Item: Field data and theory for human triggered "whumpfs" and remote avalanches
Title: Field data and theory for human triggered "whumpfs" and remote avalanches
Proceedings: Proceedings of the 2000 International Snow Science Workshop, October 1-6, Big Sky, Montana
Authors: Ben Johnson, Bruce Jamieson, and Colin Johnston, Dept. of Civil Engineering, University of Calgary, Calgary, Alberta, Canada, Dept. of Geology and Geophysics, University of Calgary, Calgary, Alberta, Canada
Abstract: Remotely triggered avalanches and whumpfs (sometimes called "settlements") are common occurrences in many mountains ranges, but have received little research attention in the past. These events are generally associated with persistent weak snowpack layers, consisting of surface hoar, depth hoar and facets. Over the past four years, snowpack data have been collected at the sites of forty skier-triggered whumpfs and thirteen remotely skier triggered avalanches in the Columbia and Rocky Mountains of British Columbia. These data are compared to data for skier triggered avalanches that were not remotely triggered. Whumpfs and remotely triggered avalanches showed significant differences in the weak layer and slab properties. Additional measurements at five whumpf sites indicated a collapse of the weak layer and downward displacement of the snow surface. At one site during the winter of 1999-2000, the speed of the propagating fracture through a weak layer under a soft slab was measured at 19.9 m/s using geophysical equipment. A theory is presented that explains propagation of a fracture in a weak layer on level terrain. This theory also explains the large difference in speeds observed for whumpfs.
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Keywords: avalanche release, persistent weak layers, fracture propagation
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