Item: Rough correlations of common snowpack stability tests
Title: Rough correlations of common snowpack stability tests
Proceedings: Proceedings of the 2006 International Snow Science Workshop, Telluride, Colorado
Authors: Mark Moore, Northwest Weather and Avalanche Center, Seattle, WA
Abstract: Because each different layer of snow can respond to applied stress in a variety of ways, and because the mechanical properties of snow layers often change dramatically over space and time, it is difficult if not impossible for one simple mechanical test to determine whether or not a slope can avalanche. Often this can only be definitively answered by actually skiing, riding, hiking, climbing or boarding the slope in question–which is not recommended as a mechanical test except in the context of slope cuts or ski testing, preferably on small safe(r) slopes. However, there are a variety of simple field tests available that can safely aid in the stability analysis process, and these include the Rutschblock, Stuffblock, Compression (or tap) Test, and Shovel Shear. When these tests are used in combination with all the other snowpack, weather and terrain factors out there–and when they are repeated often enough to appropriately sample the spatial and temporal variability of snow–then they can help to determine avalanche potential and promote safe travel. For practical purposes in many applications, common snowpack stability tests can be categorized into three basic stability levels. In addition to the internationally accepted test descriptors and classifications, these levels can be approximated in the Red-Yellow-Green or GO / NO GO rating system (Fesler and Fredston, 19942) that gives rough correlations between various tests and the estimated stability level(s) of red (NO-GO) yellow (Caution) or green (GO): • Red light (No Go) — another time, another day or try another place • Yellow Light (Caution) — be conservative, more tests recommended • Green Light (Go) — proceed, but don’t stop thinking and updating, Be aware that the strength test table below provides ROUGH test correlations, and proper application involves practice and consideration of all factors in the data triangle (snowpack, weather, terrain and the human factor). Useful snow stability information is hardly ever derived from just one test or one snowpit. It involves a process—an evolution of stability assessment—with snow profiles and strength tests being just one component. Avalanche potential is part of a strength-energy-structure continuum, and stability tests relate primarily to the strength portion. Hence other important considerations include knowledge about snowpack energy (related to the shear or fracture quality of stability tests) and snowpack structure (McCammon and Schweizer, 20023).Accident research has shown that human triggered avalanches occasionally occur with stability (Rutschblock) scores of 6 and 7 and an apparent Green/Go rating level. In these events, consideration of other instability indicators such as poor structure (more lemons) or more available energy (high quality shears) can be essential in helping to avoid or mitigate avalanche danger.
Keywords: stability test, snowpack strength, avalanche danger, snowpack structure, snow profile, shear test
Digital Abstract Not Available