/ NEW ARTICLE: Avalanche Basics Part 2: Staying Safe
Read more at http://www.ukclimbing.com/articles/page.php?id=5179
Nice article! Is the large wheel mean to be the same as the one on the map of the Ben, though (and the middle height is missing from both, and to be really nitpicky, the text extract mentions snow at 800m whereas the wheel shows no snow below 900m!).
With regard to the European Avalanche Scale, I believe it is the case that its definition of the size of avalanches means that a Very High (cat 5) is pretty much impossible in the UK. I don't think this is widely appreciated.....
Question:- How do protruding boulders anchor a slope?
Quote: 'Is the slope long and uniform or are there islands of safety like protruding boulders to help anchor the snow?'
Regards the Cat 5? Can't find any reference in the article, but yes a Cat 5 in european money can wipe out habitation.
Whereas a long uniform slope will be under more tension as the weight is essentialy being pull from the top to the bottom.
re protruding boulder...:
Depending on which research papers you read there is evidence that terrain elements like this provide discontinuities which can act as release points (ie weak points in the bonding of the 'slab'). Haven't got time right now to provide references...
I understand you are describing the mechanism which 'anchors' the lowest layer(s) of a snowpack. These are the irregularities at the ground's surface which act on the base of the snowpack.
You are also suggesting that these layer(s) next to the ground are broken by the protruding boulders, such that the ground layer doesn't slide readily.
I'm still a little unsure about how these protruding boulders anchor the layers *above* the lowest layer of the snowpack.
I'd welcome your comments on the following:-
1. Once the ground features are covered by the initial layer, subsequent snowpack layers are anchored by the bonds between snow layers and the bonds within the snowpack. The protruding rocks now become sources of weakness because they do not provide support for these upper layers and are 'holes in the snowpack'.
2. The snow around these rocks is weaker too, which means the bonds between these boulders and the snowpack are weaker. This is because of the action of the heat from the rocks acting on the snow around the boulders (the snow crystals become 'sugary' and lack strength).
3. There is a variable snowdepth, especially following the process where the terrain irregularities are covered - described here http://www.fsavalanche.org/NAC/techPages/articles/12_ISSW_Eckerstorfer.pdf
which is problematic for stability. Shallower snow is a source of instability, because weaker layers of snow are closer to the surface at these points and more easily triggered. A further issue is that propogation occurs more readily from shallower snow into deeper snow.
Putting it together, the protruding rocks can be areas of weakness in a snowpack (shallower snowpack & weaker bonds - 'sugary' snow).
Thinking about an avalanche path:-
In an avalanche path start zone, exposed and shallow buried rocks can form 'trigger' or zones of weakness.
Further down an avalanche path, as the avalanche speeds up to over 200km/h, boulders can still act as a source of weakness in the snowpack by allowing an avalanche to step-down on these weakneses in the snowpack to deeper layers. Of course, they also provide something solid to hit as a victim is carried in the avalanche flow.
Finally, in the deposition zone, rocks are terrain traps where the victim can be buried deeper.
The mechanism of 'islands of safety' being weak spots can be 3 mechanisms...
1. If the rock sticking out absorbs heat from sunlight this subtle heat can be transmitted to the snow and help trigger an existing weak layer.
2. The swallowing snowpack near an island can cause a steep temp gradient in very cold conditions and cause the growth of depth hore, thus forming a sweet spot.
3. As an obstacle which forms a cross loading feature and gets slab forming in the lee.
However, that said I still treat them as island of safety but I have triggered an avalanche when approaching one once!
Swallowing = shallowing
James (grumbling about predictive text)
The wheel diagram in the article shows the 5 point scale and there is a link to the European Scale definitions (same as on the SAIS site). My concern is that the impression given in that, for example, High (cat 4) is perceived as 4 on a scale of 1 to 5 (i.e. could be worse), rather than 4 on a scale of 1 to 4. I think it would be a good idea to avoid any mention of the Very High category in regard to UK avalanches if it can't ever happen.
The snowpack has continental characteristics, with a generally thin, highly stratified snow cover overlying a persistent depth hoar base. Its mean depth however was anywhere between 149cm and 222cm. These are volumes of snow that can are extremely uncommon in the UK except in high snow periods or areas of great accumulations, which in themselves bring up avalanche hazard anyway.
The weak layers they specifically identified in the results were depth hoar and buried surface hoar, which are extremely unstable. Again two rather rare conditions in UK snowpacks, they do still occur, we just don't get the required condition all that often.
