Item: Why measure new snow density, precipitation and air temperature?
Title: Why measure new snow density, precipitation and air temperature?
Proceedings: Proceedings of the 1998 International Snow Science Workshop, Sunriver, Oregon
Authors: H. Conway and C. Wilbour, Geophysics Program, University of Washington and Washington State Department of Transportation, Snoqualmie Pass
Abstract: The evolution of snow slope stability during storms is investigated using simple models to calculate the shear strength of a buried layer (from its density) and the imposed shear stress (from the weight of the overburden). There is a competition between the rate of loading from new snowfall and the rate of strengthening of buried layers. In theory, unstable conditions will occur when the stability index Ez(t) (the ratio of the shear strength of a buried weak layer at depth z to the shear stress imposed by the overburden) approaches 1.0. A related index of practical interest is the expected time to failure (the time when Ez(t) will become critical if the current conditions continue). The model is tested using measurements and observations of avalanche activity during three storm cycles at Snoqualmie Pass in the Washington Cascades. In two cases the avalanche activity Was high while in the other, few avalanches released. Tj (t) proved to be a better discriminator between stable and unstable conditions than Ez(t). This is because it contains information about both the magnitude and the expected changes of Ez (t) in response to the current conditions. Even if Ez (t) is close to critical, if it is not decreasing then slopes will remain stable. Results indicate the model may prove useful for forecasting avalanches during storms. The required input (hourly measurements of precipitation, air temperature and new snow density) is routinely measured at many study sites and the tractability of the model makes it attractive for operational use.
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Keywords: storm and avalanche cycles, snow slope stability
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