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Slope angle a crucial variable in determining avalanche danger

Posted: March 2, 2012 - 1:00am

The last 10 days of February were deadly on avalanche terrain. There were 10 avalanche fatalities and several other injured people in the US. Four of those fatalities were just outside ski areas, five were snowmobilers and one was a snowbiker. Several of these accidents may have been prevented with careful terrain selection, toning it down a notch and playing on mellower slopes.

When traveling in a new mountain range or on questionable snowpack, I look for answers in the terrain the snowpack lays on.  Questions like: is this slope capable of sliding? What size of avalanche is likely to occur on this slope? Where would an avalanche carry me if I get caught? These questions can be answered, to some extent, with good terrain analysis.

Traveling through different aspects, elevations, and slope steepness or under several different start zones increases the path’s complexity and can increase the challenge of managing the hazards. The larger the number and range of variables along the way, the greater the chances of traveling through or under a slope with prime conditions for avalanching.

From all the terrain variables that dictate if an avalanche can transpire on a slope, slope angle is the single most important one. Dry slab avalanches typically run on slope steepness between 25˚ and 60˚. The friction between the snowpack layers is too strong to let avalanches slide on slopes flatter than 25˚. The constant snow sloughing on slopes steeper than 60˚ prevents the snowpack from developing the snow structure that is required for a slab avalanche. From all the avalanches that run on slopes in the active range, almost half are running on slopes between 35˚ and 40˚. For us here in the Juneau area, most of the avalanches are in the upper part of the range. The good news is: we can narrow down the slopes that are likely to produce avalanches. The bad news is: people and avalanches favor the same slopes to slide on.

Slope roughness can dictate if and when a slope is likely to avalanche. Boulders or trees can anchor the snowpack to a slope. The density and size of the anchors dictate the size and types of avalanches that are possible on a slope. Clearly, these anchors lose their effectiveness as soon as they get buried. This may be one reason why some slopes avalanche only in mid-winter or during big snow years.

Throughout the winter, I start my field work on the ridge top with dry slab avalanches on my mind and as I go down in elevation, I change my focus to loose wet snow avalanches. Elevation can have a dramatic effect on many ingredients that combine in the making of an avalanche. Temperatures, exposure to winds and the amount and type of precipitation can vary dramatically within a short difference in elevation. Rain and snow can affect the snowpack and the stability differently. It is not uncommon for us in Southeast Alaska to transition into a very different snowpack with different avalanche problems within a short gain or loss in elevation.

Several winters ago, a group of old friends were skiing a bowl in the Colorado Rockies Mountains. They dug a pit and decided to ski the slope. They skied the bowl several times, traversing further across the top of the bowl into untracked snow with every run. The great day of skiing ended abruptly when one of the skiers triggered an avalanche, got caught and partially buried. When they looked into the incident, they realized that as they kept advancing across the bowl, the aspect of the slope they were skiing on changed. By the time they triggered the avalanche, they were skiing on a different aspect than where they had dug their pit and the snowpack was different. The direction a slope faces, especially with respect to the recent wind direction and the sun, can affect both the structure and the stability of the snowpack. Leeward aspects tend to be loaded with new snow and be threatened by overhanging cornices. On the other hand, slopes facing the wind tend to have thinner and weaker snowpack. I tend to be suspicious of leeward slopes after new snow and wind events, but I am even more leery of normally wind-facing slopes after unusual wind events. In these cases, we deal with additional loads on relatively weak snowpack.

Relying on a slope’s steepness, roughness, aspect, elevation, and shape can help us estimate if a slope is capable of avalanching. It can also tell us what type, size and frequency of avalanche it may produce, but only to some degree. Sometimes the vegetation on the slope can provide a better insight into the history of a specific avalanche path. Avalanches that run into forests scar the vegetation along their way, leaving a clear path of destruction.  By carefully observing the age, shape and type of trees in and around an avalanche path, we can estimate the sizes, the frequency and how far avalanches run in that path over the years.  Avalanche paths can be recognized by small, broken and scarred trees. They tend to have “disaster species” growth like: alders, aspens, willows and cottonwoods. The scarring on the trees can indicate the height of avalanche flow in the path. The age of the trees around the path and into the forest can give us an approximate indication of the return period of avalanches of different sizes. On the other hand, an old dense growth is a good sign of a slope that very rarely avalanches.

Go outside and play, but arm yourself with the knowledge of when and how to manage the terrain you play on. 

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

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