avalanches at lincoln falls?
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I'm mostly curious about the approach since there isn't really a slope above the flows. I figure persistant slabs won't be a problem in the boulder field below the flows because the depth hoar will be below the rocks. Caic says that there's risk of cohesive slabs sitting on surface or near surface hoar. I'm newly avy trained. Is this a no go situation this weekend? |
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You'll have to make that assessment when you get out there. For what its worth, I did email CAIC and they said that to their knowledge there has been no slides that they know of occurring on the approach through the boulder field . But that doesn't mean there won't be. |
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35-45 degrees is the *typical* angle for slides. Ive watched videos where slides happen on what looks like really mild slopes. |
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If historically the slope doesn't slide, there could be a reason for this worth noting. People want to climb ice there as long as it exists, and that means all winter. Avalanche mapping relies on historical slide information. |
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Stitch I agree. I would probably just stay away from any avalanche prone or potentially avalanche prone areas this weekend. |
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Well, in the case of the approach to Lincoln, I think the underlying ground surface is what matters the most. Places like Kelso mountain where slides are common have dead grass and fairly smooth terrain under the snow. Lincoln has a jagged boulder field with fairly large boulders. This I think holds the snow fairly well. If a slide is going to occur, it will have to have a snow base to slide on. But it's definitely a complex question. |
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This pic from last year shows wet slide activity on the approach slope above the trail through the trees. |
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That's historical evidence to me, then. I would get CAIC reports before heading out to Lincoln Falls. Good call by the OP to question this slope in the first place. |
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Just for the sake of follow up. |
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Stich wrote:Well, in the case of the approach to Lincoln, I think the underlying ground surface is what matters the most. Places like Kelso mountain where slides are common have dead grass and fairly smooth terrain under the snow. Lincoln has a jagged boulder field with fairly large boulders. This I think holds the snow fairly well. If a slide is going to occur, it will have to have a snow base to slide on. But it's definitely a complex question.This is the sort of reasoning that can get you into serious trouble. Yes, a grassy slope is more prone to full depth glide releases in the spring. However, a steep rocky slope can be particularly dangerous when persistent/deep slab is the problem. Near surface facets (which later become depth hoar) like to form around rocks where the snowpack is thinner (and hence the temperature gradient is larger) and buried rocks can provide a shallow spot on the slope from which it is possible to collapse an otherwise too deep weak later. Also, when it comes to surface instabilities such as wind slabs or storm slabs, it doesn't matter what the ground surface is. |
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I was not aware of that! Thanks for the additional information. |
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Mike Marmar wrote: This is the sort of reasoning that can get you into serious trouble. Yes, a grassy slope is more prone to full depth glide releases in the spring. However, a steep rocky slope can be particularly dangerous when persistent/deep slab is the problem. Near surface facets (which later become depth hoar) like to form around rocks where the snowpack is thinner (and hence the temperature gradient is larger) and buried rocks can provide a shallow spot on the slope from which it is possible to collapse an otherwise too deep weak later. Also, when it comes to surface instabilities such as wind slabs or storm slabs, it doesn't matter what the ground surface is.That's what I was most worried about since the CAIC did mention the possibility of near surface facets. I agree with Stich though that I wouldn't worry about a true persistent slab that forms early in the season because of the size of the rocks and how far down the surface hoar would be. Lincoln seems to have big enough rocks that it hasn't snowed enough yet to make a smooth surface. I did check as I went if there was strong on weak layers as I went along (with fists and ski baskets). I also did some cuts/switchbacks to test if there was cohesive/reactive snow. I don't know you but it sounds like you might know something about assessing avalanche activity. Do you believe that this was an accurate way to assess this slope? |
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Tom-onator wrote:This pic from last year shows wet slide activity on the approach slope above the trail through the trees. The fact that these sparse trees are missing limbs and bark along their upslope aspects would indicate these slides happen occasionally.BINGO! Obviously the CAIC does not check the true benchmark (tree flagging) for historical data. Classic avalanche terrain, weather, snow pack. |
