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Avalanche defences for the Trans-Canada Highway at Rogers Pass
Schaerer, P. A.
no. Ll+l e . 2 BI,D8
DIVISION OF BUILDING RESEARCH
AVALANCHE DEFENCES FOR THE TRAI{S.GANADA HIGHWAY
AT ROGERS PASS by
P . S c h a e r e r
A N A L Y Z E D
This is a joint paper of the Department of I
Public Worke and the NationaL Research Gouncil
Technical Paper No. l4l of the
Divieion of Building Research
OTTAWA November L962
AVALANCHE DEFENCES FOR THE TRANS-CANADA HICHWAY AT ROGERS PASS
by P . S c h a e r e r
TABLE OF CONTENTS
FUNDAMENTAL CONS IDERAT IONS
TERRAIN GLIMATE Snowfall Snow Cover T e m p e r a t u r e Wind AVALANCHES AVALANCHE SURVEY AVALANCHE ACTIVITY AVALANCHE CLASSIFICATION D r y S n o w D i r e c t - A c t i o n A v a l a n c h e s
Wet Snow Direct-Action Avalanches
Dry Snow Delayed-Action Avalanches
Spring Thaw Avalanches
AVAI.ANCHE DEFENCE METHODS
R e t a i n i n g B a r r i e r s
Snowfences and Windbaffles B r a k i n g B a r r i e r s
Earth rnounds W e d g e s o f c o n c r e t e
Cable nets in the avalanche Path Catching darns and benches Diverting Darns
Snow Sheds R e f o r e s t a t i o n E x p l o s i v e s
Hand planted charges P r e - p l z r n t e d c h a r g e s . . A r t i l l e r y f i r e H i g h w a y C l o s u r e Avalanche Detection Page I
z
3 3 4 4 5 5 6 I9
9
I O
l 0
I t
I I I 3 I 3 T 4 I 4 L 4 I 4 t 4 I 5 I 6 l 6 I 6r 8
I B l 8zo
zo
F i r s t S t a g e o f D e f e n c e . . . Highway location S t r u c t u r e s . . . Explosives H i g h w a y c l o s u r e s . . . Z z Avalanche detection ?'3 S e c o n d S t a g e o f D e f e n c e . . . ? 3 T h i r d S t a g e o f D e f e n c e . . . 2 3 LTST OF AVALANCHE SITES WITH RECOMMENDED
DEFENCE
z 5
3 ? 3 5 Z Tz z
? z
ACKNOWLEDG REFERENCES APPENDIX A APPENDIX B APPENDIX C APPENDIX D MENTSLIST OF OBSERVATIONS ON ACTUAL AVALANCHES
EXPERIMENTAL EARTH MOUNDS
EXPERIMENTAL BENCHES
AVALANCHE DEFENCES FOR THE TRANS-CANADA HIGHWAY AT ROGERS PASS by P . S c h a e r e r FUNDAMENTAL CONSIDERATIONS I n t h e e a r l y l 9 5 O ' s c o n s i d e r a t i o n w a s g i v e n t o t h e u s e o f R o g e r s P a s s a s a p o s s i b l e r o u t e f o r t h e T r a n s - C a n a d a H i g h w a y t h r o u g h the selkirk Range. The Pass is located between the towns of Golden and Revelstoke in British Colurnbia, with its surnmit near the town o f G l a c i e r i n G l a c i e r N a t i o n a l P a r k . I t w a s d i s c o v e r e d d u r i n g s u r v e y s preceding construction of the Canadian Pacific Railway and was narned after Major Rogers, Chief Engineer of the Mountain Division of the Railway.
T h e v a l l e y s a s s o c i a t e d w i t h t h e P a s s a r e n a r r o w w i t h s t e e p sides. Because the snowfall is heavy avalanches occur each winter and spring. Accordingly, in I953 the Departrnent of Public works, which
was responsible for the construction of the highway in Glacier National Park, began making observations on the location of each avalanche site and the conditions under which avalanches forrn. These studies were under the d i r e c t i o n o f M r . N . C . G a r d n e r . I n I 9 5 6 , w h e n t h e d e c i s i o n h a d b e e n r n a d e to construct the highway through the Pass, the Departrnent of Public'Works established an Avalanche Observation Station at Glacier. The previous observation prograrn was enlarged so that the inforrnation necessary for the specification and design of the defence systern could be obtained. An avalanche lnazard evaluation and forecast service was established for the p r o t e c t i o n o f t h e a v a l a n c h e o b s e r v e r s , s u r v e y o r s a n d c o n s t r u c t i o n c r e w d u r i n g t h e c o n s t r u c t i o n p e r i o d .
The Division of Building Research of the National Research Council co-operated in the organization of the Avalanche Observation Station. Initially it contributed inforrnation and instruments for taking n e c e s s a r y w e a t h e r a n d s n o w c o v e r o b s e r v a t i o n s , b u t w h e n t h e a u t h o r joined the staff of the Snow and Ice Section of the Division it assurned t h e f o l l o w i n g r e s p o n s i b i l i t i e s :
t o r n a k e r e c o n : r r n e n d a t i o n s o n t h e t y p e , l o c a t i o n a n d d e s i g n o f t h e d e f e n c e f o r e a c h a v a l a n c h e s i t e ;
Z . t o a s s i s t i n t h e o r g a n i z a t i o n a n d t a k i n g o f t h e o b s e r v a t i o n s r e q u i r e d f o r t h e r e c o r n r n e n d a t i o n s .
I n A p r i I o f 1 9 5 7 t h e a u t h o r w a s s e c o n d e d t o t h e D e p a r t r n e n t of Public Works and joined the Avalanche Observation Station at Glacier o n a f u l l - t i r n e b a s i s . I { e w a s e n g a g e d i n f i e l d w o r k t h e r e u n t i l N o v e r n b e r
1 9 5 9 . O n Z O c t o b e r 1 9 5 9 r e s p o n s i b i l i t y f o r s n o w a n d w e a t h e r o b s e r v a t i o n s a n d f o r a v a l a n c h e h a z a r d e v a l u a t i o n a n d f o r e c a s t w a s t r a n s f e r r e d f r o r n
the Departrnent of Public 'Works and the National Research Council to
the National Parks Branch of the Departrnent of Northern Affairs and
N a t i o n a l R e s o u r c e s .
D u r i n g t h e w i n t e r o f I 9 5 9 ' 6 0 t h e D e p a r t r n e n t o f P u b l i c 'Works
rnaintained a survey crew at Glacier which continued the avalanche o b s e r v a t i o n s a n d r e p o r t e d t o t h e a u t h o r . T h i s o b s e r v a t i o n s t a t i o n c e a s e d i t s o p e r a t i o n i n J u n e 1 9 6 0 .
T h i s r e p o r t c o n t a i n s a s u r n r n a r y o f t h e o b s e r v a t i o n s t h a t w e r e
rnade and presents recornrnendations for the avalanche defence required
f o r t h e p r o t e c t i o n o f t h e T r a n s - C a n a d a H i g h w a y i n w i n t e r . D e t a i l s o f s n o w c o v e r a n d w e a t h e r o b s e r v a t i o n s , a n d o f e x p e r i e n c e g a i n e d i n e v a l u -ating avalanche lnazard and rnaking forecasts during the period when the N a t i o n a l R e s e a r c h C o u n c i l w a s a s s o c i a t e d w i t h t a k i n g t h e f i e l d o b s e r v a t i o n s h a v e a l r e a d y b e e n p u b l i s h e d ( I ) .
