Item: Relating wet loose snow avalanching to surficial shear strength
Title: Relating wet loose snow avalanching to surficial shear strength
Proceedings: Proceedings of the 2006 International Snow Science Workshop, Telluride, Colorado
Authors: Simon Trautman, Eric Lutz, Karl W. Birkeland, and Stephan G. Custer Dept. of Earth Sciences, Montana State University, Bozeman, MT, USA Moonlight Basin Snow Safety, Big Sky, MT, USA Forest Service National Avalanche Center, Bozeman, MT, US
Abstract: Wet loose snow avalanches are a significant hazard within many ski areas. Wet snow stability changes dramatically over short time periods which typically coincide with operating hours, and few quantitative tools exist for avalanche workers attempting to predict the onset of wet snow avalanching. This study documents changes in surficial shear strength during meltfreeze cycles and relates these changes to observed wet loose avalanche activity. We conducted field work at two study sites in southwestern Montana over the course of four April days in 2005 and 2006. We used a 250cm2 shear frame to make as many as 210 surficial shear strength measurements of melt-freeze snow per day, and adjusted our results for known shear frame size effects. We also collected SnowMicroPen penetrometer profiles in conjunction with shear strength during one melt-freeze cycle. Initial results are encouraging. Changes occurred rapidly within the melt-freeze cycle as shown by highly significant changes in shear strength within half hour intervals. SnowMicroPen data shows significant positive correlations between the microstructural hardness of snow and shear strength. Most importantly, our limited data shows an apparent association between surficial shear strength and avalanche activity. On 22 April 2006 when our shear strength measurements dropped below 250 Pa we observed, and triggered, wet loose avalanches in the immediate vicinity of study slopes. Conversely, surficial stability on our study slope improved when shear strength values exceeded 300 Pa.
Keywords: wet snow, shear strength, wet avalanche
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