Weak Layer Considerations

I think this is an excellent idea Marcus. Weak layers especially PWKL, while not dominating statistics in terms of #'s of avalanche incidents, would certainly dominate statistics in terms of serious avalanche accidents.

If it were up to me, I'd think some interesting topics would include:

1) What are weak layers and why are they dangerous?

2) What snowpack structures incorporating different types of weak layers are most dangerous?

3) What makes weak layers persistent?

4) What are the effects of weak layers that allow them to be triggered long after formation?

5) What happens to weak layers in the long run? How can they stabilize?

I would think these topics, some of which are not well-covered in literature or classes could help create a repository of information for those who are early in their backcountry careers.

People could chime in with different perspectives on topics like these and could summarize incidents related to these weak layers in their skiing/climbing experience.

Food for discussion:

3) What makes weak layers persistent?

* Large grain sizes and other structural characteristics such as thickness.
* Anisotropic strength characteristics ( stronger in compression than shear ).
* Metamorphic phenomena at various spatial and temporal scales.

4) What are the properties of weak layers that allow them to be triggered long after formation?

* Their persistence is the most important factor for avalanche formation long after burial. For persistence, see above.
* Another important property of ice grains in a weak layer is the ability of the ice grains to rearrange themselves into smaller spaces through crushing.
* Poor bonding to layers above and below caused by lower number of bonds per unit volume.

Sources:

Proceedings of ISSW 2010 www.avtrainingadmin.org/pubs/2010_ISSW_Proceedings.pdf

* Some insights into fracture propagation in weak snowpack layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 716
* Fracture energy of weak snowpack layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

ANTICRACKS: A NEW THEORY OF FRACTURE INITIATION AND FRACTURE PROPAGATION IN SNOW

www.issw2008.com/papers/P__8212.pdf

5) What happens to weak layers in the long run? How can they stabilize?

* Overburden pressure and the characteristics of heat flux are the two primary factors in stabilising weak layers over the long term.
* Characteristics of heat flux drive metamorphic phenomena.
* Overburden pressure reduces the pore space, increases bonds per unit volume, decreases ice grain size.
* This is why people say that PWKL are "rounded" or "crushed" out of existence.

GaryABrill, you might like this paper ( if you haven't already seen it ):

www.geog.ubc.ca/avalanche/pubs/Book_Tsonsis_Ch24.pdf

Cookie,

At the more basic level it might be good to find the paper by Camponovo and Schweizer that diagrams how a persons body weight is transmitted to the snowpack with a cone of stress.

You mean stress bulbs? More recent work by Jamieson et al. is available on the ASARC video page hosted by Vimeo.

If I recall correctly, the research shows that it's pretty difficult for skiers to trigger weaknesses buried deeper than 1 metre. Snowmachines are another story of course.

By "at a more basic level", do you mean illustrating how dynamic force is applied to the snowpack and transmitted to buried weaknesses?

You mean stress bulbs? More recent work by Jamieson et al. is available on the ASARC video page hosted by Vimeo.

If I recall correctly, the research shows that it's pretty difficult for skiers to trigger weaknesses buried deeper than 1 metre. Snowmachines are another story of course.

By "at a more basic level", do you mean illustrating how dynamic force is applied to the snowpack and transmitted to buried weaknesses?

Yes, stress bulb or cone of stress. I think a diagram, if one could be found would be useful in this thread. I would apply it to questions 2, 3 &4, including the interesting topic of how the snowpack weak layers can stabilize and then subsequently be re-activated by additional loading or warming to the point where triggering is once again possible. I think understanding that process or those processes is crucial in the skiing game.

Here's a diagram of a stress bulb about halfway down the page: www.avalanche.ca/cac/bulletins/forecaster-blog

This is a video discussing the concepts: vimeo.com/29201289

including the interesting topic of how the snowpack weak layers can stabilize and then subsequently be re-activated by additional loading or warming to the point where triggering is once again possible. I think understanding that process or those processes is crucial in the skiing game.

