Item: STABILITY CHANGES AFTER WEAK LAYER COLLAPSE: INSIGHTS FROM AN UNEXPECTED DATASET
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Title: STABILITY CHANGES AFTER WEAK LAYER COLLAPSE: INSIGHTS FROM AN UNEXPECTED DATASET
Proceedings: Proceedings, International Snow Science Workshop, Whistler, BC, Canada, 2026
Authors:
- Andrea Bruckmeier [ Department of Earth Sciences, Montana State University, Bozeman, Montana, USA ]
- Erika Birkeland [ U.S. Geological Survey, Northern Rocky Mountain Science Center, West Glacier, Montana, USA ] [ Bridger Bowl Ski Area, Bozeman, Montana, USA ]
- Ron Simenhois [ Colorado Avalanche Information Center, Leadville, Colorado, USA ]
- Alex Marienthal [ Gallatin National Forest Avalanche Center, Bozeman, Montana, USA ]
- Erich Peitzsch [ U.S. Geological Survey, Northern Rocky Mountain Science Center, West Glacier, Montana, USA ] [ Department of Earth Sciences, Montana State University, Bozeman, Montana, USA ]
- Karl W. Birkeland [ Birkeland Snow and Avalanche Scientific, Bozeman, Montana, USA ]
Date: 2026-09-28
Abstract: While conducting avalanche mitigation or traveling through the backcountry, avalanche professionals sometimes experience a "whumpf": a collapse-mode crack accompanied by surface cracking that does not produce an avalanche. These whumpfs are a clear warning sign of instability in the surrounding terrain. But assessing the collapsed slope itself, if it is steep enough to avalanche, carries much more uncertainty. Can slopes that whumpf and are steep enough to slide still avalanche? The answer to this question supports avalanche mitigators and ski guides in assessing whether to open such slopes. Previous research on collapsed slopes is limited to one study conducted before tests that estimate crack propagation propensity, such as the Propagation Saw Test (PST), were developed. This work showed that weak layer shear strength decreased immediately after collapsing and then increased over time. However, no research has examined crack propagation propensity after a whumpf. Studying propagation propensity is key for assessing how whumpfs immediately affect slope stability. To improve our understanding of changes in propagation propensity following weak layer collapse, we conducted PSTs on a test slope in the hours and days after an unexpected collapse occurred on a layer of surface hoar. Two PSTs at our study site three days prior to this collapse provided an estimate of weak layer propagation propensity. PST cut lengths decreased immediately following the collapse, suggesting that the collapse increased the propagation propensity. However, over the next 4-5 hours, PST cut lengths steadily increased at our site, likely due to ongoing sintering and strengthening in the collapsed weak layer. By the end of day two, we were no longer able to initiate propagation, suggesting substantial stabilization of the weak layer. Our results support prior findings and observations suggesting that slopes may remain capable of producing avalanches immediately after a collapse but likely stabilize over the next day or two. Although this research does not provide answers about precisely when a slope becomes stable after a collapse, it does highlight uncertainties in our understanding and interpretation of the stability of slopes that have whumped.
Object ID: ISSW2026_O1.3.pdf
DOI: https://doi.org/10.15788/1790098473
Language of Article: English
Presenter(s): Andrea Bruckmeier
Keywords: whumpf, crack propagation, Propagation Saw Test (PST), snowpack stability, avalanche release
Page Number(s): 1316 - 1323
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