Do couloirs generally have a more stable pack?

[FiveLarge]

This question was brought forth from this TR:
http://www.tetongravity.com/forums/s...ad.php?t=80376

Decided to move the discussion here, so as to cease the hijacking I had begun.

Here was the original question:

So...generally, N-E aspects have had major instability issues over the past week in the CAIC Summit/Vail zone. I know Grizzly is at the easternmost edge of that zone, so perhaps the snowpack in that area is generally not similar to what was happening in Summit/Vail zone. If that's the case, there may not even be a discussion point here. But, assuming the pack in the Grizzly area is generally similar to that of the Summit/Vail zone. Can/does the pack consistently vary (considerably, even) depending on whether you're looking at an open slope, or a couloir (assuming similar elevation/aspect). In other words, you found excellent snowpack conditions in the chute. Most of the stability issues I was reading/hearing about on N aspects at/above TL were on more open slopes. Wasn't hearing of the same issues in chutes/couloirs. Does a couloir tend to hold a more stable snowpack? (again, in direct comparison to an open slope, assuming same elevation/aspect). Hopefully I'm making some sense. I don't think I could ask the question with any more words.

And a little more after a short reply:

Absolutely agree...and that mentality (basing decisions on what you find) is number one regardless of anything this discussion may generate. Overgeneralizations/extrapolations are dangerous...but can be helpful in planning.

Would protection from sun serve to weaken the pack (yes, too much sun = not good). For example, with a "winter" pack, south aspects tend to be more stable, because of the stabilizing effects of sun, yes?

Wouldn't you get some pretty good cross loading in a couloir? Isn't that why couloirs "last longer" in terms of holding snow (yes, protection from sun also factoring in)

I'm trying to figure out if there is real evidence here, or if it's just circumstantial evidence. It's all interesting to me...

Edit: my questions/points directly above don't go along with the thought that couloirs may generally have a more stable pack, I realize. That's why I'm asking this question. Experience seems to say yes...but, I'm not sure why it is...so, yeah...it is just circumstantial, or is it "real"?

[Tri-Ungulate]

Depends how deep/narrow the couloir is, and, as you mentioned, the aspect of course.

Shallow/narrower/more SE facing might suffer from warming before other aspects due to rock warming, leading to a propensity for wet slab releases before a wider or deeper (more shaded) would go. I made this mistake once, hopefully never again.

[smitchell333]

Depends on the couloir whether its safer or more dangerous. Often the consequences are higher if there is dogleggin so a fall or even small slide can exact heavy price for error.

Avy wise this one and others like it have one POTENTIAL advantage over some open faces - there are many cliffs, small faces, and feeder couloirs that dump a regular supply of sloughs, avalanches, and wet debris down the chute. The result can be that the snowpack is deeper and firmer in the couloir than on open faces or couloirs that do not slide regularly or have feeder situations.



I was in this couloir about 8 weeks ago so I know the underlayer was quite deep and firm from the early winter and fall storms and resulting slide deposition - as it is in certain spots high on the divide.

[FiveLarge]

Avy wise this one and others like it have one POTENTIAL advantage over some open faces - there are many cliffs, small faces, and feeder couloirs that dump a regular supply of sloughs, avalanches, and wet debris down the chute. The result can be that the snowpack is deeper and firmer in the couloir than on open faces or couloirs that do not slide regularly or have feeder situations.

I was in this couloir about 8 weeks ago so I know the underlayer was pretty deep and firm from the early winter and fall storms and resulting slide deposition - as it is in certain spots high on the divide.

This is more what I'm looking for, I think. Keep the thoughts coming...but yes, a deeper pack is generally a stronger pack...so, a combination of "feeders", good cross-loading (which would make sense), and shelter from the sun (esp on a N-ish facing couloir), would all contribute to a deeper pack.

I would think loading would be an issue directly after a storm...a couloir might load more than an open slope, no?...and less sun to stabilize. I wonder if it would actually be less stable than the open slope right after a storm...but after a "stabilization period", in large part due to a deeper pack, the couloir might be more stable?

[Powow]

Do the walls of a couloir provide significant anchoring? Surely if it is getting wider, they don't, but where the width is getting smaller/constant?

[pechelman]

this is a great question and topic for discussion and something ive thought about for a while.

anchoring wise, I think it *could* act as a support, not sure I like the word anchor in this scenario even though thats the common terminology. As always, it would depend on the snowpack, weather, and couloir geometry.

im also wondering if the couloir itself helps to segregate the snow from adjacent weaknesses and trigger points. Its like if you take a massive billet of aluminum, you are sure to find some defects, but then if you take a small coupon of it, chances are you will have much much fewer defects or trigger points.

