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

atan angleof30-40 ^º .Snowdepths of2to3mcovered
alderbrush that had been bent down slope by the weight
of the overburden.Noavalanches released on the study
slopeduringthe measurement period.The verticaland
slope parallel components of motion were measured sepa-
ratelyusingpairsofshoesmadefromlight-weightalu-
minium screening. The screeningminimized interruptions
to waterinfiltration.Pairs of shoes were placed sequen-
tially at the surface after 10 to 30 cm of snow had accumu-
lated.One shoe was constrained to slide vertically down
a fixed pole. A sliding contact mounted on the shoe made
electrical contact with a resistance wire strung along the
length of the pole. Weconfigured this as a voltage divider
circuit and shoe position could be calculated to an accu-
racy of ± 2mm.The non-vertical component was meas-
ured by running a cord froma second shoe upslope to a
10-turn rotary potentiometermounted on the vertical ve-
locity shoe (Figure 1). Although the rotary potentiometers
could resolve motions as small as 0.2 mm, additional un-
certainties arise from the experimental setup. Weestimate
theoverallaccuracyofthemeasurement ofdownslope
position to be ± 2 mm.Measurements, made at 5 minute
intervals were recorded using a data logger.

FIELD OBSERVATIONS AND MEASUREMENTS

Below we present observations and measurements during
two case histories to illustrate some salient points of rain-
on-snow avalanche activity.

(i) Case Study February 5,1996

The combined effects of intense snowfall followed by a
classicwarm-up and snow/rain transition onFebruary5,
1996 resultedin a major avalanchecycleat SnoqualmiePass
thatclosedthehighwayfor 44 hours. Just prior to theonsetof
rain, cars and peoplewere caughtandburied by avalanches.
One traveller who was out ofhis car was buriedabout 2m
belowthesurfacefor 29 minutesbeforebeingfoundby probing
anddugout alive.Avalanchesalsoreleasedattheonsetof rain.
Slides hit andpartially buried a snow blower and a vehicle
occupiedby two avalanchetechnicianswhowerespottingfor
theblower.Inall, 11 vehiclesandatleast20 peoplewerehitby
avalanches;more weredusted.Noneof theseavalancheswere
large- themaximumdepthof fracturewas20 cm.
Figure 2shows the snow stratigraphy justprior torain.
Freezingrain on January 6-7had formed a thick (5cm) ice
crust and rain on January15 added to thecrust thickness.A
total of 263 cm of snow fell betweenJanuary17to 30. Tem-
peratureswere generallycool and consistentthroughoutthe
period andthe snowpackwas remarkably homogeneousand
stable.Conditionswereclearand cold(-15 to -20ºC) duringfor
thenextfewdaysandby February 3 thenewsnowhadsettled
to 95 cm.Thegeneralstabilityincreasedwith time.Numerous
snow pits and shear tests throughout the region indicateda
strong bond betweenthe new snow and the ice layer; shear
tests through the full depthofthe snowpackyielded either
moderateto hardshears, or no distinctshearfailure.
By morning on February 4, about 12 cm of new snow had
accumulated.It cleared in the afternoon but snow showers
startedagainon themorningof February5. Snow continued
to accumulate(10 cm by 1300; 19 cm by 1620 on February 5)
andheavy snowfall continuedthroughthe evening.Natural
avalanchesfirst started to run at 1730,bywhich time just
over 20 cm had accumulated.Snowchangedto rain just be-

strung along thelength of the pole.Downslope motion was measured
by running a cord from a second shoe upslope to a rotary
potentionmeter mountedon the vertical velocity shoe. Measure-
ments, made at 5 minute intervals, were recorded on a data logger.

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