Skier Compaction: How much is enough?

[MakersTeleMark]

Disclosure: avalanche amateur stating how I think about this. Feel free to critique, someone might save my life.....

I think it's been mentioned, but the effect of skier compaction must be seen in relation to the avalanche problem. For some problems (soft slabs building up in layers over few days with intermittent snow fall and wind) I can't understand how skier compaction wouldn't work. If there are weakt layers forming during or after the skier compaction (facets close to the ground) the effect must be close to nothing. But might reduce the potential of the upper layers to propagate - due to less cohesion?

Snow crystals constantly change, either getting smaller or bigger, often with temperature gradient as the main driver. This means that most avalanche problems come and go over time, and that what was a positive effect from skier compaction on a soft slab might be irrelevant if there over time is a thick layer of facets forming near the ground or under a ice layer that the skier compaction hasn't affected. This sounds what might be the cause of the mogul slide in Lake Louise mentioned above. Would be interesting to see results from a pit on that one.

Now, tell me if I'm wrong.

From a guy who has not died in an avalanche yet, if you are thinking about compaction, you aren't thinking about shit. Focus on the basics.

Put all that laser beam shit on looking at slopes and terrain management, and if you are hungry, do an ect on your slope. And be prepared to skin the fuck out. Other than that, get after it. I do.

[sf]

From a guy who has not died in an avalanche yet, if you are thinking about compaction, you aren't thinking about shit. Focus on the basics.

Put all that laser beam shit on looking at slopes and terrain management, and if you are hungry, do an ect on your slope. And be prepared to skin the fuck out. Other than that, get after it. I do.

Absolutely.
Compaction is not a factor in any decision I make. I'm pretty much a 30-or-below-guy in the winter, since I tour solo a lot.

However, basic snow theory is good for understanding the avalanche problem at hand. If one doesn't, I'm not sure how digging a pit in any isolated spot is going to help anyone. If one use it to see/understand how the snowpack changes over time then a pit makes more sense. Note that I'm not saying "you" here.

In the same way, no understanding of how snow changes over time could easily lead someone to think that something that was "compacted" last week is safe to ski today, regardless over the weather the last week

[MakersTeleMark]

Absolutely.
Compaction is not a factor in any decision I make. I'm pretty much a 30-or-below-guy in the winter, since I tour solo a lot.

However, basic snow theory is good for understanding the avalanche problem at hand. If one doesn't, I'm not sure how digging a pit in any isolated spot is going to help anyone. If one use it to see/understand how the snowpack changes over time then a pit makes more sense. Note that I'm not saying "you" here.

In the same way, no understanding of how snow changes over time could easily lead someone to think that something that was "compacted" last week is safe to ski today, regardless over the weather the last week

If I'm digging, it's pretty much a no go until I really feel the pussy stank feeling really tasty. And if I'm diggin to ski, that is just a bad sign.

And I honestly am not like you. I ski slopes over 35 on the reg. The rest is just a catwalk. But, hear what I am saying. Know before you go is not just about reading a fucking email. Know your slopes, historical slide paths, runouts, bail out zones, plan B's. Know them all. Then get a slice at the bar and turn off your transceiver if you made the right choice. Now do that every day and get back to me.

It's the most inexact science that I have ever encountered in my life. But it's right there in front of you, so there are no excuses for death.

And I am no way disagreeing with you. I feel it, I see it. It still scares me. As it should.

[Bunion 2020]

I have a great deal of respect for Mr. Richards and would like to see his entire quote in full context.

Put simply:

Skier/boot compaction stabilizing weak layers at ski areas is no myth. It is no magical bullet either.

Skier compaction in the BC is wishful thinking especially if you are using that as a Green Light factor.

Didn't Big Sky or Bridger have a mogul run rip out to the dirt in the past decade, too?

Which time? The BB slides were wet avalanches, totally different from the discussion at hand.

I have seen moguled slops fail into basal facets on more than once occasion.

The reason most commonly credited is a layer of basal facets covered by a dense enough slab that compaction fails at reaching the weak layer under the slab and then a sufficient enough loading event to cause the weak layer to fail.

[P.A.B.C.]