If you read the research by Colbeck, McClung, Herwijnen, Heierli and others ( including McClung's extensive work on his UBC homepage ):

* At a fundamental level, avalanche formation is underpinned by a type of material failure called delamination.
* There are two models of why delamination occurs: McClung's shear fracture model and the anti-crack model proposed by Herwijnen et al.
* In any composite visco-elastic material with few flaws, the material is mostly stable across time and space during the application of dynamic force.
* Macroscopic flaws such as buried surface hoar create relatively large airspaces in the snowpack.
* From a composite materials perspective, these airspaces are flaws, which may or may not exceed the critical size required for catastrophic failure.
* For any flaw exceeding the critical size, failure is induced by applying force at a critical rate.
* Confusing the issue even more, failure of the material itself may or may not lead to catastrophic failure.

It's helpful to think of persistent weaknesses as persistent imperfections in the material. Sometimes these imperfections heal and sometimes they don't. Sometimes people trigger large avalanches, sometimes they don't. Klassen concludes that most people, including very experienced ski guides, are not very good at managing avalanche problems presented by deep imperfections. I tend to share his conclusion.

***

Do you think this clarification is helpful?

The persistence of flaws isn't particularly interesting or suprising or mysterious.

Buried surface hoar resists strength gains from overburden pressure and favourable metamorphic regimes because of its shape and its anisotropic strength characteristics. In addition, such weaknesses spend a lot of the winter being 'refrigerated' inside the snowpack.

For any flaw that exceeds the critical size, avalanche formation ( catastrophic failure ) is possible provided very simple energy requirements are met.

* en.wikipedia.org/wiki/Delamination
* en.wikipedia.org/wiki/Composite_material ( See Plywood )

If you read the research by Colbeck, McClung, Herwijnen, Heierli and others ( including McClung's extensive work on his UBC homepage ):

* Macroscopic flaws such as buried surface hoar create relatively large airspaces in the snowpack.
* From a composite materials perspective, these airspaces are flaws, which may or may not exceed the critical size required for catastrophic failure.
* For any flaw exceeding the critical size, failure is induced by applying force at a critical rate.

Do you think this clarification is helpful?

I think these three points have the most relevance for the skier who probably doesn't spend too much time thinking about "materials". The key is that there is a low bond density such that the structure of weak layers can easily be broken down. Essentially that is what these five sentences say.

As a skier I probably don't exactly care through which process the cracking begins and propagates. That is a worthwhile subject for a physicist who deals in materials.

By the way I don't think that the problem of dealing with weak layers is limited to guides. Evaluating a snowpack with known weak layers is tricky at best. All of my close calls since I had a modicum of experience have been with significant weak layers. In several cases I triggered slides at my feet, including remotely when I stopped because I didn't like the shape of the terrain. I have one friend who had a heli-operation in a dry part of the Purcells and described the snowpack as a "House of Cards". He left that operation because he considered it too risky.

This is a video discussing the concepts: vimeo.com/29201289

I think the video is very good and is something backcountry skiers on this website should look at in understanding the process of transmitting energy to the snowpack and also as regards snow testing methodology.

The bulb digram is also very good and I hope lots of people take the time to look at it.

A hoar Example: See Picture. This slide was triggered by a friend while guiding,  many years ago(this is why guides go first). Three weeks prior to this slide I had seen large crystal surface hoar growth that was confined to a certain elevation band (may have been due to stratus cloud, don’t remember).

Surface hoar crystals at specific elevations ( aka the bathtub ring ) are fairly common. Bruce Jamieson wrote a paper on this, but I can't find it.

I called my friend and we discussed the implication of these large crystals. I remember him telling me that they can become a persistent weak layer and would tend to resist overburden pressure. These crystals became buried for several weeks and persisted in this area (my speculation, fit profile, limited other likely explanation at time)  which is a protected, open to the sky, North aspect at around 5,800 feet elevation.

Yes, the persistent forms resist strength gains from overburden pressure because they have anisotropic strength characteristics. ( Weaker in shear than compression. ) This means that the pore size doesn't decrease very readily, except when the pack delaminates right before an avalanche. For more discussion, see above.

QUOTE BY COOKIEMONSTER; “If I recall correctly, the research shows that it's pretty difficult for skiers to trigger weaknesses buried deeper than 1 metre“

I thought this number was 2 meters? What about 2 or more skiers, skinning or skiing the same slope at the same time, same place? And then of course you have to consider this number invalid with new snow loading such as 1 skier+ depth and density of new snow.

Page 230 of The Avalanche Handbook ( 3rd Edition ): "However, when buried persistent layers are deeper than about 1 m, instances of skier triggering become rare, while snowmobiles may still trigger avalanches..."

