Managing Avalanche Risk in a Complex Snowpack
Posted on February 18th, 2013 by andy | 6 Comments
Another great blog post from BCA guest blogger Matt Wade of Peak Mountain Guides-
Boom! Another powerful whumpf resonated beneath our feet as we ascended through the forest. A crack propagated through the snowpack around us. We held our breaths. . . but we knew the slab would not release. We were on terrain that was only 25 degrees, an angle we had intentionally chosen because we knew this slope was likely to collapse under the weight of a person. I took out my probe and measured the snowpack depth. Leaning over to see the centimeter markings on the side of the probe, I exclaimed “Yep, another shallow area. The height of snow is only 110 centimeters.” Smiles emanated from the group. After a day of gathering observations about the snowpack we were beginning to identify patterns in snow stability. With a complex Colorado snowpack shaping up for our four-day Level 2 Avalanche Course with Peak Mountain Guides, this understanding would be essential for us to ski powder and avoid avalanches.
Our process began with an intimate look at the snowpack to gain a baseline understanding of its structure. To do this, we performed a full profile to observe the layering, grain types and sizes, hardness of layers, temperature, moisture and density, and overall depth or “height of snowpack”. Within the space of an hour we quickly discovered the snowpack was in a precarious state of balance. We found 90cm of storm slab and persistent slab sitting over a tremendously weak base of facets. Small column tests like the compression test and deep tap test revealed sudden collapse fractures at the interface between the persistent slab and the facets near the ground. Large column tests such as the extended column test and propagation saw test showed good propensity for propagation along the weak layer/slab interface. In just our first look, we had already determined there was a potential for large and destructive avalanches to be triggered by backcountry travelers.
With a puzzled look on his face, one of the students asked, “So if this snowpack shows signs of instability, how are we going to ski anything good”? It was a valid question, and one that all of us were secretly wondering about, because at the end of the day the whole point was to go skiing. “Perhaps we should look to see if the same instability observed in our snow profile exists in other places in the landscape – or not”, I suggested. It was a bone of possibility being dangled in front of the salivating jaws of the skiers. Perhaps the avalanche problem we discovered in our snow profile at treeline on a Northeast aspect didn’t exist in other places. If we could find places where there was no problem, we could have our cake and eat it too.
In our quest to understand the nature of the problem, we went ski touring. We ski toured a lot. We traveled on North, South, East, and West aspects. We looked at the snowpack below treeline and in the alpine. We measured changes in the height of snow and we dug snow profiles to see if snowpack tests produced similar results in different places. We stuck to low angle terrain so we didn’t have to worry about avalanche danger. We even skied some lovely low angle powder. It was interesting and thought provoking, and at times it felt like we were investigators on the trail of a mystery.
While we were touring we were targeted our observations of the snowpack based on the specific avalanche problems we had identified. For example, we evaluated the storm slab problem using tilt tests, compression tests, and probing since it was confined to the top 40cm of the snowpack. For the deep persistent slab problem we used deep tap tests and propagation saw tests since it was located much deeper in the snowpack. The following photo shows our list of specific field observations for the avalanche problems at hand. Having this list gave us a sense of purpose and a defined plan for gathering good, relevant observations that would actually help us make good terrain choices.
One of our most sought after observations was height of snow, or “HS”. To get this observation we simply traveled with our BCA probes in hand and periodically measured the HS as we traveled onto new aspects or when we entered new elevation bands. We were hoping to find areas where the snowpack was consistently deep, with the strategy that it would be more difficult to trigger the deep instability in those areas. Typically, a skier affects only the top 1.5-2 meters of snowpack. If we could find places where the snowpack was consistently this deep or deeper, we could avoid the persistent slab avalanche problem by skiing in these areas.
After a day of hiking around and gathering observations on a variety of aspects and elevations we discovered several things. First, we found the persistent slab avalanche problem to be widespread throughout the terrain. We found slabs over facets at all elevations and on all aspects. There was no ski terrain without the problem, period. We also found HS was widely variable from as little as 30cm and up to 240cm. Unfortunately, the places where we found the deepest snowpack we also found the greatest variability. Due to this variability we weren’t confident we could ski a consistently deep snowpack to avoid the persistent slab avalanche problem. It was almost always within 85 cm of the surface (the ideal range for human triggering). Furthermore, areas where the snowpack was especially shallow – like South aspects and windblown areas – we experienced extensive whumpfing and collapsing as we traveled. This was a direct indicator that areas with a shallow snowpack (100 cm or less) would be very easy to trigger. It wasn’t looking like a good day for the steeps!
The picture was painted. The avalanche problem was unavoidable and the only viable hazard management strategy would be terrain angle. “Well, I’m glad we figured it out and now we know exactly what to do to make sure we’re safe out here”, pointed out one group member. “Honestly, if we hadn’t assessed the snowpack so closely I don’t think I would have realized how serious the conditions are. I’m really glad we did this so we wouldn’t be tempted into the big lines when conditions just aren’t right for it”, said another. Everyone agreed that lower angle ski terrain would be the best option for the remainder of our tours. With nice, fresh snow and a variety of cool terrain features to ski, it sounded like a great plan.