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

Experience

withStabilityEvaluationforaSurfaceHoarLayer

duringwinter1995-96atRogersPass,BritishColumbia,Canada
JürgSchweizer¹,Da vidSkjönsberg²,BruceMcMahon²

1 _{Department of CivilEngineering,University of Calgary, 2500 University Drive NW, Calgary AB T2N 1N4, Canada}

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

2 _{Avalanche Control, Mt. Revelstoke and Glacier National Parks, PO Box 350, Revelstoke BC V0E 2S0, Canada}

phone: +1 604 837 7516, fax: +1 604 837 7536, e-mail: dave_skjonsberg@pch.gc.ca

ABSTRACT

THE AVALANCHE CYCLES DURING JANUARY 1996

Key words: snow crystals, snow strength, snow cover stabil-During roughly the first two weeks of January 1996a total
ity, avalanche forecasting,avalanchecontrolof 207cm of new snow was recorded at Mt. Fidelity (Fig.
1).Fourmajoravalanche cycles occurred (Fig.2).All be-
INTRODUCTION^{gan withnatural activity and resulted in acontrol program.}
The avalanche control section at Rogers Pass,British Co-^{The snowpack weakness of concern was a buried surface}
lumbia,Canada,isresponsiblefortheavalanche safety^hoarlayer.
within Glacier National Park. This involves the protection
oftheTransCanadaHighway(TCH)andtheCanadian^{SURFACE HOAR GROWTH}
Pacific Railroad (CPR)mainline and the issue ofan ava-The surface hoar crystals had formed towards the end of
lanche warning bulletin for backcountry users of the park.December1995during anticyclonicweatherconditions.
When avalanche conditions become critical the transpor-A prominent temperatureinversion (Fig. 3) due to noctur-
tation corridor is closed and avalanchesare releasedartifi-nal radiative cooling and primarily calm conditions,had
cially by artillery fire.favoured crystal growth in the valleys, and in particular in
Primary parameters forstability evaluation include up-an elevation band just below the top of the inversion layer
per elevation meteorologicaldata (e.g. to assess snow trans-where clouds (radiationorground fog)had formed (Fig.
portbywind),indexvaluesfromstudyplots(e.g.shear4).This feature of spatial distribution strongly influenced
frametestresults),stabilitytestsfromslopes(e.g.stability evaluation.
rutschblocks,hand charges) and avalanche activity(ava-
lanche observation patrol).
Theprinciple observationsiteisMt.Fidelity 1905m^DISCUSSION
a.s.l. Traditionalweather and snowpackobservation in the^{Ofparticularinterestwasthereactivity(sensitivity,re-}
study plot is supplemented by fourautomatic stations lo-^{sponse to dynamic loading) of the surface hoarlayer, i.e.}
cated in the control areathat collect additional data that is^{when andwhereavalanchesactuallyoccurredand/or were}
transmitted by radio modem to the control centre at Rogers^{triggered. General experience suggests and measurements}
Pass 1300 m a.s.l.where another study plot is located.^{show that surfacehoar layers canbe snowpackweaknesses}
for weeks (Jamieson, 1995). However, this layer seemed to
be critical only fora relatively short period of time.

Figure 1 Weather and snowpack development at Mt. Fidelity 1905 m a.s.l.

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