Science of Snow: The Deep Slab

words by Scott Thumlert

Large destructive avalanches.
Low probability, high consequence.
Big gnarly deep slabs.
Holy sh*t balls!!!
Size 3 or larger on the Canadian destructive scale (read with nasally nerd voice).

However you choose to describe them, persistent deep slab avalanches are perhaps the greatest challenge to manage when entering complex avalanche terrain.

The chance of the Grizzly slide path running full is pretty small today, but if it does and we are in the runout zone, we are all dead. And we really want to ski 8812 today, so we HAVE to cross the path. We just drove to the top of Rogers Pass from Revelstoke. We skinned for twenty minutes up the Connaught drainage and here it is, Grizzly slide path. We’ve been outside looking at the mountains for less than a half an hour, so we know almost nothing about the conditions up high in the start zone. The public bulletins say Considerable and mention a deep persistent avalanche problem. Cripes.

Well, that sure is a difficult decision, one that usually involves a steady, quick pace through the runout zone and back into the trees.

Now let’s put the problem in another context, but in the same geographic zone. You are magically transformed into the lead avalanche forecaster for Highway #1 through Rogers Pass. Sick! Bombs! Big bombs! It’s today (March 2014), and we have some deep instabilities in the snowpack that were producing large avalanches earlier in the season, but we haven’t seen much in awhile. Not all your 100+ avalanche paths ran on the deep layers. The chances of them running and hitting the highway is small, but if the snowpack fails near the base, the slide will be MASSIVE, hit the road and possibly injure the public. Should we close the highway and shoot everything? Or should we wait until we see more avalanche activity? (Side note, they have a very professional operation that uses much more information then I just spouted off to make the decision).

The point is that the “deep slab avalanche problem” affects the entire avalanche industry. It is a very challenging thing to forecast given our current tools.

The University of Calgary ASARC program has made this problem an active research topic. We are waiting for big avalanches to happen, then jumping in cars, buses, helicopters, etc. to access the site and record as much data as we safely can.

Instead of numbers, graphs and tables this article is pictures! These are all avalanches that our team has been to this season. There are indeed some weaknesses deep in the snowpack this year, so keep your head on swivel out there.

Soft cotton 1: Photo of deep tap test being performed at the fracture line. Ropes used for safety in case of further avalanches.
Soft cotton 2: The crown of the soft cotton avalanche. It was about 155cm thick.
Soft cotton 3: Panorama of the soft cotton avalanche. Sz 3.5, south facing, failed on October crust layer near base of snowpack, photo taken from helicopter.
Beat’s wall 1: Crown of avalanche about 250cm thick.
 Beat’s wall 2: Performing snowpack tests on weak layer in the fracture line… in a cave… welcome to avalanche research, here’s your shovel.
Beat’s wall 3: Sz 3, south facing, failed on facet layer near October crust.
 Rockies 1: Sz. 3, south facing, failed on basal facets, in a closed area in a Rockies ski resort, the slide failed sympathetically when the used explosives on nearby slope. Interesting to note that this slope had been skied extensively. The snowpack was P+ hard skier compacted snow on top of VERY weak basal facets. Super interesting!