On Saturday, an acquaintance of mine was caught in (triggered) a size 2.5 slab avalanche in the SelkirkMountains. Luckily, she was not killed, but, unluckily, did suffer relatively severe injuries. I know the area reasonably well, having hiked, climbed and skied there many times, and things could certainly have been worse. The terrain is typical Kootenay skiing – steep sided treed valleys with little gullies and terrain features all of which worsen the consequences of getting caught in an avalanche.
The party did many things well, apparently after the avalanche, the injured skier was rapidly reached and a rescue quickly activated (she was not completely buried), they exposed only one person at a time to the slope thus reducing the potential number of victims and leaving more people to render assistance if needed, the party had obviously made a number of snowpack observations, and were all carrying standard safety equipment. But, of course, any time we trigger an avalanche, unless it is small and intentional, we've also clearly made some mistakes.
I've triggered a number of avalanches over the course of my skiing career, so many, in fact, that I am not sure I could recall all of them if pressed. I have been, of course, terrifically lucky, in that I never triggered anything big, most were small and intentionally triggered (or at least half expected) or were triggered remotely from a safe location. Many times, I triggered smallish slabs that were unexpected, but was saved from further harm because I routinely practised safe ski practices, such as exposing only one person at a time, getting out of the way at the bottom, avoiding high consequence slopes in times of uncertain stability avoiding convex rolls, starting out on smaller terrain and all of the other fairly standard tactics that can improve ones safety margin in the mountains.
In the first year after I took my CAA Level 1 Avalanche Operations Course, I did, however, do what many new graduates of that course do, and got caught (in my case almost caught) in a big dangerous avalanche, the result of being too cocky. The episode scared the shit out of me as it had the potential to kill or at least seriously injure everyone in the party. There was one clear sign that I chose to ignore (and I take full responsibility for ignoring this sign as I was the most highly trained and experienced person in the group) and that was a moderate sudden planar failure on a compression test 40 cm down on a suncrust. I ignored this obvious red flag because we were on a ski traverse that I had wanted to do for ages and I had finally managed to get the weather and the companions to do the trip, and, we were at the base of the final avalanche slope - beyond this one slope, we had no further danger to contend with. All those things, irrelevant as they are, somehow blinded me to the danger into which we skied. We were lucky, we were in similar terrain to my friend - steep, lightly treed so that the trees could not anchor the slope but only increase the consequences of getting caught – and, while the entire slope from one side to the other cracked and shifted, some magic of gravity held the slope in place and we were able to safely “get the hell outta there.”
My friend and her party made similar mistakes but just weren't so lucky. They too had dug snow-pits, and, somewhat chillingly, found a buried surface hoar – moderate to hard sudden planar failure (but down a whopping 115 cm) on compression tests. The avalanche bulletin for the day rated treeline as moderate and alpine as considerable and noted that remotely triggered avalanches (always a scary sign) were still being triggered on the buried surface hoar (buried almost one month before). The bulletin also noted that, and I quote verbatim: “February 12th [surface hoar] is now down about 100-150 cms and continues to be triggered by light additional loads on Southerly aspects where it is sitting on an old sun crust.”
Hindsight is always 20/20. In retrospect it is clear that while the group may have thought they were in treeline terrain (moderate hazard), the snowpack characteristics (and terrain) were clearly alpine (considerable hazard), sudden planar failures should always make us think twice about the stability of a slope (even with hard test scores), a skier is a light additional load and, despite it being statistically less likely, a weak layer down 115 cm can be triggered by a skier, solar radiation and heat can rapidly destabilize the snowpack, widespread surface hoar layers can propagate large distances and release above a skier, and, finally, but most importantly, deep persistent weak layers require conservative terrain choices – skiing a 38 degree slope (even one at a time) is not an appropriate terrain choice for this classic low probability/high consequence deep persistent weak layer situation.
I have no idea how decisions were made in the group, always the most informative aspect of any accident analysis. Perhaps the party thought that they were in treeline terrain because of scattered trees and considered the hazard moderate. Perhaps they thought that a layer buried over a metre could not be triggered by a skier (statistically unlikely, but practically irrelevant), perhaps they under-estimated the effect of the sun and heat on the snowpack (it was a warm sunny day), or perhaps like me, they were too motivated by ambition and too little by prudence.
After my serious near miss, I consciously became much more conservative in my terrain choices whenever stability was uncertain. The big lines only got skied on very select days when stability was bomber and I was with a solid party. I became, quickly, abruptly, and with some chagrin, aware that, despite some education and experience, what I didn't know far exceeded what I did know, and I needed to allow some margin around my terrain choices in case my analysis of stability was wrong. I can only hope my young friends have the same epiphany.