A v a l a n c h e

D y n a m i c s

a n d

D e f e n ce

SnowAvalancheExperimentsatSkiJump
K.Nishimura¹,Y.Nohguchi²,Y.Ito¹,K.Kosugi³andK.Izumi⁴

1 _{Institute of Low Temperature Science, Hokkaido University, JAPAN 060}

2 _{Nagaoka Institute of Snowand Ice Studies, NationalResearch Institute for Earth Science and DisasterPrevention, Suyoshi,}

Nagaoka, Japan 940

3_{Sinjo Branch of Snow and Ice Studies, National Research Institute for Earth and Disaster Prevention, Shinjo, Yamagata,Japan 996}

4 _{Research Institute for Hazards in Snowy Areas, Niigata University, Niigata, JAPAN 950-21}

Key Words: Avalanche experiment, ski jump, ping-pong balls

ABSTRACT

In 1995,we have started the experiments at a ski jump to
investigate the avalanchedynamicsand internal structures
in detail. In winter, natural snow 300 kg in weight at maxi-
mum was released and flow velocities,impact pressures,
inducedwind velocities, anddynamic friction coefficients
were measured. The observation setup is almost the same
as the one installed in Kurobe Canyon where a systematic
investigation of naturalsnow avalancheis under waysince
1989.Insteadofsnowitself,insummer,wehaveused
300,000 ping-pong balls.They were stored in a container
set on the top of the inclined plane and were released si-
multaneously. Movements of the individual balls and flow
behaviors were recordedwith several video cameras. Since
the air drag gavea large effecton the ping-pongmovement,
the flow arrived at a steady state within a relatively short
distance.The frontvelocitiesstronglydepended onthe
numberof released balls.In addition,the flow formed a
distinctheadandtailstructure,whichhadbeenoften
observed not only the snow avalanchebut also other large-
scale geophysical flows in nature.

INTRODUCTION

an appropriate model which describes the avalanche mo-
tion,at least we need to know the velocities and densities
as a function of time and space. However,mainlydue to
its formidable characteristics,even the simplest measure-
ments have been difficult toperform innature. Thus,in
1995,we have started the snow avalancheexperiments at
a ski jump so as to investigate the avalanchedynamics and
its detailed structures. In winter,natural snow 300 kg at
maximumwasused.Insummer,ontheotherhand, we
have released from 2 to 300,000ping-pong balls to simu-
late the three dimensional granular flows.This paperre-
ports the above experimental procedures and gives some
results of the preliminary analysis.

MEASUREMENTS

Experiments were made at the Miyanomori ski jump field
in Sapporo,which has been used forthe normal-hill ski
jump competition; Olympic games were heldin 1972 there.
Figure 1 shows the cross section of slope.Snow flow ex-
periments in winterwere carried out along the approach
and ping-pongball experiments in summer on the landing
bahn.Both flowsdescended the slopes,the steepest part
of which was 36 deg., andcame to rest near the kante or on
the braking truck.

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