Avalanche Investigation Hector South Area
Banff Yoho & Kootenay National Park
snow.safety , Saturday 9th January, 2021 4:30PM
Parks Canada Visitor Safety field team investigated a sz 2 skier accidental slab reported on January 7 :Reported on MIN as being skier triggered sz 2 from 20m directly above crown- no involvement. https://www.avalanche.ca/map?panel=mountain-information-network-submissions/1f835ba4-e882-4216-9a59-1a120ce2b14f
This is a thin snowpack area. Height of Snow along the crown was thin at the convexity where failure occurred is no more than 60cm and the Dec 13 failure plane here was no more than 30 cm deep. Although crown depths averaged around 60cm and the corresponding height of snow along the crown was likely as deep as 150cm in places, still very thin given this is clearly a lee slope subject to loading. Investigation found well developed faceting in a weak snowpack with easy digging to the ground and low hand resistance throughout. Compression tests along the crown at an HS of 107cm in rocks found: CT7 resistant planar down 33cm on decomposing fragments plates and facets sz 1.5-3 below an old, faceted windslab (December 27th). CT21 sudden planar down 69cm on decomposing surface hoar sz 2-5 sitting on a 3mm pencil minus melt freeze crust (December 13 and failure plane of avalanche) . CT17 sudden collapse down 83cm on facets sz 3. Ski cutting just above the crown towards the right flank produced failures on the December 27 interface only.
Crown was at 2400m on a NE aspect. Quick estimate of dimensions would place this avalanche at conservatively 80m wide and 250m in length: A very healthy size 2. The crown was found at an obvious convexity in the slope that corresponded to shallower snow depths and crown height near the ski line. Rocks protruded into the bed surface at several points along the crown indicative of a small rock band or outcrop. Estimate 33-37 degree slope angle across bed surface. Crown height tapered significantly from as little as 30cm up to 80cm. Bed surface was a subtle 3mm melt freeze crust (estimate pencil minus resistance) over facets sz 2-3. This is the December 13 interface.
Source: Avalanche Canada MIN