02-19-2009, 11:17 AM #1
Correlating stability tests to slope skiability
This last weekend, my girlfriend and I borrowed some sleds and rode out into the Mt Bachelor sidecountry, toward Tumalo mtn. We parked them at Todd lake, and climbed a small ridge on skins to get a few turns in, as we were on a tight schedule to meet up with our group of snowmachiners.
Halfway up the climb, I dug a pit and found about 14" of soft to very soft on top of a firm crust, with about 3" of soft below that. This was on about a 25-30 degree slope. The temps were in the high 20's F.
The snow felt very stable, and was not wind loaded at all. I did a compression test on about a 18" high column and was able to get it to break at the crust on the 8th tap from the elbow. The bed surface was about Q2 I guess...not super smooth, but not super rough. I cut out a mini Rutchblock (only about 18" deep) and was able to get it to break on the 3rd jump, once again at the crust.
I haven't been able to put much of my actual snow science to work in the last 2 years, and I've never been very clear on correlating stability tests with actual skiability of slopes. I had watched guys highmarking on sleds for the last 2 days on much steeper slopes, but I wanted to evaluate the actual upper snowpack a little more scientifically.
We mainly skied some lower angle stuff to be safe, but what guidelines does one use for determining max slope angle to ski based on the compression test results and the bed layer interface? What I saw as definitely not a NO-GO to me, but it was a signal to stay off steeper stuff, at least from an inexperienced point of view.
02-19-2009, 12:55 PM #2
unlike terms, it seems..
the first thing you need to do to find a scientific answer to your question is to define your terms
what do you mean by "Skiability"?
how does this differ from the standard definitions for "Stability"?
can you objectify - put a number on - these individual qualities? (most of this has been done for you WRT stability)
if so, there are any number of mathematical tools to help you find the relationships you are looking for
for example, a Venn diagram of the sets of the desired qualities for "Skiability" and "Stability" can help 'co relate' them both as well as illuminate the differences, which can be just as important or even more
the goal is to be able to plug good numbers into an algebraic or differential equation
that seems the easy way
subjectivities are the problem to get around
02-19-2009, 01:04 PM #3
correlating tests to go or no-go is and was the hardest part after starting my avy education
most of the answers i got from pros was that its a good way to gauge the relative nature of slopes on different days and different places...ie a method of standardization of sorts.
The problem with this is that it takes wrong decisions to know when a certain assessment was correct or not.
ie you can get many false positives on stability with a test by skiing it and it not sliding vs saying something is safe or not safe and then having it slide to confirm or deny that.
we recently had an experience where a few in our group were certain a slope WOULD slide.
others in the group werent so convinced that it would.
we all skied it, after a couple healthy ski cuts at the top of a rollover and it didnt slide.
at the bottom of the chute, the faces adjacent on both sides of the same aspect had all slid
anyway, its about managing risks, but on a slope 25-30, you're pretty safe.
one can remove a lot of risk by managing their terrain and being aware what is above, below, and beside them.
use that to take baby steps...see how the snow reacts when you ski it, does it sluff a ltitle, crack, is it supportive etc etc etc and then relate it back to your tests.
Ill say though, there are some new tests out there to help make these go\nogo's decisions
Google the Extended Column Test (ECT) and the Propogation Saw Test (PST)
02-19-2009, 01:21 PM #4
Extended column test has become my favorite snow pit test this year. I attended a lecture earlier this year put on by Bruce Tremper and he mentioned that the ECT is proving to be more accurate than the rutchblock test.
It really allows you to see the prorogation of a failure. When added to the compression and stress tests all in one pit I think you can get a good idear of the stability in that given area that you are testing.
I'd agree with pechelman that your terrain dictated your overall level of safety. 25-30 degrees is pretty safe in moderate and below avi conditions.
Edit to add- It really is about managing YOUR OWN level of acceptable risk. We currently have a snowpack here in NW Montana that sounds very similar to the one that the OP is describing (at least within the top 1 meter). I'm personally a little worried about the faceted crystals that are forming underneath that ice lens that the OP described as "more soft snow". i bet you'll find a large temperature gradient from that ice lens down to the ground as well as from just below that ice lens up to the surface. When I have done ECT on those kinds of snowpacks I find that not only does it fail in compression but it propagates through the entire ECT at the same time. That is too sketchy for my liking. If it simply failed but didn't propagate then I would be more likely to consider a steeper line provided the terrain was good (i.e. no rock outrcrops, terrain traps, cross loading, etc.)
I've been sticking to the low angle stuff on aspects that are similar to what the OP is describing but hitting the bigger lines on the south facing aspects that do not have that ice layer and have taken immense amount of force to cause a compression failure with NO propagation during an ECT.
Last edited by AsheanMT; 02-19-2009 at 01:53 PM.
02-19-2009, 01:42 PM #5
Here is a video on what it looks like to do an ECT
Last edited by AsheanMT; 02-19-2009 at 01:50 PM.
02-19-2009, 02:31 PM #6Hugh Conway Guest
Do you feel lucky punk?
Well, do you?