TERRAIN
T h e S e t k i r k r a n g e , w i t h p e a k s r i s i n g t o 1 1 , 0 0 0 f t , i s o n e o f
the interior ranges of British Colurnbia and lies within the big bend of the
Colurnbia River. The lateral valleys on the east side are cornparatively
s h o r t a n d s t e e p , w h e r e a s t h o s e t o t h e w e s t d r o p g r a d u a l l y t o t h e C o l u r n b i d R i v e r . T h e s u r n r n i t o f R o g e r s P a s s i s t h e l o w e s t p o i n t i n t h e w a t e r d i v i d e o f t h e S e l k i r k r a n g e , r e a c h e d o n t h e e a s t s i d e t h r o u g h B e a r C r e e k V a I I e y a n d o n t h e w e s t t h r o u g h t h e I l l e c i l l e w a e t V a l t e y ( T a b l e I ) . T h e s e a r e t y p i c a l S e l k i r k v a l l e y s , n a r r o w w i t h s t e e p s i d e s ( F i g s . L , Z l . T h e v a l l e y b o t t o r n s a r e b e t w e e n 5 0 0 a n d I , 0 0 0 f t w i d e a n d c o n s i s t o f s i l t y s a n d , g r a v e l d e p o s i t s a n d b o g s . I n s o r r r e p l a c e s t h e r n o u n t a i n s i d e s a r e a k n o s t V - s h a p e d . O n e o f t h e s e n a r r o w d e f i l e s i s b e t w e e n M o u n t T u p p e r a n d M o u n t M a c D o n a l d o n t h e e a s t s i d e o f R o g e r s P a s s ; a n o t h e r i s f o r r n e d
3
-by the sides of Fidelity Mountain and Fortitude Mountain at the west b o u n d a r y o f G l a c i e r N a t i o n a l P a r k ( F i g . 3 ) .
T h e m o u n t a i n s o n t h e R o g e r s P a s s r o u t e h a v e a t e r r a c e b e t w e e n 5 , 5 0 0 a n d 6 , 5 0 0 f t a b o v e s e a l e v e l ( F i g . 4 l ) t h a t l e a d s
gradually into steep scree slopes and rock faces rising to the rnountain r i d g e s b e t w e e n 7 , 0 0 0 a n d 1 0 , 0 0 0 f t . B e l o w t h e t e r r a c e a s h a r p d r o p Ieads to talus slopes and alluvial fans and the valley floor.
Heavy tirnber and dense brush cover the lower rnountainsides and valleys, with alpine rneadows and scattered trees on the terrace at 6 , 0 0 0 f t . T i r n b e r l i n e i s g e n e r a l l y a b o u t 6 , 8 0 0 f t . T h e s t e e p s l o p e s a n d heavy timber rnake access to the mountains very difficult.
CLIMATE
R o g e r s P a s s i s l o c a t e d i n t h e s o u t h e a s t i n t e r i o r r e g i o n o f
British Columbia, popularly known as the Interior Wet Belt (2); high
annuaL precipitation and heavy snowfall are its rnost distinctive features.
Frorn the point of view of avalanche conditions, the rnountain area in
the western United States has been divideti into three different clirnatic z o n e s . T h e S e l k i r k s , i n t h e n o r t h e r n e x t e n s i o n o f t h e r n i d d l e z o n e , a r e
characterized by heavy snowfalls of moist to dry sncw, rnedium
tempera-t u r e s o n l y o c c a s i o n a l l y b e l o w z e r o d e g r e e s F a h r e n h e i tempera-t , a n d s tempera-t r o n g w i n d action on the mountains.
D u r i n g t h e w i n t e r s f r o r n I 9 5 3 t o I 9 6 0 s n o w f a l l , s n o w d e p t h a n d t e m p e r a t u r e w e r e l n e a s u r e d a t G l a c i e r , S t o n e y C r e e k a n d F l a t G r e e k . S n o w f a l l , s n o w c o v e r t e r n p e r a t u r e a n d w i n d w e r e o b s e r v e d o n R o g e r s P a s s
surnrnit and on Mount Abbott frorn 1957 to 1960. Records of
pre-cipitation and ternperature rneasured at Glacier during the years before
1953 were available frorn the Meteorological Branch of the Departrnent
o f T r a n s p o r t . Snowfall T h e a v e r a g e a n n u a l p r e c i p i t a t i o n a t G l a c i e r , r n e a s u r e d o v e r t h e 3 0 - y e a r p e r i o d b e t w e e n 1 9 Z I a n d 1 9 5 0 , i s l B . 0 i n c h e s o f r a i n a n d 3 4 2 i n c h e s o f s n o w . T h e l a r g e s t t o t a l s n o w f a l l o b s e r v e d w a s 6 8 0 i n c h e s , r n e a s u r e d d u r i n g t h e w i n t e r o f I 9 5 3 - 5 4 . O n c e i n l 0 y e a r s a s n o w f a l l o f 4 5 0 i n c h e s o r r n o r e c a n b e e x p e c t e d . T h e r e a r e n o y e a r l y r e c u r r i n g p e r i o d s o f m a x i r n u r n s n o w f a l l i n t e n s i t y . S t o r r n s w i t h h e a v y s n o w f a l l a r e i n f r e q u e n t . D u r i n g w i n t e r s of light snowfall three rnay occur that yield rnore than l6 inches in a
3 - d a y p e r i o d ; a n d f o r w i n t e r s o f h e a v y o c c u r . O n l y o c c a s i o n a l l y i s t h e r e o n e i n c h e s o f s n o w i n a 3 - d a y t r e r i o d .
snowfall, ten such storrns rnay that contributes rnore than 36
The rnagnitude and frequency of. Z4-hour snowfalls at
G l a c i e r d u r i n g t h e w i n t e r s b e t w e e n I 9 5 3 a n d 1 9 6 0 a r e s h o w n i n T a b l e I I .
S t o r r n s w i t h a h i g h r a t e o f s n o w f a l l a r e u s u a l l y o f s h o r t duration and a day with heavy snowfall is often followed by one or several w h e n i t i s l i g h t . S o r n e t i m e s , h o w e v e r , i t i s d i f f i c u l t t o r e c o g n i z e t h e end of one storrn and the beginning of another. It is possible that once in Z0 years a winter with very heavy snowfall could occur and the above-r n e n t i o n e d d e p t h s w o u l d b e e x c e e d e d . O n Z I J a n u a above-r y I 9 3 5 a 3 5 - i n c h f a l l o f s n o w i n a Z 4 - } r o u r p e r i o d w a s r e c o r d e d b u t t h i s e x t r e r n e r a t e h a s n o t b e e n o b s e r v e d s i n c e . M o s t s t o r r n s a p p r o a c h t h e R o g e r s P a s s a r e a f r o r n t h e w e s t a n d s o u t h w e s t a n d d e p o s i t a g r e a t e r a r n o u n t o f s n o w o n t h e w e s t s i d e o f t h e r a n g e . p r o r n t h e o b s e r v a t i o n s t a k e n d u r i n g t h e t w o w i n t e r s I 9 5 7 - 5 8 a n d l 9 5 8 - 5 9 i t w a s e s t i r n a t e d t h a t t h e t o t a l s n o w f a l l o n t h e e a s t s i d e o f t h e P a s s i s a b o u t B 0 p e r c e n t o f t h e s n o w f a l l r n e a s u r e d o n t h e w e s t s i d e . T h e h i g h e s t r a t e o f s n o w f a l l r e c o r d e d w a s o b s e r v e d a t G l a c i e r , w h i c h i s l o c a t e d c l o s e t o t h e s u r n m i t o n t h e w e s t s i d e o f t h e P a s s . Snow Cover
Norrnally snow covers the ground frorn I November to 20 May a t t h e 4 , 0 0 0 - f t l e v e l . A t 7 , 0 0 0 f t i t u s u a l l y l a s t s f r o r n t h e e n d o f S e p t e r n b e r until the end of June. prequent snowfalls with fairly constant ternperatures d u r i n g a n d a f t e r e a c h o n e r e s u l t i n a s t a b l e s n o w c o v e r . A t y n i c a l
p r o f i l e s h o w s l a y e r s w i t h i n c r e a s i n g s t r e n g t h t o w a r d t h e g r o u n d ; s n o w t h a t has undergone extensive rnetarnorphisrn and forrned layers with low strength w a s n o t o f t e n o b s e r v e d . T h e s n o w d e p t h h a s b e e n r n e a s u r e d a t G l a c i e r s i n c e t h e w i n t e r o f 1 9 5 3 - 5 4 ; t h e m a x i r n u r n f o r e a c h y e a r i s g i v e n i n T a b l e I I I . s n o w w a s w a s 8 9 i n . rnaxirnurn T e r n p e r a t u r e
The rnean rnonthly.winter ternperatures at Glacier for the 3 0 - y e a r p e r i o d f r o r n 1 9 Z I t o 1 9 5 0 a r e g i v e n i n T a b l e I V . T h e t e r n p e r a t u r e
D u r i n g t h e w i n t e r o f I 9 5 B - 5 9 , w h e n t h e g r e a t e s t d e p t h o f observed at Glacier, the rnaxirnurn at the Rogers Pass surnmit
F o r t h e M o u n t A b b o t t O b s e r v a t o r y , a l t i t u d e 6 , 7 O 0 f t , a d e p t h o f 1 5 l i n c h e s w a s o b s e r v e d I M a y 1 9 5 9 .
