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Author Topic: Safety of trees  (Read 3774 times)
Koda
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Safety of trees
« on: 01/18/12, 10:08 PM »


 
Just thought I would query the collective knowledge of the community about the safety of tree skiing in regards to avalanches. There is a scenario I’d like to present where I can stand to learn more, so feel free to help me out. In higher avalanche danger, I often seek ski terrain in the "safety" of trees....
 
I understand about the danger of skiing trees located below runout zones. This is not what I am after. I understand about skiing trees with mixed glades, or trees near the edge of timberline or that are sparse or spread out. This is not what I’m asking about either. I understand in those scenarios, avalanches in tress do occur.
 
What I am asking about is forested terrain well below timberline, and not exposed to the mentioned above, where the entire run is enclosed under the canopy of forest. I’ve been told that any trees or other vegetation that provides enough anchors to prevent avalanches is "too tight to ski". With regards to that I have many places in my bucket list that I ski easily, yet in my mind think there is enough trees to prevent avalanches.  Is this incorrect?
 
Let me help define what I mean, since I’ve never gone out and measured the distance between trees let me share a few pics.
 
I consider these “tight” but skiable… This is primarily what I am talking about: 1)
 
then, I consider these a bit more open… 2)
 
And then what about burn areas… where understory vegetation while growing back is thinner? 3)

3 different stashes where I find no flagging or other evidence of avalanches.
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bc_skier
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Re: Safety of trees
« Reply #1 on: 01/19/12, 01:26 AM »

Do you know the slope angle of the slopes in the photographs? Hard to tell sometimes.  Looks under 30 degrees, which if so in this case you are probably fine.

Your question is very subjective and there can be quite a few variables, however for me I avoid all terrain over 30 degrees in 'High' to 'Extreme' conditions.

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Koda
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Re: Safety of trees
« Reply #2 on: 01/19/12, 09:42 AM »

Do you know the slope angle of the slopes in the photographs? Hard to tell sometimes.  Looks under 30 degrees....
  I just got an inclinometer late last season so only one of those pics I've actually measured. The last one #3 in the burn is under 30°... 25 to 28°.

the first one #1 is steeper by my judgement, probably closer to 35°. #2 ~30°

Your question is very subjective and there can be quite a few variables, however for me I avoid all terrain over 30 degrees in 'High' to 'Extreme' conditions.

this is true, I should say I'm not looking for a definite answer or rule like yes or no... to clarify I'll never say on any given day these places are always 100% safe. I'm looking more for observations from others, maybe someone has triggered a slide under a thick forest canopy, or maybe learn more about a relation between [tree] spacing and successful anchoring. maybe there is a relation between slope angle and spacing.... ie: for #2 i would not ski that on a 'high to extreme' condition if it was steeper. I've seen small surface slabs propagate and release in similar tree environments but steeper.... there is enough open space in photo #2 for a large slab.
« Last Edit: 01/19/12, 09:47 AM by Koda » Logged

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Charlie Hagedorn
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Re: Safety of trees
« Reply #3 on: 01/19/12, 10:19 AM »

From limited experience, snow tends to be less unstable in the trees. Lots of factors, including decreased wind action, snow-filtering by the branches, tree drip crusts, etc. may help. The improved anchoring probably doesn't hurt either. Skiing inbounds yesterday, the snow was cracking as much in very tight trees (~2' between trees, at times) as on open slopes. It all depends.

I suspect that your picture #1 will slide occasionally with the right loading and layering. I'll ski things that look like that on some days and not others (Probably wouldn't touch it today, for example. The underlying icy crust is present everywhere, including the trees.).
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garyabrill
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Re: Safety of trees
« Reply #4 on: 01/19/12, 10:19 AM »

I think the key is whether or not the "treed" slope is canopied or not. Canopied means you can see little sky. #2 photo is not canopied and hence surface hoar can form. Open treed areas are "great" for surface hoar formation and maintenance.

For a canopied forest, I have only a few times released a slab. This was on steeper areas of forest above 35-40 degrees. The slides were soft slabs (with snow falling heavily at the time) and because of the forest released over a limited area. Forested areas are more often than not bumpy which makes a slope less planar for slab formation.

Canopied forests: 1) act as a reservoir for falling snow and then subsequently drop that snow as a stabilizing influence, 2) anchor the snowpack in localized areas, 3) restrict windflow, meaning windslab is unlikely.
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Koda
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Re: Safety of trees
« Reply #5 on: 01/19/12, 10:40 AM »

I think the key is whether or not the "treed" slope is canopied or not. Canopied means you can see little sky. #2 photo is not canopied and hence surface hoar can form. Open treed areas are "great" for surface hoar formation and maintenance.

For a canopied forest, I have only a few times released a slab. This was on steeper areas of forest above 35-40 degrees. The slides were soft slabs (with snow falling heavily at the time) and because of the forest released over a limited area. Forested areas are more often than not bumpy which makes a slope less planar for slab formation.

