Item: A field method for identifying structural weaknesses in the snowpack
Title: A field method for identifying structural weaknesses in the snowpack
Proceedings: 2002 International Snow Science Workshop, Penticton, British Columbia
Authors: Ian McCammon and Jurg Schweizer, National Outdoor Leadership School, Lander, WY, USA, Swiss Federal Institute for Snow and Avalanche Research SLF, Davos, Switzerland
Abstract: Recent studies have confined what experienced avalanche workers have known for years: that human triggered avalanches often coincide with specific structural patterns in the snowpack. In this paper, we examine the role of five structural parameters (weak layer depth, weak layer thickness, grain type, grain size and hardness transitions) in 145 human-triggered avalanches in the Swiss Alps and Canada, and 39 non-fracture profiles from the Teton and Snake River Ranges in the U.S. We show that, while no single parameter is a reliable predictor of instability, a simple linear sum of threshold values can provide an approximate indicator of unstable conditions. This threshold-sum method predicts the stratigraphic location of fracture planes in a majority of the cases reviewed and, based on a limited data set, appears to have predictive value when assessing false stable avalanche conditions. Because the method uses parameter threshold values that are based on field expediency as well as statistical significance, it is especially well suited for novices learning how to interpret snow profiles. As with standard stability tests, the method gives approximate results that are best used in conjunction with other tests and observations.
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Keywords: avalanche forecasting, snow stratigraphy, snow stability evaluation, avalanche education
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