5
-i s b e l o w 0 ' F o n l y a f e w t -i r n e s d u r -i n g t h e w -i n t e r , a n d u s u a l l y d o e s n o t c o n t i n u e f o r r n o r e t h a n a w e e k . T h e l o w e s t e v e r m e a s u r e d a t G l a c i e r w a s - 3 3 " F .
D u r i n g a s n o w s t o r r n t e r n p e r a t u r e s n o r r n a l l y r a n g e b e t w e e n
Z0 and 3?."F. Afterwards it is usual for cloudy weather to continue and the t e r n p e r a t u r e t o c h a n g e r e l a t i v e l y s l o w l y . p r e q u e n t l y i t r i s e s d u r i n g
a snowstorrn and the snowfall in the valleys changes to rain.
Wind
M o s t s n o w f a l l s a r e a c c o r n p a n i e d b y s o u t h e r l y a n d w e s t e r l y winds of variable speed. Large arnounts of snow are carried over the rnountain ridges and deposited locally. Sorne heavy snowfalls rnay occur with no significant wind.
AVALANCHES
Heavy snowfall and strong winds cornbined with steep rrroun-t a i n s i d e s c a u s e n u r n e r o u s a v a l a n c h e s a rrroun-t R o g e r s P a s s . T h e h a z a r d i n t h e a r e a i s h i g h , a n d c a n b e a s s e v e r e a s t h a t i n t h e r n o r e d a n g e r o u s a r e a s o f Switzerland, Austria and the United States where highways and railroads are kept open in the winter. The narrow valley, and the fact that in the w o r s t a r e a s t h e a v a l a n c h e s r e a c h t h e v a l l e y f l o o r f r o m b o t h s i d e s , r n a k e s
i t i r n p o s s i b l e t o c o n s t r u c t a h i g h w a y t h a t w i l l a v o i d d a n g e r o u s s i t e s '
Between the east boundary of Glacier National Park and the west boundary o f M t . R e v e l s t o k e N a t i o n a l P a r k a b o u t T 4 a v a L a n c h e s i t e s h a v e t o b e
c r o s s e d , ( F i g . ) . T w o d e f i l e s , b e t w e e n M t . T u p p e r a n d M t . M a c D o r r a l d and between Fidelity and Fortitude Mountains, have particularly high a v a l a n c h e a c t i v i t y . B e t w e e n a n d o u t s i d e t h e s e t w o a r e a s , w h e r e t h e valley bottorn is wider and the avalanche paths are rnore scattered, the highway can be located so that avalanches reach it only under bad conditions.
A v a l a n c h e s i t e s c a n b e r e c o g n i z e d e a s i l y b y t h e s c a r s o n t h e tirnbered rnountainsides. Rernains of snow sheds along the old railway r i g h t - o f - w a y a r e g o o d i n d i c a t i o n s a l s o o f a c t i v e a v a l a n c h e s i t e s .
Where an avalanche starts to slide and where the bulk of s n o w o r i g i n a t e s i s c a l l e d t h e r u p t u r e z o n e ( F i g . 5 ) . A t R o g e r s P a s s t h e r e is an upper and a lower ruptule zor]Ie, divided on rnost of the rnountains by t h e t e r r a c e b e t w e e n 5 , 5 0 0 a n d 6 , 5 0 0 f t . A v a l a n c h e s t h a t s t a r t t o s l i d e i n t h e l o w e r z o r r e u s u a l l y o r i g i n a t e o n o r a t t h e b a s e o f t h e c l i f f s . T h e r e i s e v i d e n c e t h a t r n a n y l o w e r r u p t u r e a r e a s a t R o g e r s P a s s w e r e o n c e
f i r e s a p p e a r t o h a v e r e r n o v e d t h e m , t h u s d e s t r o y i n g a v e r y e f f e c t i v e n a t u r a l d e f e n c e a n d c r e a t i n g t h e a v a l a n c h e s i t e s . A v a l a n c h e s f r o r n t h e u p p e r z o n e o r i g i n a t e o n t h e s l o p e s r i s i n g n e a r t i r n b e r l i n e a t a b o u t 6 , 5 0 0 f t t o t h e r n o u n t a i n r i d g e s . M a n y o f t h e s r n a l l e r a v a l a n c h e s t h a t o r i g i n a t e i n t h i s z o n e a r e s t o p p e d o n t h e t e r r a c e b e t W e e n t h e t w o z o n e s .
The avalanche path is the track followed by an avalanche d u r i n g i t s d e s c e n t . T h e r n a j o r i t y o f a v a l a n c h e s a t R o g e r s P a s s a r e c o n f i n e d i n g u l l i e s c a r v e d i n t h e r n o u n t a i n s b y r u n n i n g w a t e r ( F i g . 5 ) . A f e w r e a c h t h e v a l l e y o v e r o p e n , b a r e s l o p e s ( F i g . ) . T h e t e r r n i n u s is the place where the rnain body of an avalanche cornes to rest. Minor avalanches stop on the alluvial fan or on the talus slope at the bottorn of the mountain, rnajor ones advance into the valley and in sorrr.e cases reach t h e o p p o s i t e s i d e .
AVALANCHE SURVEY
In order to design the avalanche defence for the highway i t w a s n e c e s s a r y t o o b t a i n f o r e a c h s i t e t h e f o l l o w i n g i n f o r r n a t i o n o n b o t h n o r r n a l a n d v e r y l a r g e a v a l a n c h e s :
a) rupture zor'e, path and terrninus b ) d e p t h a n d w i d t h c ) f r e q u e n c y o f o c c u r r e n c e d ) p r e v a i l i n g c o n d i t i o n s g o v e r n i n g t h e i r o c c u r r e n c e . R e c o r d s o f p r e v i o u s a v a i l a b l e f o r t h e w i n t e r s b e t w e e n W i n t e r L 9 5 3 - 5 4 : a v a l a n c h e s w e r e p a t r o l s b y N . C .
avalanches along the highway route were 1 9 5 3 a n d 1 9 5 7 a s f o l l o w s : o b s e r v e d o n m o n t h l y P a r k W a r d e n G a r d n e r a n d a r e l i s t e d i n h i s r e p o r t . W i n t e r 1 9 5 4 - 5 5 : a v a l a n c h e s w e r e o b s e r v e d o n r n o n t h l y p a t r o l s b y T . R . W e b b , R e s i d e n t E n g i n e e r , D e p a r t r n e n t o f P u b l i c W ' o r k s ; t h e y a r e r e c o r d e d i n h i s r e p o r t a n d o n a s k e t c h f i r a p : W i n t e r 1 9 5 5 - 5 6 : a v a l a n c h e s w e r e s u r v e y e d o n r n o n t h l y p a t r o l s b y N . C . G a r d n e r , D e p a r t r n e n t o f P u b l i c W o r k s , a n d a r e r e c o r d e d o n a l o c a t i o n p l a n a n d i n a r e p o r t . W i n t e r 1 9 5 6 - 5 7 : a v a l a n c h e s w e r e s u r v e y e d b y t h e a v a l a n c h e o b s e r v a t i o n s t a t i o n o f t h e D e p a r t r n e n t o f P u b l i c W o r k s , u n d e r t h e d i r e c t i o n o f N . C . C a r d n e r a n d a r e r e c o r d e d o n a l o c a t i o n p l a n .
