Analysis of the Durrand Glacier Avalanche Accident

Durrand Avalanche Report page 4
6. In addition to the Public Avalanche Bulletin and the publicly available information from
Rogers Pass, the Canadian Avalanche Association’s private Information Exchange system
(InfoEx) provided a considerable amount of additional information about the avalanche
hazard, specifically, the nature of the deep seated and reactive early season facet layer.
This information is described in general terms in the September 23 rd , 2003 Coroner’s
Inquiry report on this accident, and in more detail in the March, 2003 report for the Coroner
prepared by Larry Stanier. According to the Coroner’s Inquiry report, Selkirk Mountain
Experience was not a subscriber to the InfoEx system at the time of the accident.
C. Formation of slab avalanche hazard
1. To understand the significance of the avalanche warnings given above, and why the
presence of a persistent weak layer and overlying snow slab creates an avalanche hazard,
one must understand the forces that act on a unit of snow resting on a slope.
weak
layer
force of
gravity
old
snow
force of
gravity on
snow slab
shear
force
snow
slab
normal
force
37°
snow shear
force
normal
force
Shear force = force
of gravity x sin 37°
Fig. 5: Formation of slab avalanche hazard. An avalanche occurs if the shear force
component of gravity pulling the slab down the slope is greater than the forces (mainly
friction) holding it up. Adding a person to the snow will increase the shear force.
2. A layer of snow that forms a cohesive (bonded) unit is called a snow slab. When such a
slab sits on a slope, the only force acting on it is the force of gravity, which can be broken
down into a component acting perpendicular to the slope (the normal force), and one that
acts to try and pull the block down the slope (the shear force).
3. The normal force creates friction between the snow slab and the underlying layer, and this
frictional force keeps the snow slab from moving down the slope; that is, it acts to counter
the shear force. There are other forces that act to keep the snow slab from moving (shear
forces on the sides, a tensional force at the top, and a compressional force at the bottom),
but the frictional force is usually the greatest, and therefore the most important.
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