Thanks for the kind words. Everyone here makes great contributions to the discussion.

From Cookie:

“If I recall correctly, the research shows that it's pretty difficult for skiers to trigger weaknesses buried deeper than 1 metre“

That is the number that is bandied about. It is thought that deeper slabs are triggered from areas where the weak layer is more shallowly buried. But I can say definitively that I've triggered a 4' slab which I described elsewhere and which I believe had as a weak layer a faceted suncrust. I've also remotely triggered a pair (nearly simultaneously) of 3 to 3-1/2' slabs on surface hoar. In both cases if I was in a location where the weak layer was buried less deeply when I triggered it, I didn't know it at the time.

I think it is a probability thing where the number of slides that can be triggered is something like three standard deviations from the norm at 3'. But there are undoubtedly outliers that are rarer that can be triggered at greater depths.

Page 152 of  Temper’s book places this number at 1.5 m(5 feet) with a caveat about triggering deeper  slabs from shallower areas. I believe that the problem with using any number is that they can become a“rule of thumb” and does not take into account the mean deviation that Gary points out above. My number will continue to be 1.5m+.5 m (error margin)=2m

I’ve also included an updated picture (bad light, no crown detail) of the Silver Star Mt. slides that I’ve been tracking this year. The path flanks were clearly visible this time and involve a large volume of snow. Also the slide tended to involve snow that was out of the path as it passed by and over the steeper rock bands. This indicates the presence of weak layers that fail as the anchors are removed by the avalanche.

Another implication of this slide path (or any slide path) is that there may be  now a shallower snow pack in the slide path, which may lead to future avalanche activity. Since the snow in path is thin, it is subject to temperature gradients that may disappear in the deeper, surrounding snow that did not slide.

Any way this is my understanding and any corrections or additional knowledge is welcome. One question I have is this; Does the energy input(frictional heat energy) generated by the avalanche destroy or modify the weak layer that was responsible for the slide?

In reply to your third point, in 1978 we had a helicopter out and were skiing north of Mt. Baker. Everything was fine until we skied a slope in the Damfino Creek drainage. The slope was a steep avalanche path and the snow structure on that one slope only was 18" of unconsolidated new snow overlying 3' of depth hoar. At that time I didn't fully grasp all the ramifications of depth hoar but did recognize it as such. I think the reason it didn't slide was that the new snow lacked any cohesion. But, obviously, skiing the slope made me very nervous. Had we been able to reasonably reverse course and walk back up, I would have done so. The main point is that the slope had obviously avalanched earlier in the winter; your point exactly.

To your fourth point, an avalanche is very likely to take out weak layers like facets or surface hoar, but the friction of an avalanche would likely replace those weak layers with a hard, smooth surface. This would be more obvious lower in the starting zone and in the track.

That is the number that is bandied about. It is thought that deeper slabs are triggered from areas where the weak layer is more shallowly buried. But I can say definitively that I've triggered a 4' slab which I described elsewhere and which I believe had as a weak layer a faceted suncrust. I've also remotely triggered a pair (nearly simultaneously) of 3 to 3-1/2' slabs on surface hoar. In both cases if I was in a location where the weak layer was buried less deeply when I triggered it, I didn't know it at the time.

Page 230 of The Avalanche Handbook ( 3rd Edition ): "However, when buried persistent layers are deeper than about 1 m, instances of skier triggering become rare, while snowmobiles may still trigger avalanches..."

I think it is a probability thing where the number of slides that can be triggered is something like three standard deviations from the norm at 3'. But there are undoubtedly outliers that are rarer that can be triggered at greater depths.

Where did you get this figure? Is this speculation, opinion, or is it sourced? This isn't a request for justification; just clarification.

Page 152 of  Temper’s book places this number at 1.5 m(5 feet) with a caveat about triggering deeper  slabs from shallower areas. I believe that the problem with using any number is that they can become a“rule of thumb” and does not take into account the mean deviation that Gary points out above. My number will continue to be 1.5m+.5 m (error margin)=2m

Using a 2 metre margin of safety is a rational individual choice, although it's probably too conservative for some people. I have a question: since 2 metres covers the entire depth of the snowpack in a lot of places, do you just assume that you can trigger an avalanche if there any weaknesses present?

