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Topic: Avy Hazard and Decision Making (from Cement Basin post) (Read 2745 times)
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CookieMonster
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Thanks for the feedback on the crust, interesting stuff. As for how density effects a skier in an avalanche, this is most definitely not simple physics, as I don't think some of the assumptions are valid: * i'm not sure the 'granular convection' referred to is valid. -i'm not sure this phenomenon exists where there is lots of turbulence -i'm not sure what the effect is when there is only one large object - when one thinks about a cereal box, small particles filter down through the cracks between large particles. But when there is only one large particle and no walls to the 'container', I would expect this effect not to exist, or at least not be as pronounced. *I would assume the density of dry moving snow, of 0.1-0.2 here, as once the snow is 'set up', the density doesn't matter as your stuck... In this low density, turbulent environment, I would expect bulk to have a bigger effect than buoyancy. Its like having two rocks of similar mass where one is larger than the other bouncing around the bottom of the river. I would argue the bigger rock is going to get more beat up. When you go to an airbag however, there is a huge reduction in density, only 1.5-2x that of moving snow as opposed to 4x-8x with pack & 5x-10x without. *Further to the last point - if a person is on their back, stroking with your arms with snow pouring over you, the frontal area top down is ~2 sq ft. With a backpack, its easily twice that, doubling the force pushing the body around. As an aside, when the avy airbag report came out last year, I did a quick analysis, and it seems their effect is so pronounced, it is statistically significant even with a tiny sample: https://docs.google.com/spreadsheet/ccc?key=0ApQmMKX-fzY_dExfdk5yNi1sYzB3em5IVFcyNEk1QXc&authkey=CJj5u-8B&hl=en_US&authkey=CJj5u-8B#gid=0
* Forgive me for being blunt: your post contains zero facts about dry snow avalanches.
* The Avalanche Handbook describes the very effect you say does not exist. ( Smaller particles ending up at the bottom, larger particles on top. )
* It's pretty hard to take your post seriously when it doesn't even acknowledge the fact that avalanches have different flow regimes and varying densities ( dense granular core, saltation layer, etc ).
* It's silly to let the non-facts, assumptions, opinion, and speculation in your post masquerade as facts whilst proposing an argument.
* Connect anything you've written to a single piece of published research on avalanches.
So if it was below freezing when it started snowing, and there was wind blowing snow onto the slope and consolidating it, what evidence was there that the bond to the crust was good? I'd like to understand why you guys think the avy danger was moderate/low when I can't figure out how its possible to tell how well the bonding was here.... I understand why a few tracks down the slope was re-assuring, but other than that, was there anything else?
* You were there n'est-ce pas?
* Arguing for the sake of argument?
* Asked and answered?
Your right jtack, not every post needs/deserves analysis, commentary, critique. But this one has evolved, maybe for everyone's benefit?
Respectfully Cookie and Amar, I differ with a couple of your conclusions.
1) Surface observations, and photo's supporting, don't tell a full story. Seeing "well consolidation" from turns might just as well suggest a soft slab forming through settlement as well as stability, lacking other signs or tests. And a slab over either a smooth or rough weak crust is not a particularly good indicator of stability.
* It's okay to differ. Here are some things to think about:
* What was the load on the weak layer?
* Avalanche formation requires catastrophic delamination, which less likely on a very rough surface.
2) Pete's CT test, a 7 with "fast failing on the rotten snow immediately under crust that the new snow had fallen on" tells me more. Regardless, or perhaps because of, the bond between the new snow and crust surface, I am weighing in in support of Pete's conservative analysis and take on instability and thus their skiing more moderate slopes given all of their observations.
* What does Pete's CT test tell you?