I was thinking about this a little longer. It seems that denser, sintered snow forms facets much more slowly than freshly fallen dendrites. I'm not a snow scientist and I didn't go too in depth into my research, but the paper "Snow in strong or weak temperature gradients. Part I: Experiments and qualitative observations" https://www.sciencedirect.com/scienc...65232X85900047 corroborates with what I am saying. Here are some quotes from that:

"the influence of a strong temperature gradient is highly dependent on density: low density, newly fallen snow (~100 kg/m 3) will evolve toward a weak assembly of "depth-hoar" crystals (large facets, steps, plates, and skeletal morphologies); higher density, older snow (~300 kg/m 3) will evolve toward firm crusts, which consist of smaller prisms;"

and

"we feel more emphasis should be given to initial density, which we believe is decisive as to whether the texture will evolve toward a firm and/or crusty assemblage of small prisms or toward a relatively weak collection of larger crystals, which are traditionally associated with growth in a strong temperature gradient. Given high enough density, it appears that traditional "depth hoar" will not grow, irrespective of the magnitude of the gradient"

Now, what does skiing powder do? It sinters the the snow, like forming snowballs, especially in zones like the OP described, where everything gets skied out. If the paper is right (it is old after all), that means that skier compaction would make it less likely for facets to form in the middle of the snowpack, which would thereby decrease the probability of an avalanche triggering on those layers.

If skiers don't ski out the first layers near the ground, skier compaction won't really help there, and its true that forming a slab on top of the layers on the ground could make deadly slab avalanches more likely on those layers. Nevertheless, as the snowpack deepens, the chance of triggering a layer at the bottom lessens, so if the rest of the snowpack is sintered by skiers, it should be stronger than if nobody skied it. This way of thinking might only pertain to areas like in Europe, inbounds in North America, and slackcountry zones in the U.S. that get tracked out consistently.

Also, I think that tracks breaking up the weak faceted snow, destroying the consistency of a potential bed surface is likely a factor that decreases the chance of avalanches.

Given two slopes in the backcountry later in the season, one that has been skied a lot during the winter and one that hasn't, and holding everything else constant, I would choose to ski the one that has seen a lot of skier traffic.

[NWFlow]

Given two slopes in the backcountry later in the season, one that has been skied a lot during the winter and one that hasn't, and holding everything else constant, I would choose to ski the one that has seen a lot of skier traffic.

Even if it's tracked out 😱?

[P.A.B.C.]

Even if it's tracked out ��?

"holding everything else constant"

[cmor]

Dave Richards said he doesn't buy into skier compaction, as cats and dozers can compact the snow but skiers disturb the upper layers. So skier "disruption" or whatever you want to call it.

Skiers or other traffic greatly reduce the future hazard by breaking up layers near the surface. How near could vary from a cm to a meter depending on what's going on. Seeing a lot of tracks on a slope does not reduce the current underlying problem, it would have been the skiers days/weeks/months prior that would have made the impact. In a ski area if you consistently have skiers in an area you can count on weak layers being consistently disturbed, if you don't and problems arise you can often go back and figure out where a closure lined up with a problem layer. Often it's worth pushing to get skiers in an area ahead of incoming weather to save yourself a lot of sweat later. In the backcountry it's a lot harder to keep track of all that, and although you're gaining some stability from past skiers there's enough uncertainty there where it's hard to use that in any decision making on a PWL.

It's common to speak in absolutes and "disprove" concepts by showing exceptions. Nothing is absolute, but keeping skiers on terrain is far more effective than any other control measure. Same story with seeing a bunch of tracks on a slope and inferring reduced risk because of that. If you're the 100th to go vs the 1st, on average the risk is going to be less. How much less will vary on a lot of variables, but that shouldn't be a controversial idea.

[Core Shot]

The OP asked about skier compaction and its net effect in a backcountry setting.

Not much “compaction” in backcountry.

Not like stomping boots inbounds for a free pass.

But traffic does lead to stability. I’m thinking hoar frost scraped down.
After a cold dry spell, a slope that’s skied heavy is wayyy more stable than a virgin slope.

Is there a better word than compaction for that?
Hoar’d out?

[whipski]

a patroller friend used to call it "letting the air out"

[John_B]

This video doesn't add a ton of dialogue to the thread since it is based upon one pit but may be worth a watch.
https://www.youtube.com/watch?v=FTF0x_PqCfI

[galibier_numero_un]

... Skier compaction in the BC is wishful thinking especially if you are using that as a Green Light factor.

^^^ this (from someone far more experienced than me) ^^^

[edit] I just dug this thread up after watching the above video and was about to post a link k to it.

... Thom

[mr walker]

Skier/boot compaction stabilizing weak layers at ski areas is no myth. It is no magical bullet either.

Skier compaction in the BC is wishful thinking especially if you are using that as a Green Light factor.

I completely agree.

In a ski area setting, which admittedly this thread is not about, skier compaction is proven to be effective. I’ll go a step further and say it’s considered essential at some ski areas.

I can’t, however, think of any BC area that gets enough early and consistent skier traffic to be considered effective compaction for the purposes of shear plane disruption. So in my mind, it doesn’t even exist.

And like Bunion said, it’s no magic bullet. There have been many failures even when compaction is applied systematically, so think about that when you see a BC zone that’s been “skied safe”.

Good thread and thought provoking question.