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Know the Snow: Can we dig snowpits in safer places?

Posted: December 14, 2012 - 1:01am

Several years ago I would scratch my head, pondering where to dig snowpits before I dove into a slope and skied it. Of course, I wanted a spot that represented the snow on the slope I was about to ski, or as some folks used to say: “dig your pit as close as possible to where you’ll trigger an avalanche without triggering one”. But risking your life trying to make sure the slope you are about to ride is safe seems to be a rather unreasonable way to go about it. Many of us in the avalanche business had been wondering if there was a way to assess the snowpack from a safe place. More specifically, can we trust stability tests results from a gentler, safer slope to apply to the steeper slope we were about to ski?

Recent insight into the mechanics that leads to an avalanche release suggest that a weak layer fracture, which undercuts the slope, advances in a collapsed wave motion. It also predicts that the load it takes to initiate a fracture is almost independent of slope angle. It was only natural to try and see if it is also applies to stability tests.

In the past, stability tests typically involved isolating a small column of snow and seeing what it takes to trigger an avalanche on that column. The Extended Column Test is a relatively new stability test that was developed by Karl Birkland and me about six years ago. It involves isolating a one by three foot column and dynamically loading one end of the column with 10 taps from the wrist, 10 taps from the elbow and 10 taps from the shoulder until a fracture develops in a weak snowpack layer. If the fracture crosses the entire column within one loading step, we consider the results as an indication of unstable snow. Otherwise, we consider the snowpack to be stable.

Since the development of the Extended Column Test it has been researched in North America, Europe and New Zealand. Clearly it is not a perfect test, but it has proven to be simple to use. More importantly, it is a better overall snow instability predictor than other stability tests and has quickly become the most used stability test by professionals and advanced recreationalists.

If our newly acquired insights into avalanche release also apply to the Extended Column Test, then neither the number of taps it takes to start a fracture nor the fashion this fracture crosses column should be affected by slope angle. In other words, as long as the structure of the snowpack remains the same, we can test the stability of a slope from a flatter, safer spot before we stick our neck on a steeper, more avalanched prone section of the slope. We collaborated with Joachim Heierli, Alec van Herwijnen and Ned Bair to test if our hypothesis holds water. Together, we did seven sets of Extended Column Tests on different slope angles, different snowpacks and different weak layers types. Two sets were in Montana, two were in the Chugach Range, here in Alaska, and three more sets were in Southeast. In each set we did one test directly upslope from the other, working our way up a small rollover with a consistent snowpack. As it turned out, as long as the snowpack is similar in structure, slope angle has very little effect on Extended Column Test results. Clearly, special care is needed to make sure the snow you are digging in is similar to the snow you are about to ski. But as long as we are confident that the snow structure at the top is similar to the snow on the steeper sections of the slope, we don’t have to risk getting caught in an avalanche when assessing the snowpack. We can dig our pit and get reliable test results on the flatter, less dangerous spot.

Although we identified a new and safer checkpoint in our snow evaluation process as a whole, things remain the same. For me, avalanche forecasting is still a holistic process. As humans, we tend to substitute hard questions with seemingly related easier questions. It is easy to replace the question “is the slope safe?” with “what are the stability test results?” This is a dangerous trap to fall into because the answer may not be the same. The stability test part of the slope stability evaluation process is not to tip the scale toward skiing when other signs suggest otherwise, but to serve as another verification point when all other signs indicate that conditions are stable.

 

• Ron Simenhois is an avalanche forecaster who lives in North Douglas. You may contact him at simenhois.ron@gmail.com.

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