S n o w

C o v e r

S t a b i l i t y,

A v a l a n c h e

I n i t ia t i o n

a n d

F o r e c a s t i n g

Effect

ofSnowTemperaturesonSkierTriggeringofDrySlabAvalanches
D.M.McClung¹andJürgSchweizer²

1 _{Department of Geography, University of British Columbia}

1984 West Mall, Vancouver BC V6T 1Z2, Canada

phone: +1 604 822 9157, fax: +1 604 822 6150, e-mail: mcclung@unixg.ubc.ca

2 _{Department of CivilEngineering,University of Calgary}

2500 University Drive NW, Calgary AB T2N1N4,Canada

phone: +1 403 220 7479, fax: +1 403 282 7026, e-mail: jschweiz@acs.ucalgary.ca

Keywords: dry snow, snow temperature, snow strength, slab

avalanche,avalanche formation,skier triggering

ABSTRACT

Fieldobservationsandexperience showthat snowtem-
peraturescan havea stronginfluence ondrysnowslab
instability.Experience showsthattherearetwogeneral
categoriesof importantcompetingeffects: 1. metamorphism
(dependingon temperature, temperature gradientandother
snowproperties)andcreep;2.mechanicalproperties
(excludingmetamorphismeffects) includingsnow stiffness
(hardness),fracturepropagationpotential(failure
toughness)andstrength.Thereare twogeneral features
whichseparate thesecategories:1.theymayoperateon
different time scales and 2.fora given snow temperature
they usually operate in opposite directions with respect to
stability.Forexample,warmersnow temperatures imply
faster bond formation due to metamorphism in a potential
weaklayerthereby increasing stabilitybutwarmertem-
peratures in the weak layeralso decrease snow stiffness,
failure toughness and strength.

TEMPERATURE EFFECTS

McClung (1995,1996) described the effects of snow tem-
perature on hardness, failure toughness andstrength. Fig-
ure 1 shows a schematic ofthe effects and definitions of
the terms based on experimental results from slow, direct,
simple sheartests on alpine snow. The important effects
are: 1.stiffness (hardness) is highly temperature depend-
ent.The increase in stiffnessisabout 100 % as the tem-
perature decreases from -2ºC to-15ºC.Thisisthe most
important temperaturedependentproperty of alpine snow.
The stiffness isdefined as the initialresistance to defor-
mation and this property is closely related to hardness as
determinedwiththehand hardnesstestinthefield.2.
Failure toughnessdefined as the work input neededto fail
the material(reacha peakon the stress-strain curve) is tem-
perature dependent. It is equivalent to the area under the
stress-strain curve when the peak is reached. The failure
toughnesstypicallyincreasesbyabout20-40%asthe
temperaturedecreasesfrom-2ºCto-15ºC.3.Failure
strength(defined aspeakonthestress-straincurve)in-
creases with decreasing temperature. Over the range -2 ºC
to -15 ºC,we estimate that the strength increases by about
25 %.We are somewhat uncertain about this lattervalue
due to scatter in our shear testing results (McClung, 1977;
Schweizer, 1996): natural strength variations in a layer are
similarto the effects of temperature on strength.

curve until a peak is reached. Stiffness is defined as theinitial slope
of the stress-strain curve. (b) Schematic of stress-straincurves for
alpine snow of two different temperatures: strength, toughness and
stiffness decrease with increasing temperature.

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