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DensityandFrictionMeasurementsinaFlowingDrySnowAvalanche
K.J.Burrell1,J.D.Dent1,M.Y.Louge2,D.S.Schmidt1,E.E.Adams1,T.G.Jazbutis1
1Department ofCivilEngineering, Montana State University, Bozeman, MT 59717
2Mechanicaland Aerospace Engineering,CornellUniversity, Ithaca, NY 14853
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ABSTRACT
UsingtheRevolvingDooravalanchefacilitynearthe
Bridger Bowl Ski Area (Dent et al 1994),measurements of
density and dynamic friction in a flowing avalanche were
made.Correspondingflowdepthswereconcurrently
measured using a floating arm with a small skid that rode
on the surface of the snow.
Measurementsof density were made using two devices;
a capacitanceprobe (Louge et al 1996),and calibrated op-
tical sensors that serve to measure flow velocity.The ca-
pacitanceprobe was constructed to measure the dialectric
constant of any material that passes in front of it.Through
a calibration procedure, the dialectric constant of a given
type of snow can be related very accurately to the density
of that snow.The capacitanceprobe was used in one ava-
lanche last winter to determine the density of the snow at
1 cm and 6 cm depths within a 1.5 m deep avalanche.In
addition, optical sensors were used to measure the reflect-
ance of the snow as it passed the sensors.The reflectance
measured is determined in part by the density of the snow.
By proper calibration, the signals from the optical sensors
can be related to the snow density.Signals from adjacent
optical sensors arealso cross-correlatedto determine snow
velocity.Last winterdensity and velocity measurements
were made in several slides at various depths using these
optical sensors.The results indicate that snow density is
largest at the bottom of an avalanche, upwards of 400 Kg/
m3 ,and in the first few centimeters from the bottom,falls
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off rapidly to about 300Kg/m3 .The density then contin-
ues to decreaseslowly to about 250 Kg/m3at the surface of
the avalanche.
The dynamic friction coefficient at the base of two ava-
lancheswasfoundbymeasuringtheshearand normal
forcesonaroughened23cmx28cmaluminumplate
mounted flush to the running surface.Deflections of the
plate were measured using strain gauges, from which the
forces on the plate could be found.The ratio of the shear
force to the normal force on the plate provides a measure
of the friction coefficient at the base of the slide.Of the
two slides measured, thedrier anddeeperslide hadalower
coefficient of friction which allowed it to travel faster and
farther.
INTRODUCTION
Improved observations of the dynamics within avalanches
is required before an accurate model fordetermining ava-
lancherunoutdistancescan bemade.Theidea thatan
avalanche may move as a single mass while riding almost
passively upon a thin shearing layer at its base comes from
themodelingofgranularmaterials.To completethede-
scription of the dynamics within this shearing layer,it is
necessarytoobtainaccuratemeasurementsoftheflow
parameters such as velocity,the solid volume fraction or
density and the stress ratio within this layer.This paper
documentstheimplementationofinstrumentationused
to provide accurate measurements of these parameters.
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