Snow instability patterns at the scale of a small basin

Reuter, Benjamin; Schweizer, Jürg

doi:10.1002/2015jf003700

Cited by 22 publications

(30 citation statements)

References 88 publications

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“…The presented framework offers a way to assess snow instability at a location in the field. However, we are lacking a reference for snow instability which in part is a consequence of slope scale effects (Gaume et al, ) and spatial variations (Reuter, Richter, & Schweizer, ). Moreover, to validate snow instability model results subjective field observations are typically only partly suitable.…”

Section: Discussionmentioning

confidence: 99%

Describing Snow Instability by Failure Initiation, Crack Propagation, and Slab Tensile Support

Reuter

Schweizer²

2018

Geophysical Research Letters

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Snow instability is a generic term describing the propensity of a snow slope to avalanche. In need of a concise mechanics‐based concept we suggest a framework based on failure initiation, crack propagation, and slab tensile support. Following these three steps we modeled three metrics from mechanical data, which we derived from snow micropenetrometer signals. Verifying the metrics with field measurements confirmed that slab thickness and weak layer strength typically influence failure initiation, elastic modulus and weak layer fracture energy largely control crack propagation, and slab thickness and tensile strength provide the required tensile support. For all three metrics, considering slab layering was essential. Validation with signs of instability showed that the most accurate model includes all three steps – suggesting that snow instability can be described by failure initiation, crack propagation, and slab tensile support. Further validation is needed to assess the framework's potential for operational use.

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Section: Discussionmentioning

confidence: 99%

Describing Snow Instability by Failure Initiation, Crack Propagation, and Slab Tensile Support

Reuter

Schweizer²

2018

Geophysical Research Letters

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“…The main sources of uncertainty in forecasting are the unknown temporal evolution and the spatial variations in instability in the snow cover. For these reasons predictability of snow avalanche occurrence is limited; it is inversely related to scale, i.e., a probability of occurrence can be given at the regional scale, but not at the scale of a single avalanche path (Schweizer, 2008). In forecasting of natural systems, in which variations may or may not be random, a distinction is often made between forecasting and prediction.…”

Section: Introductionmentioning

confidence: 99%

On the relation between avalanche occurrence and avalanche danger level

Schweizer¹,

Mitterer²,

Techel³

et al. 2020

The Cryosphere

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Abstract. In many countries with seasonally snow-covered mountain ranges warnings are issued to alert the public about imminent avalanche danger, mostly employing an ordinal, five-level danger scale. However, as avalanche danger cannot be measured, the characterization of avalanche danger remains qualitative. The probability of avalanche occurrence in combination with the expected avalanche type and size decide on the degree of danger in a given forecast region (≳100 km2). To describe avalanche occurrence probability, the snowpack stability and its spatial distribution need to be assessed. To quantify the relation between avalanche occurrence and avalanche danger level, we analyzed a large data set of visually observed avalanches (13 918 in total) from the region of Davos (eastern Swiss Alps, ∼300 km2), all with mapped outlines, and we compared the avalanche activity to the forecast danger level on the day of occurrence (3533 danger ratings). The number of avalanches per day strongly increased with increasing danger level, confirming that not only the release probability but also the frequency of locations with a weakness in the snowpack where avalanches may initiate from increase within a region. Avalanche size did not generally increase with increasing avalanche danger level, suggesting that avalanche size may be of secondary importance compared to snowpack stability and its distribution when assessing the danger level. Moreover, the frequency of wet-snow avalanches was found to be higher than the frequency of dry-snow avalanches for a given day and danger level; also, wet-snow avalanches tended to be larger. This finding may indicate that the danger scale is not used consistently with regard to avalanche type. Even though observed avalanche occurrence and avalanche danger level are subject to uncertainties, our findings on the characteristics of avalanche activity suggest reworking the definitions of the European avalanche danger scale. The description of the danger levels can be improved, in particular by quantifying some of the many proportional quantifiers. For instance, based on our analyses, “many avalanches”, expected at danger level 4-High, means on the order of at least 10 avalanches per 100 km2. Whereas our data set is one of the most comprehensive, visually observed avalanche records are known to be inherently incomplete so that our results often refer to a lower limit and should be confirmed using other similarly comprehensive data sets.

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“…Yet, all these mechanical properties are not simulated independently in snowcover models, but density parametrizations are used to estimate them. However, simulated vertical density profiles are often smoother than observed in nature (Reuter et al, 2016) and in addition, using density alone is insufficient due to influences of snow microstructure such as its anisotropy or bonding (e.g. Hagenmuller et al, 2014).…”

Section: Introductionmentioning

confidence: 99%

Shear failure of weak snow layers in the first hours after burial

Reuter

Calonne

Adams

2019

Preprint

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Abstract. In a dry stratified snowcover slab avalanches release following failure in a weak layer below the slab. Typically, such weak layers consist either of persistent grain types or precipitation particles. Experience suggests that non-persistent instabilities often crest during or towards the end of a storm – probably because weak layers of precipitation particles strengthen rapidly. Studies so far have mainly focused on persistent grain types providing only sparse data to describe non-persistent weak layer failure. To understand differences between persistent and non-persistent weak layers we measured fracture mechanical properties relevant for avalanche release in a temporal series of laboratory tests. At defined lag times we tested small layered samples containing a weak layer of surface hoar, facets or decomposing fragmented particles in shear. Highspeed frames from the failure zone and image correlation analysis confirm that weak layers concentrate the shear strain. Failure consistently occurred after 20–30 % of strain energy was dissipated – despite shear strain rates as high 10−2 s−1. Our results of shear modulus and shear fracture toughness compare well with published data. The values for surface hoar and decomposing fragmented particles increased due to sintering. In the first hours after burial both weak layers had similarly low values, indicating they are equally fragile. Only for surface hoar and decomposing fragmented particles could we calibrate a formulation which allows for estimating the shear modulus from SMP signals.

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Snow instability patterns at the scale of a small basin

Cited by 22 publications

References 88 publications

Describing Snow Instability by Failure Initiation, Crack Propagation, and Slab Tensile Support

Describing Snow Instability by Failure Initiation, Crack Propagation, and Slab Tensile Support

On the relation between avalanche occurrence and avalanche danger level

Shear failure of weak snow layers in the first hours after burial

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