3) Is Bullion Peak, with its north face exposure, really that similar to East Peaks east and north east exposures at EPk's slightly higher altitude? Had the "moderate" winds gusting to 65 mph, out of the southwest to west, over on top of Crystal the day before, effected Bullion Basin the same as the east slopes of East Peak. Granted, as Cookie pointed out, none of the pictures showed evidence of wind affect, although from the snow flying from skis, it wasn't light powder, but somewhat heavier. Could you know that ahead however, before pushing off onto those east slopes? Maybe, maybe not.
* Yes, you could know beforehand.
* Look at the snow on the trees.
* Look at the snow surface.
* Look for drift lines.
* Check ski penetration for variation in snow depth.
4) Does telemetry alone ever give us 'good to go'? With new snow and 'moderate' winds, gusting to 65, over some hours, why wouldn't one expect instability and look for it. Only if all your observations indicated otherwise, right?
* The telemetry correlated with conditions in the field.
3)Both are tests. Both subject to spatial variability. Both subject to the variability's that go with location, aspect, contour, tester, etc. It all depends on what you're looking for the 'tests' to do - give you the go ahead to ski the slope, or give you pause to consider. The problem with using skiing as a test, is that if your test indicates instability, well, you're in an avalanche. Most experts look for test evidence to verify possibility instability, rather than stability. Pete chose to back off rather than do more tests after he found suggestions of instability.
* Some people have more uncertainty than others. This is perfectly normal.
* It's perfectly valid to issue a backcountry avalanche forecast of "LOW" danger.
4)I can't differ strongly enough on this point. Old tracks, new tracks, if there is an instability question, the newness of tracks means nothing. It could mean you're more likely to find a weak spot triggering a slide, if for no other reason than we tend to want new tracks, not over or on top of the previous, so we are unintentionally testing more terrain for instabilities, increasing our probability of finding them if they exist.
* Or the snow could be well-settled powder that doesn't want to move.
If you remember, it wasn't that many years ago, in December on today's date, and not far away on the same greater slope, that the 7th skier in a group of 7, as the last skier to start his descent, triggered a slab catching four skiers, partially burying 3 and totally burying 1 - who, despite heroic efforts on the part of a partially buried partner to dig himself free, locate, probe and begin digging for that skier, was not recovered alive. And this is the only reason we're having this discussion - to maybe help prevent it from happening again, anywhere. We're really only skiing to have fun.
* It sounds like your uncertainty would have been higher. No problem, just make conservative decisions.
I want to add something else. It's been interesting to watch the more conservative folks argue directly against very plain facts. This indicates a form of bias that can be best be summed up by "reluctance to change one's mind in light of new evidence". Please think about how this could affect your decisions in the field.
And no, I'm not joking when I make that suggestion. The Avalanche Handbook specifically discusses this form of bias.
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Eli3
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Dearest Dr. C Monster;
Please see highlighted passages below... I was agreeing with amar for the most part, I wasn't clear in that I was trying to indicate where I thought his assumptions don't hold up in pointing out that slight differences in density of a skier can't be modeled with 'simple physics'. I was very clear in that I was challenging assumptions, and by definition, assumptions are not facts. Furthermore, these 'challenges' are along similar lines to your post in that avalanches are an incredibly complex flow phenomenon, and making postulations about what may happen with slightly varying densities is probably not sound.