[smitchell333]

This is more what I'm looking for, I think. Keep the thoughts coming...but yes, a deeper pack is generally a stronger pack...so, a combination of "feeders", good cross-loading (which would make sense), and shelter from the sun (esp on a N-ish facing couloir), would all contribute to a deeper pack.

I would think loading would be an issue directly after a storm...a couloir might load more than an open slope, no?...and less sun to stabilize. I wonder if it would actually be less stable than the open slope right after a storm...but after a "stabilization period", in large part due to a deeper pack, the couloir might be more stable?

Yeah - they can be less safe both after significant snowfall and right at periods of major warming as the stuff above slides into the couloir. But in the long run after things generally stabilize they can be safer than other places. Also this can be true for other aspects as well - S, E, W if they are getting the slide deposition they can be deeper and firmer.

im also wondering if the couloir itself helps to segregate the snow from adjacent weaknesses and trigger points. Its like if you take a massive billet of aluminum, you are sure to find some defects, but then if you take a small coupon of it, chances are you will have much much fewer defects or trigger points.

Interesting thought.

[FiveLarge]

iskibc posted some thoughts in the TR thread. I'll copy them here:

Good questions, good topic. I'm tired and sick right now, so my response will be a bit short, but hopefully it helps a little.

1. The Grizzly couloir is directly east facing. No northerly aspects at all.

2. Couloirs hold much more snow than open faces. More snow + smaller area = stronger pack (obviously not in all cases, but a pretty good generalization). A couloir with vertical rock walls from top to bottom will anchor the snow much better than an open face or a slope with spaced out anchors. Again, not always the case, but somewhat of a generalization.

3. This couloir is narrow and doesn't get the cross-loading that a lot of other shallow couloirs in the front range get. First looking at the couloir I noticed the apron had a healthy bulge in it, which indicates the couloir has either slid big or sloughed out frequently throughout the winter. This is a good sign as chances are the couloir isn't holding a thick and deep slab, just waiting for a trigger. It had looked like the couloir sloughed either from the storm 14 days ago or from the smaller 10" storm 9 days ago. Good sign. No signs of any other aspects, slopes, faces, couloirs sliding either. I saw only one very small point release from a few days ago at about 11K.

4. As far as testing, we had already skied a similar slope and from observations and poking around we decided it would be alright to ski. The similar aspect we hiked up was rock solid and showed no signs of instability.100% positive? Of course not. Given the recent weather, stability was in our favor. We were skiing at 9am at 13K, so the snow was nowhere near wet. The warm temps from the past week and the cold nights up around that area has locked up the higher elevations pretty good. At the bottom of the chute we were skiing corn on a super hard and compacted base. The worst that could have happened when marshal dropped in would be for the slope to rip below him. He dropped in at the high point of the couloir. That cornice wasn't going anywhere. His drop into the couloir was more of a ski cut than anything. Once we got into the chute I probed and found a really nice hard, compacted, springlike layer underneath the 6" or so of pow. Ideal conditions in my opinion.

My question centers more around his thoughts in points 2 & 3. I'm really not questioning their decision...just sort of wanting to know if there is some science behind what I'm finding in experience/accounts of others...

[Camel Toad]

Here are a couple of couloirs with associated conditions etc.

This couloir had a wide are in the middle. The slope had a crown fracture running completely across it just about where the skier is. It faces pretty much directly north, there is a ridge to the east that blocks light and winds. On the day we skied this the south sides were occasionally whoomping. We cut a cornice bomb and absolutely nothing happened in this couloir. The middle part had already slid so with the upper bombed and the lower slid we skied it with no problems. The middle had a slight roll over that needed to be treated more like a slope than a couloir in that it could rip fairly wide so it would be easier to get caught up in a slide if it cut loose. We would not have skied the south side of this peak on that day due to south side avy concerns.



This couloir faces northeast, is completely blocked from the sun and from cross winds. The entrance to the couloir is the only place that is of major concern due to the high walls and steep angle, the couloir sheds any heavy snow loads, the bottom had a huge (soft) debris cone. The upper section is setup in a way that slabs can build, not to big a deal usually, but the only way up this thing was from the bottom.





This couloir faces south, it gets direct sun through various parts of the (dependent on rock walls blocking). We approached it from the north/west where we had experienced whoomping at various points in the day. The south face had an entirely different snowpack, with great snow up top (colder less direct sun), leading to sun affected snow lower.