The photo of the slope and the text makes me think they were not neccessary look at what happens with protruding boulder, by that I refer to ones that are actually visibly sticking out of the snow pack and are not buried. The photo of the test slope is certainly not what I have in mind when talking of protruding boulders and islands of safety. The text also makes no mention of protruding boulder and instead seems to look at buried underlying variation in snow depth. Where the thin parts of the snowpack cause by features like buried boulders push the weak layer closer to the surface making them more likely to fail on loading.
Hopefully that answer 1 & 3? I think the problem may have been your interpretation of 'protruding boulders'?
The second point is true, heating of rocks can cause point release avalanches and quite commonly mares tails. Again unless the rock is in the sun this effect is minimal, which is why it is more of a problem in contintential areas as they do have more blue sky days than Scotland. However whenascending at least, most people in the UK are on North facing slopes, away from any direct sunlight.
Perhaps you need to add a paragraph or so explaining the different characteristics of British vs continental snow packs and why they may be different. Also maybe clarify exactly what you mean by an island of safety and why not all perceived islands of safety are what they appear to be. Maybe explain depth hoar, buried surface hoar, propagation etc.
I'm pretty sure folk are going to come away thinking < boulders in slope = islands of safety > and not even question why they may not be.
That's certainly the impression I got, as a snow novice.
It is an avalanche basics article and sadly there was not enough space for everything, as you can apreciate the topic is huge. As such we tried to stay to the main points and guide people to seek either further training or reading. These subjects are covered in the book which is now available on iPad as well.
The reasons were actually different to the 3 you mention, and involved some moderately complicated physics. Will dig up the refs when I have spare time.. Or possibly if Dr Fyffe passes by here he may enlighten us!
Picking up on the boulders as islands of safety point. It is fairly basic mechanics - such points act as stress loci hence they act as release points. JIB refered to one article. There are many more
Also a rock does not have to be in the sun to cause heating. Typicaly the ground (i.e. the rock in question) will be warmer than the snow above (this is because the snow is a good insulator.
Your point about the article being focused on the UK and very shallow snowpack presupposes that the reader will make that distinction as he/she heads overseas. I doubt that they will.
I was also intrigued by your comment that most people in the UK will be on north facing slopes! Not sure that could be substantiated.
To make matters worse the process of release only occurs once for each avalanche, and occurs in the weak layer which will be covered by a layer of windslab, and is hard study due to safety issues and weather issues (most avalanches release during periods of bad weather).
Also the physics of release is very complex, especially when is spatial variability is considered.
There is a number a of variables which affect stability, some of which vary through time due to processes which occur in the snowpack. Some of these variables/processes may promote stability around boulders, some promote instability. Which set of processes is dominant,depends very much on many different factors. As well at the likely-hood of triggering, consequences of triggering an avalanche should also be considered. Being close to rocks, does that mean you are more likely to be on the edge of anything that you trigger for example.
There are so many variables that, like much avalanche stuff, I feel that the answer is not black or white, but it is somewhere in-between.
The highest danger rating definition ("Cat 5" or "Very High" or "Extreme" depending on which country's system you're using)says that the snowpack is generally weakly bonded and unstable, widespread natural and human-triggered avalanches are certain. It doesn't attempt to specify the likely avalanche size.
A Size 5 avalanche is one that could destroy a village.
So whilst "Cat 5" conditions may or may not produce a size 5 avalanche (and yes -it is pretty hard to imagine one of those in the UK!) they're going to produce plenty of smaller slides.
One other quick thing on the size scale - it is logarithmic - there's a factor of ten difference between each point on the scale so size 2 is ten times as big as a size 1.
Hope I understood you correctly and that clears up the question.
I probably am confused but I do understand there are two different scales here!
> The highest danger rating definition ("Cat 5" or "Very High" or "Extreme" depending on which country's system you're using)says that the snowpack is generally weakly bonded and unstable, widespread natural and human-triggered avalanches are certain. It doesn't attempt to specify the likely avalanche size.
I think this the crucial point: the European Avalanche Danger scale (which is the one SAIS uses) says:
"Very High: The snowpack is poorly bonded and largely unstable in general.
Numerous large-sized and often very large-sized natural avalanches can be expected, even in moderately steep terrain."
where "large" and "very large" correspond to Size 4 and Size 5. (from http://www.slf.ch/schneeinfo/zusatzinfos/interpretationshilfe/interpretationshilfe_e.pdf )
So it's still not really clear to me what cat 5 means in a UK context.
A search of the SAIS archive forecasts reveals that a cat 5 was issued on about 12 occasions between the adoption of the European scale in the winter of 1994/5 and 2007 (more recent forecasts aren't easily searchable). This is in line with the the 0.2% frequency of Cat 5s in Switzerland (see above link). I still think it is a matter of debate whether having such a rarely used category is useful in Scotland (I can see the point in the Alps as it indicated when the general public may be at risk).
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