Facets & slope failure
18th January 2013
(Above) Beinn a Chaorainn: Slope failure on facets at 720m on easy angled ground. Note the long crack centre right of the photo. We experienced a much bigger slope failure at lower altitude on the way back to the forest with cracks in excess of 15m.
Slope failure is when a weak layer (which is usually relatively thick) collapses underneath the snow. If it happens fast enough, and the slope angle is steep enough, the slope will avalanche; if it’s a relatively slow collapse the slope will collapse down ( & sometimes emit the classic ‘whumpf’ noise) but not avalanche. The failure in the weak layer is technically a ‘shear-fracture’ and this failure has to propagate across the slope in a split second if the slope is to slide. Slope failure is often a pre-cursor to avalanche activity (on nearby steeper slopes at higher altitude, perhaps) so is one of nature’s little warning signs that the Big White Bogeyman wants to come out to play.
Huge amount of snow has been scoured and shifted in the last 24hrs by strong SE winds. Lee slopes are not totally white but where the new windslab has accumulated it’s underlain by a layer of very weak facetted crystals. This layer was 10cm thick in places today. The recent prolonged cold temperatures have encouraged the growth of these sugary-looking crystals and if the cold conditions continue they’re likelyto be with us for some time.
(Above) Pit site slope failure with long deep cracks top and bottom. Bitterly cold in the strong wind today.
(Above) On the way back. A different world in the woods…a calm sylvan scene
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