Thanks for the feedback on the crust, interesting stuff.As for how density effects a skier in an avalanche, this is most definitely not simple physics, as I don't think some of the assumptions are valid:* i'm not sure the 'granular convection' referred to is valid. -i'm not sure this phenomenon exists where there is lots of turbulence -i'm not sure what the effect is when there is only one large object - when one thinks about a cereal box, small particles filter down through the cracks between large particles. But when there is only one large particle and no walls to the 'container', I would expect this effect not to exist, or at least not be as pronounced. *I would assume the density of dry moving snow, of 0.1-0.2 here, as once the snow is 'set up', the density doesn't matter as your stuck... In this low density, turbulent environment, I would expect bulk to have a bigger effect than buoyancy. Its like having two rocks of similar mass where one is larger than the other bouncing around the bottom of the river. I would argue the bigger rock is going to get more beat up. When you go to an airbag however, there is a huge reduction in density, only 1.5-2x that of moving snow as opposed to 4x-8x with pack & 5x-10x without. *Further to the last point - if a person is on their back, stroking with your arms with snow pouring over you, the frontal area top down is ~2 sq ft. With a backpack, its easily twice that, doubling the force pushing the body around. As an aside, when the avy airbag report came out last year, I did a quick analysis, and it seems their effect is so pronounced, it is statistically significant even with a tiny sample: https://docs.google.com/spreadsheet/ccc?key=0ApQmMKX-fzY_dExfdk5yNi1sYzB3em5IVFcyNEk1QXc&authkey=CJj5u-8B&hl=en_US&authkey=CJj5u-8B#gid=0
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~Link~
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This is simple physics, which is probably the reason that it is so poorly understood (blame the terrible state of science education in the US)
Hear he, hear he.
There is also a secondary effect, which is that larger objects tend to sort themselves to the surface of a shaken mixture of solid particles of similar density due to packing (like shaking a box of cereal, the large unbroken pieces end up on top, smaller bits go down). So anything which makes you bigger and bulkier (like a backpack) will help slightly in this manner too, although this effect is less important than pure flotation (since an avalanche behaves more like a flowing fluid, and less like an agitated mixture of solid particles).
...So the daypack reduces the density of the human+pack combo by 20% relative to the human alone, the overnight pack by 30%, and the airbag pack most of all. All of them will float better in flowing avy debris than a human body alone.
Joe Gnarly Powder-Pig is not only after first tracks, but also Wheaties cereal sponsorship! It all makes sense now!
Continue your conspiratorial undermining of public education within the US, McGraw-Hill. You Multinational Corporation SOB, you...
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natefred
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I am incredibly surprised by the arm-chairing in this thread.
Conditions were obviously stable. The evidence shows this: from telemetry to field obs to the skiing and the photos. How much explanation is required when the essential difference is that some people are more uncertain than others?
All I've noticed in this thread are some reasonable questions raised and some really informative responses. It was stable, OK. I don't think anyone's disputed that, people are just asking some questions to see how that conclusion was drawn. Your assertion that from the telemetry and some pictures that anyone could see that it was stable is the worst case of arm chairing I have noticed in the thread.
Everyone posting seems interested in furthering their understanding, and thankfully are willing to speak up when they disagree with the advice or reasoning offered, or have been taught something different. I haven't noticed a critical tone in any of the posts, just attempts to understand the responses offered and a healthy dose of questioning some very confidently made assertions. I think most of us are in the position of not being nearly as confident in our stability assessments and we are very interested in learning how others are able to increase their confidence.
If some of the statements made had gone unchallenged, I would have started wondering if anyone was paying attention. I doubt that Amar or expected his responses were going to end the conversation, I hope we can let it play out among those who are interested in it.
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CookieMonster
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Dearest Dr. C Monster;
Please see highlighted passages below... I was agreeing with amar for the most part, I wasn't clear in that I was trying to indicate where I thought his assumptions don't hold up in pointing out that slight differences in density of a skier can't be modeled with 'simple physics'. I was very clear in that I was challenging assumptions, and by definition, assumptions are not facts. Furthermore, these 'challenges' are along similar lines to your post in that avalanches are an incredibly complex flow phenomenon, and making postulations about what may happen with slightly varying densities is probably not sound.
I prefer Mr. C Monster! I wasn't trying to be an *** ( although I'm certainly good at it ), I was simply pointing out the absence of facts in your post. Nothing personal; I do this all the time.
I was trying to indicate where I thought his assumptions don't hold up in pointing out that slight differences in density of a skier can't be modeled with 'simple physics'.
Sure... and it's perfectly fine to do so.... I'm not the thought police, but it's silly to use non-facts to point out where someone's assumptions don't hold up. Here are some non-facts:
i'm not sure the 'granular convection' referred to is valid.