7 -D u r i n g t h e t h r e e w i n t e r s 1 9 5 7 t o I 9 6 0 p e r s o n n e l f r o r n t h e A v a l a n c h e O b s e r v a t i o n S t a t i o n c a r r i e d o u t t h e s u r v e y . T h e i n f o r r n a t i o n I i s t e d i n A p p e n d i x A w a s r e c o r d e d a n d a v a l a n c h e s t h a t d e p o s i t e d s n o w n e a r o r o n t h e h i g h w a y r i g h t - o f - w a y w e r e t r a c e d o n t o l o c a t i o n p l a n s . T h e r u p t u r e z o n e s a n d p a t h s o f s o r n e o f t h e r n a j o r a v a l a n c h e s w e r e n o t e d o n p h o t o g r a p h s o r s k e t c h e s ( F i g . 6 ) . T h e p r e v a i l i n g c l o u d y w e a t h e r a n d a s s o c i a t e d p o o r v i s i b i l i t y , h o w e v e r , d i d n o t p e r r n i t f u l t o b s e r v a t i o n o f t h e rup[ure zone and the avalanche path. The recording of every avalanche, even when it terrninated far frorn the highway, proved to be useful for l a t e r s t u d i e s , R e f e r e n c e r n a r k e r s s e t o n t r e e s a n d p o l e s a l o n g t h e highway right-of-way permitted an acc\lrate survey of the avalanche L o c a t i o n s . P a t r o l s b e t w e e n G l a c i e r ( M i l e l ? ) a n d M t . T u p p e r ( M i t e l l . 5 ) w e r e r n a d e d a i l y , a n d t w i c e d a i l y w h e n a v a l a n c h e s o c c u r r e d ; t h r o u g h t h e s i t e s o f t h e r n a j o r T u p p e r a v a l a n c h e s ( M i I e I 0 . 2 t o M i l e I l . 5 ) t h e y w e r e n o t f e a s i b l e e v e r y d a y , o w i n g t o t h e h i g h h a z a r d i n t h i s a r e a , b u t p a t r o l s w e r e r n a d e a t l e a s t o n c e a w e e k . T h e h i g h w a y w e s t o f G l a c i e r a s f a r a s t h e I I I e c i l l e w a e t r a i l w a y s i d i n g w a s p a t r o l l e d a t I e a s t o n c e a w e e k and whenever conditions indicated that avalanches rnight have occurred. D u r i n g t h e w i n t e r o f I 9 5 9 - 6 0 o c c a s i o n a l p a t r o l s w e r e r n a d e i n t o t h e B e a v e r V a l l e y b e t w e e n t h e e a s t p a r k b o u n d a r y ( M i l e 0 ) a n d M i l e 1 0 . 2 . A I ' T u c k e r S n o c a t r r w a s u s e d f o r t h e p a t r o l s b e t w e e n M o u n t T u p p e r ( M i l e I l . 5 ) a n d F I a t C r e e k ( M i l e 2 5 ) . W h e n s n o w i n t h e v a l l e y w a s l e s s t h a n Z f e e t d e e p , a t t h e b e g i n n i n g a n d t h e e n d o f w i n t e r , h o w e v e r , i t c o u l d n o t b e u s e d a n d p a t r o l s w e r e r n a d e o n s k i s o r o n f o o t . AVALANCHE ACTIVITY T h e f o l l o w i n g g e n e r a l a v a l a n c h e a c t i v i t y w a s o b s e r v e d d u r i n g w i n t e r s w h e n t h e a v a l a n c h e s w e r e s u r v e y e d a l o n g t h e p r o p o s e d l o c a t i o n the highway: W i n t e r 1 9 5 3 - 5 4 : A l a r g e t o t a l s n o w f a l l o f 6 8 0 i n c h e s w a s o b s e r v e d a t G l a c i e r ( T a b l e I I ) , b u t d i d n o t c a u s e u n u s u a l l y ' l a r g e o r f r e q u e n t a v a l a n c h e s . f u t a j o r a v a l a n c h e s o c c u r r e d i n F e b r u a r y ; t h o s e i n s p r i n g w e r e l a t e a n d n o t s i g n i f i c a n t .
Avalanche activity was low during the winter and spring. W i n t e r L 9 5 4 - 5 5 : W ' i n t e r I 9 5 5 - 5 6 : F r e q u e n t a v a l a n c h e s w e r e o b s e r v e d i n s o r n e p a t h s ; g e n e r a l l y s p e a k i n g a c t i v i t y w a s b e l o w a v e r a g e d u r i n g b o t h w i n t e r a n d s p r i n g . the f o r
W i n t e r 1 9 5 6 - 5 7 A v a l a n c h e a c t i v i t y w a s l o w d u r i n g t h e w i n t e r m o n t h s ; b u t n u r n e r o u s r n a j o r s p r i n g a v a l a n c h e s t h a t o r i g i n a t e d i n t h e u p p e r r u p t u r e z o n e o c c u r r e d . W i n t e r 1 9 5 7 - 5 8 : . B e c a u s e o f l i g h t s n o w f a l l a n d w a r r n w e a t h e r a v a l a n c h e a c t i v i t y w a s v e r y l o w d u r i n g t h e w i n t e r a n d s p r i n g . ' W i n t e r l 9 5 8 - 5 9 : T o t a l s n o w f a l l o b s e r w e d a t G l a c i e r w a s 4 4 2 i n c h e s ( T a b l e I I ) , a n a r n o u n t t h a t i s r e a c h e d o r e x c e e d e d o n l y o n c e i n a b o u t l 0 y e a r s , V e r y f r e q u e n t b u t r a t h e r s r n a l l w i n t e r a v a l a n c h e s w e r e o b s e r v e d . A p r i l b r o u g h t e x t r a o r d i n a r i l y l a r g e e a r l y spring thaw avalanches as well as sorrle caused by two h e a v y s n o w f a l l s .
' W i n t e r
1 9 5 9 - 6 0 : A v a l a n c h e a c t i v i t y w a s a v e r a g e . H e a v y s n o w f a l l s c a u s e d rnajor avalanches in Novernber and Decernber.
T h r o u g h t h e c o - o p e r a t i o n o f t h e C . P . R . D i v i s i o n a l E n g i n e e r at Revelstoke, a study was rnade of all the avalanches that affected the r a i l w a y l i n e f r o r n S t o n e y C r e e k t o R e v e l s t o k e b e t w e e n l t l 0 a n d L 9 5 2 .
U n f o r t u n a t e l y t h e r e c o r d s a v a i l a b l e w e r e i n c o r n p l e t e , a s t h e r a i l w a y h a d n o t r e c o r d e d n u r n e r o u s a v a l a n c h e s t h a t w e r e s u c c e s s f u l l y c o n t r o l l e d b y s n o w s h e d s . S o r n e s c a n t y i n f o r r n a t i o n o n a v a l a n c h e s i n e a r l i e r y e a r s w a s o b t a i n e d frorn employees of the railway and the Parks Branch who had lived in the a r e a a t t h e t i m e .
