Joined: 06 Dec 2004 Posts: 362 Location: Defiance, CO
Posted: Mon Jan 03, 2005 11:55 pm Post subject: Settlement
I have some questions regarding settlement, and I’m speaking from a continental snowpack perspective (Colorado). Collapsing and settlement typically occur in the days that follow a storm. In avalanche reports shortly after storms around here, the buzzwords of “widespread collapsing and settlement” seem to always crop up. Many times I’ve felt and heard the settlement of a new slab, and it’s quite an eerie sound.
I know the answer is, “it depends”, but my question is this: After you hear collapsing and the new slab has settled, does that indicate the slope is safer? I'm guessing that since the new slab did not break away and fall down the slope, that it fell vertically in place and is staying put.
I’d love to hear some discussion regarding people’s thoughts on settlement. The reason I ask is I was out at my local stash last Thursday after we received upwards of a foot of ‘atypically dense’ snow, meaning wetter than the usual champagne powder. I was on an open slope of about 30 degrees below treeline when I heard the settlement occur. After the settlement occurred, I dug a hasty and found a weak layer that was much different than the weak layer that I witnessed just prior to the settlement (in my study pit). The weak layer prior to settlement was about a half inch of hoarfrost that likely formed last week during the clear cool nights and warm days. After the settlement occurred, the weak layer was compressed by the collapsing layer and did not measure as ‘weak’ as it was prior to settlement, confirmed by several shovel sheers.
Another reason this topic comes to mind is in watching AT Apostle’s video last week, there was a point in mid descent at which his partner said “Did you hear that settle”, at which AT replied, “Yup” or “watch it” (or something to that effect). Then without hesitation, his partner makes 20+ aggressive jump turns right down the center of the couloir. To me, this sequence implies that settlement without a slide may make the route safer? I realize settlement could potentially occur on a number of different layers in the snowpack. I appreciate any discussion you knowledgeable folks may have. Thanks.
Joined: 06 Dec 2004 Posts: 5270 Location: 4-corners, NOT BOZEMAN!
Posted: Tue Jan 04, 2005 12:45 am Post subject:
My 2 cents. It is not a bad thing but should be taken for what it is. That whumph is the widespread release of stress and tension. The slab or area may have stayed in place, but the whumphing is a sign of localized instability. Attention should be given and steeper slopes should probably be avoided. A whumph on a 30 degree slope probably wouldn't be followed by a release, on a 34 degree slope???? On a 37 degree slope!!!!!!
might be best to describe rapid settlements of the snowpack as 'whumpfs'. The word settlement can get confusing. Check out these two sentences:
1. 'We saw a good settlement while out skiing today'
2. 'We saw good settlement while out skiing today'
The first sentence would imply observing an indication of snow instability, but by adding just the letter 'a' the second sentence implies a observation that is typically associated with improving stability.
Joined: 06 Dec 2004 Posts: 791 Location: Ask Heisenberg
Posted: Tue Jan 04, 2005 2:05 am Post subject:
fbksskier is right about the terms. Settlement is a commonly misused term. When there is a sudden collapse of a weak layer the best terms are collapse, whompf, or perhaps compressive/compression failure.
Settlement really refers to the slower settling of snow over time, somewhat more rapidly immediately after new snowfall then slowing with time. This is due to forces of gravity as well as metamophism on the scale of the ice grains.
Settlement usually indicates improved stability, whoompf or collapses are usually red flags for instability.
Joined: 06 Dec 2004 Posts: 2203 Location: not suited for office work
Posted: Tue Jan 04, 2005 2:30 am Post subject:
I've experienced numerous collapses while walking ridges before watching the whole hillside avalanche.
Collapsing is an indicator of instability and should be given the greatest attention particularly on slopes steep enough for avalanche. The next collapse heard may be the one initiating the avalanche.
I have never heard a snow settlement. _________________
Quote:
All the cagey avalanche folks I know, me included, have sworn off slopes steeper than about 33° for the rest of the season.
One exception to settlement as a indicator of stability is a snow storm that arrives cold and leaves warm. As the storm progresses the new snow will settle creating an upside down layer of new snow. New heavy over light typically settles as it falls. If it clears and get cold too quick, we get death crust, sucky skiing on top
Just like here today...started snowing at 17F last night and we got a quick 5" and my snowstake said 31". it contuinued to snow all day and has warmed to 32F. My snow stake now reads.....31"...still. All this new snow has consolidated into one cohesive layer at least here...might be different up there.
