|
Key Words: Avalanche,History, Mitigation,Survival
ABSTRACT
Scott Flavelle will trace the history ofavalanche damage
to the Alcan transmission towers and the various mitiga-
tion methods recommended by ouravalanche forefathers
since the powerline was first builtin 1953to tworecent
towerfailures in 1992 & 1993.
Hector MacKenziewill then describe his psychological
epic of being trappedby weather in aplywood coffee shack
inalarge avalanche track withmodernlinesmanwhile
avalanches are running...
INTRODUCTION
TheAluminumCompanyofCanada(Alcan)has oneof
Canada's oldest avalancheproblems. Since the first winter
ofoperationninetransmissiontowershavebeencom-
pletely destroyed by avalanchesandmany others damaged.
Severalsnowavalancheconsultantshaveprovided
recommendations overthe last fourdecadesin regards to
theavalanche hazardand implementationofprotective
measures.The intent of this paper is to provide a histori-
caloverviewoftheconsultantsinputontheperennial
avalanche hazard.
PURPOSE OF THEPOWERLINE
An extensive lake system was dammed on the east side of
theCoastMountainRange toprovideelevated fallfora
hydro electric generating facility at the head of a long sea
level fiord.Two single circuit transmission lines conduct
the electricity from this site,Kemano,80 kmoverland to
the aluminum smelter located in the town of Kitimat at the
head of another fiord.
TERRAIN
The transmissionlines follow theKemanoRiver Valleynear
sea level for 15 km before ascendingandcrossing the 5000'
Powerline (or Kildala) Pass through10 km of alpine before
descendingthe 7 km Hanging Valleyto the Kildala Valley,
again nearsea level.Finally,the linescontouraround a
bay before crossing a sub-alpine pass to access Kitimat.
The transmission towers in the river valleys are almost
all locatedon thealluvialfans below steepgulliesandcreek
beds,taking advantageofthe high spotstomaximize the
linespan.However,giventhat avalanches ontheNorth
Coast frequentlyreachsea level, many of these towers have
been affectedby avalanches running out onto the alluvial
fans, particularly ones consisting of wet snow mixed with
debris. The avalancheterraintraversedin thealpine(3000'-
5000')consistsoftheusualcoastalmixofterrain
configurations-lowangle wideopen bowls,steep open
slopes,and well definedcouloirs and chutes of all aspects
and ground roughness.
|
CONSEQUENCESOF POWER OUTAGE
If power to the aluminum smelter is lost for more than six
hours the aluminum "freezes" in the pots requiring a $1
million start up and refurbishing as the aluminum has to
bejack-hammeredoutofthepots.Duetothepotential
extent of industrial setback, Alcan placed a second trans-
mission circuit alongside the first in the hazardous areas,
and on the same towers in safe areas. In this way, as long
as onlyone side of the parallel linesare lostpowerstill
gets to the smelter. As the second circuit is not used by the
smelterexcept inemergencies itnow conducts surplass
powerdestined fortheBCHydropowergrid.So pres-
ently,ifeitherofthelinesarelost,significantfinancial
loss will be incurred.
AVALANCHE HAZARD TOTOWERS
In the alpine regions it is normal to have a 5 m snowpack,
whilethe depth ofsnowatsealevel mayvarydramati-
callydepending onthewinter.Stormsareusuallyvery
severewith strongwinds,fluctuatingtemperatures,and
heavy precipitation.
Each tower exposedto an avalanche hazard hasits own
unique avalancheterrain situation, this combined with the
fullspectrumofmountain weatherand snowpacks over
the 42 yearlife of the line, has producednumerable unu-
sual,rare,and insomecasesunprecedented avalanche
events causing tower,work camp or forest destruction.
Fourreports on the estimated snow avalanche risks to
the towers were written during the past 36 years.
In1955&1960,Dr.M.R.de Quervain expressed that
there wasa "regrettable lack of information on currentava-
lancheactivity"and that"systematic observations on snow
and avalanches"wereneeded. Based on avalanche events
he concludedthat it was the lower elevations which were
mostatriskdueto"wetandmuddy avalancheswhich
advance totowersitestakentobereasonablysafe".He
further noted that Alcan's previously accepted "1955 cal-
culated risk" was reduced, warranting the construction of
additional earth deflectors. All towers were given a rating
of the hazard based on an unprotected site.
In 1973 an avalanchedestroyed tower 105L which was
located mid track of a large moderate angled alpine bowl.
Thistowerhad received areasonably saferating -"ava-
lanches only in exceptional and unexpected situations."
And in 1985 another tower 124L with the same rating was
also destroyed along with several hectares of forest.
In 1985,P. Schaererrated the avalanche hazard to all
the towers in the alpine. In using a similarrating criteria
as de Quervain he agreed and concluded that there were
stillmanytowersatsignificantriskwhichshouldhave
protective measures taken, as well as many towers which
would remain exposed onlytorare but large avalanches
|
|
1
2
3
