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 1 Avalanche Control, Alaska Railroad,Box 11-1492, Anchorage, AK 99511
phone: 907-344-8220, fax: 907-522-7672
2 Department of Earth Sciences, Montana State University, Bozeman, MT 59716
phone: 406-585-9222, fax: 406-994-6923, e-mail: uesmd@gemini.oscs.montana.edu
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Key Words:Frequency, Magnitude,Chugach,Runout
ABSTRACT
Since1946, theAlaska Railroad hassystematicallyrecorded
naturalavalancheactivitytothetraintracklevel.
Parameters recorded include date, location,and apparent
volume computed by length and width on the train track.
This relatively long term record shows a significant re-
lationship betweenfrequencyandmagnitudeon individual
avalanche paths and the Chugach Range as a whole.For
each avalanche path, thedistance between Beta and Al-
pha angles are calculatedand the train track position ex-
pressed as a percentage of Beta in orderto normalize the
runout distance expression. The %Beta term standardizes
avalancherunoutsso that asignificantdataset cancompare
frequency and magnitude fora large number of avalanche
pathsina singlearea.Aregressionanalysisdefinesthe
overallfrequency relationshipforallavalanche paths at
any given location relative to the measured Beta angle.
This information can be used in the design of fixed fa-
cilities in runout zones to determine the likely frequency
of majoravalancheevents at any location. A more refined
analysis of risk levels in these locations is thus possible.
INTRODUCTION
Avalanche workers throughout the world are intrigued by
large avalanches associatedwithperiodicavalanche cy-
cles. A strategy in engineeringavalanchedefensesis to use
a 10yearor100yearavalanche event return period and
modeltheforcesassociatedwiththeseunusuallylarge
avalanches aswellasmaximumrunoutdistance (Bovis
and Mears,1976;Lied and Bakkehoi,1980;Bakkehoi,et.
al., 1983; McClung and Lied, 1984; Lied and Toppe, 1988;
Mears et.al.,1988;Mears,1992).Field workers are con-
stantly trying to recognizecircumstancesthatproducesuch
large events and cycles.
Since1991,theChugach Mountainsonthesouthern
coast of Alaska have been the location of the annual World
Extreme Ski Championships.Many recent ski media arti-
cles have characterized the Chugach snowpack as stable.
However,theChugach Range issubject tobothextreme
maritime and continental climatic influences which com-
binetoproduce oneofthe mostcomplexsnowpacksin
North America. At different times throughout the winter,
each climates regime dominates.The result is a complex,
stratified snowpack that producesavalanchecycles which
are not easily characterized. Tocharacterize the snowpack
as stable is not founded. As in every mountain range, there
are extended periods of relative stability,followed by ex-
treme instability.
It is helpful for both practitioners and scientists to look at
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historical influences to try and extract the pertinent infor-
mationtosolvetheirparticularproblems.Dataonava-
lanche occurrences are of particular interest in establish-
ing the periodicity and magnitude of avalanche events. In
an areaas remote and unpopulated as the ChugachRange,
relevant data is mostly anecdotal.The two exceptions to
this are thesystematic avalanche occurrencerecordsavail-
able from the Alyeska Ski Resort and the Alaska Railroad.
Events have beenso recordedat Alyeska Resort since 1976,
and on the Railroad since 1946.
Thispaperusesthisdata set todescribethe relation-
ship between the frequency and magnitude of avalanches
in the Chugach Range.
METHODS
The Alyeska data base was used to describe events uphill
of Point P (the 10 degree point) with a periodicity of less
than 10 years.In 1985,the resort began recording runout
distanceonavalanchepathsby percentagewith100%being
maximum runout distanceaccording to existing vegetative
parameters.We haveanalyzedthisdatausingmoving
averages to derive the average numberof events at given
locations in the avalanchetrack and runout zone.
Since 1946,the Alaska Railroad has systematically re-
corded allavalancheoccurrencesthat reachedthe railroad
track.We havesortedthedataaccordingtovarious
parameters.The analysisconducted onthisdata base is
more relevant tolong term,large magnitude events.The
Railroad data parameters analyzed include:
*Frequency by avalanche path
A distribution of events sorted by milepost orlocation
*Apparent Volume of selected paths
Apparent volume of all avalanche paths was sorted by
size,then characteristicvolumecurveswere selected
forregression analysis.
*Analysis oflargest 5 events on eachpath
The largest 5 events foreach path were selected, then
thevolumeofthelargestevent ineach path wasdi-
vided by the volume of the smallest of the 5 event sam-
ple to produce a ratio. The averageratio was derived in
ordertoisolatethosepathsthatproduced unusually
large events.
*Analysis oflargest 2 events
The same analysis asin #3except with onlythe top
two events.
*Volume curves of 5 largest events on select paths
A similar analysisto #2 but with only thelargest5 events
included.
*Chronological distribution of 5 largest events
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