Avalanche risk assessment - Ministry of Forests, Lands and Natural ...

McClung and Mears (1991) define a runout ratio (∆ x /X β ), a dimensionless
measure of extreme avalanche runout, for the prediction of zones
affected by high-frequency avalanching as:
∆ X
X β
tan β–tan α
=
tan α–tan δ
Tables 18 and 19 shows how extreme runout summary statistics vary with
the terrain properties found in different mountain ranges (Smith and
McClung 1997; McClung 2001b).
table 18 Avalanche runout summary statistics: mean values (source: Smith and McClung 1997)
Canadian Coastal B.C. Coast Columbia
Rockies Alaska Range Mtns*
Mean values n = 127 n =52 n =31 n =46
α 27.8 25.4 26.8 32.5
β 29.8 29.6 29.5 34.2
δ 5.5 5.2 5.5 34.2
H 869 765 903 538
L – – – 8702
∆x 168 302 229 38
∆x / Xβ 0.114 0.25 0.159 0.064
*Note: The Columbia Mountains data describe high-frequency avalanche runout (i.e., less than
100-year events).
table 19 Avalanche runout summary statistics: extremes (source: Smith and McClung 1997)
Canadian Coastal B.C. Coast Columbia
Rockies Alaska Range Mtns
Values n = 127 n =52 n =31 n =46
α min 20.5 18.9 20.4 25.4
β min 23.0 23.0 22.8 27.0
δ min -21.5 0.0 -5.0 -25.0
H min 350 320 426 125
L max – – – 2372
∆x max 542 790 1150 217
∆x / Xβ max 0.40 0.66 0.56 0.32
Note: A negative delta angle indicates upslope avalanche runout (i.e., run-up on the opposite
valley wall).
Chapter 3 Avalanche risk assessment 79