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  • Writer's pictureHenry

Anatomy of a Fall: Learning From Avalanches in the Field

Updated: Apr 9

We woke up to our last skiing day with a beautiful layer of about 20cm fresh snow down. Completely taken by surprise, we wolfed down breakfast and headed for the slopes.

Skiing on a Saturday is always fun as it's transfer day - many people are not skiing as they are making their way home after their week away, with a new wave of holiday makers arriving later in the afternoon.

We made hay whilst the sun shined (or didn't, more accurately). Heading straight for Cugnai lift, we discovered no crowds and picture perfect powder over and beside the piste.

After a few laps, the cloud cleared and we headed off the back of Cugnai down one of Val d'Isere's best off piste runs.

It was heaven.


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First Impressions

The run out of Cugnai involves a traverse towards Ferme Hut, then beyond down the access road and out onto Manchet.

As we settled onto the traverse, we found our way blocked. A huge avalanche had slid down and buried almost all of the path out.

We saw the top half first. A small slab avalanche had run down the side of the slope in towards the valley, carrying for some distance and disappearing behind a mound.

As we rounded the corner, however, the full scale of the slide became apparent. The slide had carried over 100m down to the valley floor, and had gathered significant amounts of debris along the way, to create a foot probably close to 60m and standing three metres high in places.

We weren't able to access several bits of the avalanche to give it a proper post-mortem; however, from our perspective on the valley floor we could learn a huge amount about how this avalanche happened and why it became so large, and it nicely highlighted elevated dangers to look out for.


Small Beginnings

This was a slab avalanche; somewhere about a metre or so down in the snowpack the upper layer of the snow had collapsed down onto the lower layers, separated by a thin layer of faceted snow.

The failure between layers had spread, although not by a huge amount. From our position, it appeared as though the failure had propagated across about 10m or so. This was a sign of a generally stable snowpack - if it fails, it wouldn't spread into a large avalanche, although as with all aspects of avalanche safety, "it depends" is the watchword as we saw here.

The avalanche failed on snow really close to the ground, indicating that this was a result of early season snow from December. Warm weather in the last few weeks, especially afternoon sun on this west facing slope, had melted the snowpack down, increased the density of water in the snow pack, and had then loaded it with new snow when it arrived in late February and early March. This is a perfect example of why early season weak layers can cause havoc even this far removed from when the snow fell, as the snow melts and brings these layers back into play.

We could see the slab that had failed because there was still a piece of it that had broken away from the main snowpack, hanging on for dear life. As well as simple accessibility and time, this was a big reason why we didn't go in for a closer look. This last bit of slab needed a gnat's sneeze to follow the rest of the snowpack down the mountain.

The very top of the slide, showing the slab hanging on for dear life. This photo was taken from a couple of hundred yards away, and doesn't necessarily showcase scale very well.

Typical Terrain Traps

A terrain trap is one of two things that exacerbates the impact of any avalanche that does occur.

It could be a feature in the path of an avalanche that makes it inherently more dangerous; a cliff to carry you over, gullies for you to be carried down, or rocks to slide you into. Alternatively, it is an area of non-avalanche risk terrain that could be avalanched into.

This was a classic case of the latter; the valley floor sat far below the avalanche risk terrain which was then avalanche into.

Alongside the more traditional terrain trap in play, the profile of the slope played another role in growing a small avalanche into something enormous. A convex slope - that is a slope that starts steep then shallows out - provides the ideal angle for avalanches to start (remember that avalanches only start on slope angles between 30° and 40°). As it falls into the snow below it, it re-triggers the failure of the weak layer growing and growing the avalanche.


Learning Lessons

The whole point of studying avalanches that have already happened is to understand the circumstances in which they happened. Using this knowledge and experience, we can then apply it to future situations to avoid triggering similar slides.

Normally when you carry out an avalanche post-mortem, you get right up close to the crown wall and investigate the snow from which the avalanche failed on. You then add in known external influences and the resultant size and strength of the avalanche to create a "formula" for why it happened and similar scenarios in which it can happen again.

We couldn't do this. We didn't have access to the crown wall up high nor did we have the time to dig into the slide and work out exactly where it went wrong.

However, even this passing glance of the slide and the study of a few pictures tells us a lot. Triggered probably overnight on a weak slab from early in the season based on how close the slide occurred to the ground and the fresh layer of snow from overnight snowfall, this matches other spontaneous slides that were impacting the resort that morning. Following both near term and long term bursts for warm weather, and especially on a west facing slope prone to the afternoon sun, this tells us that the snowpack in these areas is weakening sufficiently to cause large spontaneous slides. And, always useful, a reminder to watch out for terrain traps.

Moving forward, this was a useful cue to pay attention to certain faces, especially west facing steep slopes. Avoiding these, and the terrain traps that lay beneath them, would be the key to skiing safely moving forward.


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