Item: Integrating shear quality into stability test results
Title: Integrating shear quality into stability test results
Proceedings: 2002 International Snow Science Workshop, Penticton, British Columbia
Authors: Ron F. Johnson and Karl W. Birkeland, Gallatin National Forest Avalanche Center, Bozeman, Montana, U.S. Forest Service National Avalanche Center, Bozeman, Montana Department of Earth Sciences, Montana State University
Abstract: This study investigates whether collecting shear quality data in conjunction with stability test results improves snowpack evaluations. Over the past six seasons we have consistently evaluated shear quality when evaluating snowpack stability. Shear quality is subjectively evaluated on a 3-tiered scale from Q1 (clean, fast shears) to Q2 (average shears) to Q3 (irregular or dirty shears). Our method is a formalization of what ski patrollers and others have been doing in the U.S. and elsewhere for at least several decades. We used a dataset of nearly 700 individual stability tests (rutschblock, stuffblock and compression tests) collected by seven observers on slopes from Alaska to Chile. In addition to stability test results, observers noted whether slopes they felt were similar to their snowpit location had avalanches, or collapsing or cracking snowpacks, on that day. Results suggest that shear quality provides important stability information, especially when stability test results appear to indicate relatively stable conditions, but the shear quality is rated Q1. This might be because stability test results are often spatially variable, while our experience indicates that shear quality is more homogeneous. Given these results, we believe formally integrating some description of shear characteristics into stability assessments may be important for avalanche workers and backcountry enthusiasts.
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Keywords: avalanche forecasting, snow stability, stability tests, stability evaluation
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