A v a l a n c h e s t h a t a f f e c t e d t h e r a i l w a y l i n e b e t w e e n S t o n e y C r e e k and the east portal of Connaught Tunnel and the west portal and Illecillewaet s i d i n g d u r i n g t h e p e r i o d f r o r n 1 9 l 0 t o 1 9 6 0 a r e p l o t t e d i n F i g . 7 . T h i s g r a p h s h o w s t h a t t h e r e a r e p e r i o d s o f h i g h a v a l a n c h e a c t i v i t y a b o u t o n c e i n B y e a r s ( r n o r e t h a n f o u r a v a l a n c h e s o n t h e g r a p h ) . T h e o b s e r v a t i o n s r e q u i r e d f o r the planning of the defence for the highway appear to have been rnade during a p e r i o d o f l o w a c t i v i t y . I t i s p o s s i b l e , t h e r e f o r e , t h a t t h e n u r n b e r a n d s i z e o f t h e a v a l a n c h e s o b s e r v e d i n t h i s s u r v e y a r e n o t r e p r e s e n t a t i v e o f t h o s e t h a t r n i g h t o c c u r d u r i n g a p e r i o d o f g r e a t a c t i v i t y . S n o w f a l l a n d t e r n p e r a t u r e r e c o r d s f r o r n 1 9 3 0 t o I 9 6 0 s h o w a l s o t h a t t h e l a r g e a r n o u n t o f n e w s n o w , c o r n b i n e d w i t h h i g h t e r n p e r a t u r e s , a s s o c i a t e d w i t h t h e p e r i o d s o f f r e q u e n t a v a l a n c h e s i n t h e y e a r s b e t w e e n 1 9 3 0 a n d I 9 5 2 d i d n o t o c c u r d u r i n g t h e p e r i o d o f t h e a v a l a n c h e s u r v e y . T h e a v a l a n c h e s u r v e y w a s c o r n p l e t e d b y s i t e s t u d i e s d u r i n g both surnrner and winter. Rupture zorres and avalanche paths were observed f o r e x t e n t , s l o p e a n g l e , e x p o s u r e a n d s o i l c o n d i t i o n s . T h e a g e o f t r e e s a n d t h e a r e a w h e r e t r e e s g r o w i n t h e t e r r n i n u s z o n e i n d i c a t e d t h e f r e q u e n c y a n d e x t e n t o f c l i r n a x a v a l a n c h e s .
. 9
-AVALAN CI{E CLASSIFICATION
A r n e a n s o f c l a s s i f i c a t i o n i s n e c e s s a r y f o r a v a l a n c h e s t u d i e s . V a r i o u s c l a s s i { i c a t i o n s h a v e b e e n s u g g e s t e d s i n c e a v a l a n c h e s w e r e f i r s t d e s c r i b e d i n t h e l a t e I 9 t h c e n t u r y ( 3 ) , a n d e a c h s e r v e s a p a r t i c u l a r p u r p o s e , e . g . e v a l u a t i o n o f s a f e t y f o r t o u r i s t s , t h e e f f e c t o f a r n o v i n g a v a l a n c h e o n s t r u c t u r e s o r v e g e t a t i o n . F o r t h e d e s i g n o f t h e o v e r - a l l d e f e n c e s y s t e r n a t R o g e r s P a s s a c l a s s i f i c a t i o n b a s e d o n t h e r r l a n n e r i n w h i c h a v a l a n c h e s a r e f o r r n e d p r o v e d t o b e r n o s t p r a c t i c a l . A v a l a n c h e s w e r e d i v i d e d i n t o t h e f o l l o w i n g c l a s s e s : C I a s s Main Cause D r y s n o w d i r e c t - a c t i o n a v a l a n c h e s S n o w f a l l , w i n d W e t s n o w d i r e c t - a c t i o n a v a l a n c h e s S n o w f a l l , i n a s s o c i a t i o n w i t h r a i n o r t e r n p e r a t u r e s a b o v e 3 2 " F
Dry snow delayed-action avalanches Unstable snow cover, snowfall,
and wind. over a long period of tirne
Spring thaw avalanches (or wet snow High ternperature s, unstable
d e l a y e d - a c t i o n a v a l a n c h e s ) s n o w c o v e r .
Each group can produce avalanches of rninor, rnediurn or rnajor p r o p o r t i o n s . T h e y r n a y a l s o o c c u r a s c o r n b i n a t i o n s o f t h e a b o v e c l a s s e s and in this case mav have unusual size.
T h e y e a r s o f t h e a v a l a n c h e s u r v e y i n d i c a t e d t h a t t h e r e a r e t w o a v a l a n c h e p e r i o d s e a c h y e a r . D u r i n g t h e f i r s t , b e t w e e n e a r l y N o v e r n b e r a n d M a r c h , d r y a n d w e t s n o w d i r e c t - a c t i o n a v a l a n c h e s a n d d r y s n o w d e l a y e d
-action avalanches predorninate. In exceptional years avalanches forrned
before Novernber rnay reach the valley if the snow in the rupture zorre e x c e e d s 2 B i n c h e s . T h e s e c o n d a v a l a n c h e p e r i o d b e g i n s i n t h e l a t t e r h a l f
of March and continues until the rniddle of May. These avalanches are
rnainly spring thaw avalanches, although direct-action avalanches rnay also
o c c u r .
D r y S n o w D i r e c t - A c t i o n A v a l a n c h e s
T h e r n a j o r i t y o f a v a l a n c h e s f a l l i n t o t h i s c a t e g o r y a n d o c c u r d u r i n g o r i r n r n e d i a t e l y a f t e r a s n o w s t o r r n . T h e y c o n s i s t , r n a i n l y , o f n e w s n o w s l i d i n g o n t h e s u r f a c e o f o l d s t a b l e s n o w , s o t h a t t h e i r s i z e i s
usually governed by the arnount of new unstable snow in the rupture zor.e
and in the avalanche path. Avalanches rnay becorne airborne when falling
o v e r c l i f f s o r d e s c e n d i n g v e r y s t e e p t e r r a i n . T h e a s s o c i a t e d s n o w c l o u d c a n h a v e a h i g h s p e e d .
D u r i n g t h e y e a r s o f t h e 6 u r v e y r n u c h i n f o r m a t i o n w a s o b t a i n e d on the location and frequency of occurrence of both rninor and rnediurn dry s n o w , d i r e c t - a c t i o n a v a l a n c h e s , b u t l a r g e a v a l a n c h e s o f t h i s t y p e w e r e o b s e r v e d a t o n l y a f e w s i t e s . B e t w e e n 1 9 5 3 a n d I 9 5 0 n e v e r r n o r e t h a n t e n reached the highway on the sarne day. The records indicate that rnajor a v a l a n c h e s w i l l p r o b a b l y n o t o c c u r a t a l l s i t e s s i m u l t a n e o u s l y e x c e p t o n c e i n l 5 t o Z 0 y e a r s . T h e p o t e n t i a l h a z a r d c r e a t e d b y d r y s n o w d i r e c t - a c t i o n a v a l a n c h e s c a n b e r e c o g n i z e d b y o b s e r v i n g s n o w c o v e r a n d w e a t h e r . T h e d e p t h o f n e w s n o w i s t h e r n o s t i m p o r t a n t g o v e r n i n g f a c t o r , a n d i s u s e d i n t h i s r e p o r t t o i n d i c a t e t h e c o n d i t i o n s t h a t c a u s e d r y s n o w d i r e c t - a c t i o n a v a l a n c h e s t o o c c u r a t e a c h s i t e . I t s h o u l d b e k e p t i n m i n d , h o w e v e r , t h a t s n o w d e p t h i s o n l y a g e n e r a l g u i d e ; o t h e r f a c t o r s s u c h a s t e r n p e r a t u r e , w i n d , s n o w density, and snowfall intensity rnay also influence the hazard. Experience h a s s h o w n t h a t i f t h e t e r n p e r a t u r e i s b e t w e e n Z 0 a n d 3 Z " F n e w s n o w w i l l b e c o m e s t a b l e a f t e r t w o o r t h r e e d a y s . U s u a l l y i t i s t h e s n o w t h a t f a l l s d u r i n g t h i s s t a b i l i z a t i o n p e r i o d t h a t i s r e s p o n s i b l e f o r a n a v a l a n c h e .
U n d e r c e r t a i n c o n d i t i o n s , h o w e v e r , t h e s n o w d o e s n o t s t a b i l i z e s u f f i c i e n t l y and the snowfall over a number of days rnay be responsible for creating
c o n d i t i o n s f a v o u r a b l e f o r t h e o c c u r r e n c e o f d r v s n o w d i r e c t - a c t i o n a v a l a n c h e s . W e t s n o w , D i r e c t - A c t i o n A v a l a n c h e s
A v a l a n c h e s o f t h i s c l a s s a r e c a u s e d w h e n s n o w f a l l s a r e irnmediately followed by rain or by warrn weather with ternperatures h i g h e r t h a n 3 2 " F . T h e y c o n s i s t u s u a l l y o f w e t n e w s n o w , b u t c a n a l s o contain wet old snow. The wetter the snow the rnore avalanches rnove on the surface, with little or no snow cloud and accornpanying windblast.
