Item: Formation and forecasting of large (catastropidc) new snow avalanches
Title: Formation and forecasting of large (catastropidc) new snow avalanches
Proceedings: 2002 International Snow Science Workshop, Penticton, British Columbia
Authors: P. Foehn, M. Stoffel, and P. Bartelt, Swiss Federal Institute for Snow and Avalanche Research, 7260 Davos Dorf, Switzerland
Abstract: The catastrophic avalanche winter 1998/99 in the Alps demonstrated that still larger deficiencies exist within the themes: "formation" and "forecasting" of large, new snow avalanches. It is well known that the two themes are interconnected and that without knowing the proper formation processes of such avalanches a timely and locally accurate forecasting remains an illusion. In a first part the state of knowledge of the various formation processes is assessed. Based on the indirect evidence of observed catastrophic avalanches and on the existing theories of the mechanics of snow slab release, several variants of release mechanisms are reconnoitered. It will be shown that the load rate, the terrain and the mechanical behavior of new snow are major factors for the initiation of very large avalanches, whereas the instability of previously existing snow surface layers may rather hamper the formation of large avalanches. In a second part the forecasting procedures and their relevances are discussed. A major point is the fact, that the imminent amounts of new snow are seldom known, because large snowfall events may only be forecasted quantitatively with a success rate of about 60%. Hence, if forecasters apply the usual methods of stability evaluation - in default of more adequate procedures - they face the problem that only a few of the so-called contributing factors are really known. Moreover the ones which are best known from previous field work (stability factors) are - as mentioned above - not the most deciding ones.
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Keywords: snow mechanics, slab avalanche fonnation, avalanche forecasting, catastrophic avalanches
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