Item: Wet snow diurnal evolution and stability assessment
Title: Wet snow diurnal evolution and stability assessment
Proceedings: International Snow Science Workshop, Davos 2009, Proceedings
Authors: Frank Techel, Chrisitine Pielmeier, WSL Swiss Federal Institute of Snow and Avalanche Research SLF, 7260 Davos Dorf, Switzerland University of Berne, Institute of Geography, 3012 Berne, Switzerland
Abstract: Wet snow avalanches pose a threat to snow recreationists and to infrastructure such as mountain roads. The timing of wet snow avalanche formation is difficult to predict. The loss of strength is more important than stress imposed by additional loads. Processes leading to weakening, such as water infiltration can be gradual or sudden. Therefore, the time scale of observations is essential. A questionnaire among 40 international avalanche professionals aimed at determining common practice in assessing wet snow stability during first and repeated wetting events. It turns out that only few respondents use differently stability tests for dry and wet snow. 14 respondents consider tests in wet snow useful. In view of the responses, we sampled 35 locations during first and repeated wetting cycles in the Swiss Alps. We compare the results of one rutschblock test (RB), four extended column tests (ECT) and two shovel shear tests from the morning and the afternoon. Additionally, vertical liquid water content profiles and snow profiles are analyzed. Moist and very soft persistent weak layers most often produce unstable test results, where the failure character is a collapse. Observed ponding horizons did not produce failures in the tests. As in dry snow, the RB is comparable to the ECT in wet and moist snow. The influence of surface melt-freeze crust is investigated by comparing ECT fracture propagation with a modified version of the ECT, where the surface melt-freeze crust is removed. A procedure to assess deep, wet snow instabilities is suggested.
Keywords: wet snow, snow stability, water content, rutschblock test, extended column test, shovel shear test, persistent weak layer
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