During the years of the survey only rninor and mediurn
a v a l a n c h e s o f t h i s t y p e w e r e o b s e r v e d . R e c o r d s f r o r n t h e r a i l w a y c o r r l p a n y i n d i c a t e , h o w e v e r , t h a t m o s t o f t h e b i g a v a l a n c h e s , w h i c h b l o c k e d r a i l t r a f f i c f o r l o n g p e r i o d s o f t i r n e , w e r e t h e r e s u l t o f h e a v y s n o w f a l l s f o l l o w e d b y w a r r n w e a t h e r ( e . g . I 7 J a n u a r y I 9 3 0 , 9 / l O J a n u a r y 1 9 3 3 , 2 4 / 2 6 J a n u a r y 1 9 3 5 , l 0 F e b r u a r y l 9 5 I ) . I t w o u l d a p p e a r t h a t r n a j o r o c c u r r e n c e s o f w e t s n o w d i r e c t - a c t i o n a v a l a n c h e s w o u l d b e t h e r n o s t u n f o r t u n a t e s i t u a t i o n f o r the highway also; they could cover it with deep snow sirnultaneously at
rnany sites. They would also cause rnaxirnurn loading conditions for avalanche d e f e n c e s t r u c t u r e s . S t r u c t u r e s s h o u l d , t h e r e f o r e , b e d e s i g n e d f o r c o n d i t i o n s t h a t a r e w o r s e t h a n t h o s e o b s e r v e d d u r i n g t h e p e r i o d f r o r n I 9 5 3 t o 1 9 5 0 . D r y S n o w , D e l a y e d - A c t i o n A v a l a n c h e s
T h e s e a v a l a n c h e s a r e a l s o c a l l e d c l i r n a x a v a l a n c h e s , a n d a r e caused by a cornbination of factors that build up an avalanche trazard over a
I I -p e r i o d o f t i r n e , e . g . s e v e r a l r n i n o r s n o w f a l l s o n a n u n s t a b l e s n o w l a y e r , o r s t r o n g w i n d f r o r n t h e s a m e d i r e c t i o n f o r s e v e r a l d a y s . D r y s n o w d e l a y e d - a c t i o n a v a l a n c h e s r r t a y o c c u r u n d e r r r l a n y d i f f e r e n t s i t u a t i o n s . B y c a r e f u l s t u d y o f t h e s n o w c o v e r a n d t h e w e a t h e r i t i s p o s s i b l e t o r e c o g n i z e w h e n t h e y w i l l p r o b a b l y o c c u r . T h e s e a v a l a n c h e s h a v e t h e s a r n e e f f e c t o n t h e h i g h w a y a s d r y s n o w d i r e c t - a c t i o n a v a l a n c h e s , b u t u n l i k e t h e o t h e r c l a s s e s t h e y o c c u r s p o r a d i c a l l y a n d d o n o t a f f e c t l a r g e s e c t i o n s o f t h e highway s irnultaneous Iy.
Spring Thaw Avalanches
S p r i n g t h a w a v a l a n c h e s o c c u r a s a r e s u l t o f l o s s o f c o h e s i o n o f s n o w a s i t r n e l t s . ' I h e y rnay be divided, generally, into two groups. T h e f i r s t , e a r l y s p r i n g t h a w a v a l a n c h e s o r i g i n a t i n g i n t h e l o w e r r u p t u r e z o n e , u s u a l l y o c c u r o n a f e w c o n s e c u t i v e d a y s i n t h e f i r s t h a l f o f A p r i l , b u t t h e y b e g a n t o s l i d e a s e a r l y a s I 8 M a r c h 1 9 6 0 a n d a s l a t e a s 3 M a y i n 1 9 5 4 . T h e y r u n d u r i n g t h e d a y t i r n e b e t w e e n 9 . 0 0 a r n a n d 6 . 0 0 p r n . A v a l a n c h e s f r o r n n o r t h e r l y a n d e a s t e r l y s l o p e s r r r a y o c c u r u p t o t w o w e e k s l a t e r t h a n t h e r n a j o r i t y . E a r l y s p r i n g a v a l a n c h e s c a n o c c u r w i t h i n t w o o r t h r e e d a y s s i r n u l t a n e o u s l y a t n u r n e r o u s s i t e s ; i n s p r i n g 1 9 5 9 t h e y c o v e r e d at the sarne tirne 6,000 ft of the future highway to a depth of between I and
l 5 f r .
T h e s e c o n d g r o u p , l a t e s p r i n g t h a w a v a l a n c h e s , a r e t r i g g e r e d i n t h e u p p e r r u p t u r e z o n e a b o v e 6 , 5 0 0 f t a n d u s u a l l y o c c u r d u r i n g t h e f i r s t two weeks in May. Late spring thaw avalanches reach the highway only a t a f e w s i t e s w h e r e l o n g s t e e p p a t h s f r o r n t h e u p p e r r u p t u r e z o n e c r o s s i t ( . . g . o n M t . T u p p e r a n d M t . A b b o t t ) .
I t i s c o n s i d e r e d t h a t t h e e a r l y s p r i n g t h a w a v a l a n c h e s i n I 9 5 9 and the late spring thaw avalanches in 1957 u'ere rnaxirnurn events, and
t h a t s i r n i l a r a v a l a n c h e s w o u l d n o t o c c u r r n o r e f r e q u e n t l y t h a n o n c e i n I 0 y e a r s .
AVALANCHE DEFENCE METHODS
I t i s a l r n o s t s e l f - e v i d e n t t h a t t h e h i g h w a y s h o u l d , t o t h e g r e a t e s t p o s s i b l e d e g r e e , b e c o n s t r u c t e d t h r o u g h a r e a s s a f e f r o r n a v a l a n c h e s . W h e r e a n a v a l a n c h e s i t e c o u l d n o t b e a v o i d e d , t h e f o l l o w i n g R o i n t s w e r e c o n s i d e r e d i n c h o o s i n g t h e I o c a t i o n : A t s o r n e s i t e s a v a l a n c h e s r e a c h t h e t e r r n i n a l a r e a i n f r e q u e n t l y . P l a c i n g t h e h i g h w a y a t t h e e n d o f i t w i l l r e d u c e t o a r n i n i r r r u r n t h e a r n o u n t o f a v a l a n c h e s n o w t h a t w i l l r e a c h i t . a )
b)' Higher up the rnountainside the avalanche path i s o f t e n n a r r o w . A h i g h w a y l o c a t e d t h e r e w i l l require the rninirnurn length of snow shed for p r o t e c t i o n . T h e s p e e d o f t h e a v a l a n c h e a t s u c h a p l a c e w i l l u s u a l l y e n s u r e t h a t t h e s n o w w i l l s l i d e a c r o s s t h e h i g h w a y . D e p o s i t e d s n o w c a n b e r . e -moved easilv to the downhill side.
c ) A t s o m e s i t e s t h e h i g h w a y r n a y b e p l a c e d o n
fill. The embankrnent rnay stop the bulk of the
avalanche snow so that only light snow and wind-blast affect the highway.
d) If highway cuts cannot be avoided at avalanche
s i t e s , t h e y s h o u l d b e a s s h a l l o w a s p o s s i b l e . The bank on the downhill side should be levelled i n o r d e r t o a v o i d d e e p s n o w d e p o s i t s a n d f a c i l i -tate snow rernoval.
