Avalanche Assessment in Canada
In Canada, two ways in which avalanche danger
can be assessed is by using the hazard rating system or the snow
stability system. The hazard rating system is taken from a
regional perspective (mesoscale) and is based on weather and snowpack
analyses. It is conducted by the Canadian Avalanche Association
(CAA) who present the ratings in a public avalanche bulletin throughout
the winter. The avalanche danger is rated low to extreme.
Low or green indicates that any kind of avalanche is unlikely and that
it is safe to travel in the backcountry. Extreme ratings indicate
that natural and human triggered avalanches are certain and that travel
should be avoided. These types of bulletins are important for
backcountry travelers to be aware of but they can not be relied
upon. The information posted is generalized for an entire
region and therefore, is not detailed enough to determine the avalanche
danger of a particular slope. One way to determine the
probability of avalanches for a local area is by using the snow
stability rating system. This system is performed by patrollers
of a local ski resort or by the outdoor enthusiasts themselves.
From field observations and data, the stability of the snowpack is
rated “very well” to “very poor”.
Assessing your own slope
When determining the snow stability in the backcountry, one must note evidence of past avalanches, the wind and sun effects, the amount of precipitation, the air temperature and the sound of the snow under foot. In the field, pits are dug on a similar aspect and inclination to the slope that will be traveled. A test profile is done by poking up the wall of the pit; the layers are identified and the snow crystals and their density are examined within each layer. The strength of the bonding between these layers is tested by performing a few different tests on the snowpack. One is the compression test; it is done by applying a downward force on an isolated a column of snow in order to see if some layers compress and fail. It is measured in terms of how much force is used to get results; it is graded easy to hard. To test the shear strength of the weak layers identified, a shear test is done. Shear strength of a snowpack is the ability of layers to resist sliding along a plane (Allaby, 1999). Two ways in which this can be examined is by doing a shovel shear test and a rutschblock test. A shovel shear test is when a shovel is place vertically in behind a column of snow and pulled until the weak layer or layers slide (Jamieson, 1997). Like the compression test it is graded easy to hard. A rutschblock test is used to test the shear strength of a slope using the actual weight of a person. Usually on a 30° slope, a large block of snow, the length of a pair of skis, is isolated and tested. It is scored 1to 7 based on the amount of loading. For example, if a shear fails just by having a person stand on top of the block then it is rated a 1. If it doesn’t fail with several hard jumps then it is rated a 7.
By piecing together the snow pit results and other contributing factors, the stability can be rated. Typically, a very poor stability rating means there is obvious signs that the slope is unstable. Signs that suggest this are tracks from recent avalanches, cracks in the snow, rutschblocks score between 1 and 3 and the compression or shear tests are easy. Stable and safe snowpacks are associated with rutschblock cores of 6 or 7, the compression test has ‘hard’ results and there have been little effects from new precipitation, loading or buried weak layers. The snowpack is especially stable when temperatures are cool after warming. For the most part, instability isn’t that clear; between ratings of good and poor the stability factor is somewhat vague. There are other things to take into consideration: the accumulation rate of recent precipitation, the rate of wind loading and the change in temperatures. A slope is typically unstable if it has snowed 3cm per hour, if it has snowed more than 20cm in 48 hours, or if it has rained. Also, wind loaded slopes always have the ability to fail. Furthermore, drastic changes in temperatures (>10° Celsius), or no freezing at night can cause unstable conditions. Easy to moderate shear and compression tests means the slope has the potential to slide as does a rutschblock test between 1 and 5 (Jamieson, 1997). In unstable conditions, backcountry enthusiasts should avoid slopes greater than 30°, and travel on top of ridges or in dense trees. If the stability rating is fair, then one can venture onto slopes up to 35° if your party has enough experience. Most terrain is safe to be on top of in very stable conditions (Jamieson, 1997).
The following is a list of good
books about avalanches:
-The Avalanche Handbook (David McClung and Peter Schaerer)
-Backcountry Avalanche Awareness (Bruce Jamieson)
-Avalanche Accidents in Canada (Bruce Jamieson and Torsten Geldsetzer- vol 4)
-Mountaineerin, Freedom of the Hills (The Mountaineers)
References
Allaby, Ailsa, & Allaby, Michael. (1999). Dictionary of Earth science. (2nd ed.). New York: Oxford University Press.
Jamieson, Bruce. (1997). Backcountry Avalanche awareness. Revelstoke, B.C.: Canadian Avalanche Association.
Tremper, Bruce. 2001. Staying alive in avalanche terrain. Seattle Washington. mountaineers books.