Trying to derive a concrete value from a probability is an impossibility
02-20-2009, 12:08 AM #7
snowpacks know nothing of probabilities
the trick is to man up and use a more concise mathematical tool
it's just too damn easy to cop out and lay it off on statistical mechanics and other 'imponderables' at this point
what's gonna eventually bust this cherry is for someone to identify the proper metric(s) for observation and modeling so we don't have to lay it off on chaos theory and perturbations and other such metaphysical shit
02-20-2009, 12:11 AM #8Hugh Conway Guest
02-20-2009, 12:16 AM #9
02-20-2009, 12:25 AM #10
btw, the ECT rawks
Lemon Boy sent me a pdf about 4 years ago, that's how i learned my 'standard'
it works very very well
it's one of the reasons i think better metrics are the key to objectifying things
02-20-2009, 09:19 AM #11
Sometimes you have to save that epic line for the corn season when the snow has become bombproof.
There is never a concrete answer. There is only a varying degree of risk available for your assessment. How much risk you take is entirely up to you.
02-20-2009, 06:26 PM #12
classic case of 'is/ought'...
02-22-2009, 07:38 AM #13
>Correlating stability tests to slope skiability
I have no answer other than dig every single time you ski, even if you know what you are standing on, dig anyway. Except perhaps in late spring.
I nearly always dig and sometimes get the hurry-up from ski partners who know that I know what we are about to ski on, and what's under it. But I dig anyway and 99% of the time I learn something from it. Over the years all this digging will pay off. Kind of like kissing your girl long after you married her and became lazy.
More seriously, digging a seemingly unrevealing pit can become instantly meaningful the next day when you dig another seemingly meaningless pit. Put the two in relative terms and they have a whole lot of meaning.
Also look for likely instability in small contained test slopes and dig them, get and easy result with a clean sudden shear, then try and trigger with a ski cut. I'm yet to pull it off, but have not been trying nearly hard enough. I tried today, got repeated CTE(2) down 20cm in wind slab with a planer sudden pop when it sheared. I then tracked-out the little convex feature trying to get something to move. Nothing.
As for you example: CTM(18) down 14"... I have skied plenty of supported slopes with results in that area and had great stability on multiple descents all day long. But on those days I also chose to avoid anything over 35 degrees+convex+wide propagation area above a long steep slidepath. But don't go using that as a benchmark just because I am still alive.
I think building your own personal database of stability tests versus actual stability is almost a full time commitment. One reason I quit my job for the backcountry.
In addition to the ECT have also started using another more simple test recently: an r-block with no back or side cuts at all. So not really an r-block. Just a clean pit wall a little longer than your skis with clean side walls. I am most interested in how it breaks, how deep and whether is propagates down the sidewalls. It is my proposition that if it propagates down the side walls after a small jump then I should be able to trigger an avalanche on a small test slope. If I can build a database supporting that correlation then I will be happy. The ECT would be just as good or better test for this purpose, but I like the simple and direct cause-effect nature of the test I described (and was taught by an avalanche professional generous with his time and knowledge).
Last edited by neck beard; 02-22-2009 at 07:49 AM.Life is not lift served.
02-22-2009, 03:08 PM #14
02-22-2009, 10:41 PM #15
Don't think of pits and tests as ways of making a go/no-go decision. Instead, think of them as a way to confirm your plan for the day. Based on forecasts, weather, prior experience, first-hand reports, and the like, you should have a good idea of what your going to ski before ever digging a pit. If its sketchy and you plan on doing mellow slopes, use the pit to show you why you don't want to get on steeper terrain. If you know it's more stable and you plan on something more serious, use the pit to affirm your confidence in the pack.
Pits are not the end all, be all. They are highly variable and can often be judged very subjectively. That's why you have to use other tools (forecast, etc.) to help guide you in the right direction before ever deciding where to ski.
A point I've had stressed many times in my avy education is this: remember that pits should be used to show you what you already know.
Skiing, whether you're in Wisconsin or the Alps, is a dumbass hick country sport that takes place in the middle of winter on a mountain at the end of a dirt road.
02-25-2009, 10:21 AM #16Registered User
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03-15-2009, 05:21 PM #18awake1563
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"Based on forecasts, weather, prior experience, first-hand reports, and the like, you should have a good idea of what your going to ski before ever digging a pit."
The hardest thing is going to a new range, with different variables. but local knowledge of the aforementioned things from a reliable source I'd say is more valuable than digging a pit. Not that it's useless to dig, but pits are only reliable for that 30x30 or 30x90 piece of the hill that you just tested... snow depth, temps, wind loading, aspect, angle, and elevation cause to many vast differences to apply data from one pit to even the rest of the slope it's on. You'll be able to predict stability much better with massive amounts of knowledge of local weather, aspect, temps, slope angle, recent events, and snowpack history and the ilk. But you'll still see me digging 18-24" deep "hand pits" or whatever you call them to test the upper layers if I'm questioning myself, which happens a lot. If I'm worried about the base of the snowpack, well I probably won't be skiing anything over 30.
there's a lot of interesting theories out there, but I like to use some of the methods in this book, using things you can quantify, such as general avalanche hazard, slope angle, group size, aspect etc. to make most of my calls, and if things aren't feeling right or I want a verification, dig a quickie.
Well, I probably just mudded up the murky waters, but as far as correlating you quick pits, it's all a matter of time and experience, but if you give a column a tap or two and the block slides onto you and knocks you over... GTFOOT!!!but I know we can't all stay here forever, so I wanna write my words on the face of today...