Joined: 06 Dec 2004 Posts: 127 Location: Fernie BC
Posted: Tue Jan 04, 2005 5:33 am Post subject:
Whumphing to me sets the red light. About a week ago I was on some mellow slopes about 25 degrees on a NE aspect. There was about 30 cm of light pow on the dec 10 raincrust, after couple of pole probes I suspected there was a weak layer between that and the previous rain crust. I skied the slope and it was whumphing every 100 feet or so, sometimes I really felt it drop. I observed no windslabs in the vicinity, but I'm sure if the slope was 30 to 40 degrees I would have triggered sluffs.
When I hear or feel it settle I pay attention!
Joined: 06 Dec 2004 Posts: 2370 Location: Colorado
Posted: Tue Jan 04, 2005 5:56 am Post subject:
Reading the original post, it kind of sounds like the question is if a whumphted slope is more stable and skiable? My guess is that a person generally doesn't affect a big enough area of the slope to stabilize the whole run. You might stabilize it right under you and a few yards around you, but not the whole slope. Plus, if it was unstable enough to really settle a large area you probably stand a good chance of having already set a slide off.
I would be really worried that even if the snow directly under you was more stable because the weak layer was collapsed, the area not far in front of you would still be weak because the weak layer has yet to collapse.
Joined: 06 Dec 2004 Posts: 268 Location: Beautiful British Columbia
Posted: Tue Jan 04, 2005 11:50 am Post subject:
Snowman wrote:
Settlement is a commonly misused term. When there is a sudden collapse of a weak layer the best terms are collapse, whompf, or perhaps compressive/compression failure.
Settlement really refers to the slower settling of snow over time, somewhat more rapidly immediately after new snowfall then slowing with time. This is due to forces of gravity as well as metamophism on the scale of the ice grains.
Settlement usually indicates improved stability, whoompf or collapses are usually red flags for instability.
Bang on, as usual there Jim!! Here is settlement illustrated graphically from one of my original avalanche course diagrams.
Things to note:
1. Initially, snow falls as individual flakes or snow crystals.
2. But pointy crystals and sharp angles are not as thermodynamically stable as more rounded crystals. So once the snow lands, there is a movement of water vapour from the points to the central part of each snow crystal; this is called rounding.
3. Now some of the moving water vapour molecules also form bonds between snow crystals. This is called sintering.
2. Rounding and sintering stabilize the new snow and, along with gravity, cause it to settle. Thus, there is a net loss of height- which causes settlement.
3. Rounding and sintering happen over time, but they are strongly temperature dependent- the colder it is, the slower these processes are. That is why hazardous conditions linger around longer in Continental snowpacks, compared to Coastal ones. Typically, at 0°C, the snowpack may stabilize over a day or two; at -40°C, sintering and rounding virtually stop and hazard may persist for days or even weeks.
4. Sintering also creates bonds between the new snow and the underlying snow. Again, this process is highly temperature dependent. However, it is also very dependent on the type of snow crystals that exist at the interface; for example, if the underlying snow surface consists of surface hoar crystals (facets), then bonding between new and underlying old snow may be very poor.
5. So a potentially hazardous situation is created when the layer of new snow sinters and bonds, and becomes more dense and forms a snow slab., while at the same time, there is poor bonding of the new snow to underlying snow.
6. A worst case scenario comes about when:
(a) the new snow consists of smaller, more rounded snow crystals to begin with, which can produce a denser snow layer.
(b) wind speeds pick up as the storm progresses, which causes additional snow build-up, and also breaks down larger crystals into smaller grains, which increases snow density even more. Note that the smaller the grains are, the more contact points there are, and the faster sintering progresses.
(b) the underlying snow surface is cold and consists of facets. The combination of a cold interface, and one with facets, may really inhibit bonding of the new snow to the old. Here is an illustration:
Anyway, hopefully some of the pro's will give their two bits worth and then we will all have a really good understanding of these processes.
From an avalanche safety point of view, here are a few things to pick up:
1. new snow is most unstable right after it falls and has not yet had time to form good bonds. The more snow, the more unstable it will be (remember, shear stress is directly dependent on the density and height of snow).
2. wind increases the hazard by causing increased snow build-up, and breaking down crystals so that they form a more dense new layer.
3. cold temperatures, or cold places (such as a northerly aspect), inhibit sintering and therefore stabilization of new snow.
4. how stable the snowpack is depends not only on how good crystal to crystal bonding is, but how well the new snow bonds to the underlying snow. _________________ There are no easy solutions, only intelligent choices
AB and Snowman addressed the normal (and usually slow) settlement process.
Several have pointed out that sudden collapse, properly a whumpf, is an entirely different animal. Whumpfs, as WOW and others have said is a classic Red Flag.
