H a z a r d

M a n a g e m e n t

a n d

P u b l i c

Wa r n i n g

specific avalancheterrain usingtheriver scaleas its model.ship.Each risk level increases with the increasing prob-
ability fordestructive avalanche forces (i.e., long-running
IDENTIFICATIONOFSPECIFIC AVALANCHE^{avalanches),theincreasingprobabilityforcontactwith}
TERRAIN RISK FACTORS^{potentially injurious terrain obstacles, and/or the decreas-}
Avalancheslopeshavea varietyof configurationsfrom short^{ing probability fora successful rescue.}
test slopes of ten to twenty meters to entire mountain sides^{Mosthumantriggered avalanches have crown depths}
havingathousandmetersormoreinverticaldrop.In^{of less than 1.5 meters.Therefore, in assigning a risk level}
addition, avalancheslopes vary in the number and type of^{to a slope, a slab depth of approximately 1.5 meters or less}
potentially injurious or fatal terrain features.Some slopes^{should be assumed.Theelements listed in eachclasslevel}
may be confinedwithrock outcrops, cliffs, gullies, benches,^{are independentconsiderations, one or all of which, when}
and individual trees orstands oftrees,while others may^{compared,mayprovideasufficientfoundationfor}
be unconfinedandrelativelysmooth from the startingzone^{assigning the appropriate risk level to the slope being con-}
to the runout.^{sidered. The ATR Scale is represented in Table 6.}
In considering the basis for the ATR Scale, slope length
was found to be a potentially significant factor.The length^{TABLE 6.AVALANCHE TERRAIN RISK SCALE}
of the avalancheslope effectively determines the distance
(ortime)overwhich onemaybeexposed topotentially^{Class 1}
injurious avalancheforces and terrain features.It follows^{*no injury ordeep burial is likely;self rescue is highly}
from this that the riskofinjury ordeath (R) increases as^probable
the magnitude of forces (F) generated bythe moving de-^{small potential startingzone-minimal destructiveforces}
bris,and the numberofpotentiallyinjurious terrain fea-^{(less than 15 meters of vertical)}
tures (S) with which the victim may come into contact in-^{*no potentially injurious terrain features}
creaseover the lengthof the slope (x).Restated, thegreater*small,consistent runout (no deepburial potential)
the distance(or time), over whichavictim could encounter
potentiallyinjuriousavalancheforcesand/orterrain^{Class 2}
features, the greater the opportunity forsuch encounters^{*shallow burial potential only; rescue is highly probable}
will be,and thus the greater the probability of severe in-^{*small potential starting zone}
jury or death.This relationship is expressed in Figure 1.^{*small destructive forces (15 to 50 meters of vertical)}
*no potentially injurious terrain features

Figure 1.R = F(x) + S₁(x) + S₂ (x)+.....+S
n^{(x)*gradual, consistent runout (no deepburial potential)}

Internal forces within the moving avalanche are not con-^{*small deposition area for beacon search}
stant as debris runs from the top of the path (T) to the bot-^{Class 3}
tom (B).Norare the number of contacts with potentially^{*deep burial potential; rescue is questionable}
injurious terrain featuresnecessarily constantover thepath^{*moderate sized starting zone}
length.Considering such variability, a more accurate way_{*moderate destructive forces (50 to 100 meters of verti-}
of expressing Figure 1 is represented in Figure2.(In both_cal)
equations,only terrain features aligned along the descent
path areconsidered.Featureslocated across the slope at a^{*no potentially injurious terrain features}
given elevation present no more risk than a single feature^{*abrupt transition in flat ordepressed runout}
becausetheprobability of contactingmore thanoneis zero.)^{*moderate sized deposition area forbeacon search}
+^{nClass 4}
Figure 2.R(%) =_T[!] ^BF(x)dx[!]{_T[!] ^BS
i=1^{i(x)dx}*deep burial and/orsevere injury is probable;rescue is}
doubtful

• large potential starting zone
• large potentialdestructiveforces (morethan 100me-
  ters of vertical)
• potentially injurious terrain features on less than 50%
  of slope
• large deposition area forbeacon search
  
  Class 5
• deep burial, severeinjury and/or deathis virtually cer-
  tain;rescue is highly improbable
• large ormultiple potential starting zones at various el-
  evations
• potentiallydevastatingdestructiveforces(more than200
  meters of vertical)
• potentiallyinjuriousterrainfeaturesonover50%of
  slope
• large ormultiple deposition areas forbeacon search

Note that slope length only addresses the potential forse-
vere injury ordeath from internal forces and from contact
withobstacles,notfordeep burial.Care mustbetaken
whenconsideringslopelength,nottooverlookthefact
that a deep burial is not dependenton slope length.Both
adeep release ona short slopeand/ora shallowrelease
above a creek bed can cause death from a deepburial.

THE AVALANCHE TERRAIN RISK (ATR) SCALE

Assigningspecificvaluestoavalanche forceand terrain
feature variables could be very difficult if not impossible
before the actual avalanche event occurs.In any case, as-
signing such values isnot the purpose ofthispaper.Of
greaterimportanceisunderstanding therelationshipof
these variables to the probability for severe injury or death.
The proposed ATR Scale attempts to express this relation-

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