Posted on: 2015-01-28 17:06:30 by davidof

Persistent Slab Avalanches

With a major storm cycled heading over the Alps this video is a reminder of the danger of persisent weak layers in the snowpack. Now we don't tend to get the snow depths that they see in Utah but this year there has been a problem in the French/Italian border areas of the Alps since late November.

Now what is really interesting is to see the avalanche researchers triggering slides 2 to 3 meters deep just by sticking a ruler into the fragile layer. This causes the weak layer to collapse like a house of cards. Now you can think of your skis or snowboard as the ruler. Ok you are unlikely to punch down through a 2 meter slab but you only need an area of 50 cm depth somewhere on the slope, say caused by a rock or rollover and some wind scouring and you are through and the whole slope can go. That's a factor in spatial variablity. It is worth reflecting that slides of this depth are probably going to kill you due to trauma, possibly even if you have an Airbag system.

There are two principal types of weak layers: depth hoar or gobelets and surface hoar. One depth hoar crystals reach about 1mm they persist throughout the season until the thaw. One of the researchers in the video says that they can be "crushed" but as you can see from the test pits if 3 meters of snow depth doesn't crush them nothing will, for me this is an error or misconception. What can happen is enough snow falls that skiers can go right over the weak layers without triggering them. As a general rule around a meter of snow depth over the entire slope is sufficient but remember, if that slope does go, it will go big.

Avalanche researchers talk about the Bridging Index; this term was originated in 2003 by Schweizer and Jamieson. The bridging index is calculated from the hardness of each snow layer above a potential weak layer and its depth. Basically the harder the snow and the deeper the snow the more it insulates a skier from the weak layer.

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