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Gorms
Joined: 06 Dec 2004
Posts: 381
Location: Under the Qwest Building
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 Posted: Thu Dec 23, 2004 5:42 pm Post subject: |
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| Thanks Snowman....I got it. |
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tele mark
Joined: 06 Dec 2004
Posts: 770
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| Posted: Thu Dec 23, 2004 9:22 pm Post subject: |
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| Ava Blanche wrote: |
It is very important to note that the graph above refers to slope angle versus avalanche accidents; not slope angle vs avalanche occurrence, which is what the original question asked.
I don't have access to the Tremper graph, but it documents over 800 avalanche occurrences and, although avalanches are rare below 30 degrees, they certainly aren't non-existent. So the general rule is start to use caution if you are on a slope of more than 25 degrees, or exposed to one that has more than a 25 degree angle. |
The graph I referenced correlates actual av occurences to slope angles. Probably the same one Tremper references, however since I haven't read his book I can't say for sure (the scatter plot in the Av Handbook comes from Ed LaChapelle's research in Alta circa 1950s-60s).
Good point Gary on Considerable meaning human avs. likely, naturals possible. I've found in my own experiences traveling on considerable hazard days I will experience collapsing on slopes down to 30 degs., however the combined load is still not significant enough to produce a slide unless the slope angle exceeds 35 degrees. On high hazard days adding a human trigger drops that angle down to 30 and significantly increases the frequency of slides as well. Not that this gaurantees safety, just an observation around reducing risks and minimizing the odds that you'll trigger a slide.
Good discussion of Alpha angles as well. WHile spending a day touring w/Tremper he mentioned different alphas depending on continental or maritime packs. I could see other factors such as vegatative coverage impacting the alpha as well. |
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Ava Blanche
Joined: 06 Dec 2004
Posts: 268
Location: Beautiful British Columbia
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| Posted: Thu Dec 23, 2004 9:48 pm Post subject: |
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On the subject of Bruce Tremper, if any on you have not yet gone through the superb avalanche tutorials that the Utah Avalanche Centre has produced, go to http://www.avalanche.org/~uac/ava_edu.html right now!!
The specific graph of slope angles vs avalanche occurrences is available at http://www.avalanche.org/~uac/Common-questions.html#DANGER ; if possible, could someone drop that graph directly into this discussion (I can't figure out how to do it on the computer I'm using).
The business of really focussing on slope angles, and carrying a clinometer to do this more accurately, has been known for a long time. The reason for this is simple: the force that is trying to pull the snow slab down the hill is equal to the weight of the slab x the sine of the slope angle; so the greater the angle, the greater the force. This may seem simple enough- but for you physics types, it is worth noting that there is a very real physical reason for carefully watching slope angles.
The business of using alpha angles (specifically, an alpha angle of 25 degrees) is relatively new and isn't mentioned in many of the current avalanche books, or taught in courses. I'm not saying you heard it here first, but it is a very useful guide for staying safe that has been introduced to a lot of backcountry skiers on this forum.
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frank
Joined: 06 Dec 2004
Posts: 917
Location: München
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| Posted: Fri Dec 24, 2004 12:55 am Post subject: |
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Here you go!
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Kees
Joined: 06 Dec 2004
Posts: 324
Location: Nederland
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| Posted: Fri Dec 24, 2004 7:55 am Post subject: |
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This graph of course doesn't mean that you are a lot safer on 45 degree slopes, then 37 degrees! There are just a lot more skiers doing 37 degree stuff, so they show up more in the statistics. (And in practice it is rather difficult to find slopes that are only 45 degrees...)
It is a bit weird that you see such a dramatic increase in risc in such a small change in steepness. I read on the SLF forum that it is not just the sinus factor of the force that destabilises the slope, it also has to do with how the snow settles and strengthens on steep slopes. On steeper slopes, the snow creeps more and thus stengthens less easily. These two factors combined make this dramatic increase in risc on just a few degrees more steepness. |
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snowdynamics
Joined: 12 Dec 2004
Posts: 618
Location: Wasilla, Alaska
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| Posted: Fri Dec 24, 2004 9:19 am Post subject: |
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Actually, the aplha angle concept isn't really new...
And it is in the new McClung/Schaerer Avalanche Handbook, Pages 115-117.
Carry On.
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Ava Blanche
Joined: 06 Dec 2004
Posts: 268
Location: Beautiful British Columbia
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| Posted: Fri Dec 24, 2004 4:02 pm Post subject: |
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The concept of alpha angles is nothing new; but the use by ski tourers of a 25 degree alpha angle, or a 25 degree avalanche shadow zone, to keep themselves far enough away from slopes that could potentially avalanche, is relatively new.
