Spontaneous Avalanche Releases in Svalbard, Influence of Climate Parameters on Snow Mechanical Properties
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With the development of the city of LONGYEARBYEN in SVALBARD, the exposure of the people and the infrastructures to snow avalanches is becoming a concern. The cold, dry and windy climate of SVALBARD produces a very dense snowpack with frequent occurrences of avalanches. There is currently a consensus in the scientific community that the prospect of climate change with predictions of higher temperatures and precipitation levels will probably affect the size and number of avalanches released in the Arctic. To allow for a better understanding of these anticipations, the intricate relationship between the snowpack temperatures, the layering, and the snow mechanical properties needs to be investigated with regard to the release mechanisms of spontaneous avalanches. Temperature influences both the weak layer development and slab mechanical properties, and therefore it plays a key role in the formation of unstable snow profiles. A monitoring of the snow temperature profiles has been used in order to investigate the global role of temperature on the snowpack development and its stability. Three avalanches have occurred close to the instrumented site. This led to the development of avalanche release scenarii in light of the temperature field evolution. The key role of the dependency of slab mechanical properties to air temperature on the initiation of failure has been pointed out. Indeed, the data collected before the three avalanche releases suggests that the last event leading to an avalanche release is a modification of the slab temperature. In order to specify the relative influence of the processes taking place in the slab during a temperature change, a series of triaxial tests on snow were performed in the cold laboratory. The goal of those experiments was to evaluate the relative influence of temperature and snow grains shape on the viscous properties of the snow. Results have shown a clear distinction between the viscous flow rates of rounded and faceted grains. They implied that faceted snow slabs tend to exhibit a more fragile behavior than rounded grained snow slabs, thus promoting the weak layer failure propensity. This weak layer failure propensity needs to be measurable in order to understand the development of an unstable snowpack. Traditional methods provide results closer to indexes than usable parameters values, so an innovative testing device has been developed with the intention of being able to accurately measure the weak layers shear strength. This method allows us to measure the shear stress - displacement curve at a given strain rate and at different depths along the snow profile. It is based on the vane test which is more well known in soil testings, and is used to measure the strength of thin layers (down to 1 mm). It has been tested in high arctic conditions and the first results are correlated with snow pits and rammsonde data. The tested geometry has been modeled, providing some first insights towards the validation of the test. The results presented and the shear measurement device developed may be used as a basis for further tests in order to better understand the evolution of the weak layer mechanical properties during its development and the role of the slab’s microstructure on the stability of the snow slope. The insights, and analyses presented in this thesis are used as a basis for discussion on the role of climate change and its most probable effects on the stability of snow slopes.