-i'm not sure this phennomenon exists where there is lots of turbulence
-i'm not sure what the effect is when there is only one large object - when one thinks about a cereal box, small particles filter down through the cracks between large particles. But when there is only one large particle and no walls to the 'container', I would expect this effect not to exist, or at least not be as pronounced.
If you're not sure about granular convection, and you're not sure if granular convection applies to turbulent fluids undergoing strong shear disturbances, and you're not sure if a boundary changes the physics in an appreciable fashion... then on what exactly is your postulate or proposition based? The original proposal was that a backpack changes the buoyancy of a skier caught in a flowing avalanche.
Shouldn't one understand the basics of a flowing avalanche before proposing that someone else's argument is wrong? You'll notice that I haven't weighed in on the backpack proposition at all... why do you think that is? ( HINT: It's because I don't know. )
Again, it's just a question!
All I've noticed in this thread are some reasonable questions raised and some really informative responses. It was stable, OK. I don't think anyone's disputed that, people are just asking some questions to see how that conclusion was drawn. Your assertion that from the telemetry and some pictures that anyone could see that it was stable is the worst case of arm chairing I have noticed in the thread.
Everyone posting seems interested in furthering their understanding, and thankfully are willing to speak up when they disagree with the advice or reasoning offered, or have been taught something different. I haven't noticed a critical tone in any of the posts, just attempts to understand the responses offered and a healthy dose of questioning some very confidently made assertions. I think most of us are in the position of not being nearly as confident in our stability assessments and we are very interested in learning how others are able to increase their confidence.
If some of the statements made had gone unchallenged, I would have started wondering if anyone was paying attention. I doubt that Amar or expected his responses were going to end the conversation, I hope we can let it play out among those who are interested in it.
Yes, there are two sides to every armchair. I'm certainly not against curiousity, but again, it's interesting to watch people argue in the face of very plain facts.
In re: CookieMonster's alleged arm-chairing. Pointing out observations and facts isn't arm chairing; you'll notice I didn't question the OP or Amar's decision making, reasoning processes, or choice of objectives for the conditions.
It's worth asking: how much explanation is required when the essential difference is that some people are more uncertain than others?
Think about that question... no really... THINK about it.
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« Last Edit: 12/30/11, 02:00 PM by CookieMonster »
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GerryH
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Now that its again snowy, we can all go back to some great skiing. Looking forward to the outrageous posts in the next few days.
Of couple of quick last comments. It seems your last several questions Eli3 were well answered by Amar. Amar is spot on in describing the value of keeping the pack on in an avalanche. The increased volume hypothesis is also called the 'mixed nut' event. Shake the can of mixed nuts and the big ones always end up on top, regardless of their density/weight; so too in a running avalanche, which is considered a granular laminar flow, rather than a liquid (as we float when carried down the river). So keeping the pack on makes us a bigger nut in the can, in the flow, hence much more likely to end up on top. This is the exact theory of the airbag deployment, which both makes us rather instantly larger, and provides protection - to a limit. The people I've talked with and read of who've survived avalanches when carried into trees have usually spoken well of having their pack on to help reduce the impact - if they were lucky enough to hit back first. And most needed the supplies in their pack for first aid, survival and rescue, as well. There is a growing body of incident evidence showing likely improved survival rates in Europe of those caught in avalanches and deploying airbags. They won't help you however if blasted through the trees, caught in a really big avalanche or carried over, into & through other terrain traps (over cliffs, into rockfall or crevasses; there were a number of Euro's, including an entire family, who died wearing airbags last season in France.
Amar, the points you make re avalanche gear and other potential helpful survival gear are spot on. I think many of us wrestle with the same questions. Myself, as a habit, I always wear my beacon, even during the spring-summer cycle. I don't know, maybe I figure it will help someone find me if I plunge through a bergshrund, fall in a moat or creek, etc. I usually go to a shorter probe (lighter), and may even drag out my old 'plastic' Life Link shovel (whose blade has never failed, but handle has bent), because it's sometimes nice to dig a perch or lunch table. Your comments re other pack contents are well said.
