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Know the Snow: Glide avalanches

Posted: March 30, 2012 - 12:01am
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Glide avalanches can be distractive as they mobilize a large amount of snow. In this photo Alan Gordon standing next to a crown wall of a large glide avalanche.    Photo courtesy of Ron Simenhois
Photo courtesy of Ron Simenhois
Glide avalanches can be distractive as they mobilize a large amount of snow. In this photo Alan Gordon standing next to a crown wall of a large glide avalanche.

Sometime late November, I was sitting and talking about the heavy early season snowfall with Brian Davies, Eaglecrest patrol director. At some point Brian said that this could be a big glide avalanche season. Like in many other cases when we have talked about snow and avalanches, Davies’ prediction was right.

Currently, more and more signs of gliding activity are clearly visible on the mountains around us. Although not unique to Southeast Alaska, they are relatively uncommon in other snow climates.

Glide avalanches have kept me from a good night’s sleep more than once since I moved to Juneau.

But, I am not alone.

They are known as a nightmare for avalanche professionals worldwide. They are extremely difficult to forecast, they are almost impossible to control with explosives and are often very destructive as they mobilize large volumes of snow. To my knowledge, glide avalanches destroyed at least two chair lifts in Europe this year. One of these avalanches got caught on tape and went viral over the internet (You can see it here: http://www.youtube.com/watch?v=og5h-E-Ntsk&feature=related).

Glide avalanches occur when the entire snowpack glides over the ground. The relatively slow gliding process can take place over a few hours and up to several months. If the gliding snowpack breaks free from its supporting boundaries, an avalanche will transpire. An obvious clue of snow gliding are the cracks in the snowpack that appear on steep slopes this time of year. Excessive snowpack gliding and its associated glide cracks tend to form on specific slopes. These slopes are typically steep and covered with smooth gliding surfaces like rock slab, weeds or bent over alders. Active glide avalanche paths can sometimes produce more than one avalanche in a winter.

Forecasting glide avalanches is still an unanswered challenge for the avalanche community. The difficulty of forecasting glide avalanches is not due to lack of understanding glide processes. Glide processes have been researched over the years and are relatively well understood. The problem lies with the distance of the glide process from the snow surface. The gliding interface is at the base of the snow cover. Thus, gliding rates are hard to gauge and traditional forecasting methods like snowpack and weather observations have proven inaccurate.

Two years ago, I was trying to see if there was any correlation between glide avalanche events, stream flow below the avalanche slope, time of day and weather events like temperatures, snow and rain. I came up empty. I found no obvious direct correlation between glide avalanches and environmental or climates events.

In my personal experience, controlling glide avalanches has been just as challenging as forecasting them. I have had mixed results trying to control these avalanches using explosives. In many cases, I used very different strategies than for other types of avalanches and the gliding seemed to stop. But I could never tell for sure if that was due to the avalanche control or other reasons.

To my knowledge, there is very little knowledge on artificial control methods for glide avalanches. Other folks that deal with glide avalanches also agree that they are extremely hard to trigger with explosives. In fact, the Swiss resorted to excavators and snow cats to trigger or break the gliding snow, rather than using explosives, whenever they can.

When I travel in the mountains, I rely on my knowledge and experience to keep myself safe. However, when the avalanche problem of the day is glide avalanches, my forecasting abilities are admittedly somewhat limited. On these days, I rely on the fact that glide avalanches occur in very specific places. I maintain and update a mental map of where gliding occurs and avoid those areas altogether.

This year is a good year to pay attention and recognize these places where glide cracks and avalanches develop.

• Ron Simenhois is an avalanche forecaster who lives in North Douglas. You may contact him at simenhois.ron@gmail.com.

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