C a s e

H i s t o r i e s

a= -3.0º+0.79b +0.0036X
(r ²= 0.75;s= 1.4º).^b
When applied totheOverthe Rainbowavalanche path,
the predicteda = 21.0º (Figure 8). Statistically 21º is a rea-
sonable a angle for Colorado avalanchepaths(McClung et
al.,1989).McClung's study found the mean value of b=
27.4º and the mean valueof a = 22.6º for 98 Colorado paths.
In the case of Overthe Rainbow the predicted a = 21.0º
means future avalanchescould cross the parking lot. How-
ever,extreme avalanches would not be expected to travel
beyondtheparkinglot,northedistancenecessaryto
achieve a21ºaangle (assuming a flat runout). At the north
edge of the parking lot the slope climbs steeply for24 ver-
tical meters,effectively creating a barrierthat would stop
avalanches. It is the opinion of this author that it would be
unlikely for avalanches to over run this slope. Theaangle
at the north edgeof the parking lot is 23º (Fig.8).

CONCLUSION

The newly-createdOver the Rainbow avalanchepath poses
a real, and at certain times a serious, threatto the Loveland
Basin Ski Area parking lot. From the February 2 avalanche
eventseveralconclusions canbedrawn eitherfromde-
duction orinference.

STATISTICAL ANDDYNAMICAL ANALYSIS

Statistical and dynamical analysis shows that future ava-
lanches in the Overthe Rainbow path can be expected to
travel further than the February 2 avalanche event.Large
avalanches can be expected to travel into and even across
the ski area parking lot.This analysis is also important as
itprovidesnumericalinsighttothedynamicforcesin-
volvedintheavalanche. Intermsofavalanchesizethe
February 2 event was not a large avalanche(299 m),but it

Table 3. Summary statistics for Over the Rainbow avalanche,

February 1996.

larger diameter treeswere mostly uprooted when the root/
soil system failed.

AVALANCHE RUNOUT DISTANCE POTENTIAL

The runout distance potential was computed using Mear's
adaptation of the Norwegian Geotechnical Institute (NGI)
method (Lied and Bakkehoi,1980;Bakkehoi et.al., 1983;
Lied and Toppe, 1989).Mears performed the same multi-
ple regression analysis on 112 extreme avalanchepaths in
Colorado (Mears,1992)and found that the parameterX
(horizontal distance from starting point to 10º point,orb^b
,
in the runoutzone) in additiontob(10º point in the runout
zone) producedthe best regressionequation. For Over the
Rainbow,bwas measured and calculatedat 27.9º.Mears'
Colorado equation is:

Figure 8. Statistical runout prediction model. Runout distance is predicted to extend beyond the parking lot.

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