Can you test a snow bridge with avalanche probe?
May 1, 2023 at 10:24 am #3780061
Does anyone test snow bridges with avalanche probe before crossing? What length probe would you use if yes? Have some spring trips coming up so wanted to check what y’all do for snow bridge safety. Thanks!May 1, 2023 at 11:52 am #3780068
Visuals, instinct, experience, hard jab w small basket poles in front of each step attempting more force than my steps… any doubt, pick a better spot even if it involves a work around hikeMay 1, 2023 at 12:55 pm #3780070
stomp with your foot ahead of you to see if you can get it to collapseMay 1, 2023 at 3:07 pm #3780074
I would think an avalanche probe has such a small cross section that it could punch thru something that would support your weight… I prefer a trekking pole basket… but whatever… test your method on less serious bridges first… and be mindful that warmer days an afternoons bridges will be weaker… and… sometimes what looks like a thick bridge has been seriously erroded/melted from underneath and you can’t see that…May 1, 2023 at 4:04 pm #3780080
Thanks for the info guys. I’ll try to find something low consequence to test the approaches.
Regarding the avalanche probe I think the basic idea I’ve heard about is that the resistance reduces when you punch through the bridge thereby giving you an idea of the thickness. I was just pointed to this paper that describes this also https://arc.lib.montana.edu/snow-science/objects/issw-2006-193-202.pdfMay 1, 2023 at 4:50 pm #3780082
they’re talking about snow bridge over a crevasse where you might die if it failed
I was thinking more about crossing a creek where I don’t want my boots to get wet
A couple things in that article:
“Snow is one of the most brittle common substances, meaning it doesn’t stretch much
before it breaks. Fracture toughness of snow is directly proportional to the square of the density— twice as dense, four times as tough”
“One of the author’s biggest crevasse falls occurred trying to get a rappel rope unstuck. He had the brilliant idea of repeatedly flicking a loop of slack upwards, inadvertently bouncing on a snow bridge over a bergschrund. Dynamic forces matter! Skiing is an advantage in this regard, but walking or crawling gently can make up for that. When a climbing partner says, “Think light!” she means, “Place your boots down gently (slowly) and flat to reduce the impact”May 1, 2023 at 5:27 pm #3780089
Right … its mostly a mountaineering focussed paper. My intended scenario is avoiding falling into creeks flowing under snow bridges which is roughly similar I guess (except on a smaller scale)May 1, 2023 at 5:44 pm #3780090jscottBPL Member
@bookLocale: Northern California
There’s a true story about an experienced ranger plunging through a Sierra snow mass into a creek belowe–he may not have even known it was there–and then being washed some ways under the snow and perishing. After reading that, I started noticing similar scenarios around the edges of alpine lakes and snowed over meadows with fairly deep streams meandering through, etc. Sometimes there’s an extensive mound of snow that’s eroding from beneath hiding a lake edge, or again, one of those creeks or rivulets.
One Spring I recall running into a group heading over a very snowy approach to Muir Pass–snowed in for miles before and after the pass. The next day I followed on. There was a beautiful snow arch high up over a creek with a large empty space in the middle. I wondered if someone from that group had plunged through, creating the gap. It was about an eight foot drop. Not pleasant with a pack on.May 1, 2023 at 6:15 pm #3780099
Yea, it’s not just about getting your boots wet… a, you can break a leg on the rocks below, b, you can get swept away by a raging ice water torrent below, c, you can get trapped in a ice water bath under there where no one can see you… that has happened as well as the ranger story above… a ranger that I once met:(((May 1, 2023 at 6:29 pm #3780101
Question… say the avalanche probe tells you how thick the snow bridge is…. do you know how thick the snow needs to be to hold your weight??? And how that will vary with the current temperature and the overnight temperature the night before, and how long since the current temp has been above freezing and sun exposure?? Seems easier to me just to use my poles out ahead of my steps to test…May 1, 2023 at 8:02 pm #3780105
It would help if there was a rule of thumb to follow. So let’s say we know with good likelihood that >6 ft is always safe and < 2ft is almost certainly not safe … the range of uncertainty is not that much. We could cross with good confidence of safety just with a quick probe (in theory)May 1, 2023 at 11:22 pm #3780120Bruce TolleyBPL Member
@btolleyLocale: San Francisco Bay Area
“My intended scenario is avoiding falling into creeks flowing under snow bridges which is roughly similar I guess (except on a smaller scale)”
Since you are asking this question in spring, I assume this is a spring, early summer question about backpacking snow travel over backpacking trails or cross country terrain. I don’t think an avy probe will help or any rules of thumb, You could try reading the terrain and determine where a stream/arroyo might be. Or look carefully at your map for streams under the snow.
If you are really worried about this 1) carry snow shoes to increase flotation or 2) always cross snow during the morning when it is firm.
One Memorial Day weekend in the Snow Mountain Wilderness in northern Calif I started to post hole in the late afternoon, then fell through the snow bridge into a creek. I had to quickly find a place to dry out to avoid getting cold as the sun started to get lower on the horizon….May 2, 2023 at 7:12 am #3780129
my rule of thumb is
1… in the Sierra this spring, don’t do trips that involve major crossings…
2… if I must do crossings, don’t do them on extra warm days… don’t hike those days this spring
3… if you absolutely must cross, do it in the morning… and check every step before with poles vigorously…
Somebody is going to die out there this spring… avoid these conditionsMay 2, 2023 at 7:37 am #3780132
“1) carry snow shoes to increase flotation”
that would help to avoid postholing into the creek
in that article, he gave an example of someone putting on their skis to spread the load out but it was counter productive – it didn’t help avoid the large structure from collapsing, then the skis made it more difficult to survive the fall or arrest the partner’s fall. If you’re being washed down a stream, having snowshoes on would be more difficult.May 2, 2023 at 7:41 am #3780133
“he may not have even known it was there”
“After reading that, I started noticing similar scenarios around the edges of alpine lakes and snowed over meadows with fairly deep streams meandering through, etc. Sometimes there’s an extensive mound of snow that’s eroding from beneath hiding a lake edge, or again, one of those creeks or rivulets.”
That seems key to me – constantly try to figure out where the dangerous places might be. When there’s a snow bridge take your time to find best spot to cross, see how thick it is,…May 2, 2023 at 8:01 am #3780135Bruce TolleyBPL Member
@btolleyLocale: San Francisco Bay Area
@ DWR D
You are absolutely on target. I was thinking of streams you could not see. But upon re-reading the first post, the OP indicates using snow bridges to cross streams that are visible.
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