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clearly demonstratedhow powerful anddestructivea rela-
tively small avalanche can be.
Land-use Planning
In land-useplanning,directmethods suchasobservational,
historical, geomorphic andvegetativerecords typicallypro-
vide thebest indicationof thearea affectedby anavalanche.
Conventional wisdomholds that when these records are
notavailable,theindirectmethodsofstatisticaland
dynamicalanalysis shouldbeusedtodeterminerunout
distance.However,thisincidentdemonstratesthe
importance of doing statistical and dynamical analysis to
determinerunoutdistanceofavalanchepathsthatcan
potentially affect developmentseven when conventional
wisdom suggests otherwise.
At Loveland Basin the observational record (50+ years)
and the vegetative record (200-500 year-old conifers) indi-
cated avalancheswould not reach the parking lot.Nature
does not abide by man's rules or follow his reasoning. Had
statistical and dynamical analysis been done priorto the
February 2 event,ski area personel would have known it
to be possible foran avalanche to reach, and even travel
across,theparkinglot.Canadianavalancheconsultant
Chris Stethem (1992) said it well: "Our knowledge tells us
thatifweuseexplosivecontrolforprotectionofvalley
developments,soonerorlaterwe'llshootdownthebig
one...." Knowingthe calculatedrunout distancefor therare
event can aidthe avalancheforecaster so that an extraordi-
nary avalanchedoes not become an unprecedentedevent.
FORECASTING
The February 2, 1996, Over the Rainbow avalanchewas an
extraordinary event.However,it was caused by a combi-
nation of ordinary factors.The weather conditions in the
months prior to theavalancheweretypical for theLoveland
BasinareaandtherestoftheNorthernMountainsof
Colorado: several storms separatedby days or weeks of dry
conditions.Snowfall during January was almost a record,
andsnowfall from January23 to February 1 was impressive.
But snowfall during the storms of February 1995 and 1986
(and otherstorms)were even greater, and no destructive
avalanches occurred at Loveland Basin.
The weather hadcontributed to a weaksnowpack; how-
ever,localknowledgesuggestedthestrengthofthe
snowpackwasno worsethan typicalmid-wintercondi-
tions.Ifanythinglocalexperiencesuggested thelower
portion of the snowpack may have been slightly stronger
than usual.The initialreleaseof theFebruary 2, 1996, event
did not seem unusual. (A year earlier during the February
1995 storm an even larger slab released on an evenweaker
snowpack,butit,likepreviousevents,plowedintosoft
snow and stopped in the trees far above the parking lot.)
Since the snow conditions in the starting zone do not
explainthe avalanche'sextreme runout, that leavesthecon-
ditions in the runout zone as the likelyculprit.Unfortu-
nately detailed snowpack observations in the runout zone
were nevertaken,butthe smallavalanche (describedin
theSnowpack conditionssection)on January 1 gives an
importantcluetoconditionsintherunout zone,which
leads by inferenceto one further conclusion.
Previous avalanches had always stopped before, or just
into, the thick conifers on a bench at about 3,414 m. Small
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avalancheswouldloseenergyandmomentumonthe
bench, andlarger avalancheswould meet resistance when
the debris plowed into stable, fresh snowin the track. On
February 2,1996,it seemsplausible the surface hoar and
upper-levelkinetic crystals may havereducedthefrictional
resistance of the moving avalanche and allowed its lead-
ing edge to move furtherand travel across the old runout
zone. The avalanche, once across the bench spilled down
ontothesteeperslope(segment5)whereitreleased
additional snow that crashedinto the parking lot. (Though
the average gradient of segment 5 is about 25º, it includes
several very short but steeper pitches.)
Forthe avalanche forecaster thelessonslearned from
the February 2,1996,avalanche include the obvious and
hackneyed lessonsof"expecttheunexpected" and "ex-
pect avalanchesto run further than expected,"but there is
anotherimportantlesson.Whenlargeavalanchesare
expected, snow conditions in the runout zone can be just
as important,orperhaps more important,in determining
runout distance than the volume ofsnow in the starting
zone. Beingaware of snow conditions in runout zonesmay
reducethe surprise when ordinary conditions result in ex-
traordinary avalanches.
REFERENCES
Bakkehoi,S.,Domaas,U.andLied,K.,1983.Calculationof
snow avalanche runout distance.Annals of Glaciologyv. 4, p.
24-29.
Lied,K.andBakkehoi,S.,1980.Empiricalcalculationsof
snow-avalancherun-outdistancebasedontopographicpa-
rameters.Journal of Glaciology , v.26, no. 94, p. 165-178.
Lied,K.and Toppe,R., 1989. Calculationof maximumsnow
avalancherunout distance basedon topographic parameters
identifiedbydigital terrain models.Annals of Glaciology ,v.
13.
McClung, D.M., and Schaerer, P., 1993.The Avalanche Ha nd-
book .Seattle, WA: The Mountaineers, 271 p.
McClung,D.M.,Mears,A.I.,and Schaerer,P., 1989.Extreme
avalancherun-out:Data fromfourmountainranges.Annals
of Glaciologyv. 13, p.180-184.
Mears, A. I., 1992. Snow-Avalanche Hazard Analysis for Land-
Use planningand Engineering.Denver,ColoradoGeological
SurveyBulletin 49,55p.
Mutel,C.F. andEmerick,J.C.,1984.FromGrassland toGla-
cier:ThenaturalhistoryofColorado .JohnsonPublishing
Company, Boulder, Colorado, 238p.
Perla,R.,Lied,K.,andMcClung,D.M.,1980.Atwo-param-
etermodel of snow-avalanche motion.Journal of Glaciology
v. 26, no. 94, p. 197-208.
Stethem,C., 1992. The AlpineMeadows Avalanche:One ex-
pert's recollections.The Avalanche Review . v. 10, no. 5, p. 11.
Voellmy,A., 1955.On the DestructiveForces of Avalanches.
(English translation) U.S. Forest Service. Alta Avalanche Study
Center Translation no. 2, 1964.
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