Any way this is my understanding and any corrections or additional knowledge is welcome. One question I have is this; Does the energy input(frictional heat energy) generated by the avalanche destroy or modify the weak layer that was responsible for the slide?

It's just about impossible to say whether or not an avalanche will eradicate the weakness in which it formed. Sometimes it happens; sometimes it doesn't happen. Here's what I wrote in the April 2010 issue of The Avalanche Review ( for an article on this subject ):

***

The size of the forecast region is the most important factor, with precise answers only available for very small areas. However, even for small areas, the chaotic interaction between terrain and weather makes it difficult to predict the effects of widespread avalanching on future snowpack instability. The following scenario, which is just one possibility out of many, hints at the overall complexity of this forecasting problem.

Instability will persist when a bed surface composed of faceted crystals is immediately reloaded during a storm. On the other hand, future snowpack instability on that slope will be very different if the faceted crystals exposed by avalanching are subjected to multiple melt/freeze cycles prior to the next storm. Melt/freeze activity is often limited by aspect, so it is possible for the faceting process to continue on cold aspects, while faceted crystals on warm aspects undergo rounding as a result of melt/freeze metamorphism. In this highly general scenario, the weather builds new patterns of snowpack instability that are difficult to uncover without careful observations.

Therefore, for most recreational skiers, knowledge of a recent avalanche cycle is a very general and imprecise piece of information. General information often has a dangerous and unwarranted influence on individual or group beliefs about the presence of instability and its parameters. Without abundant information, expert knowledge, and significant experience (Randy and Nick provide great examples of this), a recent avalanche cycle should not exert undue influence on recreational travel choices and decision-making at any operational scale.

More than anything, incremental changes to the snowpack caused by synoptic scale weather events will alter the characteristics of the danger but won’t eliminate it.

* The chaotic relationship between terrain and weather is a primary source of uncertainty.
* Incremental changes to the snowpack are a primary source of uncertainty.
* Avalanches remove weak snow from some, but not all, slopes.
* Avalanches may or may not remove all the weak snow from a specific slope.
* Use multiple sources of information to determine the likelihood of avalanche formation.
* An avalanche cycle over a large area certainly does not mean a specific slope is safe.
* Proactively managing uncertainty is essential to safe decisions.

Wind reaches its maximum velocity at the ridge top, which gives it the most scour power. Unless I missed something, I'd expect to find sastrugi in those locations, not consolidated powder.

Your hypothesis about the relationship between energy and the apparent stability of consolidated powder over sugar snow is probably correct, but the line can be exceptionally fine.

Cookiemonster. Thank you so much for your reply to my question. Reading your stuff is like eating candy. In answer to your question. I always assume that I can trigger an avalanche in powder on a steep avalanche prone slope.

If the pack is only two meters deep and a weak layer is present, especially so.

Also, thanks for the kind words. Most of what I write is just a combination of research from other people!

I think regarding weak snow near cols that the aspect is what usually matters. Of course east aspects and most northerly aspects usually would be lee, so that leaves the other aspects. While southerly and southweasterly slopes near the tops of gullies and couloirs are often blown down to a shallow snowpack,  in my experience more often than not there is considerable strength even in faceted snow there because of solar influence and in the Cascades by wind driven rain that once refrozen adds strength. Westerly and northwesterly aspects get more interesting because the sun has less affect while the wind influence remains. Particularly in the Rockies and Purcells northwest slopes when they are loaded can be very problematic as they are often underlain by depth hoar. They would of course be loaded by easterly (less likely) or southeasterly (more likely) winds. The gullies and thin snowpack areas can also easily be cross-loaded by more southerly winds. Those aspects scare me in regions where depth hoar is often a factor or there can be a unique situation elsewhere.

I would also add that moraines because of the porous nature of the rock and because of the way they (moraines) stick out into the wind are classic locations for depth hoar in almost all mountain climates. The large moraine on the southerly portion of Spire Gully for example, has avalanched to the ground several Springs when the snowpack is not particularly deep.

Freeskiguy, a gut feel is that although these west and south facing gullies (as in the first of your last four photos) would slide with new snow instabilities they are probably unlikely to go as depth hoar slides until springtime when the faceted snowpack becomes very weak and isothermal. The gut feel is just based on the width of the starting zones and the amount of lateral anchorage in the gullies.