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Different avalanche types present their own challenges

Posted: February 17, 2012 - 1:00am  |  Updated: February 20, 2012 - 4:45pm
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A loose snow avalanche starts in one spot and fan out as it goes down the slope.   Courtesy of Ron Simenhois
Courtesy of Ron Simenhois
A loose snow avalanche starts in one spot and fan out as it goes down the slope.

On Feb. 1, I was working on Mount Roberts with a group of students taking an advance avalanche course. The weather above the tram was windy with sub-freezing temperatures. At the same time, in town, it was warm and rainy. While we were traveling we could see several different types of avalanches run at the same time on different elevations. Later that day we found out that one of those avalanches ran across Thane Road and closed it for several hours.

When I plan a trip into the backcountry, I try to identify the avalanche problem of the day. Different avalanche types have different characteristics and demand to be negotiated differently. Here in Southeast Alaska, we are “blessed” to occasionally encounter two or more different types of avalanches in a single ski run.

Avalanches come in different sizes and shapes, and they can be classified into a number of different types, including cornice, ice or glide avalanches. But in this column I’ll touch on the four main avalanche types: loose dry and loose wet avalanches, and dry slab and wet slab avalanches. Every different avalanche type develops under different conditions, has different causes, clues to look for and ways to negotiate the hazard they present.

Loose snow avalanches are easy to start with skis, snowboard or snow machine. Natural loose snow avalanches usually start in one point on the slope, then fan out in a triangle shape and pick up more snow as they go. They typically involve only small amounts of soft surface snow. However, occasionally they can be quite big and destructive. Loose snow avalanches tend to run on steeper slopes and usually start on slopes with irregularities like rocks or trees. Dry loose snow avalanches most commonly occur after new low density snowfalls, especially if the sun pops out and slightly warms the slope.

A slab avalanche is when one or more layers of cohesive snow breaks loose from the slope and slides as a unit, eventually breaking into smaller blocks as it slides down the slope. Slab avalanches transpire when a weak snowpack layer can no longer support the additional weight like more snow, a skier, a snowboarder or a snow machine. As result, it collapses and undercuts the bonds that hold the upper snowpack layers to the slope. Out of all types of avalanches, this is the big killer. It often involves large volumes of fast-moving snow, and the upper boundary of the avalanche (crown wall) often breaks above the victim and makes escaping very difficult.

Wet and dry avalanches are different beasts all together and most avalanche professionals make a clear distinction between wet and dry avalanches. Their mechanics are different, they differ in the way they flow and the way we manage the hazard they present. As a result, forecast considerations for wet and dry avalanches are also very different.

As a whole, dry snow avalanches occur when the load on the snowpack exceeds its ability to withstand any additional load. On the other hand, wet snow avalanches occur when water flows through the snowpack and dissolves the bonds connecting the snow grains as a cohesive unit. As more and more bonds dissipate, the snowpack weakens to where it can no longer support its own weight and avalanches. When we deal with avalanches, we pay attention to new snow on the slopes, we increase the load on an isolated piece of snowpack to see how much additional load it can take and we heavily load it to trigger controlled avalanches. Dealing with wet avalanches is a whole different story.

The name of the game when dealing with wet avalanches is timing. A stable slope can turn deadly in a matter of a few hours. Since wet avalanches are hard to artificially trigger, we diligently track the water flow through the snowpack to find out when it reaches sensitive layers. We carefully choose on what slope to play and we make sure we’re back for an early happy hour in our favorite pub before our ski slopes start to bake in the hottest hours of the day.

You may wonder what type of avalanche blocked Thane Road on Feb 1. The avalanche ran down to sea level where the weather was above freezing temperatures and rainy for several days, so it had a wet slide component to it. The big question is: How did it start? Looking at the slope a couple of days later and by the shape of the crown wall suggests that the avalanche started in elevation where temperatures were below freezing and the snow was dry. That being said, without actually being on the slope and looking at the crown wall closely, I would classify my analysis as bar talk, based on an educated guess rather than a fact.

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

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