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It slid last week so it's good…right?

  • peteyboy
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11 years 11 months ago #130110 by peteyboy
It slid last week so it's good…right? was created by peteyboy
Hoping to start a knowledgeable discussion about decision making regarding the safety of a slope that has knowingly avalanched during a recent storm cycle on top of a resilient crust. Here's our recent representative scenario: New snow on an old crust. Cold temps throughout. Known poor bond; as storm total is building, the new snow is gaining cohesion. Avalanche rating risk is high, all kinds of signs of the layer being highly reactive. Shooting cracks in the trees, tons of whoomphing, even large remote triggers. The whole tree of red lights. We skied (on the very knowledgeable advice of the owner guide of the area we return to every year) a zone where he had ski cut and slid each vulnerable shot earlier in the storm cycle. But now 40 to 80 or more cm on top of that in less than a week. Skied every shot with no signs of instability whatsoever.
For our decision making, I wonder about slide activity on a mature crust: how well and for how long/much new snow at what speed/wind/temps can we trust that we won't just have a shallower cohesive new slab on the same poor bond? Really an issue for when there is rapid reloading like we just had. I realize that buried surface hoar crystals will be wiped out by avalanching the slope, but the crust remains. Not looking for mathematical answers, just to generate thought and comment.

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  • wolfs
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11 years 11 months ago #130131 by wolfs
Anecdotally, I have had experiences in the Crystal BC where it was areas that had recently slid that were more prone to releasing their most-recent slabs than were areas very nearby that had not slid. Could see slightly buried debris piles beneath such slides as clues. Hypothesis is the smooth scour of the previous slide had made the underside of that slab even less likely to bond to the base, and more able to obey gravity's call when it was separated or tweaked by a ski cut. On the plus side chances are lesser that it would step down to next layer beneath scour. Maybe, subjective on how much "new" slab there is, as to whether a slid slope is safer, or not?

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  • T. Eastman
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11 years 11 months ago #130146 by T. Eastman
Replied by T. Eastman on topic Re: It slid last week so it's good…right?
A slide can create a slick bed surface for subsequent snow falls. This is likely why some tracks are frequent performers.

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  • maximusj
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11 years 11 months ago #130153 by maximusj
Replied by maximusj on topic Re: It slid last week so it's good…right?
I think that you have to be wary with this... avalanching can (and often does) hit the reset switch, but if a slide occurs on a crust and the next snow event does not bond well to the crust, you are back where you started.

Here is a short discussion from The Avalanche Review. It references a continental snowpack, but the points are valid here as well - there is no silver bullet.

www.avalanche.org/moonstone/Forecasting/...Snowpack.TAR28.4.pdf

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  • rlsg
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11 years 11 months ago #130201 by rlsg
Ten four that frsg...

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  • peteyboy
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11 years 11 months ago #130223 by peteyboy
Replied by peteyboy on topic Re: It slid last week so it's good…right?
That's why I feel we got lucky. But I posted this because I have found this to be a fairly pervasive opinion point of slope safety - much like "it gets skied a lot".

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