Daniel Bonzi: Recognizing avalanche terrain
Continued. See previous post: Powder snow and avalanche awareness
Even if the snowpack instability is high and triggers are available, avalanches will only occur on terrain with certain features and characteristics. For backcountry recreationists, recognizing avalanche terrain is a crucial component of avalanche risk management; this requires understanding how terrain features and characteristics influence the avalanche danger.
Among the numerous terrain factors that affect avalanche danger, the most significant are: slope angle, orientation to wind, and terrain traps.
Slope angle, or incline, is critical in identifying avalanche terrain. The most deadly avalanches (slab avalanches) generally start on slopes over 30° of incline. When the snowpack is highly unstable, it is wise to stay away from slopes steeper than 25° to 30°.
Wind exposure, the orientation and exposure of a slope with respect to the wind, is another primary factor in avalanche formation. Winds distribute falling snow unevenly and also transports loose snow fallen during previous storms. Unstable wind slabs often form on the lee slopes (down-wind side of a ridge); these wind packed layers have the potential to release as large slab avalanches.
Terrain traps are terrain features that amplify the consequences of getting caught in an avalanche (i.e. increase the odds of traumatic injuries or/and of a deep burial). For example, gullies and deeply shaped bowls are generally more dangerous because they make the escape almost impossible; also, trees, large rocks, cliffs, etc. in a slide path are likely to cause serious physical injuries to persons caught in the avalanche.
Other factors to consider when identifying avalanche terrain are:
- sun exposure (variations of temperature affect weak layers);
- shape of slope (convex features are triggered more easily);
- size of slope (large slopes produce larger, more powerful avalanches);
- terrain roughness (trees, dense bushes and large rocks tend to act as natural anchors for certain layers of snow);
- elevation (loading of slopes is increased in the high alpine due to more intense snowfall and winds);
- potential of natural triggers (for example, cornices overhanging along a ridge which can break and trigger an avalanche on the slopes below);
- evidence of past avalanches (like classic avalanche paths easily recognizable by the vegetation damage in the forest, fracture lines or avalanches debris in the alpine zone, etс).
For any powder snow enthusiast, when selecting slopes to ski or ride, it is critical to recognize terrain features that will most likely produce an avalanche; by minimizing or eliminating exposure to these features, one can significantly reduce the avalanche risks.
Keeping your powder adventure safe
Free-skiing and free-riding in the uncontrolled mountain environment (backcountry) are exciting activities, but without proper knowledge and skills, enthusiasts often take unnecessary risks. Selection of the appropriate terrain on any given day is the most efficient way of avoiding avalanche involvements.
For backcountry recreationists, assessing the avalanche danger and making sound decisions about the risk involved on a particular slope, at a particular time, is a difficult challenge. No safety formula exists that will provide simple "go - don’t go" answers when it comes to select safe snow slopes. Practical aids have been developed in Europe and in Canada to help amateur recreationists evaluate the avalanche risk and select terrain through a structured decision making process. These avalanche decision support tools are helpful, but are not a substitute for avalanche training and experience.
It takes years of experience to gain the expertise of professionals that are out in the backcountry on a daily basis, constantly observing conditions and making decisions to reduce avalanche risk. For many backcountry recreationists, being accompanied by a professional mountain guide is the best way to ensure a low-risk powder adventure. Trained mountain guides have the skills required to make the critical terrain decisions that will reduce the avalanche risk to an acceptable level.
Finally, no matter how much you know about avalanches and how careful you are at reducing exposure to avalanche hazard, the mountain snowpack can still surprise you from time to time. Using a wide margin of safety is the key for backcountry skiers and snowboarders to keep enjoying their powder experience year after year.
Great thanks for your attention!
ACMG / UIAGM Mountain Guide
Member of the Canadian Avalanche Association