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Key Words: Avalanche, Design-Avalanche,Slab,Densities,
Thickness
ABSTRACT
Observations of forest destruction throughoutvarious snow
climates suggest that design avalanches([!]100-yearreturn
periods) usually result from fracture and release of thick,
widespreadslabs of newdry snow.Suchavalanchesusually
travelthe longestdistances intorunout zonesand prob-
ably achieve the largest velocities and impact energies.
Dry, new snow slab densities and thickness were esti-
matedfromanalysisofsustainedstormsofseveraldays
durationatelevensitesintheUnitedStates.Thesites
chosen representcontinental(Gothic, CO; YuleCreek, CO;
Elkton, CO; WolfCreek, CO)intermountain(Alta, UT; Jack-
son, WY)andmaritime(Mammoth, CA; Alpine Meadows,
CA;Paradise, WA; Stevens Pass, OR;Mt. Hood,OR) snow
climates. The 18 storms selected for study were all charac-
terized byasteady increase in snowpackdepth through
the storm period (e.g., accumulationexceededsettlement),
were below freezing,anddid not have rain associated with
the storm.Data were collected at standard high-elevation
snow study plotsthat represent starting zone conditions
wherewind effects were not important. Mean densities in
the new snow layer wereestimated by the relationshipr=
HW/DH, wherer= averagedensity of the new snow layer,
HW = the water equivalentduring the storm, and DH = the
snowpack depth increase during the storm.
The following conclusions result from the storm analy-
sis: (1) mean slab densities and thickness did not vary sig-
nificantly fromone snow climate toanother;(2) average
daily precipitation rates were greater within the maritime
climates,(3) storms were colder and of longer duration in
continental climates.
INTRODUCTION
Aspointedoutbyde Quervain (1972)majoravalanches
affecting valley-bottomlocations in Switzerland are usu-
allyassociatedwithstormsofseveraldays'duration in
whichnewsnowfallexceedsabout1.2m.Salm,et.al.
(1990),defined a parameterd
othat is used to estimate the
thickness of new snow slabsresulting from majorstorms
of several days duration. Slab thickness was found to vary
with climateregionin Switzerland.Theassumption in both
theolderand morerecent Swissworkisthatthemajor
"design" avalanches with return periods,T, on the order
of 100 years (30 years < T < 300 years) are associated with
thick slabs of new,dry snow.The resultingavalanchesreach
high velocities, producelarge impactpressures, andtravel
thelongestdistancesintotherunoutzoneswhereland
development and engineering works may be planned.
Direct observations of damage to forests and man-made
structuresfromavalancheswithlongreturnperiods
throughoutvariousclimateregions oftheUnitedStates
(Mears,1992)confirmssomeoftheSwissassumptions.
Long-return period avalanchesthat affect the largest areas
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intherunoutzonesand produce thelargest destructive
forces are usually associated with dry snow regardless of
thesnowclimateofagivensite(McClung,1990).Such
avalanchesachieve thelargest velocities and energies, and
coverthelargestareas.Givenalong returnperiod,dry-
snowavalancheswillproducethedesign,longreturn-
periodavalanche inmaritimeaswellasincontinental
climates.Large avalanches ofwetsnow,orcourse,also
occur andmay producethelargestdepositdepthsandstatic
avalanche loads,particularly in maritime climates.Such
avalanches, however,because of lesservelocities,runout
distances and areas covered,usually do notproduce the
designcase formostland useplanning and engineering
applications.
When structuresmustbeexposedtothedesignava-
lanche, information about avalanche-release volumes and
flow densities are necessary so the engineer can compute
impact pressures, forces, and moments on exposed struc-
tures.In some avalanche-dynamics models calculation of
avalanche flow thickness alsodepends on released snow
thicknessand/orvolume.Suchinformationcan beesti-
mated,as is done in Switzerland,from data on new slab
thicknessand density.Avalanche velocitiescan be com-
puted independentlythroughapplicationof statistical and
physical modeling,as discussed byMcClung (1984) and
Mears, (1992).
THE DATABASE AND "STORM" CHARACTERISTICS
The entire database of the Westwide Reporting Network,
consisting of 125 stations that reported weather recordsin
theUnitedStatesduringtheperiod1967-1995,was
searchedfor this study.Some of the record begin in 1944.
In addition, detailed snow and precipitation records from
Yule Creek,Coloradoweremadeavailable byMr.Chris
Landry of Yule Creek AvalancheServices. Of the reporting
stations available, 11 hadrecords of sufficientlength,detail,
and qualityto be useful in this study. Four stations (Gothic,
CO;Yule Creek,CO;Elkton,CO;andWolf Creek,CO)
represented the continental sites,two stations(Alta,UT;
Jackson,WY) represented the intermountain climate, and
fivestations(AlpineMeadows,CA;Mammoth,CA;
ParadiseRanier, WA; StevensPass, OR; and Mt. Hood, OR)
represented the maritime sites.As noted,many othersta-
tions have reported data to the Westwide Network,how-
ever, these data were not sufficient in detail and/or length
to be used in this study.
"Storms"weredefinedasanycontinuousperiodin
which (1) the snowpack increased in depth(i.e. newsnow
accumulation>settlement),(2)temperaturesremained
below freezing throughout the period, and(3) rain did not
fall. Condition "1" defined the storm period andthe thick-
nessofthenewsnowslab.Conditions"2"and"3"are
important because eitherwarm temperatures orrain will
tendtoinducedensity increases.When above-freezing
temperatures are followed by cold temperatures melt-
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