Much valuable experience is available frorn countries such as
Switzerland, Austria and the United States on various rnethods of defence
against avalanches. Some of it could be apptied directly for the highway
c i e f e n c e a t R o g e r s P a s s ; o t h e r m e t h o d s h a d t o b e t e s t e d f i r s t u n d e r Iocal c o n d i t i o n s . T h e v a r i o u s d e f e n c e m e t h o d s a r e l i s t e d b e l o w , f o l l o w e d b y
a description of each rnethod and a surrrrrlary of observations rnade at
R o g e r s P a s s b e t w e e n 1 9 5 7 a n d 1 9 5 9 . A c t i v e D e f e n c e :
S t r u c t u r e s a n d v e g e t a t i o n ( t r e r r n a n e n t m e a s u r e s ) R e t a i n i n g b a r r i e r s
Snowfences and windbaffles B r a k i n g b a r r i e r s D i v e r t i n g d a r n s S n o w s h e d s R e f o r e s t a t i o n E x p l o s i v e s ( t e r n p o r a r y r n e a s u r e s ) H a n d p l a c e d c h a r g e s P r e p l a n t e d c h a r g e s A r t i l l e r y f i r e
_ r 3
P a s s i v e D e f e n c e : A v a l a n c h e w a r n i n g H i g h w a y c l o s u r e s A v a l a n c h e d e t e c t i o n S t r u c t u r e s a r e p e r m a n e n t a n d e x p e n s i v e r n e a s u r e s , b u t t h e y require little rnaintenance. Explosives and avalanche warning are flexible d e f e n c e s t h a t r e q u i r e l i t t l e i n v e s t r n e n t , b u t s p e c i a l p e r s o n n e l r n u s t d e t e r -rnine the tirne when explosives have to be applied or when dangerous areas should be closed. A11 rnethods rnay be applied either alone or in combination. R e t a i n i n g B a r r i e r sR e t a i n i n g b a r r i e r s d r e e r e c t e d i n t h e r u p t u r e z o n e t o p r e v e n t s n o w f r o m s l i d i n g ( F i g . B ) . T h e y r n u s t c o v e r t h e w h o l e z o n e w h e r e d a n -g e r o u s a v a l a n c h e s r n a y s t a r t t o s l i d e a n d m u s t b e a t l e a s t a s h i -g h a s t h e rnaxirnurn expected snow. This type of defence has been studied in
S w i t z e r l a n d a n d i s u s e d i n n u r n e r o u s a v a l a n c h e a r e a s . T h e S w i s s G o v e r n -r n e n t h a s i s s u e d d e s i g n s p e c i f i c a t i o n s , w h i c h c a n b e u s e d a s a g u i d e ( 4 ) . R e t a i n i n g s t r u c t u r e s a r e f r e q u e n t l y u s e d i n c o r n b i n a t i o n w i t h r e f o r e s t a t i o n .
R e t a i n i n g s t r u c t u r e s r n a y b e b u i l t f r o r n t i m b e r , s t e e I , c o n c r e t e and alurniniurn or of nets of steel cable or nylon. If they are designed
correctly no avalanche can start and the area is cornpletely safe, but con-s t r u c t i o n c o con-s t con-s a r e h i g h a n d r e t a i n i n g con-s t r u c t u r e con-s a r e e c o n o r n i c a l l y
f e a s i b l e o n l y w h e r e t h e r u p t u r e z o n e i s s r n a l l o r w h e r e i n h a b i t e d a r e a s r n u s t b e p r o t e c t e d .
Snowfences and Windbaffles
Deep snowdrifts in the rupture zone cause rnany avalanches. S n o w f e n c e s e r e c t e d o n t h e w i n d w a r d s i d e o f r n o u t a i n r i d g e s r e d u c e t h e arnount of drifting snow, and wind baffles burlt irnrnediately above or in the avalanche slope influence the rnanner in which the drifting snow is deposited ( F i g . 9 ) . A s t a b l e s n o w c o v e r c a n b e c r e a t e d t h i s w a y . S t a n d a r d s n o w { f e n c e s a n d w i n d b a f f l e s , r n a d e o f w o o d a n d a b o u t l 5 f t h i g h c a n b e b u i l t a t c o r n p a r a t i v e l y l i t t l e c o s t . E x p e r i e n c e i n A u s t r i a , S w i t z e r l a n d a n d t h e U . S . A . i n d i c a t e s , h o w e v e r , t h a t t h e e f f e c t i v e n e s s o f t h e s e s t r u c t u r e s i s l i r n i t e d . S n o w f e n c e s w e r e o f t e n c o r n p l e t e l y c o v e r e d w i t h s n o w a n d w i n d -baffles only reduced the avalanche }razard but did not elirninate it. I-urther-rnore, this defence rnethod is ineffective when a snowfall is not accornpanied b y w i n d , s o t h a t i t i s g e n e r a l l y c o n s i d e r e d u s e f u l o n l y i n a s s o c i a t i o n w i t h o t h e r d e f e n c e r n e a s u r e s ( 5 ) .
B r a k i n g B a r r i e r s B r a k i n g b a r r i e r s a r e o b s t a c l e s d e s i g n e d t o r e d u c e t h e e n e r g y o f a r n o v i n g a v a l a n c h e a n d h a l t i t s a d v a n c e . T h e y r n a y b e e a r t h r n o u n d s , w e d g e s o f c o n c r e t e o r s t o n e r n a s o n r y , s t e e l c a b l e s , c a t c h i n g d a r n s o r b e n c h e s b u i l t p e r p e n d i c u l a r t o t h e a v a l a n c h e d i r e c t i o t t ( 5 ) B r a k i n g s t r u c t u r e s a r e u s u a l l y r n o r e e f f e c t i v e a g a i n s t a v a l a n c h e s o f w e t , h e a v y s n o w t h a n a g a i n s t t h o s e o f f a s t - r n o v i n g d r y s n o w , a n d a r e u s e d t o b e s t a d v a n t a g e n e a r l h e t e r r n i n u s w h e r e a v a l a n c h e s h a v e a l r e a d y I o s t r n o s t o f t h e i r f o r c e . A f l e r a f e w a v a l a n c h e s h a v e o c c u r r e d , h o w e v e r , t h e s t r u c t u r e s r n a y b e f i l l e d w i t h a v a l a n c h e s n o w a n d t h u s b e c o r n e i n e f { e c t i v e . E a r t h r n o u n d s E a r t h r n o u n d s a r e u s e d t o a b s o r b e n e r g y f r o r n t h e a v a l a n c h e a n d s t o p i t b e f o r e i t r e a c h e s t h e a r e a n e e d i n g p r o t e c t i o n . T h e y a r e r n o s t e f f e c t i v e o n a n y f l a t s e c t i o n i n t h e a v a l a n c h e p a t h w h e r e t h e i n c l i n e i s l e s s t h a n 2 0 d e g r e e s : t h e y s h o u l d b e b e t w e e n I 5 a n d Z 5 f . t i n h e i g h t a n d b u i l t i n a c h e c k e r e d a r r a n g e r n e n t a s c l o s e t o g e t h e r a s p o s s i b l e . A t I e a s t t w o r o w s o f r n o u n d s a r e r e q u i r e d ( F i g . i 0 ) . E x p e r i r n e n t a l e a r t h r n o u n d s w e r e b u r l t o n M t . T u p p e r i n O c t o b e r 1 9 5 7 ; r e s u l t s o f t h e o b s e r v a t i o n s r n a d e d u r i n g t h e f o l l o w i n g t h r e e winters are given in Appendix B, and indicate that rnounds are a suitable t y p e o f d e f e n c e f o r R o g e r s P a s s ( F i g s . 1 0 , I I , 1 2 ) . W e d g e s o f c o n c r e t e W e d g e s b u i l t w i t h a v e r t i c a l w a l l f a c i n g t h e a v a l a n c h e a c t i n t h e s a r n e w a y a s e a r t h r n o u n d s , b u t a r e r n o r e e f f e c t i v e i n s p l i t t i n g t h e a v a l a n c h e a n d s l o w i n g i t d o w n . T h e y a r e , h c w e v e r , e x p e n s r v e . C a b l e n e l s i n t h e a v a l a n c h e p a t h A v a l a n c h e s r u n n i n g i n n a r r o w g u l l i e s r n a y b e s l o w e d b y a n e t w o r k o f s t e e l c a b l e s s p a n n i n g t h e p a t h . T h i s d e f e n c e h a s b e e n u s e d f o r e x p e r i m e n t a l p u r p o s e s o n l y a n d n o c o n c l u s i v e r e s u l t s a r e a v a i l a b l e t o d a t e . I t w a s n o t t e s t e d a t G I a c i e r . C a t c h i n g d a r n s a n d b e n c h e s D a r n s b u i l t p e r p e n d i c u l a r l i r n i t e d e f f i c i e n c y b e c a u s e f a s t - r n o v i n g f i l l t h e s p a c e i n f r o n t o f i t w i t h s n o w . i n t h e a v a L a n c h e p a l h i s e v e n l e s s t h a n t o t h e a v a l a n c h e d i r e c t i o n a r e o f a v a l a n c h e s j u r n p the darn and soon T h e e f f i c i e n c y o f b e n c h e s o r t e r r a c e s t h a t o f d a r n s .
