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YAA_CAC_tech Posted: 03-21-2012 7:10 PM

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We thought we would start this tread as a place to put down some of our thought and ideas that don't really fit in our regional conditions threads.  Please feel free to engage in discussion and share your thoughts, questions or comments.

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YAA_CAC_tech replied on 03-21-2012 9:50 PM

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A key part to understanding any avalanche problem and planning how to manage it is to understand the distribution of the offending layer or interface in the terrain.  We often talk of avalanche problem being widespread, confined to specific terrain or isolated.  From the questions people have been sending us recently it seems there may be some confusion about what these terms actually mean.

Any avalanche problems weather widespread, specific or isolated will be distributed across a wide area

• Widespread layers are simply layers that are found almost everywhere.  A good example of a widespread layer from this year is our December 5^th rain crust, which we find consistently 50cm from the ground from valley bottom to about 1500m across the White Pass region.
• If an avalanche problem is not widespread but can be found in fairly predictable places we say it is confined to specific terrain.  An example would be a windslab problem developing after S winds found on N aspect ridge top wind-loaded features. 
• A problem layer that may be lurking in some terrain with specific characteristics but not everywhere with those same characteristics has an isolated distribution.  Isolated problems are the hardest to manage, I think of them as landmines and finding them in snow pits can be hit or miss.  The February 21^st surface hoar layer in the White Pass is perfect example of such a problem.

Digging multiple pits in a small area today reminded us just how variable the distribution of the Febuary 21^st surface hoar layer can be.  We managed our hazard by choosing a low angle slope to dig on..  Within 100m we dug three pits on a North East aspect.  In our first pit we found a well-preserved layer of surface hoar 80 cm down and reactive.  50m across the slope the surface hoar was there but smaller and starting to settle into the slab above, here it was reactive only under very hard loads.   50 m in the other direction the surface hoar was nowhere to be found.

While all these pits were dug on a feature with fairly uniform terrain characteristics I suspect that the presence or absents of the surface hoar in the individual pits is due to small variations in the amount of wind and sun the surface the snow was exposed to back in mid February when this layer was developing.  Characteristics impossible to identify now.

In the last avalanche cycle we saw that where this layer exists it is capable of producing large and destructive avalanches.  Nothing since then has made me think that this is not still the case.  At an average depth of 80cm this layer is approaching the limits of what is susceptible to human triggering.  However, given its isolated nature sledding or skiing on these N - NE aspects is analogous to walking through a mine field:  you might make it through no problem; on the other hand you might not; and then, BOOM!

After weeks of chasing this layer down I’m still not confident I have it figure out.   As hard is it is to not jump onto those tempting north facing slopes avoiding them is the only technique I know of how to mitigate this problem.  Digging a pit and not seeing surface hoar is no guarantee of safety.  Personally I look forward to may years of skiing and high marking chalky North aspects so resisting the lure of the minefield is going to by my strategy until I start seeing strong evidence that this layer has been assimilated into the mid pack wherever it exists.

- Eirik