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Key words: automatic snow-depth profile, erosion rate, wind
effect,
ABSTRACT
For2 years, the CEN (Centre d'Etudes de la Neige - Snow
Study Centre) has beenmaking snow depth measurements
on a location of avalanchestarting zone which undergoes
snow transport by wind. The goal of this work is a refined
study of the temporal evolution of a snow profile.
The experimental site, alreadyusedfor studies on blow-
ing snow periods characteristics,is located in the French
Alps near Grenoble, at an altitude of 2800 meters. A slope
has been instrumented with a set of 7 snow-depth sensors
usingultrasonicwaves.Thesesensorshavebeen setup
betweentwopylons.The device deliversmeasurements
every15minutes.Concurrently,monitoringofmeteoro-
logical conditionsisachieved on a nearby site.The first
snow-depthrecordsshowthat,inmostcases,this
equipment does not suffer from heavy snowfalls, hoar frost
and strong windvelocity. However, statisticalpost-process-
ing is neededto eliminate bad values.
This experimental equipment is expected to provide a
link between fieldobservations of snow characteristics and
our theoreticalunderstandingof thenecessary time to erode
or accumulatesnow quantityaccordingto snow-grain mor-
phological features.
This paper describes the development and experimen-
tal testing of the installation, then the observations used to
correlateweather conditions, snow particles featureswith
evolution of snow pack are presented.
Studyframework
The effect of wind on snow grains begins during precipita-
tion. The weakstructuresof crystals arequickly fragmented
andtheir size decreases.Immediatelyafter a snowfall, snow
flakesbegintomoveassoonas wind velocityreaches a
thresholdcharacteristicofeachsnowparticle.The
thresholdusually increases withtime,depending on air
temperature, crystal characteristics and the quality of the
cohesion between snow particles [Guyomarc'h and et al.,
1994].So, these snow grains produceaccumulations with
more orless high cohesion and hardness.Redistribution
of the snow by the wind is essential forsnow-slab forma-
tion.Snowparticlesarepickedupinwindwardzones
where the windnear thesnowcoversurface isefficient
and deposited in leeward zones.This deposit is often at
the origin ofan overload that can lead to slab avalanche
release.
Research directions
Aftermuchresearch worksonblowingsnow,snowdrift
accumulations and, more generally, on wind effect on the
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snow distribution in high mountainous regions [Castelle,
1994], it appearsthat understandingtheevolutionof a snow
packwhich undergoeswind effect shouldalsoresultin
takingintoaccountmoreeffectivelyoftheerosionor
accumulation rate of an amount of snow thickness at the
starting zone of avalanches.
In orderto improve ourcurrent understanding, at the
present time, based on empirical knowledge, visual obser-
vations of calibrated woodensnow poles (apart from about
5meters)havebeenfirstacquired.Howeverunderbad
weatherconditions,itwasimpossibletohaveaccess to
the location and to obtain data regularly. In other respects,
for our studieson snow transport by wind, weneedto focus
on real time snow depth evolution data. These data need
to be compared to real time weatherinformation.Such a
correlation would provide a much more accurate picture
of snow evolution due to wind effect at the snow surface.
Unfortunately,it seems that such a set-up does not ex-
ist,especiallytofollowtheevolutionofthesnowpack
surface. For these reasons 2 years ago, the CEN has started
to develop and experiment a new device based on a set of
snowdepth sensors.These sensorshave beenset upon
two cables stretched between two pylons on a east-faced
slope. A first test of this apparatus, named"Profileur", was
carriedout duringthe last35 days of 1994-95 winter season.
Overthe winterseason of 1995-963 sensorswere added
and data of the whole season recorded.
What difficulties mustbe resolved?
Themostimportantpointwastobeabletohavegood
working order whatever the weather conditions. Secondly,
ourinstallation has to deliverregularinformation in real
timeandifpossible,beeasilytransmittedtoGrenoble.
Interaction between the installation and the environment
mustnotoccurand itsaccessmustnotbetoodifficult
during winterseason.Lastbutnotleast,the costofthe
installation has not to be too high.
What kind of sensor can be used?
ForseveralyearstheCENhasattemptedtoelaboratea
sensor basedon ultrasonic waveto measure thesnow depth
at high mountainous locations.The basic principle is the
following : the travel time of ultrasonic pulses emitted by
atransducer fixed about6metersabove theground and
reflectedbysnowsurfacebacktothetransduceris
measured. This transducer works successively as a trans-
mitterand then as a receiver.The microprocessorcalcu-
latesthesnowthicknessaccording tothreeparameters:
travel time, height of the sensor above ground level andair
soundvelocity.Asthelastparameteristemperature
dependent, theair temperature is measuredin a ventilated
shelterandtakenintoaccountinthecalculation.This
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