What does it mean to break an avalanche?
In the mind of the skier breaking the avalanche, his first reaction is to in terror, realize gravity of the situation and consequences. To him, it means that unless he does not escape the path of the avalanche, it is likely that he will be caught, buried, and with substantial probability, die.
In the minds of his companions, it means that what was once a fun day, has now turned into their worse nightmare and the fight to save their friend’s life has only begun.
Dealing with the consequences of the subsequent tens of seconds following breaking an avalanche is about half of what an avalanche class is about. This principally involves companion rescue and medical care. Roughly, the other half is spent understanding everything prior to skiing that slope. This includes the classic avalanche triangle: weather, terrain, snowpack, and human factors. Then, using that information to select and navigate through safe terrain.
What is shocking though, is how extraordinarily little time is spent on answering the actual question: what does it mean to break an avalanche?
And by that, I mean, what sequence of events, explainable by Physics, have unfolded from the time the skier stepped on the slope, to the time the avalanche began to run? That is precisely the question I want to answer today.
Broadly speaking, three events occur: trigger, propagation, and slip. Typically a skier will start descending a slope, triggering a local failure on a weak point. Then, the local failure propagates to the rest of the slope. Finally, if the slope is steep enough, the slab will slip and the avalanche will run.
In order to use a real world example, we will be using an avalanche video and analyze each step with photo stills. The video is an avalanche from a ski resort in Argentina called Bariloche in 2008. You can read about the accident here: http://www.backcountryaccess.com/2010/08/16/saved-by-a-tracker-in-bariloche-argentina/ and watch the embedded video.
The first step in the creation of an avalanche is the trigger. This is the localized failure and subsequent failure of a weak layer. It is the point at which the stress on the weak layer is too great and it fails. For this reason, a primary terrain consideration when mitigating trigger likelihood is to avoid stress concentration areas. This can include trees, rocks, convexities, thin areas, and many other features as well. Below is an illustration of where the avalanche triggered in the Bariloche accident followed by a stability test showing a layer before and after it failed on a layer of surface hoar.
The next step in an avalanche is propagation. This is the most observable part of the process when avalanches are not running naturally. This is the process by which the local failure expands causing a near simultaneous widespread failure and subsequent collapse of the entire snowpack at the weak layer.
This is often accompanied if not with an avalanche, with shooting cracks and whumpfs. These are all signs of the same phenomena: the local failure was communicated to the entire snowpack causing a widespread failure and collapse. Below are some photos of shooting cracks and an illustration of propagation:
At this point, the avalanche has not begun to run. One last question must be answered. Before that, I want to talk about what the current situation is: There is a slab onto of a bed surface. The layer in between has failed and the slab has collapsed. There is nothing but friction preventing the slab from sliding down due to gravity. This is a classic problem in a high school Physics course: a block on an incline. Here is a depiction of that problem.
On the left you can see Fg which is gravity pulling the block down and Ffs which is friction holding the block. The other diagram is the same except after breaking apart gravity into the component along the ramp and into the ramp. Now, the only thing remaining to do is that for a given slope angle calculate if the force of gravity overcomes friction. To illustrate that, here is a plot of friction and gravity across a range of angles using a coefficient of friction related to typical snow.
This plot was actually made a number of years ago and has an amusing story attached. It was around the time that I was beginning to learn about avalanche education, but my father, Santiago Rodriguez, had been teaching for some time and still worked at HP. While in a meeting, all of us instructors got an email along the lines of: “meeting is very boring, so I decided to make this plot to keep myself entertained.” So out of that meeting comes one of the most illustrative plots used in avalanche courses I teach now.
Above you will see three lines. We are most interested in the two intersection points. The first is at thirty degrees: this represents the critical angle for snow with the given friction force. This is precisely the angle and coefficient of friction for surface hoar. This elegantly explains why surface hoar instabilities tend to break at lower angles. The second intersection point occurs right around 37 degrees: precisely the typical starting range for avalanches (37-42). This answers the basic question: is this slope steep enough so that friction is overcome by gravity and the avalanche runs?
So, what does it mean to break an avalanche? It means that someone has been able to trigger the avalanche: cause a local failure and collapse. That the failure is propagated through the rest of the snowpack. And finally, there is sufficient steepness that friction is overcome by gravity.
Trigger, propagation, and slip. These are the three parts of an avalanche breaking.
Before ending, I want to take the time to give credit where its due. Everything that I have just talked about was not widely accepted and at times violently rejected by the avalanche community not more than 5 to 10 years ago. If not for the work, dedication, and persistence of very good scientists and people, it might have taken much longer for a theory based on science and not historical precedent to be accepted. Most notably, I want to thank Joachim Heierli who wrote his PhD dissertation on the, “Anticrack Model for Slab Avalanche Release,” which formed the foundation for modern understanding of avalanche dynamics, and the model presented in this article. His dissertation can be found here: http://d-nb.info/101372173X/34.
Next week, I will go into depth talking about how to evaluate each component to asses stability.
Summary of what it means to break an avalanche. The article focuses on the events in time that occur to create an avalanche.
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