I n s t r u m e n t s

a n d

M e t h o d s

GlideAvalancheForecasting
A.Wilson¹,G.Statham²,R.Bilak²,B.Allen³

INTRODUCTION^{returns toan upright positionready tomonitorthenext}
The forecasting of glide avalanches is particularly impor-^{avalanche cycle.}
tantwhen human lifeorstructuresarethreatened.The
bestwaytoreduce theimpactofsuch avalanches isby^METHODOLOGY
suitable controls.Control requires that closures are in ef-_{The starting zonedimensions (Figure 1) are75 meters wide}
fect during unstable periods and/or explosives are used to_{by 250 meters long with a ground surface of grass through-}
minimize any risks.However, gliding, and the stability of_{out.Theremoteweatherstation,fromwhichweather}
a glide slabs are not easy to predict rendering the control_{correlation's were taken, is located a short distance from}
processa challengingtask.Until recently it was thought_{the 3.7 start zone.}
that water had to be present forgliding to occur [McClung
1987].However,a two yearstudy showed that in 88% of
the cases of gliding water was present at the snow ground
interface; theother 12%were duringcoldperiods [McClung
1990].This,along with the fact that glide avalanchesdo
notnecessarilyoccurduringstormcycles,leadstothe
conclusion thatforecastersrequire moreinformation if they
are to make accurate forecasts.
It is generallyagreedthat rapiddisplacementof the snow
pack occurs before glide avalanches[McClung 1987].The
physical measurement of displacements in the snow pack
has been attempted in the field [Akitaya 1988, In der Gand
1966,Lackinger 1986]but allofthese methodscan only
measureoneavalanche cycle.Thus,unfortunately,the
technologytoprovideforecasterswithcontinuousglide^{Figure 2. Schematic of theProbe used to measure}
monitoring throughout the a glide cycle season has not as^{tilt angles on the glide slope}
yet been developed.
This paper presents a new approach which has the po-^{Glidemonitoringwasachievedthroughsprungprobes}
tentialtomeasuresnowmotionthroughoutthewinter^{(Figure2)mountedinthenativegroundsurface ofthe}
season.The avalanchestarting zone chosenwas path #3.7^{avalanche starting zone.Three probeswere installedin}
at Galena Pass, British Columbia.Path 3.7 is a southeast^{the 3.7 starting zone, in a downslope, inline array with 50}
aspect witha verticalfall tothehighway of700meters.^{meters between each sensor (Figure 3).The probe located}
The instrumentation was a sprung probe which tilts as the^{nearthetopoftheslopewasapproximately20meters}
snowmoves.Oncetheslopehas avalanched the probe^{below the highest glide crack location.One inch threaded}
rod was grouted into the ground, andwhen the probe was

Figure 1: Glide Path 3.7 at Galena Pass, British Columbia.

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