Technical information, news, research, and opinion on avalanches, snow safety, and winter backcountry travel.

Thursday, December 3, 2009

Terrain Observations: Choosing Terrain Appropriate For Conditions

I'm learning to fly, around the clouds ... what goes up, must come down—Tom Petty

NOTE: Backcountry avalanche forecasting is composed of four interconnected elements: goal, people, awareness, and uncertainty. Four types of observations, people, terrain, snowpack, and weather are used in backcountry avalanche forecasting.

CAPSULE: Recreational backcountry skiers are taught to analyse terrain for sources of exposure by using static variables that don't account for the dynamic nature of avalanche phenomena. This article discusses a more general model for analysing terrain that accounts for the dynamic nature of avalanche phenomena with respect to size, phase, and velocity. This model is non-technical and can be effectively applied by most people without any specialised training.

THANKS: To my father for his invaluable input.

Why would you ski in one place instead of another place? How do you choose terrain appropriate for conditions?

Recreational backcountry skiers are taught to choose terrain using a simple list of shape parameters such as slope shape, angle, openness and size. This information is then combined with a simple list of terrain traps such as trees, rocks, cliffs, gullies, benches, and depressions. Collectively, we can refer to all these elements as slope shape parameters because ultimately all these elements do refer to the static shape of the terrain.

Unfortunately, the use of static parameters has some significant drawbacks. For example, using a simple list of slope shape parameters can limit perception, and that can lead to dangerous gaps in analysis. Doesn't something very specific come to mind when you hear the word gully or convexity or cliff? Perceptual traps are often associated with rules of thumb, and the relationship between rules of thumb and serious errors is well documented. In the real world, using static slope shape parameters teaches recreational backcountry skiers to find very specific hazards, but it also produces analysis that contains many blank spots. I refer to this phenomenon as pin the hazard on the donkey, or with the more technical term hazard diffusion.

Hazard diffusion occurs because a list of static slope shape parameters does not account for the existence of multiple spatial scales in the relationship between terrain and avalanches. To this point, consider the Strathcona-Tweedsmuir tragedy which occured in the Connaught Valley drainage of Glacier National Park, British Columbia in February 2003. For an avalanche of sufficient size and velocity, the Connaught Valley is a giant terrain trap. If you are willing to indulge in some semantics, then we can say that for a large, fast-moving avalanche, the term Connaught Gully would be a much more appropriate description.

The heart of the problem is that determining the degree of exposure for a given section of terrain depends largely in part on the size, phase, and velocity of an avalanche. Since avalanches occur at a variety of spatial scales, and since terrain exists at a variety of spatial scales, analysis of exposure must account for a variety of spatial scales. This type of analysis requires a more general set of parameters that integrate the spatial and temporal elements of avalanching with multiple scales of terrain. Such integration allows recreationists to very effectively analyse exposure across multiple spatial scales of terrain for a variety of important aspects of avalanche phenomena.

In The Beginning
You might be wondering how this model arose and whether or not it's backed up by any hard evidence? It's a good question with a very complicated answer, but I'll try to be as concise as possible. For the past couple of years I've been working on developing computer-based tools to analyse terrain for exposure to avalanches. It's rough business, especially since computers are mind-numbingly unaware. When a human looks at a digital elevation model, all the terrain features are staggeringly obvious. Unfortunately, a computer sees nothing but a list of elevation values. To get an idea of the problem, imagine if you had to perform route selection from a grid of elevation values. It's quite literally like trying to ski with your eyes closed.

To get around these problems, you can develop algorithms that the computer can use to perform analysis on the elevation values. For example, you can tell the computer about the relationships between the values, and then the computer can use those relationships to reconstruct data such as slope angles. This is all very simple stuff. But let's take things a step further and imagine that you want the computer to identify an avalanche starting zone, or a runout zone linked to multiple avalanche paths. Modern computers are incredibly powerful, but they cannot easily be programmed to solve certain types of problems. This is doubly true when you are faced with real world constraints such as time and money. After a few months of dead ends, I began to realise the depth of the problem and the futility of approaching it head on.

I found the first part of the solution in an old document that discussed Peter Schaerer's work on avalanche terrain along the Trans-Canada highway. Peter Schaerer discusses surface area as an important attribute of an avalanche starting zone, and at that very moment I became intensely curious. Could surface area be used as a statistical signal of avalanche terrain? Needless to say, I returned immediately to my computer and began to calculate surface area for different sections of terrain. The values were startling. It turns out that surface area is a very useful statistical measure of exposure inherent to any section of terrain. Eventually, I developed other geostatistics that made the original problem very tractable. For this reason, I feel comfortable saying that this model is backed up by hard data, and in fact this model is currently being used successfully in the real world.

In the avalanche community, there is a spectrum of complexity, from complex mathematical models to textual hazard communication tools. Regardless, everyone agrees that simplicity is an extremely effective strategic-tactical approach to education and communication. The value of simplicity far exceeds the scope of this article, but it's safe to say that simplicity is an effective technique because people cannot absorb material they do not understand. In the words of an esteemed technical writer who is a friend of mine, it's a lot of work, making things simple.

