Geosciences, December 2019, MDPI AG,
DOI: 10.3390/geosciences10010002.
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