To me one of the most important takeaways (in addition to what Cookie Monster just posted and said so well, while I was writing this) from this series is the communications, analysis and considerations that went on within Pete and Amar's parties, in making the decisions they did. Both made good decisions. If I was with Amar and party, I probably would have argued for digging a quick pit on a good spot before taking the plunge down the eastside of East Peak, but, that might just be me. I've certainly become a little more conservative, ski-wise, as I've gotten older. So I look forward to when we can ski together again Amar, we've always had fun and come back intact. Thanks for your patience with me in asking my questions, and for taking the time to respond and ask yours! Its been a good dialogue. I'll not start this again....... Cheers and a Happy New Years skiing to all. Gerry
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rj
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We were up there that day and and agree it was very stable, unconsolidated fluff on hard pack with no slab formation, as stable as I've seen it in 30+ days in the area. RJ
Well, if we're going to analyze the avy hazard and decision-making here, might as well link to the available forecasts and data for reference: Danger rose for all zones near and west of the Cascade crest:  |
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Scotsman
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I'm also not convinced how a wearing a pack would help flotation in an avalanche. I see how it would protect the spine, but the density of the pack is probably going to be denser than the snow in motion - while this would slightly reduce the skiers average density, it makes them heavier and bulkier, making it harder to stay on top of the snow. I guess i'd have to see statistics similar to the airbag ones to believe this.
You are wrong and Amar is right. It's physics and has nothing to do with being denser and heavier and sinking...it's all about being larger when wearing a pack. Research the physics of why avy air bags work to keep you on top and you will understand.
Amar is also right IMHO that previous ski tracks can be a sign of stability....good for you Amar for flying in the face of convention and dogma. Welcome to the dangerous skier club. 
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skimac
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Here are my thoughts they are mine alone. Some of us take less risk and are conservative. Some of us take more risk and are fine with it. I have been in both camps. As long as we don't injure each other life is good. As a member of this community I feel a responsibility not only to my skiing partners but to the rest of the community to carry my avy gear at all times. Most of my avy gear is not for my safety it is for the safety of others i.e. the community. I want to be able to help out if someone else happens to step on that sweet spot in a slope and gets an unexpected ride.
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« Last Edit: 01/01/12, 08:56 AM by skimac »
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Jeff_Ward
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This guy probably thought the recent ski tracks right next to his line were a good sign of stability. http://www.youtube.com/watch?v=6qVwIuznFW0
It's obvious that the skier triggered this slab from a shallow point in the snowpack (look at the video at 1:25) but it would have been difficult to know the buried rock rib was there prior to the avalanche.
In my opinion, recent tracks may be a good sign of stability when dealing with storm slab or loose snow but applying this logic to other types of instability could get you into trouble.
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garyabrill
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This guy probably thought the recent ski tracks right next to his line were a good sign of stability. http://www.youtube.com/watch?v=6qVwIuznFW0It's obvious that the skier triggered this slab from a shallow point in the snowpack (look at the video at 1:25) but it would have been difficult to know the buried rock rib was there prior to the avalanche. In my opinion, recent tracks may be a good sign of stability when dealing with storm slab or loose snow but applying this logic to other types of instability could get you into trouble.
Good thoughts, Jeff.
I wouldn't argue that this slope, the "Hourglass" wasn't reasonable on this day given first hand accounts of a relatively shallow amount of recent reported "unconsolidated snow" overlying a crust that likely limited the depth of any potential slide. I've skied it once myself in similar conditions, though later in spring.
I would say, though, that this slope is a great example of an extremely high consequences slope, one that if one were to make an error in judgement, is likely to be injurious or deadly. It is a gereat example of a slope that one wouldn't want to make a habit of skiing.
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