UKC

Yesterday I witnessed a spectacular cornice collapse come down the gully to the right of where we were planning to climb (White Nile on the Braeriach). The debris covered an area about 30m wide and 15m from the front edge to the back, and was probably getting on for 2m deep in places. I think there is a good chance that you would have been fully buried had you have been walking on that slope. The blocks in it were as big as fridges and microwaves (I am sitting in a kitchen as I write this) and had you have been climbing under it I am certain the pummelling would have been extremely severe.
So we decided to turn back and give White Nile, and any other routes in the corrie, a miss. They all had similarly large cornices looming over them.
It got me to thinking, how do you predict a cornice collapse? My theory was that if conditions were right for that one to go, then they were right for all the rest of them to go too, is that a reasonable assumption? Also had we have not had that warning, what other signs should have let us know that cornice collapse was possible at that time.
The day before the temperature had been high, that morning it was dropping, and had possibly dropped 3 or 4 degrees in a few hours. I would have thought that lowering temperatures would have stabilised the cornices, is that a false assumption?
Keen to learn from this one.

In reply to Phil Ingle:

Some thoughts:

When ice fall climbing, quick drops to low temperatures can create weaknesses in the ice - including fracture lines. So it is often wise to NOT climb ice falls (esp. icicles) after sudden drops to low temperatures. Maybe relevant here?

The warm temps would cause the cornice to sag, possibly creating (or widening) a fracture line. On re-freezing, the contraction/expansion could further weaken the fracture, resulting in collapse. (thinking of the expansion of water as it freezes)

So, just coz it got colder does not mean it got more stable, possibly the opposite.

In reply to Phil Ingle:

It's a good question Phil, and one I suspect some of the SAIS people might be able to offer more info about. You could email them.

I have to admit, I've never considered falling temperatures as being likely to increase the likelihood of cornice collapse (I'd always considered them more likely to be a version of wind-slab or wet snow avalanches) but I am no expert. Was it below freezing when you were there?

I believe that the risk of cornice collapse is not always included in the overall 'avalanche hazard' rating, so posts like yours are useful. It's worth logging it with the SAIS too, if you haven't already. There's a little more about this subject here: http://meagaidhblog.sais.gov.uk/2008/02/old-cornice-and-avalanche-debris/

All the best,
Danny.

In reply to Phil Ingle:

Interesting. I was planning to go into have a look at the white nile area at the weekend but thought that the temperature was too high so called it off.
I have been hit by falling cornices before which then triggered an avalanche which then swept me several hundred metres and over a few short bluffs. So yes it is something to bear in mind.
In my case the sudden rise in temperature of several degrees in less than than an hour was the culprit. Just before the incident I remember seeing all the signs such as sun pillows (can't remember their actual name - the snow balls that roll down steep snow slopes) and hore frost and rime falling from the cliffs. I was in exactly the wrong place at the right time and if I were ten metres to the side in the corrie it would have just gone passed me like an express train.
Anyway, did you see howmmuch ice there was in white nile ? I'm very interested.
Kind regards
James

In reply to Phil Ingle:

If you do email SAIS and get a reply, would you post it up please? I would be interested to hear what they have to say on it too.

In reply to girlymonkey:
cornices prone to collapse is regularly mentioned in the forecasts and is highlighted by the hazard graphics at the bottom of the forecast as well.

It is independent of the avalanche hazard. Not sure if they have a standard wording for major cornice collapse hazard or not. Not easy to predict.

In reply to alasdair19:

I know that, but I also would have thought that dropping temperatures would make it less likely. Some of the suggestions further up about the expansion and contraction in fracture lines could make sense, but it sounded from the wording that these were guesses rather than definite answers. So the suggestion was that the OP could ask SAIS team for an explanation, and I would be keen to hear their explanation also

In reply to girlymonkey:

I checked the forecast archive and for the Saturday there was no cornice icon.

They may be like seracs ie really random apart from definitely falling down by August! I've heard people considering full.moons as influencing seracs collapse and it was fullish on Saturday I think....

In reply to Phil Ingle:


I don't know that I'd read too much into the temperature variation. Cornices are intrinsically unstable, it's possible it would have collapsed anyway regardless of the weather on the day. Not a very satisfactory answer, I know.

In reply to Phil Ingle:
I'm not sure I can offer much more than has already been aired...but I'll try.