I like what skibum said:
Quote:
Reading the original post, it kind of sounds like the question is if a whumphted slope is more stable and skiable? My guess is that a person generally doesn't affect a big enough area of the slope to stabilize the whole run. You might stabilize it right under you and a few yards around you, but not the whole slope. Plus, if it was unstable enough to really settle a large area you probably stand a good chance of having already set a slide off.
I would be really worried that even if the snow directly under you was more stable because the weak layer was collapsed, the area not far in front of you would still be weak because the weak layer has yet to collapse.
In some cases, a whumpf may just mean failure of the weak layer so that the area that collapsed is now dependent on surrounding fragile snow structure to pin the failed area in place. Stress is transfered to the surrounding areas, making them more likely to fail. (In fact the redistribution of shear stress and strain following localized failure is thought to be the likely mechanism of slab failure in most cases).
There is a big difference in the short and long term effects of settlement and whumpfing. Settlement and strength gain is a slow process, and even the whumpfing you described, Telee, will, as you suggested, likely strengthen the snowpack. But at the time you observed it, the whumpfing is not a positive sign. For a period of time after your observation, it is sign that there is something dangerously wrong in the snowpack.
Finally, sometimes, especially on surface hoar, shallow new snow, lacking cohesion, may actually become more hazardous as bonding of the new snow makes it more cohesive. Stabilization may not take place for a week or two and often not until sufficient new snow falls to compress the surface hoar into adjacent layers (per AB's diagram).
Joined: 16 Dec 2004 Posts: 1342 Location: them thar hills yonder
Posted: Tue Jan 04, 2005 9:20 pm Post subject:
Snowman wrote:
fbksskier is right about the terms. Settlement is a commonly misused term. When there is a sudden collapse of a weak layer the best terms are collapse, whompf, or perhaps compressive/compression failure. Settlement really refers to the slower settling of snow over time, somewhat more rapidly immediately after new snowfall then slowing with time. This is due to forces of gravity as well as metamophism on the scale of the ice grains. Settlement usually indicates improved stability, whoompf or collapses are usually red flags for instability.
Seems to me that "settlement" is a catch-all term for any event that causes the snow to shift, usually resulting in improved stability in the long run. Avalanching is a form of settlement; the snow settles in the runout zone. Treebombs are a form of settlement too, and so are whumpfs and slow settlement.
IMO what we really need is a different term for the slow settlement that Snowman and Ava describe. Something more descriptive as to the actual process and less nebulous than "settlement". How about "densification" or "deflation" or something like that.
Settlement has been microscopically shown to be grain rearrangement under the pressure of overlying new snow layers, and/or from warm temperatures, the bending and shrinking of arms and branches of snow forms, and consolidation that is the result of decreasing grain size and increased bonding between grains.
Snotsicle,
In your last post your are confusing sudden events that subsequently lead to increased snowpack consolidation and settlement.
Joined: 06 Dec 2004 Posts: 268 Location: Beautiful British Columbia
Posted: Tue Jan 04, 2005 11:48 pm Post subject:
Well, we could go back to the very original term that I learned back in the Dark Ages: rounding and sintering was called destructive metamorphism because it broke down crystals (as opposed to constructive metamorphism, or temperature gradient metamorphism, which creates new crystals and is now being referred to as "faceting").
But Gary is right: "settlement" used in this context usually refers to the slow rounding and sintering process which tends to stabilize new snow.
Whumpfing is the sound made during sudden settlement of the snow, and is considered a "bad" sign, or a sign of potential instability.
Avalanching, although technically also a form of settlement, is not usually referred to as a benign event. "Sloughing", though, is usually considered benign and a sign that the snowpack is stabilizing.
Why don't we have a little contest to see how totally confusing we can really make things. In German there is a good saying "warum einfach wenn ist auch ungeschickt geht"; which literally translates as "why do it the easy way when there is a more difficult way".
--------------------------
Anyway, here are four good glossaries that have some definitions that may be helpful.
The CAA WEB site has an excellent glossary, but for some reason, it is "hidden" under the Weather Information section and is difficult to find: http://www.avalanche.ca/weather/glossaries/index.html Unfortunately, the glossary doesn't include the term "settlement".
"The slope-perpendicular decrease in thickness due to gravity and metamorphism. Often misused to refer to a rapid collapse or "whumpf" of the snowpack. Significant settlement (25%) of new snow is generally favorable for the stability of that layer.
The rate at which snow settles, calculated from the ratio of new snow depth to change in total snow depth, is the settlement rate.
Deposited snow which has increased in density after deposition is called settled snow."
It defines settlement as: "Slow decrease of the snow depth due to rounding, sintering and densification of snow." _________________ There are no easy solutions, only intelligent choices
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