Posted: Mon Dec 20, 2004 2:43 pm Post subject: Minimum angle for avy danger
I took an avalanche course yesterday and in it htey said that the minimum angle for an avalanche to happen was 30 degrees. However, in my reading on the posts here it seems that the minimum angle is more like 25 degrees. Now this class was given by a patrol at a small hill in the midwest, and even though it is all man made snow and the steepest area is around 26 - 28 degrees, it has slid at least two times in the past 5 years or so. 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?
Joined: 07 Dec 2004 Posts: 1116 Location: HELLsinki, Finland
Posted: Mon Dec 20, 2004 2:51 pm Post subject:
The usual reference I have heard that there is a high risk for AVY on slopes between 27 and 45 degrees. If the slopes is steeper there usually isn't time to accummulate snow before it slides off (moist maritime enviroment is a different story), and if the angle is less the snow wont get enough speed to start an avalance.
Anyways you must always remeber that even though it is unlikely that the slope you're on (less than 27 deg) you would trigger an avalance, you might be still at a risk zone sa an avalance may start somewhere above you from a steepers slope.
Joined: 06 Dec 2004 Posts: 13192 Location: People's Republic
Posted: Mon Dec 20, 2004 3:08 pm Post subject:
We usually say that most avalanches start on slopes between 30 and 45 degrees. However, in my courses I always caution that this is based on large numbers of aggregate data and that avalanches can and do start on slopes less than 30 degrees under the right conditions. Measuring slope angles is a very important skill for backcountry travell. However, if that is the only thing you do, and your litmus test is 30 degrees, I would not count you as safe as if you were to endeavor to learn more about snowpack and stability every time out and to begin to identify those times when the instability is such that slopes in the 25 to 30 degree range would be suspect as well. It is a lifelong process. You won't get there in a level 1 course and you won't get there by looking for statistical answers, because there are so many outliers in the data that have basis in physical reality.
A personal case in point. I was ascending the west side of Mount Yotei (Hokkaido) in 1991. There was about 20 cm of new snow well bonded to the old on the ascent. At tree line I stopped to change to crampons (the remainder of the climb is on hard, windblasted rime and sastrugi) and was looking for a good place to sit doen. I peered over a small ridge onto a neighboring slope and suddenly realized I was moving. I ended up buried up to my waist in debris from a thin hard slab that ran through sparse trees for several hundred feet. I was able to extricate myself before my partner reached me and then went back up to measure the angle of the slope that I had triggered. It was 28 degrees. The small difference in aspect--maybe 4 or 5 degrees to the south of the ascent slope--had placed me on a cross-loaded slope with a bed surface of sun crust that did not exist on the slope we had ascended. This is certainly not the only condition that can create an avalanche on a slope of less than 30 degrees. It really underscores the need to be looking at the terrain and the snowpack with a closer eye than just measuring slope angles. _________________ that sounds like a sure-fire way to get bitch-slapped by devil's club -- dschane
The main thing after just taking a Level One is to get out and use it or lose it. First thing though is to make sure you have a slope meter, if not your lost. Its your compass to safety. It's basic skill I hope they over stressed at your course. Once you apply the course to expereince and continued learning, you can answer your question yourself within a short time. Get out ski as much as possible as soon as possible. I had many question after Level one, that only expereince could answer.
Once you are properly measuring slopes then you can do the pit stuff along with pole probes. Doing these things will eventually bring you to micro-management of terrain on any given route. Any slope in excess of 28 degrees can slide or be a hazard during when natural activity is happening . If no natural activity is happening or predicted, I typically work 30-32 as the "zone". But this is the coast and things are different in other areas.
I have seen slides occur at 28 degrees that could wipe out a small town. Found some car size debis recently from a slide that were sitting at 10 degrees
It is certainly true that the lower the angle, the less likely an avalanche is to occur, the slower it is likely to run, and the less the runout distance is likely to be. But the slicker the bed surface, avalanches run at lower angles, faster, and farther. So, when conditions are atypical, don't bet the house on the minimum avalanche release angle.
Case in point, Alpental on surface hoar, ski patrol triggered slides on 22 degree angles a couple of years ago. Around 1980 a fatal avalanche in depth hoar took place near the Columbia Icefields in a 15 degree boulder field. The depth hoar is said to have flowed like water around the rocks.
Tremper has a good section in his book on avalanches as related to slope angles.
The other thing to consider, of course, is that when the snowpack is atyical, has weak layering, remote triggering can release avalanches from flat ground at considerable distance (a few hundred feet to a mile or more?) from surrounding slopes.