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Kevin Nicastro wrote: That's what I was most worried about since the CAIC did mention the possibility of near surface facets. I agree with Stich though that I wouldn't worry about a true persistent slab that forms early in the season because of the size of the rocks and how far down the surface hoar would be. Lincoln seems to have big enough rocks that it hasn't snowed enough yet to make a smooth surface. I did check as I went if there was strong on weak layers as I went along (with fists and ski baskets). I also did some cuts/switchbacks to test if there was cohesive/reactive snow. I don't know you but it sounds like you might know something about assessing avalanche activity. Do you believe that this was an accurate way to assess this slope?The extent of my experience is an aiare avy 1 course, a couple of years of backcountry skiing experience and a nerdy interest in avalanches. I would not say I am an expert by any measure. If the rocks are above the surface then they can act as anchors on the slope. They have to be pretty densely distributed to prevent slabs from forming however. Close enough that the slope would be annoying to ski is a good rule of thumb. Evaluating the snowpack as I go with hand pits, pole probes and switchbacks is exactly what I do to asses surface instabilities. If persistent slab is your problem then those tests give you little information. Unfortunately, short of a cornice drop or a bomb it's really hard to evaluate persistent slabs. That picture certainly looks like prime avalanche terrain. I am maybe more conservative than many but I would probably avoid that area entirely on a considerable day for that aspect or even moderate if the danger is persistent slab. |
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Mike Marmar wrote: That picture certainly looks like prime avalanche terrain. I am maybe more conservative than many but I would probably avoid that area entirely on a considerable day for that aspect or even moderate if the danger is persistent slab.Look, I'm not telling you all not to be vigilant, but if Lincoln Falls had avy danger on even the high-moderate end, you would know about it. Why? Because that ice gets climbed for sure every weekend day and probably damn near every day, snow or shine, all season. That doesn't mean it can't, but the probability is pretty small. I have certainly been there when slides were happening on a good chunk of terrain surrounding the approach and falls area, but nothing on the approach itself. In fact, considering the death trap conditions that regularly occur nearby on the routes near the Shroud etc., I think Lincoln Falls is considered by many to be fairly safe area. |
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Just because it hasn't killed anybody doesn't mean that it won't. |
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Kevin Nicastro wrote: I figure persistant slabs won't be a problem in the boulder field below the flows because the depth hoar will be below the rocks. Caic says that there's risk of cohesive slabs sitting on surface or near surface hoar.Huh?? I'd go back to the book on this one. I think you have some confusion on snowpack. Do you you have "staying alive in avalanche terrain"?' Also I would place some concern on what is ABOVE you as well. It's my understanding that the Lincoln Amphitheater above the falls is the slide path. |
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Hiro wrote:Just because it hasn't killed anybody doesn't mean that it won't. Last season I was up there trying to approach on a day when everything on the CIAC report was red and it was storming. I retreated.Yeah, this is reasonable. If it dumps 5 feet up there in a day and the avy danger everywhere is extreme, then yeah, go ski somewhere safe. But days like this are in the huge minority. Again, I am not trying to tell people not to be vigilant, but I sort of feel like the tone of this discussion is treating Lincoln Falls like its the amphitheater above the Ribbon or something similar, which its not. Scott McMahon wrote: Also I would place some concern on what is ABOVE you as well. It's my understanding that the Lincoln Amphitheater above the falls is the slide path.You are sort of correct there. That is, the big avy hazard on the mountain itself is up in the bowl. But the danger is waaay up in the bowl. The little slope above the falls is too low angle and with not enough loading of snow to slide over the falls themselves. |
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Scott McMahon wrote: Huh?? I'd go back to the book on this one. I think you have some confusion on snowpack. Do you you have "staying alive in avalanche terrain"?'Do you mind explaining more? |
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Kevin Nicastro wrote: Do you mind explaining more?How how are you getting depth hoar below the rocks? In the rocks? Are you saying it won't slide because it's a boulder field with soft snow on top? It also doesn't have to be just hoar to slide. It can be a weak layer between strong layers (slab), facets, ice crust, sun crust, rain crust, hard pencil wind layer etc. What are you basing the lack of persistent slabs on, it being fluff on top of boulders? Does it have to be persistent to slide (NO). Also even when you can see grass, you can still be slid on. Just because a slope is anchored doesn't mean their isn't a slab issue, it just means you will have a more difficult time triggering it. Regular slabs will be the majority of your problems on a given day, not just persistent. Persistent layers can be difficult to trigger, and sometimes have to step down to make that even occur. If you are finding 2 feet of fist on top of large boulders then there won't be a persistent layer deep down. Also I didn't understand this "I hadn't noticed this the last time I was up there but the angle of the slope (exponentially steeper) also led be to think that it's a pretty well supported slope, even without the boulder field anchoring it" Why would an exponentially steeper slope be better supported? Also remember that rocks and trees can also be trigger points since they retain heat from the day. Plus they need to anchor up through the layer if it's there. If anything a steeper slope will run more. I'm no expert by any means, just some things to think about. Maybe I'm reading your analysis wrong. fsavalanche.org/anchors |