Canopied forests: 1) act as a reservoir for falling snow and then subsequently drop that snow as a stabilizing influence, 2) anchor the snowpack in localized areas, 3) restrict windflow, meaning windslab is unlikely.


excellent input on how a canopy affects surface hoar. The tree environment I am primarily interested in is a canopied forest. #1 photo is such a condition, those trees are tight and under a heavy canopy. I'd like to hear more input on this environment because much of the terrain in my home court is heavily forested canopy.

But not withstanding photos 2 and 3..... in regards to the #3 "burn" photo, I think the heavily treed (albeit burnt dead trees) should not distract from the danger of a complete lack of canopy. Thanks for sharing how the canopy affects stability, when I was focusing on other elements of the study like spacing.... hence my comment on the burn photo.
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Schenk
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Re: Safety of trees
« Reply #6 on: 01/20/12, 11:08 AM »

Let us not forget, trees can weaken the snowpack in many situations.
Think: perforations in a sheet of paper...the paper (layer of snow) usually tears (breaks) at the perforations when stressed enough .

Getting swept through trees can be traumatic to anyone caught in a slide like that. We have all seen slides that propagated into and through tree stands...Always do stability tests on a representative slope and don't count on trees increasing snowpack anchoring.
« Last Edit: 01/20/12, 11:11 AM by Schenk » Logged

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Marcus
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Re: Safety of trees
« Reply #7 on: 01/20/12, 11:51 AM »

Two anecdotes, worth what you paid for them (and neither quite addressing the type of terrain in your pictures, Koda)...

Right before the slide broke on the Phantom last year, I remember looking just uphill at the trees I was perched near, thinking that they were a dubious safe spot at best.  The slab broke right across the base of the trees, fracturing in a broken line down to the next small cluster.  Obviously in the aftermath we were flushed through trees, with the expected results.  This was all much more open terrain than you're talking about.

Second one -- a few years back, a friend on a cat skiing trip was skiing a heavily treed slope.  It was the second lap on that slope of a full guided cat and she was second to last, or something.  Probably skier 16 or 17 on that slope?  Anyway, the slope fractured when she skied it and flushed her through the trees, with very serious (thankfully not fatal) injuries.  The guides had never seen that slope show any sign of activity in many years, so it was clearly an uncommon release.

Pretty sure that last slope was more "christmas trees" and not more established old growth or second growth like in your photos.

More in line with your pictures, I've definitely cut loose small slabs on steep rolls in terrain show in your pictures -- at Yodelin, specifically, on the north slopes that everyone skis.  Didn't travel very far or propagate much, but in the right circumstances I can see it flushing you into the trees or breaking out into a larger section of the forest.  Not common, probably, but very high consequence if it did happen.
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Koda
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Re: Safety of trees
« Reply #8 on: 01/20/12, 01:36 PM »

Two anecdotes, worth what you paid for them ....

There is definetly some good information trickling in, your two ancedotes here are helpful... I'm curious to learn more about the second one, first hand details of that would be helpful to this discussion. I have also kicked off small surface slabs in heavy trees, but they were always in locations of more open spaces... I think "open spaces" here are the weak link. I'm curious if said open space had anything to do with your second anecdote incident?

Your reference to cat skiing recalls a memory of cat skiing at Mt Bailey years ago on a very unstable day, the guides kept us confined to low angle glades, but tested fate on a steeper tree run. The guide ski cut the top and a large slab resulted and destroyed the area below... in a split second. IIRC, this "tree run" was similar to my photo #2... (notice the open fall line in otherwise heavy timber slope + the open space I'm standing taking the photo), the location of ski cut the guide performed was an open space worth a few turns below not unlike my photo #2. I don't recall the propagation spreading farther than the open space....  but this was many years ago.

What I am gathering here is there is a relation between tree spacing, slope angle, and overhead canopy. I can say that tight trees often offer much greater stability in otherwise unstable conditions but this thread is helping my define or measure those elements.
  • fall lines that stay well under the canopy contribute the most stability
  • burn areas with no canopy only offer marginal stability only in the snags themselves... there is also more likely less ground vegetation
  • "open spaces" contribute to instability and offer larger distances of propagation. The larger the open space, the greater the danger

I think at some point it might help to define "tree spacing" with an actual measurement in order to help define "really tight tree skiing" like I am referring to. Right now all I can describe how I define "tight trees" is spacing short enough to constantly disrupt your fall line....
« Last Edit: 01/20/12, 01:41 PM by Koda » Logged

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Koda
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Re: Safety of trees
« Reply #9 on: 03/17/12, 02:13 AM »

this thread came to mind on a recent tour this week we noticed a very reactive surface layer. Lots of creep and fractures propagating in about the top 2" of snow. It was snowing heavily the latter half of the day, and we concluded a thin layer of surface hoar under the recent 2" layer.

Well our objective was under a heavy canopy of old growth and after a few hours we came back around hooking up with our original skin track through the timber were a natural point release (maybe from a tree bomb) had slid across our skin track. This reactive surface layer was present... even under the heavy canopy of 'tight trees'.  Now I am imagining this suspect layer preserved a few days with more loading....

« Last Edit: 03/17/12, 02:17 AM by Koda » Logged

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