Evolution of snow slope stability during storms

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 Sigma(z)(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 t(f)(t) (the time when Sigma(z)(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. t(f)(t) proved to be a better discriminator between stable and unstable conditions than Sigma(z)(t). This is because it contains information about both the magnitude and the present trend of Sigma(z)(t). Even if Sigma(z)(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 model is suitable for operational use because the required input (hourly measurements of precipitation, air temperature and new snow density) is routinely measured at many study sites. (C) 1999 Elsevier Science B.V. All rights reserved.