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Key words : Snowdrift, Local Avalanche Forecasting,Instru-
mentation
ABSTRACT
Wind can create even greaterunstableaccumulations of
snowinmountainousareasthanheavysnowfalls.But
knowing wind conditions is not sufficient to predict these
accumulationsbecause theirformationalsodependson
the snow qualityofthe snowpack surface all around the
measurementstation.Inconsequence, the assessmentof
snowdrift is requiredto improve avalancheforecasting. Ac-
cordingtothisassumption,averysimple(andcheap)
instrument has been designed to take snowdrift measure-
ments,andtestedinawind-tunnel.Thisinstrumentis
called 'driftometer'. The driftometer can catchsnow parti-
cles blown by the wind, whichmakes a daily evaluation of
thedirectionandintensityofthesnowdriftpossible.
Driftometers have been used by several safety services at
European skiresorts during three winterseasons.Snow-
drift data has been recorded as well as avalancheactivity.
This experiment has shown that there is a strong correla-
tion between snowdrift and avalancheoccurrence. The ex-
periment also demonstrates that the driftometer is a useful
tool foravalanche forecasting.
INTRODUCTION
Everyone agreesthat snowdrift is responsible for plenty of
avalanches. As a matterof fact, winds can create huge ac-
cumulations (fig.1) which are quite often very unstable.
As already mentioned, wind measurements do not give
very reliable information about snowdrift intensity which
also depends on the quality of the snow cover.Let us re-
memberthat snowdrift can only occurwhen the snow is
mobile;there may be not at all,even during strong wind
situations (fig.2)!
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Instead of using wind data forsnowdriftassessment and
resulting avalanche risks,a direct measurement of snow-
drift would be better!
This idea of measuring snowdrift is not new and a lot of
scientists andtechnicianshavebeenworking onthis subject
during the last 35 years.They built many sensors,but,as
far as weknow, none canrecordsnowdrift direction during
24 hours without energy: an instrumenthad to be designed
forsuch a purpose.
We called this instrument a 'driftometer'.
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DESIGN
The design ofthe driftometerwas based on 2basic con-
straints imposed by practice:
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the instrumentshould record snowdrift directionas well
as snowdrift intensity.
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theinstrumentmustbecheap and workwithouten-
ergy
In
1990twoprototypeswerebuiltatCEMAGREF
(R.Bolognesi, F.Naaim, F.Ousset)andone of themwas tested
in field conditions during winter1990-91.This first trial
was not successful: the prototype was not able to indicate
the snowdrift directionwith sufficient reliability. After this
experiment, the developmentof the instrument continued
at
SFISAR(R.Bolognesi,O.Buser)incooperationwith
CEMAGREF.
Twoproblems hadstill to besolved: separationair / snow
particles and determination of snowdrift direction.
1.1.SEPARATION AIR /SNOW PARTICLES
Learningfrom variousexperiments (Mellor, Foehn, Meister,
Castelle), we decided to catch snow particles blown into a
tube by the combined effects of filterand pressure fall.
Then we built some new simple prototypes in order to
check in the field that the tube sections and the volume of
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