Henry Schniewind has published this interesting video of the Foglieta avalanche on the 5th January 2015. The "Fog" is one of the Ste Foy trade routes. Using ski lifts and with a bit of climbing skiers get over 1500 meters of vertical. It is very popular with guided groups as it is a big face and powder can often be found days after the last snowfall.
(note, original video set to private - but you can see the video with discussion by Henry Schniewind and Liam here http://www.youtube.com/watch?feature=player_detailpage&v=QVr_FghYmgY#t=894 and this is possibly a lot more useful and a copy of the original video here: http://www.skipass.com/news/113886-avalanche-a-sainte-foy.html)
The entrance at 2700 meters altitude is steep, around 45 degrees and frequently wind loaded (and rocky). Skiers normally traverse across before starting skiing on a North-West facing slope with sections of 35 degrees. The avalanche risk on the 5th was 3/5 - Considerable risk, which means localized avalanche risk. This is what the avalanche bulletin said for the sector:
"Above 2400 meters, fresh snow and north wind are the origin of some new instabilities which have poor bonding with a sublayer that has evolved little and consists of angular grains. One or more skiers can trigger a slab avalanche particularly on North West to North East slope aspects"
This was a major slide that took the whole face. Some people have expressed surprise at the extent of the slide but if the whole face has a weak layer with fresh snow on it there is no reason why the collapse cannot propagate over a wide area.
Here is what risk 3 is classified as in Europe
"The snowpack is moderately to poorly bonded on many steep slopes... Triggering is possible, even from low additional loads, particularly on the indicated steep slopes. In some cases medium-sized, in isolated cases large-sized natural avalanches are possible."
So it certainly doesn't entirely rule out a large avalanche and the Foglietta has some form, on the 18th February 2014 most of the face went on a risk 2 day, in this case slide depth between 1 and 3 meters. The slide ran from 2875 meters to 1950 meters.
"A large slab on the north face of the Foglietta after a snowboarder broke part of the cornice on the ridge to give access to the face before dropping in. Luckily there were no other skiers on the face at the time. The snow went on one of the weak layers formed earlier in the season. The slide occurred during a whiteout which made checking the debris complicated for rescue workers. (11h45, 18/2/2014)."
Here is a link to a photo so you can judge for yourselves: http://lh4.googleusercontent.com/-4lcOeBbmfVk/UwjUhuYB5eI/AAAAAAAAS0g/S9DGUprEbkg/s1000/IMG_1853_4%252520a.JPG
It perhaps demonstrates something Alain Duclos has talked about. The first skiers don't trigger the slab but progressively punch down to the buried weak layer. Finally a skier knocks over the first domino in the pile and the collapse propates out extremely rapidly. The video puts to bed the idea that, unless you are a super strong freerider, you will be able to outski the slide. You've also only got a few seconds to use an airbag if you are wearing one. Also note that the skiers are following a single track down to the waiting area. This is a risk management technique based on the ideas of spatial variability, the guide sets the "safe" track and clients follow. This indicates that the guide believed the slope to be potentially dangerous as you would normally ski from much higher up.
The slide wasn't that deep, around 30cm, and this was probably the difference between a number of nasty injuries and one or more deaths. The result was 5 skiers taken by the slide with 2 suffering light injuries (cuts, leg fracture) and another more seriously with back injuries. There have been six avalanche fatalities this season despite, or because of, the poor snow depths.
Henry raises the question of what is a safe distance to stop at. The group seems to be behind, or just to the side of, a small rock outcrop. This is quickly overwhelmed. I know that guides like to keep their clients in visual contact but there is maybe an argument for skiing the whole slope and keeping in contact with radios. This requires a good lead and tail gunner to help with anyone who gets into difficulties, climbing up to help look for a lost ski or a fall could take a lot of time.
Liam Luke points out that the group were entirely autonomous, despite being spread over 1km distance and 300 m. of vertical they were able to independently start rescuing other group members having visually observed clues as to where to search. All members were dug out before the exterior rescue services arrived.
http://pistehors.com/backcountry/wiki/Avalanches/Slab-Avalanches
http://pistehors.com/backcountry/wiki/Savoie/Ste-Foy-Off-Piste
http://www.henrysavalanchetalk.com/avalanche-foglietta-north-january-5th-2015-ste-foy-tarentaise
http://www.leadingedgeski.com/team/liam-luke
