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ABSTRACT
First,this paper succinctly describes ournumerical pow-
dersnowavalanche model.Then,an application ofthis
model ispresented in the frame ofan avalanche zoning.
Thisapplicationisanexampleofa practical useofthe
numerical model in avalanche risk engineering. Foreach
study the model was used to simulate different avalanche
scenarios. It allows to give a coherencebetween inputdata
(production conditions) and the effects of the avalanche.
The use of the model can be made according to the follow-
ing process :
*First it beginswith searchinginthe archivesfor themost
important powdersnow avalanchehaving taken place
on the site. This search concerns the conditions of pro-
ductionaswellasthe effectsoftheavalanche inits
flowing and run-out zones.Forthis purpose,different
indicators can be used. All caused damages "indicate"
that the dynamic pressure developed by the avalanche
wassuperiortothethresholdstressofthedamaged
objects (tree,pylon,etc.).Effects are mapped in terms
of pressure fields.
*Secondly we use the numerical model tosimulate an
avalanche scenarios whose entry conditionstake into
account allthe availableobservationsconcerningthe
productionfactors(meteorology,snow,startingzone
etc.). It is important to note here that such type of data
is very often sketchy forseveral technicalreasons ( ac-
cess to starting zone ...). Severalnumerical simulations
allow to specify and complete the model input condi-
tions. The purpose is to define a "referenceavalanche"
whichproduceseffectssimilar to theobserved one(pres-
sure field, velocities,height etc.).
*Last we define the "observed-extreme-avalanche" and
simulate it. The model gives access to a large spectra of
scenarios by increasing orreducing production condi-
tions orby modifying the topography of the flowing or
run-outzones.Therefore,we can buildadatabase of
avalanches that can be used in defining the protection
forthe desired site.
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INTRODUCTION
An analysis of the meteorological conditions causing the
main powder snow avalanchestakingplacein FrenchAlps
shows that this type of avalanches occurs most often after
a
strongsnowfallbycoldweather(T<-5ºCduring 24
hours precedingtherelease). Thepowder snow avalanches
are characterized by a low density, high velocities and by
the structure of its particularflow. Whirlwinds are one of
the remarkable characteristics that differentiate it from the
dense avalanche. The intense turbulence inducesa strong
incorporation of air and causes a hugeincrease of the vol-
ume.Thiskindofavalanchesishereconsidered asthe
flowofaheavy fluid(particles +air)ina lightone (the
air).
The powdersnow avalancheis similarto the gravity
current from the visual point of view and also in term of
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physical process.
Theequationsgoverningthisflowarefluidmechanics
conservation laws. The strong turbulence requires the use
of a closure model that allows to calculate both its inten-
sity and scale. The interaction between the avalancheand
thegroundis |
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