“Riding the dragon” is a term used to describe the concept of riding on a known weak layer and hopefully getting away with it. Why is it possible to ride these weak layers and not trigger them? And then what makes them fail — all of a sudden?
As previously described, unsettled powder has minimal cohesion and just doesn’t transfer enough energy to cause failure. Secondly, luck definitely has a hand in some folks’ fortunes. Though there are many reasons for “riding the dragon,” safely, we should look at bridging or a hard protective layer of snow that forms over the suspect layer and protects it from failure.
How much snow is enough snow to protect a layer from failing? This is an important question, as riding the dragon isn’t the safest endeavour in the world and the decision-making process becomes much more detailed and observations of the snowpack much more critical.
Skier-triggering becomes much more difficult as more snow falls and protects a layer but the avalanches also get bigger. Confidence increases so slope selection becomes more exposed and aggressive. Weight becomes important as a single skier may not trigger a layer but several skiers or a big crash might. Then there is snowmobile use with its added weight and deeper penetration of the snowpack.
Snow depth, hardness, slope angle and shape all play a role in bridging layers’ ability to protect a layer. With all the variables its hard to give a definitive answer as I sit at my desk but you have to keep checking the snowpack to increase your knowledge and experience. And always check out the avalanche bulletins.
Remember: if you don’t dig to check the snowpack, you cannot know what is in it. Half of the equation of gaining experience is then lost and a right answer cannot be achieved.
One reason bridging layers fail is because of weakness called deficit zones which are tricky spots of thin snowpack that have caught many people off guard, causing avalanches in strongly bridged snow packs. Let’s use a 250-centimetre bridge of strong snow over a weak layer. If this bridge is uniform, no problem. But the fact is these layers are never uniforme.
The layer starts to taper to nothing on couloir walls and around wind exposed rocks as well as isolated tree islands and wind scoured areas. As you’re recreating away, assuming that you’re on a strong snowpack except in these thin zones, the bridging layer maybe less than 10 centimetres and if the layer fails it may transfer under the 250-centimetre thick slab and start a slide.
This example is taken from an avalanche that killed seven people a few years ago. The combined weight of the skiers approaching a zone of thinning snowpack adjacent to a rock outcrop triggered the whole bowl into sliding.
Thin snowpack areas are more prone to a variety of weaknesses as well as the deficit concept. A simple way to check the snowpack is to use your probe to measure the depth of snow and to start developing an understanding as to where the thin areas are.
Jim Markin is an avalanche forecaster certified with the Canadian Avalanche Association. He is also certified as an assistant ski guide with the Association of Canadian Mountain Guides.