Complex spring day
28th March 2016
(Above) Cornice-triggered dry slab avalanche in Coire Chriochairein today at about 10am. Steep NE aspect at 1000m. 50m wide crownwall & 45-50cm deep, running out 300m.
(Above) Recent drifting has ‘refreshed’ cornice development over N to E aspects. Quite a large chunk fell off and triggered the slope below. Cornices aren’t quite as ubiquitous as they were a week or so ago but are still a consideration over some gullies and scarp slopes. My guess is that warming from the sun this morning weakened the newly formed cornice, which collapsed taking with it some deeper old cornice. In combination that would be quite a high loading on the slope below and obviously enough to trigger the slope.
(Above) A poke a round high on the scarp slope near the avalanche site revealed this. Found significant failures in a quite moist (moist enough to wet my glove) and very soft layer of ‘wet grains’ beneath a thin buried crust. On top of all this was relatively new, dry, consolidated windslab. In the picture, my ski pole is closest to the moist wet grains layer. If you look carefully you’ll see a colour difference between the different snow types. Pretty certain this was the failure layer for the nearby avalanche, though higher up of course with much deeper windslab overburden. This is a fairly localised, since deeper windslab is confined to the highest reaches of affected gullies and scarp slopes.
(Above) Looking down across the avalanche and cornice debris tip to Aberarder and Loch Laggan. 3cm of fresh snow down at 400m this morning but the snow line retreated up the hill to 600m by lunchtime. The sun has a big effect on snow at this time of year, occasionally dramatic.
(Above) The sun did its thing on the surface layers of the snow on this quite steep sun-affected ESE aspect in Coire Chriochairein. About 800m altitude at the top of the picture. Rollerballers down lown and single point releases & wet sluffs (…ahem!) higher up where the slope angle increases. There was a covering of relatively low density fresh snow in many places which responded quickly to warmth from the sun. Meanwhile elsewhere….
(Above) …..Bellevue Buttress in Coire Ardair. In the shade until lunchtime and just receiving sun here. Note the sluffing from the crags onto the crag apron: dry sluffs in this cold and shaded place. Again, low density snow and beyond its ‘angle of repose’, so sluffing off in this very steep location. (Click on the photo for better resolution).
(Above) Looking towards the upper section of Raeburn’s Gully (far left in distance), Pinnacle Buttress and the top end of Easy Gully (centre of shot in distance).
(Above) Looking up towards the crownwall from near the end of the debris tip. Easy to see where the chunk of cornice emanated from. The debris was cold, dry snow and quite a contrast to the moister surface of the untouched snow near the rucsac.
(Above) Part of the Post Face of Coire Ardair showing (R to L) North Post, Centre Post, South Post and a snowed up bit where Last Post normally develops. Noticed two climbers (arrowed) ascending the first part of Centre Post. They paused to have a good long look at it, decided otherwise and descended using safe travel techniques.
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