What can happen while digging a snowpit.

[tsondaboy 0]

 

 

 

 

 

 

Sooo, how did we get to that point.

 

That day the last day of the ARAC course, the plan was originally to go up Goryu, but since there have been blowing some really strong winds that morning we decided to head towards the Tsugaike area instead. We met our instructor Dave at the bottom of Tsugaike gondola, along with an　other guided group led by Evergreen. Due to strong winds, the top station at gondola was not operating yet (it did later on in the day) so we used the lift to go up. We made it at the top of the Tsugaike #2 lift at about 10:30 and stated getting ready to go up. Some of the members of the tour had problems with their gear, so we took 2 of the tour guys that were ready and started hiking up. We were 5 persons taking the ARAC plus our instructor and the 2 guys from the tour, so in total 8 people going up at that point.

All along our path there were strong indications of wind loading (wind direction probably NW), with wind scoured areas alternating with wind deposited pillows.

We made it up the first top of the ridge at about 12:00 and decided to take a sort brake and decide what to do next.

In front of us lay an 50~70 m wide NE facing slope, that had very few trees on it, in contrast to the neighbouring slopes. Most of the people in the group were so exited about the seemingly safe tree area, that were almost about stating to strap their boards/skis.

Dave of course said that we are here to study the wind loading effect on the slopes and no one gets down before we assess the avalanche danger on the slope. He instructed us to wait were we are wile he was going to do a ski cut test, before we all go down to start digging pits and do various other observations. He ski cut the slope just below the convexity and stoped about 50 m away from where we were and 2 m far from a cluster of pine trees that were coming down from the ridge above him. The slope didnt show any signs of instability so he called us to go down were he was to start doing our pits.

I went down first, but for some funny reason I had the feeling that something is wrong. Instead of going down along were he ski cut the slope, I chose to walk on the ridge top a bit towards the SW side, so that I dont cut the slope above Dave, and came down next to him trough the tree cluster on his other side. After me, an other guy with his board on his back (pick 1) started coming down on snowshoes along the path that Dave did. He had gone down half the way 5 m after the lonely standing tree when.... ssssssssshoooot!

A fracture propagated in a split of a second, from were he was standing along the ski cut line Dave did earlier, right next to my right foot that was down slope and along the path I did to reach were Dave was and went on for 20 more meters till it reached an area that the trees were denser. After that, it didnt take more than 2~3 seconds to start moving downwards, faster than anything I had ever seen or imagined. “Slide!” I screamed wile feeling that someone is pulling the carpet under my feet! The guy that was coming down grabbed upon the crown part and held tight watching the debris moving fast away from him. Needles to say, he almost soiled himself.

 

 

 

 

After things calmed down a bit, both on the slopes and in our heads, we started thinking what might have caused the slope to fail.

The first thing to observe was that 5 mm big hoar crystals covered the smooth bed surface that the avalanche slid on. The first thing that came in mind was that Dave wile on skis didnt cut the slope deep enough to reach the week layer and make it fail, but the guy that came later along the path loaded with his board was cutting through that layer on his snowshoes and did. The results from the compression tests we did subsequently were even more intriguing.

We didnt get any failure on the first 2, then a hard 3 @ 95 cm on the next one and finally a moderate 4 and 3 again on the same surface at the last 2. Someone that might have done only 1 compression test, didnt got any failure and didnt notice the buried surface hoar, might had considered the slope safe and decided to go down on it...

The temperature readings along the profile, revealed much about the history of the snowpack and what danger lay beneath us. The reading @ (90 cm) in the hoar layer was -8 deg, just above it -9.3 going progressively towards -10.5 @ the top of the snopack. In other words, we had a strong temperature gradient that meant that moisture was being sucked up from below towards the surface leading to further enlargement of the hoar layer and degrading of the slope stability.

 

Summarising everything I have to say that this event humbled me more than anything else we came across during the ARAC course. Just made me realise not how much I ve learned during the 5 days of the course, but how few things I actually know about avalanches. The course itself exceeded any of my expectations and cant but recommend it to anyone that is doing/plans to start doing BC riding or anything else that has to do with being on the snowy mountain.

Finally, a big thanks to Dave/Rodney/Bob for all the knowledge that have passed me during these days.

 

 

PS:Sorry for the long post, but I thought I should post us much information as possible.

[soubriquet 0]

Excellent post. Very informative. Thanks.

[Kumapix 0]

very scary

[dizzy 0]

wooops.

 

well, good that no one was hurt. you would have gotten more than you bargined for during an ARAC course had it slid.

 

but awesome you had such fresh data to look at. looking at the snow in the crown and the flanks must have been cool to see.

[bushpig 0]

damn!

 

Very interesing to read Tsondaboy. Cheers for posting it

[mattlucas 0]

Good pictures Tsondaboy

Amazing feeling when the snow slides away eh

And it picks up speed fast

 

That is almost on the same aspect as the slide FT and I triggered late last March

Yours is northeast

Ours was east from the top of #2 lift

 

Do you know how far the slide went?

[tsondaboy 0]

Amazing to watch it moving away from you, I don’t know if I would feel the same if I was watching it coming towards me......

 

I am not sure how far it went but I estimate that it went all the way down to the bottom of the valley. The slope becomes steeper downwards so I couldn’t see from were I was standing what happened below. I went a bit further down the slope, through the tree area of the third pick to get an idea how far it went. It seems that the debris of the avalanche we triggered, triggered 2 more areas below, because I could clearly see 2 more crown fractures. I was not able to see were the debris from these 2 where deposited. The thing went down like a domino, so I wouldn’t be surprised if it triggered more avalanches on its way downwards.