CAA North Columbias March 2003:
3/17 NORTH COLUMBIA REGION
The top 20-60 cm of the snowpack is moist up to the 2000m elevation. The snowpack remains fragile, with numerous shears observed within the storm snow Sunday. Wind slabs have been reactive at treeline and in the alpine. An easy to moderate shear persists down 75-120 cm on the old mid February surface hoar. The surface layers will weaken on sunny aspects as the day warms. Expect a crust to form in the moist layers which will increase strength below treeline. The avalanche cycle continued through the weekend with numerous size 2.0 to 3.5 avalanches from natural, skier, and machine triggered events. Slabs triggered in the storm snow have stepped down to the November crust and even to ground in some events. One observer remotely triggered five separate size 2 slabs 60 cm deep while digging at a snow profile site at a safe location!
The avalanche handbook has a scatter plot of slide observations arranged by slope angle, very similar to Gary's bar chart of accidents (by accidents do they imply human involvement?) The scatter plot forms a nice bell curve starting @ 25 deg. and going up to 50 deg. Between 25-30 deg. and 45-50 deg. very few slides occur (the tail ends of the bell curve). Between 30-35 deg. and 40-45 deg. more occur, and the top of the bell curve is 35-40 deg. w/ 37-38 deg. being the apex of the curve. As such few slides happen <30 deg., my empirical observations suggest this occurs only on high or extreme hazard days (although it seems like some noteworthy exceptions are referenced in this thread).
My logic is that if the hazard is rated high or extreme one should stay under 25 deg. to be safe. The catch 22 being that you have a hard time moving on those low angle slopes in deep snow. As such going to a resort on high hazard days is a good idea.
my empirical observations suggest this occurs only on high or extreme hazard days (although it seems like some noteworthy exceptions are referenced in this thread).
My logic is that if the hazard is rated high or extreme one should stay under 25 deg. to be safe.
I don't know that I'd agree with that analysis. In a way you are mixing apples and oranges. High and Extreme avalanche hazard is supposed to mean that natural avalanches are likely as opposed to Considerable when, although there is some possibility of naturals, most avalanches are expected to be triggered artificially. The same weak layering may exist in Considerable Hazard as in High or Extreme, but the amount of loading or the stress distribution may be such that the threshold for naturals has not been met. However, the sudden loading by humans may be as significant to the concentration of stress at the weak layer as slow loading under snowfall. The net effect: Slopes may release on as low or even lower angles under human loading in Considerable hazard as under High hazard under certain conditions.
I think, when thinking about releasing avalanches at low angles, the key consideration, probability - wise, is the existence of weak layers with low coefficients of friction (facets, surface hoar and ice layers).
Obviously, in all situations, consequences are the other key element.
Joined: 06 Dec 2004 Posts: 268 Location: Beautiful British Columbia
Posted: Wed Dec 22, 2004 3:02 pm Post subject:
Gary Brill 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. _________________ There are no easy solutions, only intelligent choices
Joined: 06 Dec 2004 Posts: 4135 Location: psssttt, over here...
Posted: Wed Dec 22, 2004 10:20 pm Post subject:
One of the things I consider while traveling in the bc, is the alpha angle. I consider the runout from a natural avalanche and whether it can intersect my path. this is a bit different than standing on a slope considering if it might slide when I ski it.
I don't know if you were considering that in your original post. _________________ It's all about fun....
Joined: 06 Dec 2004 Posts: 536 Location: Squamish, BC
Posted: Thu Dec 23, 2004 1:02 am Post subject:
Ava Blanche wrote:
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.
And what is Ava Blanche's favorite method of determining if the slope is more or less than 25 degrees? In case you missed last year's thrilling discussion on this, it's the T-test (which actually gives you a benchmark for 50 percent, which is 26.6 degrees).
Maybe when AB gets back from his Christmas vacation on the beach, he'll post some photos of the T-test in action (he's so good at it, he does it while actually skiing). _________________ We need wilderness because we are wild animals. Every man needs a place where he can go to go crazy in peace.
- Edward Abbey
Joined: 06 Dec 2004 Posts: 791 Location: Ask Heisenberg
Posted: Thu Dec 23, 2004 2:33 am Post subject:
Not sure if it worthy of its own thread......but could someone please explain what an alpha angle is for me. Thanks
If you stand at the very end of a runout and sight to the top of the starting zone that is the alpha angle.
If you stand at the first point in the path (first from the top coming down) which has a local angle of 10 degrees and sight to the starting zone that is the beta angle.
Different slides in the same path can have dramatically different alpha angles. Usually the alpha angle is referenced to a particular return period, such as 100 years. There are statistical methods for obtaining the alpha angle from the beta angle but they are different for every region, with no clear dependence on whether it is maritime or continental. And these statistical relationships come from collecting alpha and beta angles for maximum runout events with a given return interval, typically 100 years.
Note that obtaining the alpha angle can be done on a map with some accuracy, but measuring it directly requires standing at the top and sighting to the end of the debris or maximum known extent, or else standing at the bottom and sighting upwards.
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