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