When Models Collide
I can't find anyone who really knows where the list of shape parameters arose, but it seems likely that it coalesced over time from the collective wisdom of mountain folks. During my time in the mountains, I have observed a very interesting phenomenon with respect to terrain selection. The list of shape parameters provided to most recreationists is extremely useful, but most don't seem to realise that it's just a partial list. To this point, the Avalanche Handbook specifically states that it's important to consider combinations of features in addition to individual features. Yet there is a lack of simple tools that people can use to accomplish this type of multi-scale, multi-feature analysis.

Finally, it's incredibly important to note that this model is not designed to help people outsmart terrain, which is a form of abuse frequently undertaken with travel technique. This model is designed help recreational backcountry skiers by providing simple, scale-invariant tools to determine the real degree of exposure inherent to a route or objective.

The Model
LOCALE. A model for analysing exposure.

Line-Of-SightIs line-of-sight limited? Do local terrain features obstruct your line-of-sight to terrain above or below? Is your uphill view blocked by rocks or trees? Can you see all the terrain below or only some of it? Pay attention and double-check your decisions when line-of-sight is limited for any reason. Limitations to line-of-sight can reduce your reaction time to zero.
OpenIs the terrain open enough to produce small, medium, or large avalanches? Are there open breaks in the forest that allow snow to travel unobstructed to your location? How much open terrain is present and where is the open terrain relative to your location? Large quantities of snow can accumulate in open areas near mountain tops before an avalanche and in valley bottoms after an avalanche.
ConfinedIs the terrain confined relative to the size or speed of an avalanche of any size? Could a large, fast-moving avalanche overrun the valley floor? Are you in a gully where escape from a small but fast-moving avalanche could prove impossible? Small avalanches form deep deposits in confined terrain, such as hollows or depressions, where snow can accumulate. Estimate your reaction time before something goes wrong and double-check your decisions if reaction time is short. Reaction time is a very important part of your margin of safety.
AccumulationHas snow accumulated above or below? Avalanches often start where snow accumulates, and then run downhill where they deposit snow on the valley floor.
ObstacLesAre there obstacles above or below? This includes cliffs, crevasses, rocks, and trees. Obstacles above may block your line-of-sight, inducing hazard blindness, and can cause traumatic injuries during any phase of an avalanche. On the other hand, obstacles can block or redirect flowing snow and may offer protection at your current location.
StEepIs the terrain steeper than 30 degrees? Avalanches start and rapidly accelerate on steep terrain. Steep terrain produces fast-moving avalanches that can release above you and travel toward your location. If you trigger an avalanche on steep terrain, you may be unable to escape from the flowing snow. Steep terrain can limit your line-of-sight, and indicate locations suitable for sluffs, cornice drops, serac fall, or rock fall—all of which can start avalanches above or around you.

During Trip Planning
You can use this model during trip planning as well.

Two examples are provided, along with relevant markup. According to The Avalanche Handbook, it is important to consider combinations of features rather than simply identifying separate gullies, benches, and cliffs. These parameters find specific traps and diffuse sources of exposure for terrain of varying size. At the bottom, several unmarked examples are included for large, medium, and small scale terrain. Feel free to evaluate the unmarked images by using the LOCALE parameters.

Image 1.1. Goat Island Mountain. At a large scale, these parameters are mostly concerned with large avalanches. This type of analysis would be suitable for general route-finding. It's worth noting that areas with line-of-sight limitations and obstacles often present significant route-finding challenges.
CONCLUSION: This analysis helps you develop a big picture understanding of exposure and traps for this area. Travel through terrain on the left side of this image takes place above and below accumulators on steep, open slopes with numerous obstacles and many limitations to line of sight. Travel through terrain on the right side of this image takes place above and below accumulators, on steep, confined slopes with numerous obstacles, and many line of sight limitations.

Image 1.2. Chinook Pass. At a small scale, these parameters are mostly concerned with small avalanches. Several possible backcountry ski runs are illustrated. You can envision other backcountry ski runs in the context of the hazards. Despite its relative simplicity, some of the backcountry ski runs in this area have numerous obstacles, line-of-sight limitations, and and significant accumulation potential. Intricate terrain with complex groundcover makes beacon searches difficult.

CONCLUSION:The ski runs in this area are steep, with numerous obstacles, but line-of-sight is generally good, except in a few areas. Still, small avalanches could have very bad consequences in many of the ski runs. Intricate terrain increases time for search and rescue.

Try It Out
Use the LOCALE parameters to evaluate the terrain in the following photographs. Feel free to use your own photographs as well.

Image 1.3. Baker Backcountry. Line-of-sight, Open, Confined, Accumulators, Obstacles, Steep.
Image 1.4. Baker Backcountry. Line-of-sight, Open, Confined, Accumulators, Obstacles, Steep.

Image 1.5. Baker Backcountry. Line-of-sight, Open, Confined, Accumulators, Obstacles, Steep.

Image 1.6. Snoqualmie Pass Backcountry. Line-of-sight, Open, Confined, Accumulators, Obstacles, Steep.

Image 1.7. Snoqualmie Pass Backcountry. Line-of-sight, Open, Confined, Accumulators, Obstacles, Steep.

Image 1.8. Snoqualmie Pass Backcountry. Line-of-sight, Open, Confined, Accumulators, Obstacles, Steep.

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