Skier Compaction

[Z]

Just wanted to start a thread/hear some opinions on compaction in slide terrain.

Say a slope is totally tracked out, no pockets of fresh snow left...Open bowl, 45 degrees, prime slide terrain...14 inches of fresh fall quickly, just to increase the danger....is it possible to trigger anything more then soft snow/sluffs, could any deeper layers be involved in a slope that is totally tracked??

Is there any amount of compaction/tracks throughout the winter that you would consider a slope to be stable no matter of additional loading...for example you ski the same slope all winter long, and see the amount of skiers/compaction.....

I know you should still dig your pit/see whats going on for yourself....just wanted to hear some opinions.....

[AltaPowderDaze]

good one Z. i hear all sorts of theories on skier compaction and we'll get back to that in a second. for now, i'll try to answer the question asked. a layer of facets or surface hoar can remain dormant under the snow until it gets enough weight on it to slide. that means that no matter how many people have skied the slope before the storm you can still trigger a slide if the snow was heavy enough to affect that weak layer. a good way to test this one is to dig a pit, isolate a column and add 12"x12" blocks of cohesive snow on to the column until a layer fails (estimate the density/weight of the blocks). it should give you an idea of how much snow you need to reactivate the layer. as a for instance, look at the dutches draw avy over here. it gets tracked out all the time but two layers that have been under the snow since november and december respectivly ended up failing. mostly it was the december layer bit it did step down. those layers would have settled out again if they were given time to adjust to the added weight of the new snow. it may have taken a week or so but it would have gained strength with every passing day.

ok, now back to the skier compaction part of your question. my theory on this one is that as the snow falls most wind slabs get cut up and can't strenthen to the point of sliding immediatly. also some skiers may affect the buried surface hoar or near surface facets and lessen the chance of propagation across the whole slope. with skier compaction i'd think that near surface facets will also have less chance of really getting started because of the loss of porous space for them to form. instead of a fist hard layer on a bc slope you may find a 1 finger hardness on a skier compacted slope. this means it is a denser slab sitting on the interface and less space for those suckers to form. that part is always hard for me to think through so if it doesn't make sense then i'll try to explain it better. maybe someone else from a patrol background would have better knowledge of this.

[splat]

What's the time needed for a hoar to form that can get covered with new snow and then be triggered later, APD?

[AltaPowderDaze]

splat, it depends on how strong the temperature gradient is. i am a little unclear on the time line for some of this but basically around here i see a noticable layer of near surface facets on the north aspects within a week of clear cold nights. i think this process is highly variable with a lot of factors to consider but typically anytime you see surface hoar forming you can bet that the near surface facets are starting up. this has a lot to do with water vaper and such but usually if you are getting a temperature gradient of more than 1*c per 10cm they you have facets in the making. if you have some more questions on this i can look some of it up to give you a better answer.

[AltaPowderDaze]

oops, i just reread your post. looks like you were talking about surface hoar. that's an easy one. one night of clear skies and cold temps with some humidity can take care of that. if no wind, sun or rain destroys the frost/hoar then the snow falls and you will see a weak interface between the new snow and the hoar. if it holds in place and the new snow settles out you'll notice the surface hoar during a shear test or compression test but not likely with pole probes and such. next storm that has about 1" of water weight should get that layer reactivated and moving with some skier help.


look here (http://tetongravity.com/forums/showthread.php?t=22896) also.

[splat]

That's kinda what I was talking about - whether one night's fomation of hoar on the surface, covered by fresh snow on a well-skied run could be enough time for a potential weakening that would cause that new snow to release.

In a week of cold temps, you could be skiing the hoar, listening to it do the crystal tingle and it would be pretty obvious what happened. But I think what Z is asking is if a regularly-skied slope could get dangerous in the course of a day or two, or overnite. Z?

[powderfarmer]

Just witnessed the surface hoar formation this week while on a hut trip. Three days of 4-6 inches a day, nice snow. one clear night, got down to around zero. on the skin out with sherpastyle, there was surface hoar realatively advacned, everywhere. it was snowing at dinner time the night before mind you. I would say the crystal size was 15 to 25 mil. Pretty cool stuff, but scary since every pit we dug in the days before was sliding on a surface hoar layer buried under the 12-18 inches of fresh.

[AltaPowderDaze]

14 inches of fresh fall quickly, just to increase the danger....is it possible to trigger anything more then soft snow/sluffs, could any deeper layers be involved in a slope that is totally tracked??

Is there any amount of compaction/tracks throughout the winter that you would consider a slope to be stable no matter of additional loading...


to answer question 1, 14 inches of average density snow (6-10%) is about 1" of water weight. that is when you start to think about the layers that are already down there and any wind that may have put more weight in any one spot.

qustion 2, just look at a resort and watch patrol bomb slopes that are constantly skied but still produce slides. we had high rustler fully moguled come down one early spring.

there are always places that you can make continental/intercontinental a snowpack fail when enough weight is added (relative to dry snow failure). the trick is to wait for it to adjust to the new weight or let it run naturally.

[splat]

A case to consider might be the slide that took craig kelley. I believe everyone thought it would have consolidated, given the time it sat. But a bad layer down low finally went. Correct me if I'm wrong, because I don't recall the specifics and could be reaching here.

[AltaPowderDaze]

Durrand PDF (http://webpages.charter.net/pmillar/Durrand%20Report.pdf)

[splat]

That's right, a lot of stuff was sliding up there that year and was prime to slide. A lot of lessons in that report.