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

HeadFormationinLightGranularAvalanches YasuakiNohguchi1andKoitiNishimura2 1 Nagaoka Institute of Snowand Ice Studies, Suyoshi, Nagaoka, 940 Japan Tel:+81-258-7522, Fax:+81-258-0020, e-mail:nhg@nagaoka.bosai.go.jp 2 Institute of Low Temperature Science, Hokkaido University, Sapporo, 060 Japan

Key Words: Avalanche,Head, Tail, Similarity

ABSTRACT

A sufficiently developed natural avalanchealways has a
head at the front end anda tail at the rear end.To experi-
mentally simulate avalanches with such a head-tail struc-
ture,avalancheexperiments were carried out using table
tennis balls,golf balls and styrene foam particles, etc. as a
model material for snow.As a result, only in the granular
avalanchesoflightmaterials,clearheads wereformed.
Fromtheseexperimentsandtheoreticalanalysis,itwas
found that the similarity of the formation of this structure
relates to the chute length L and the accelerlation of grav-
ity g and the terminal velocityof the granularavalanche
on the chute V
e^{through a dimensionless numberV}e ^2/Lg.
When V
e ^{2/Lg << 1,the head-tail structure is formed.}

1. INTRODUCTION
   
   Mostofavalanche researchershaveneverbeendirectly
   caught up intoactual avalanches.If such an experience
   were not very dangerous,it might be very signifficant for
   the advance of the avalanche science.To dothissafely,
   using light particles such as table tennis balls(TTB) and
   styrene foam particles (SFP) instead of snow orsand,we
   are conductingmodel experiments of avalancheson small
   slopes (Kosugi et al.1995and Nishimura et al.1996).In
   these avalanches, observers can obtain scientific informa-
   tion of the avalanchecloseat handwithout danger.In these
   experiments,the similarities between natural avalanches
   and model experiments are essentially significant to esti-
   mate any results of the model experiments.
   One of the mostimpressiveproperties of the shape of
   large scale natural avalanchesis that they have a head-tail

structure like a teardrop as is well known in gravity cur-
rent.Savage and Noguchi (1988),Nohguchi et al.(1990)
and Hutter andNohguchi (1991) examined the behavior of
granular avalanches by qualitative analysis of the similar-
itysolutions.Asa result,a bodyofgranular material al-
waysspreadsonaeven slope,butahead-tail structure
couldnotbeformedusing theirtheoretical framework.
Water tank experiments have also been used to study pow-
dersnowavalanches.Inwatertankexperiments,heads
similarto gravity currents always appear.
In this paper, we will show the mechanism of the head
formation and the similarity of the head-tail structure for
model experiments on a reducedscale slope.

2. MODEL EXPERIMENTS
   
   Chutesandparticlesusedintheseexper imentsare
   summerizedinTables 1and 2and Figs1and 2respec-
   tively. The long chutes of 10 m order (No.1-4) are for TTB,
   PB (plastic ball) andGB (golf ball) avalanchesandthesmall
   chutes of 1morder(No.5-10) are forSFP and PP (plastic
   particle) avalanches.
   In each avalancheexperiment, all particles before start
   are at restand by opening a gatestart to flow down along a
   chute.The avalanches are recorded by video cameras to
   obtain theirshapes and velocities during flowing down.

Figures 1 Chutes used in these experiments.

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