Publicado en 04/03/2014 a 05:59 PM por Pedro

In the last article I addressed the three dynamic components of an avalanche: Trigger, Propagation, and Slip. As a quick recap, when a slab avalanche runs, those three are the critical events in time. The trigger causes a localized failure and collapse, propagation communicates that failure to the surrounding snowpack, and if gravity overcomes friction, the slab slips and runs as an avalanche.

The goal of any stability assessment should be to identify when these sequence of events is likely to occur. 

Traditionally, despite this being perhaps the most important skill a person could learn from an avalanche course, it is consistently one of the more poorly taught concepts, if taught at all. It is commonly presented as a checklist of concerning conditions or a mechanical series of steps. While I do agree that tools such as alptruth, yellow flags, red flags, avaluator, and others are helpful, they are certainly no replacement for a stability assessment which is intuitive, based on the physical model for fracture mechanics, and allows for growth through experience.

The fundamental problem with these methods is that they do not encourage understanding of the underlying conditions present in the snowpack.  This lack of attention to the underlying reasons for instability contributes to degraded terrain selection decisions. Alternatively, better integration of information on the snowpack with terrain choices would lead to more accurate stability evaluations and help individuals make better terrain choices.

A number of years ago, the realization that current tools lack integration between advances in science and teaching in the classroom and field gave birth to the primitive version of what has since developed into the Stability Wheel. This assessment tool, created by Santiago Rodriguez, combines the advances in snow fracture mechanics by Joachim Heierli with many years of teaching experience. For about 6 years now, the stability wheel has been in use and development in courses across the Northwest including Lake Tahoe, Oregon, Idaho, Nevada, and Silverton Avalanche School in Colorado. It has also been used in avalanche courses in Argentina and Chile, including teaching the guides and patrollers of Las Lenas Ski Resort, the largest resort in Argentina.

As with many great ideas, it is eloquently simple. It poses the question for each of trigger, propagation and slip: how likely is the event to occur? Combine those in the simple graphic below and you have the stability wheel.


The concept is to evaluate each of the necessary components for a slab avalanche and assign a likelihood: low, moderate or high. These likelihoods are then used to make terrain decisions based on which components are most problematic.

The likelihoods are drawn directly from modern stability tests, and observations of current conditions. I will leave discussing stability tests in detail including how to integrate test results in the next article.

Instead, I want to focus on how this enables intuitive decision making and risk mitigation through terrain selection. When making terrain decisions, you can choose to mitigate one or more of the three factors in an avalanche: Trigger, Propagation, and Slip. After all, for a slab avalanche to release, all three are necessary components. So, how can terrain decisions mitigate each factor?

The first factor you can mitigate is Trigger Likelihood. This, of course, varies greatly on the type of instabilities, but here are a list of possible terrain decisions:

-Avoid shallow areas where it is easier to impact the weak layer

-Avoid rocks and trees where it is easier to impact the weak layer

-Avoid convexities which concentrate stress

-Avoid travel on wind loaded areas

All of these terrain selection decisions are meant to make it less likely that a given traveller impacts and triggers the weak layer.

The second factor you can mitigate is Propagation Likelihood. Of all the factors, this is probably the most difficult to negate since it depends greatly on the snowpack. When that isn’t possible, the second best choice is to select terrain with lower consequences. Some examples might include:

-Avoid terrain favoring formation of persistent grain types.

-Avoid large open slopes in favor of smaller slopes with smaller consequences.

Lastly, you can mitigate the Slip Likelihood. Of all elements, this is the factor which you have the most control over. The reason for this is simple: slip is highly reliant on slope angle. Simply choosing to travel on slopes that are below typical avalanche slope angles greatly decreases the probability of a slab slipping and the avalanche running.

The Stability Wheel was designed to allow for intuitive integration of observations and tests into a decision making framework based on the physical model for avalanche dynamics. Just as importantly, it is intuitive to make terrain selection decisions to mitigate risk based on the components of the stability wheel.

In the coming weeks I plan on writing more from different perspectives on the Stability Wheel and publishing videos of its use like the one posted below. For the moment I have also left discussion of structural weaknesses at the center for a future post. If you have any feedback or comments we would love to hear them in the comments below, on our Twitter page, or on our Facebook page.




Pedro Rodriguez

SnowGeek Founder

                                                                                                 Stability Wheel Demo Video


The Stability Wheel was submitted and accepted as an abstract to the 2013 European International Snow Scientist Workshop. We were unable to make it to Europe to present, but have resubmitted the abstract for the 2014 North American International Snow Scientist Workshop in Banf, Canada.

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