C a s e

H i s t o r i e s

with low return frequencies and uncertainties in the pre-recommend further improvements and the obser-
diction of theircharacter.vationsofavalancheoccurrence,snow,andwx
In1992oneofthetowers,113Rwhichwas rated asex-data.
posedtorare avalanches wasdestroyed.Afterthisthird
event of relatively low risk towers being destroyed in the^{1960-61 First winter of avalanche occurrencepatrols, con-}

past 20 years, Alcan waskeen to have a further risk assess-^{tinued to 1974.}
mentundertakentoattempttodeterminewhichtower^{1961Addendumto Tower Reliability Report (M. Parker)}
would be next.^{1973Dry snow avalanchein TwinPeaksbowl destroyed}
In 1993,C.Stethem & S. Flavelle grappledwith the ob-^{Tower 105L. de Quervain recommendsprotection}
jectiveofthistaskand concluded as thosebeforethem,^{in the form of a steeldeflector andsplitting wedges}
that it is not possible to predict when and where the next^{forthe individual towerlegs.}
rare,exceptional,unusual,orunprecedented avalanche^{1978deQuervainsitevisit.Evaluatesavalanche pro-}
would strike an unprotected tower.Instead a risk matrix^{tection on existing lines and avalanchehazard on}
summary was developedto examineon a larger scalewhich^{proposed new coastal route.}
towers have the most risk from those types of avalanches,1985Feb. Dry avalanchedestroyedTower 124L. Unprec-
what additional risksordifficulties may be encounterededented avalanche event.HerbBleuerprovided
during reconstruction,and where the risk of both parallelsome avalanche safety during winterreconstruc-
linesbeing knockedoutsimultaneouslybya singleava-tion.PeterSchaerer wrote reports:Rating of haz-
lanche event may exist to create the worst economic sce-ard exposuretotowersand recommended addi-
nario.tional protection.
Insummary,itappearsthatoriginallysnow
avalancheswerenot reckonedwith as a hazardto the trans-^{1992Dec. 27 Dry slab avalanche destroys Tower113R}

mission lines. Though after thefive tower disaster in 1954,^{at Kildala Pass. Damageddeflector at Tower 116R.}

the caternary suspensionwas built andprotective measures^{ColinZachariasand ScottFlavelleprovideava-}

were emphasized forthe accessible and predictable slide^{lanche safety during reconstruction to Feb. 8.}
pathsin thelower valleys. This seemedto solve theproblem^{1993Spring Chris Stethem and Scott Flavelle produce}
for the next 20 years. However, eachsubsequent damaging^{avalancheatlas and author a further report on the}
avalanche incident: 1973,1985,& 1993,involved the rare^{Avalanche Terrainand Avalanche Hazardtothe}
and exceptional avalanches that the consultants had rec-^{powerline. Its objective was to estimate the degree}
ognized as a low but potential risk.This risk of damage to^{of risk to the various towers from avalanches, and}
the towers must be an inherentacceptablerisk unlessmajor^{the risk encounteredduring winter repairs, as well}
mitigation work iscarried out forall 21exposed towers.^{as suggesting additional protective measures.}
As it stands, it is only a matter of time before another tower^{1993Nov.21 Strong outflow winds knocked down the}
is hit,but which one, is the million dollarquestion.^{replacement Tower 113R.}
Avalanche safety was provided through Stethem
CHRONOLOGY OF AVALANCHE RELATEDEVENTS^{& Associates, with Herb Bleuer, ScottFlavelle, and}
HectorMacKenzieasonsiteavalanchetechni-
YEAREVENTS^{cians. Powerwas restored Feb. 10.}

AVALANCHE SAFETY PROGRAMS DURING
TOWER 113R RECONSTRUCTION

Tower 113R is located on the SE side of Powerline Pass in
the Glacier Creek drainage of the Kemano Riverin an al-
pine cirque. Thesite is threatenedby avalanchepathsfrom
both valley sides, specifically the track of a SE aspect ava-
lanche path (Gc4)and the runout zone of NE aspect path
(Gc3).
Numerous avalanche paths affect the routes approach-
ing Powerline Passfrom both the Kemano and Kildala Val-
leys.Winter access is feasible via helicopter.
The avalanche safety programs during the two winters
of tower113Rreconstructionconsisted of the following
components:
Preparation of AvalancheSafety & Rescue Systems
Survival Plan, Equipment and Shelter
Daily HelicopterReconnaissance & Site Access
Daily Weather Forecasts, Base &Field Weather Obser-
vations
Snow Stability Observations
Daily Avalanche Hazard Evaluations

1951

Transmission line surveyed. Dr. M.R. de Quervain
commented on emergency sites.

1952-53 Transmission line construction.

1954

1954

1955

1956

1957

1958

1960

258

Spring Second transmission circuit constructed.

Aug.1 Transmission lines energized.

Jan.25Avalanches inGlacierCr.destroyed five
transmissiontowers.Powerwasrestoredseven
days lateron a patched left circuit.During sum-
merand fall that year the caternary was rigged to
suspendthe trans mission lines 500' above theGla-
cierCr.valley.(deQuervainrates hazard totow-
ers, recommends protection)

Wood& steel deflector built for Tower 116R,earth
deflectors built wherever material and cat access

Dec.7Mixedwetsnowand rockavalanche de-
stroyedT230inKildalaValley. (reportbyDr.V.
Dolmage)

TowerReliabilityReport-protectionmeasures.
(M.Parker)

de Quervain evaluates the avalanche risk for tow-
ers,appraises protection work being carried out,