I don't have the Avalanche Handbook here with me, so my apologies if this is not correct. The only book I know that specifically deals with this is Peter Weir's Avalanche Handbook.
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Valdez Telehead
Joined: 06 Dec 2004
Posts: 1568
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| Posted: Fri Dec 24, 2004 4:55 pm Post subject: |
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Here's a debris pile I found under SOdsyssey Gulley last week The start zone is a 30 degree and the debris is sitting at 10 degrees. (I measured it accurately with a slopemeter) It is obviously a hard slab that broke due to rapid loading, broke on a very hard and slick surface layer and slid. I assume that some of these car sized blocks were pushed further down the incline in a "pin ball" effect by subsequent blocks in one release event. It broke during the storm, evidenced by new snow surrounding the blocks.
I ski up and down this little slope lots and it's the first time I have seen it break.
So yes avyies can occur at 30 degrees and yes runouts can go to 10 degrees. But in this example conditions were High, so odd things happen. Funny though, at this angle of 10-15 degrees, you could outwalk the speed of the approaching blocks or jump on one and ride it.....but don't 
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Gary Brill
Joined: 07 Dec 2004
Posts: 1717
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| Posted: Fri Dec 24, 2004 5:07 pm Post subject: |
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AB:
| Quote: | | It is very important to note that the graph above refers to slope angle versus avalanche accidents; not slope angle vs avalanche occurrence, which is what the original question asked. |
While this is factually correct, reading lagdawg’s original question,
it is clear that “not needing to worry about a slope under 30 degrees” is a suspect statement and an implication was made with respect to risk.
| Quote: | | They said that for a slope less than 30 degrees to slide it would have to have to be a "perfect avalanche", so you don't have to really worry about it. So, for those of you who have much more avy experience, what are your opinions? |
This graph: depicting average angles of some 809 human triggered avalanches in Switzerland and Canada includes both the cases of avalanche release in new snow layers and avalanche release on PWKL (persistent weak layers ofsurface hoar, facets, bonds to ice layers). The avalanches released on storm snow layers, one would think are at the mid and upper end of the range, whereas those with weak layers should be at the lower end. However, this graph from the CAA differs in that it is for some 300+ Canadian avalanches only. It’s data set is likely part of the Tremper graph.
The latter graph differs in that it is for Canadian statistics only . Further, the last twenty years of Canadian statistics, shows that nearly all fatal avalanches where documentation exists have occurred with PWKL’s. 
PWKL’s are particularly hazardous because they are unpredictable, release at unexpectedly low angles, sometimes remotely, and at times may involve deeper snowpack layers (deeper burial - disastrous consequences). Furthermore, Jamieson’s work suggests that compression tests and RB’s are more likely to give “false stable” results when there are PWKL’s.
It is worth noting in the recorded Canadian data, fully 16% of the avalanche accidents were on slope angles of 26-30 degrees and 6% under at 25 degrees or less. The CAA suggests that remaining on slope angles of 25 degrees or less substantially lowers the risk of avalanche release. |
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Ava Blanche
Joined: 06 Dec 2004
Posts: 268
Location: Beautiful British Columbia
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| Posted: Sat Dec 25, 2004 9:43 pm Post subject: |
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Excellent comments, as usual, there Gary. You'd be a much richer man if everybody that learned something from your posts paid you a dollar per idea (hey, what's a life worth anyway?).
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TeleAl
Joined: 25 Dec 2004
Posts: 5718
Location: Switzerland
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| Posted: Sun Dec 26, 2004 8:05 pm Post subject: |
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Those bar charts and pie charts and graphs are getting this Math teacher all excited. 
Great info and links. |
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Gary Brill
Joined: 07 Dec 2004
Posts: 1717
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| Posted: Sun Dec 26, 2004 8:28 pm Post subject: |
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The last pie chart just made me think of dessert.  |
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k
Joined: 10 Dec 2004
Posts: 199
Location: supranivean environment
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| Posted: Sun Dec 26, 2004 11:47 pm Post subject: |
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Re: bar graph.
there's a BIG difference between 31 degrees and 35 degrees.[/quote]
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frank
Joined: 06 Dec 2004
Posts: 917
Location: München
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| Posted: Mon Dec 27, 2004 8:52 am Post subject: |
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| Gary Brill wrote: | | PWKL’s are particularly hazardous |
I agree ... but what the heck is a PWKL?  |
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lagdawg
Joined: 13 Dec 2004
Posts: 49
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| Posted: Mon Dec 27, 2004 2:39 pm Post subject: |
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| I beleive a PWKL is a Persistant Weak Layer. |
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