S n o w

C o v e r

S t a b i l i t y,

A v a l a n c h e

I n i t ia t i o n

a n d

F o r e c a s t i n g

Tosummarize,weenvisagethefollowingsequenceof
events:

*beforerainstartsthedownslopestressesfromthe
weight of the slab areclose to the averageshear strength
of a buried weak layer. At some locations, stresses from
the overburdenexceedthe basal shear strength (a"defi-
cit zone" or"super weak spot");the slab is in tension
at the top end of such a deficit zone.

*raininduced shrinkageoccurringalmostinstantane-
ouslyat the surface alters the distribution ofstresses
throughthe snowpack.Thiseffectivelyincreases the
energy available to extend the length of existing zones
of deficit.In addition,wetting occurs overthe entire
slope and perturbs all existing zones of deficit simul-
taneously.

*slope failure at the onset of rain is more likely in cases
where the slab is less than ~ 1 mthick,and when the
transition from snow to rain is rapid. Rapid strain rates
needed to cause strain-softening and propagation of a
zone of deficit are more likelywhen the transition is
rapid. Whenwarm-ups are slow, grains are more likely
to sinterbond and the associated densification causes
the snow strength to increase rapidly.

CONCLUSIONS

Avalanche activity often increases at the onset of rain and
decreases with continuedrain. Theavalanchesthat release
immediately usuallyreleaseasslabswellbeforeliquid
water has penetrated and lubricated the basal layer.To be
effective with control work we have found it preferable to
control slopes at the onset of rain when the slope stability
ismostsensitive.Ifwewait,many pathswillhavere-
leased naturally. Furthermore, control becomes less effec-
tive afterrain has penetrated the snowpack, although we
have had success by using 10 to 20 kg charges elevated a
few meters above slopes.
Creep behaviorof alpine snow isstrongly influenced
by metamorphic processes and capillary forces (when liq-
uid waterispresent).Both processescause the snow to
shrink independently of gravity, and the rate of shrinkage
increaseswithtemperatureand isparticularlyrapidin
the presence of liquid water.The rate of deformation de-
creases rapidly as the snow densifies.It is likely that the
rapid shrinkage of the surface snow during firstwetting
contributes to the instability. The capillary induced strain
reduces thedepth ofslabthatcansupportlongitudinal
stresses and effectively increases the energy available to
drive an existing shear band to instability. A rain induced
surface alteration occurs rapidly overa wide region and
perturbs all existing zones of deficit simultaneously, thus
increasing the possibilityof slope failure.

ACKNOWLEDGEMENTS

This research was funded by the U.S. Army Research Of-
fice (GrantNo. DAAH04-95-1-0172). Wealso wish to thank
BobBenedict,Rob Gibson,Lee Reddon,Tim Schaub and
Joe Wilson forassistance in the field.

80

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