As someone has already mentioned, cornice collapse hazard is independent of the avalanche hazard and falls into the same territory as full depth avalanches. We just don't fully understand the mechanics and forces involved + there are too many additional variables compared to 'regular' dry/wet slab and loose snow avalanches. Unstable snow lying on inclined ground is easier for us to test and, generally speaking, much less threatening to one's personal safety than a large, gnarly snowy cantilever overhanging a steep 300m runout over crags and boulders!

However, 'experience' does tell us something:-

New snow cornices:

Soft cornices formed in low wind speed conditions can collapse spontaneously once they have reached their own (low) critical threshold where snow weakness overcomes snow strength. Often relatively small cornices.
Higher sustained wind speeds can mean smaller sintered snow particles, denser (stronger) snow and larger cornices before they too succumb to the disequilibrium in strength/weakness and collapse, often some way back from the edge.
Both of these types of cornice do not like sudden rises in temperature, particularly if the rise takes the snow through 0 degrees C. The faster the transition to warmer temperatures the less the cornices like it. To compound this, if the cornice is very recently formed then it is even more prone to collapse than one that has had a day or two to consolidate (age harden) at cooler temperatures. Add some rain to the mix and it's game-on for cornice collapse.

Older cornices (made of coarse-grained snow)

These are more problematical. They'll almost certainly have been through several melt-freeze cycles and become stronger over time (at least during each successive freeze event). In sustained melt phases (spring-time, for instance) the free water between grains weakens the cantilevered snow structure quite often near point of greatest stress - the 'root' (not a technical term!) of the cornice, which may well be some way back from the edge. Deep fissures result which open progressively, part company with the rest of the snow and the cornice collapses.

If one of these types of cornice collapses, experience informs us that other similar cornices nearby will be in a similar, if not the same, condition and should be avoided. But.....each cornice is different - even those in close proximity to ones that have collapsed - and may hang on for days or weeks in thaw conditions. The prudent approach of course is to avoid these cornices in thaw conditions since you cannot be 100% certain of their stability. A sudden refreeze (over just a few hours) of a thoroughly wet cornice is unlikely to stabilise it much - it'll still be too wet. Sustained cold temperatures are needed for some (notional) degree of stability to return.

Common-sense advice would be that if there are large, old overhanging cornices nearby and you're walking around in soft wet old snow on a sunny warm/wet mild day, then chances some cornices will collapse. It's not a golden rule but it's quite a good rule.

Cornices are really fickle features of our winter mountains. When I worked in the Cairngorms we quite frequently cut car-sized relatively new cornices with 20m or so of knotted accessory cord. I was always surprised by the results. Some would collapse with just a couple of sawing-action pulls of the cord when I assumed we'd be there working up a sweat for 10 minutes. Quite often the reverse was true, too. What I did learn from this was that snow is a complicated and largely unpredictable material with widely varying subtle (and not so subtle) engineering properties that can differ from one place to another, one day to the next.

Same with cornices.

Caution is key when dealing with them.......

.....always.

In reply to depthhoar:

thanks for that.

In reply to Phil Ingle:

Large cornices are like seracs - unpredictable. If weather/temps are fluctuating (especially warming up) and variable treat with caution.

In reply to girlymonkey:

Hi. Sorry for the slowness, I have had no internet for the last couple of days. I did get a reply from SAIS. Here it is...

[Yes cornices are pretty unpredictable and suspect most of the time, especially when newly formed and during warm periods, However it is often very surprising in that some you would expect to collapse dont and just 'shrink' and some collapse spontaneously.
We consider cornice hazard separately to avalanche hazard as cornice collapse involves much more of a mechanical or structural failure as opposed to an avalanche which is a shear failure between layers and for this reason we are not able to provide information on cornice collapse with any level of certainty.

Recently the snowpack has warmed to depth both from the air temperature influences and ground temperatures which are always zero.

From our experience it would take many days for a deep snowpack that is isothermic (0℃ throughout) to be affected and refreeze.
Therefore I would only expect the cornice to have refroze a few inches from the surface - this would have had no effect on the stabilisation of the cornice structure which will have been in a unstable condition - nb the weight of moist snow would be around 800k/cu metre.

I always treat cornices in much the same way I treat seracs and consider them as an objective and unpredictable hazard and minimise exposure to the threat as much as possible.]

From this reply, and others, I think the main thing is that they are unpredictable, and any variation in conditions may be a trigger. So best to do routes under them in the middle of settled periods of weather, if at all.

In reply to Phil Ingle:

Great, thanks for posting that Always good to get more snow knowledge!

In reply to Phil Ingle:

Phil - thanks for posting (And thanks to SAIS for the info!)