Now that I am done with the pretty photos on to your questions. On a given slope a couloir is SOMETIMES a safer place. The amount of snow that can slide is reduced (cannot propagate across a slope), there are potentially more anchors (rocks can also maintain facet layers / melt moats), if the couloir is blocked from the sun it can have less variability in the snowpack temps (sometimes good, sometimes needed). If you can start from the top that is helpful as you can trigger a good sluff and "cleanout" the couloir. On the other hand if you get caught in a slide in a couloir you are a pinball for rock walls and the debris can pile up very deeply. Hope that helps.

[lemon boy]

couple of thoughts:

need iski/marshal confirmation but it appears that particular shot goes at >45*s for much of its length.

If you look at the pictures it appears that iski was able to pull well out of the fall line for picture taking at a variety of points (eg: safe zones).

the other thing we could do is armchair the obvious clues method:
Rating: considerable or higher <-- looks like moderate climbing to consid in the afternoon so this gets a no.
Avalanches: reported or observed in the last 48hrs <---Yes
Unstable Snow: colapsing/cracking deep trailbreaking etc.. <----No
Path: obvious path, recognizable by a novice <---?*
Loading: new snow, wind or rain last 48hrs <---No
Trap: terrain traps <----?*
Thaw: melting of the surface and accompanied by sinking into the snow as high as bootops (skis on) <---No

By the numbers:
(clues then &#37; of avy accidents)

2 or less, 2%
3 or less, 8%
4 or less, 23%
5 or less, 53%
6 or less, 90%
7 or less, 100%

*'s: I'm not so sure how to evaluate the OCs w/respect to this situation, this is not an "avalanche path" in the classic sense although clearly it gets a lot of sluffing at the least. As for the terrain trap part, the apron is a nice wide fan where debris isn't going to be collecting in on top of you or straining you through trees but you be stuck getting ragdolled through the thing.

So depending on how you call em you're looking at 2 to maybe 4 at the outside. Now...who remembers how to analyze the OC by the numbers

all that is to say, I think it depends but then...what do I know? Not much I can tell you, not much.

[snoboy]

Herr Munter talks about evaluating slope angle in his Reduction method. What he referes to is the _steepest_ part of the slope, in a couloir that can be a lot steeper than the skiable line if you look to the sides...

[iskibc]

Just through experience, not scientific facts, I've observed that couloirs typically "slough" out more frequently and typically do not fracture (soft or hard). Steeper couloirs will slough frequently during a storm, a few days after the storm, and will then lock-up and settle sometime within 4-7 days after the storm. Most of the time the sloughing acts as a way to stabilize the couloir by allowing the snow to settle and harden. Large open faces tend to hold storm snow and energy much longer, thus creating unique and distinctive layers.

There also needs to be a distinction between what classifies as a couloir and what classifies as a gully, chute, etc. There's a difference in the terminology and terrain as well as a big difference when it comes to stability between these classifications. I would feel uncomfortable skiing a 35 degree gully flanked by willow scrubs and the occasional rock moreso than skiing a 45 degree couloir with vertical rock walls up and down the entire length.

This is a good topic. There's not a whole lot of literature or science out there concerning couloirs, gullys, chutes, etc.

All of my beliefs concerning stability in a couloir comes from experience and first-hand knowledge.


I like snoboy's reference too. There are some couloirs where the slope angle will change 5 degrees or more from when you measure in the gut to when you measure from the sides of the couloir. Aspect is also a factor in the situation of deep, halfpipe-like couloirs as well.

[cololi]

The general consensus here seems to be that there are a number of factors tha contribute. Slope angle is a big one, if it is generally over 40 degrees, the snow will tend to sluff off more as mentioned. The underlying surface is key too. If it a slab of rock, you may have to worry about water percolating through the pack and providing a glide layer depending on aspect, temperature, moisture falling, time of year, etc.

I do think that couloirs require more research leading up to the time you are skiing it. Are there escape routes, does it cliff out, what is the underlying surface like, what has the wind done this season, etc. If you can do get caught in something, I think the risks go up dramatically because the snow is concentrated in one relatively narrow chute (dependant on the chute) and can flow rather quickly and slam you against the sides. Think of river running through a narrow gap and how much more power the water has compared to a wide stretch of river. I don't know of any scientific research that has proven that snow acts like water when flowing in terms of where in the flow the snow is, but it seems that the less non moving surface there is, the less friction there will be and the more power the flow will have.

When I was doing snow safety we had one small couloir that had a nasty tendency to pull out after just about any southwesterly wind event over 10 mph. It was a good place to see how dangerous even a small sluff can be if caught in a narrow place with no escape. The depo piles even from a 6 inch fracture would be at least 6 feet deep. Safely ski cutting the chute seemed to be pretty effective and ir rarely propogated out too much once it reached the open bowl.