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Table 1. Avalanche dimensions, Over the Rainbow.
THEAVALANCHE
Description
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Table 2. Segment descriptions and calculated avalanche velocities at the top and
bottom of each segment, for the Over the Rainbow avalanche path.
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Thecomputedvelocitiesatthetopandbottomofeach
segment of Overthe Rainbow are given in Table 2.
The value assumed for m depends upon factors such as
snow type,path roughness and trees orrockoutcrops in
the avalanchepath. In the upper portion of the avalanche
path a valueof m=0.2 was assumed in segments 0-3. Lower
internal friction of the avalancheincreased whenthe mov-
ing snow impacted and removed the thick,heavy timber;
in segments 4-5,m=0.30 was used; and m=0.35 was used
insegment 6.(Theavalanche did notreach intothe7th
segment.)
TheM/Dvalue was chosen fromsuccessive iterations
forcing the model to match the actual runout distance. In
practice,typicalvaluesforM/Drangefrom100mto
10,000m. The PCM model performed well as thecomputed
stopping position was within 3 m of the actual position.
VEGETATION
Vegetationcanprovide cluesto pastavalancheoccurrences,
and priorto February 2,a thick stand of mature conifers
stood above the Rainbow ski run.Englemann spruce and
Sub-Alpinefirsfrom15to61cmindiameterand 12to
24+ m had withstood previous avalanches. Sawedsections
from trees broken by the avalanche showed agesof almost
200 years and older (189 years, 15-cm diameter; 239 years,
24-cm diameter).A section taken from a large Englemann
spruce thatwasuprootedinthelowertrack revealed it
was over500 years (61-cmdiameter).
The small diameters and old trees are consistent for the
spruce-fir forest where thefrost-free growing seasonis only
about 2 months (Mutel and Emerick, 1984). The thickness
of the timber stand and the ageof the conifers suggest that
there have been no avalanches of similarmagnitude--to
remove timber--formore than 200 years.
Investigationalongtheeasternperimeteroftheava-
lanche path revealed past avalancheshad also stopped in
the conifers.Trees higher on the slope showed scars and
broken branches, but loweron the slope evidence of past
avalanchesendedas brancheson the uphillsides remained
intact.
The calculatedmaximum impact pressures (Table3) are
consistent with values required to destroy mature forests
(Mears, 1992).Looking at the carnageof downed timber it
appeared that the avalanche broke trees up to 30 cm.The
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The avalanchewas classified as a HS-SS-AE-5-O. A small
explosive initiated a hard-slab release in the upper start-
ing zone and a soft slab in the lower portion.As the ava-
lanche sweptdown the mountain it entrainedsofter snow
andstrippedtheentiresnowcovertotheground.The
crown faceand flanksaveraged1.4 m deep. Table1 shows
the dimensions of the avalanche.
The PCM model (Perla et. al.1980)was used to com-
putevelocityand acceleration along theOvertheRain-
bowavalanchepath.Terrainmeasurementsweremade
from the USGS7.5-minute topographicmap(1:24,000) and
were field checkedforgeneral accuracy. The PCM model
isan extensionofthe originalVoellmy model(Voellmy
1955).
ThePCMmodelconsistsofthreeequationsthatde-
pend upon slope angle,q;length,L ;dynamic friction,m ;
and a mass-to-drag ratio,M/D. Figure 6 shows a centerline
profile of the Over the Rainbow avalanche path extending
from starting zone to the runout. The avalanche path has
been subdivided into seven segments whereqcan be con-
sidered consistant within each segment of lengthL.
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