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Bozeman, MT, United States

Bair E.H.,University of California at Santa Barbara | van Herwijnen A.,WSL Institute for Snow and Avalanche Research SLF | Birkeland K.W.,Us Forest Service National Avalanche Center
Cold Regions Science and Technology | Year: 2015

Extended Column Tests (ECTs) are used to assess crack initiation and propagation. Previous research shows that tests 90. cm in length may propagate, suggesting instability, while tests 2. m in length may not propagate, suggesting stability, for identical snowpacks. A practical question is: are 90. cm ECTs optimal for assessing stability? To test the added value of 2. m ECTs for stability evaluation, we collected data on 220 ECTs, with 136 side-by-side standard length ECTPs (full propagation indicating instability) followed by 2. m ECTs. We only performed 2. m ECTs after a standard ECT propagated because we assumed 2. m ECTs would not propagate if standard length tests did not. These tests were preceded by an a priori stability assessment. Our results show imbalances for both tests. The ECT had a similar probability of detection (0.88-0.92, POD), i.e. the ability to detect unstable conditions, as in previous studies, but a much lower probability of null events (0.54-0.75, PON), i.e. the ability to detect stable conditions, with variation due to the binary classification of "Fair" stability as stable or unstable. Adding a 2. m test after an ECTP result lowered the POD (0.49-0.58), but substantially raised the PON (0.88-0.98) of the combined tests. The proportion of tests in agreement, i.e. ECTP and 2. m ECTP, increases with decreasing stability. We conclude that an ECTP followed by a 2. m ECTP is a clear red flag, indicating instability. The interpretation of an ECTP followed by a 2. m ECTN/X (no propagation) is not clear. Though this result suggests stability, a high potential for a false stable result means we cannot recommend the 2. m ECT for binary stability assessments. © 2015 Elsevier B.V. Source


Bair E.H.,University of California at Santa Barbara | Bair E.H.,U.S. Army | Van Herwijnen A.,WSL Institute for Snow and Avalanche Research SLF | Birkeland K.W.,Us Forest Service National Avalanche Center
Cold Regions Science and Technology | Year: 2015

Extended Column Tests (ECTs) are used to assess crack initiation and propagation. Previous research shows that tests 90. cm in length may propagate, suggesting instability, while tests 2. m in length may not propagate, suggesting stability, for identical snowpacks. A practical question is: are 90. cm ECTs optimal for assessing stability? To test the added value of 2. m ECTs for stability evaluation, we collected data on 220 ECTs, with 136 side-by-side standard length ECTPs (full propagation indicating instability) followed by 2. m ECTs. We only performed 2. m ECTs after a standard ECT propagated because we assumed 2. m ECTs would not propagate if standard length tests did not. These tests were preceded by an a priori stability assessment. Our results show imbalances for both tests. The ECT had a similar probability of detection (0.88-0.92, POD), i.e. the ability to detect unstable conditions, as in previous studies, but a much lower probability of null events (0.54-0.75, PON), i.e. the ability to detect stable conditions, with variation due to the binary classification of "Fair" stability as stable or unstable. Adding a 2. m test after an ECTP result lowered the POD (0.49-0.58), but substantially raised the PON (0.88-0.98) of the combined tests. The proportion of tests in agreement, i.e. ECTP and 2. m ECTP, increases with decreasing stability. We conclude that an ECTP followed by a 2. m ECTP is a clear red flag, indicating instability. The interpretation of an ECTP followed by a 2. m ECTN/X (no propagation) is not clear. Though this result suggests stability, a high potential for a false stable result means we cannot recommend the 2. m ECT for binary stability assessments. © 2015 Elsevier B.V. Source


Bair E.H.,University of California at Santa Barbara | Simenhois R.,Southeast Alaska Avalanche Center | Birkeland K.,Us Forest Service National Avalanche Center | Dozier J.,University of California at Santa Barbara
Cold Regions Science and Technology | Year: 2012

Storm snow often avalanches before crystals metamorphose into faceted or rounded shapes, which typically occurs within a few days. We call such crystals nonpersistent, to distinguish them from snow crystals that persist within the snowpack for weeks or even months. Nonpersistent crystals can form weak layers or interfaces that are common sources of failure for avalanches. The anticrack fracture model emphasizes collapse and predicts that triggering is almost independent of slope angle, but this prediction has only been tested on persistent weak layers. In this study, dozens of stability tests show that both nonpersistent and persistent crystals collapse during failure, and that slope angle does not affect triggering (although slope angle determines whether collapse leads to an avalanche). Our findings suggest that avalanches in storm snow and persistent weak layers share the same failure mechanism described by the anticrack model, with collapse providing the fracture energy. Manual hardness measurements and near-infrared measurements of grain size sometimes showed thin weak layers of softer and larger crystals in storm snow, but often showed failures at interfaces marked by softer layers above and harder layers below. We suggest collapse often occurs in crystals at the bottom of the slab. Planar crystals such as sectored plates were often found in failure layers, suggesting they are especially prone to collapse. © 2012 Elsevier B.V. Source

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