Analysis of the Durrand Glacier Avalanche Accident

Executive Summary page 2
The destination chosen for this fatal ski tour first required going down into the narrow valley of
Cairns Creek, and then up to a sub-alpine area called Swiss Meadows. This part of the route
required crossing potentially high risk and unavoidable avalanche terrain on the south face of
Tumbledown Mountain during a period of considerable avalanche hazard. The overall slope of
this face is about 38° to 40°, which, statistically, is within the most common range of slope angle
where avalanches occur. An avalanche occurring on this face would carry skiers down over
bluffs or through gullies and into the terrain trap at the confined bottom of the Cairns Creek
valley.
From Swiss Meadows, the group originally intended to go up low-angled and relatively safe
slopes to the summit of Fronalp Peak, as shown on Selkirk Mountain Experience’s guiding map.
Instead, they toured up below the west ridge of Tumbledown Mountain on a route that was so
close to the base of the slope that it was subsequently overrun by the avalanches that occurred
later that day.
The La Traviata couloir, where the fatal accident occurred, is a steep, wind-loaded, 310 metre
long shallow gully with a 37° upper slope where the avalanche started. All the avalanche
reference books and other sources consulted warn that such couloirs have a high probability of
being potential sites of avalanche activity. According to the references, the specific features of
this couloir that were indicators of high avalanche hazard are:
(a) The slope steepness. Avalanche accidents most commonly occur on slopes of 31° to 35°;
this one is even steeper at 36° to 37°.
(b) Lee slope build-up and cross-slope loading. Most avalanches occur on slopes where
winds cause snow to accumulate and build up to form dense and potentially dangerous
snow slabs. In this case, more than a metre of additional snow was present in the gully.
(c) A convex upper slope. At such places of convex curvature, a snow slab resting on a
persistent weak layer would be in tension due to the difference in the rate of snow creep
above and below the break in slope. A skier crossing over such a convex slope is more
likely to trigger a release compared to other slopes.
(d) Relatively smooth underlying terrain. This would not provide the anchors that a more
coarse, rocky slope would have, and means that the slope would have a relatively small
surface roughness depth. Compared to other areas, this slope would be more prone to
slide with lesser amounts of snow.
(e) Connectivity to an area of shallow snow. In such areas, the weight of a person skiing over
into the more shallow snow pack can set-off a skier remote triggered avalanche. This is
more likely to happen if the snow slab rests on a persistent weak layer, such as a layer of
faceted crystals, as was the case at Durrand.
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