An investigation of the spatial variability and efficacy of the Extended Column Test

Abstract

Most avalanche accidents are the results of an avalanche triggered by the victim, or a member of the victim's party. Many of these accidents are the result of uncertainty regarding the stability of the snowpack. Spatial variability of snow stability is a significant cause of this uncertainty. There has been significant previous work on the spatial variability of snow stability at multiple spatial scales, but most of these studies have focused on measures of fracture initiation. This study investigates the spatial variability of Extended Column Test (ECT) results (an index of fracture propagation). We measured the spatial variability of ECT results in 23 grids across southwest Montana over the course of two winters. These slopes were all topographically uniform, wind sheltered clearings, with snowpacks relatively undisturbed by skiers or snowmobiles. Twenty eight ECTs were spaced across each grid in a standardized layout with a 30 m x 30 m extent. Our results are consistent with previous work, with some grids showing high levels of variability as well as other grids with relatively homogenous results. We found no consistent spatial pattern to our test results. We tested slopes with a variety of weak layers (surface hoar, depth hoar, new snow, and near surface facets), slab characteristics (slab hardness, slab depth), and snow depths and found no correlations with ECT results. We found a relationship between the forecasted regional avalanche danger and the percent of ECTs showing propagation in a grid. As the regional danger increases the percent of ECTs propagating in a grid does as well. ECT results are most variable under moderate danger. When the regional avalanche danger is either considerable or low, results are likely to be more consistent. The key practical implication of our results is that ECTs, like all other stability tests, should be interpreted with an appropriate level of caution and in consideration of all other relevant variables. The spatial variability of this test has the potential to be high on some slopes, while on other slopes test results will be entirely in agreement.