. l 5
-D u r i n g t h e f i r s t y e a r s o f t h e a v a l a n c h e s u r v e y i t w a s o b s e r v e d that the bench forrned by the abandoned railway grade retained small
avalanches. Accordingly, experirnental benches were built in four avalanche
p a t h s a t M t . T ' r p p e r b e t w e e n M i l e I I . 6 a n d M i l e 1 2 . 3 d u r i n g t h e s u r r t r n e r s o f 1 9 5 6 a n d L 9 5 7 . D u r i n g t h e t w o w i n t e r s b e t w e e n 1 9 5 7 a n d 1 9 5 9 c a t c h i n g d a r n s w e r e b u i l t o f s n o w o n t h e o u t s i d e e d g e s o f t h e b e n c h e ' s . T h e b e n c h e s a r e d e s c r i b e d a n d t h e r e s u l t s o f t h e o b s e r v a t i o n s a r e g i v e n i n A p p e n d i x C .
Observations showed that benches are not su-fficient as an
a v a l a n c h e d e f e n c e a t G l a c i e r a n d s h o u l d b e u s e d o n l y i n a s s o c i a t i o n w i t h c a t c h i n g d a r n s , w h i c h c a n b e c o n s i d e r e d i n t h e f o l l o w i n g s i t u a t i o n s : a ) b ) c ) d ) A S A S 1n at
defence against rninor avalanches
s i t e s w h e r e n o t r n o r e t h a n t h r e e a v a l a n c h e s o c c u r p e r w i n t e r
the terrninal zone of an avalanche site
s i t e s w h e r e t h e r e i s i n s u f f i c i e n t s p a c e f o r t h e c o n s t r u c t i o n o f r n o u n d s .
Catching darns require about the sarne earth quantity as two rows of rnounds with equal height, but rnounds are rrrore efficient in stopping avalanches, D i v e r t i n g D a r n s
Diverting darns or walls in the avalanche path divert an avalanche
l a t e r a l l y s o t h a t i t c a n r u n o u t h a r r n l e s s l y . T h e a n g l e b e t w e e n t h e d a m a n d t h e d i r e c t i o n o f t h e a v a l a n c h e r n u s t n o t e x c e e d 2 0 d e g r e e s ; a n d t h e d a r n should be higher than the depth of the avalanche plus the head of water c o r r e s p o n d i n g t o t h e l o s s o f e n e r g y w h e n t h e a v a l a n c h e h i t s t h e d a r n . T h i s r e q u i r e s h i g h s t r u c t u r e s i n r n a n y c a s e s . A s e r i e s o f t w o o r t h r e e catching dams behind each other rnight prove to be r,:rore economicaf (7). T h e s i d e o f t h e d a r n f a c i n g t h e a v a l a n c h e r n u s t b e a s s t e e p a s p o s s i b l e i n o r d e r t o p r o d u c e a g o o d d e f l e c t i n g e f f e c t . A v e r t i c a l w a l l w o u l d b e
rnost efficient, but in view of its high cost an earth darn with a natural
s l o p e o r o n e c o v e r e d w i t h r i p - r a p l n a y b e c o n s i d e r e d . D i v e r t i n g s t r u c t u r e s b u i l t i n A u s t r i a a n d S w i t z e r l a n d h a v e c o n c r e t e o r r n a s o n r y w a l l s 1 0 t o Z O f . L h i g h , b a c k f i l l e d w i t h e a r t h . F o r t h e d e f e n c e o f t h e r a i l w a y l i n e o n R o g e r s P a s s , s t o n e f i l l e d I o g c r i b s B t o Z 0 f t h i g h w e r e u s e d . B i n t y p e r n e t a l w a l l s o r w a l l s w i t h s h e e t p i l i n g r n i g h t a l s o b e c o n -s i d e r e d , b u t n o c a -s e i -s k n o w n w h e r e t h i -s h a -s b e e n d o n e . D a r n -s b u i l t i n o r n e a r t h e t e r r n i n u s r n a y p r o v e i n e f f e c t i v e f o r l a t e r a v a l a n c h e s b e c a u s e o f b a n k i n g o f e a r l i e r d e p o s i t s .
S n o w S h e d s S n o w s h e d s d i v e r t s n o w o v e r t h e h i g h w a y a n d w h e n p r o p e r l y d e s i g n e d p r o v i d e f u l l s a f e t y . T h e y r n u s t c o v e r t h e w i d t h o f t h e u s u a l a v a l a n c h e p a t h a n d b e s t r o n g e n o u g h t o c a r r y t h e l o a d o f d e p o s i t e d s n o w a n d r n o v i n g a v a l a n c h e s . T h e y c a n b e b u i l t o f c o n c r e t e , s t e e l , w o o d o r a c o m -b i n a t i o n o f t h e s e r n a t e r i a l s , -b u t a r e a n e x p e n s i v e d e f e n c e e c o n o r n i c a l o n l y w h e r e t h e r e a r e f r e q u e n t a v a l a n c h e s o r w h e r e a h i g h d e g r e e o f s a f e t y i s r e q u i r e d . W i t h t h e f i r s t p l a n n i n g o f t h e a v a l a n c h e d e f e n c e a t R o g e r s P a s s i t w a s r e c o g n i z e d t h a t s n o w s h e d s w o u l d b e n e c e s s a r y a n d t h a t r n o s t o f t h e m w o u l d b e l o c a t e d i n t h e t e r m i n u s o f a v a l a n c h e s w h e r e a h e a v y s n o w l o a d w o u l d o c c u r . I t w a s i r n p o r t a n t t h e r e f o r e t o e s t i r n a t e t h i s l o a d , a n d o b s e r -v a t i o n s w e r e c a r r i e d o u t t o s u p p l y i n f o r r n a t i o n o n t h e p o s s i b l e d e p t h a n d d e n s i t y o f t h e s n o w o n t h e s h e d s . T h e o b s e r v a t i o n s a r e l i s t e d i n A p p e n d i x D . R e f o r e s t a t i o n D e n s e f o r e s t c o v e r i n g a r u p t u r e z o n e a c t s a s a r e t a i n i n g b a r r i e r , a n d t r e e s o n t h e w i n d w a r d s i d e o f a r i d g e r n a y a l s o b e e f f e c t i v e a s a s n o w f e n c e t o c o n t r o l d r i f t i n g s n o w . T i r n b e r i n a r e a s w h e r e a v a l a n c h e s c o u l d o c c u r s h o u l d b e p r o t e c t e d f r o r n f i r e a n d d i s e a s e a n d o n l y I o g g e d c a r e f u l l y ; a r e a s w i t h a c t i v e a v a l a n c h e s c o u l d b e r e f o r e s t e d . S u c h p r o j e c t s , h o w e v e r , r n u s t a l w a y s b e a s s o c i a t e d w i t h t h e c o n s t r u c t i o n o f t e r n p o r a r y r e t a i n i n g s t r u c t u r e s , b e c a u s e s n o w - s l i d e s c a n d e s t r o y t h e g r o w i n g t r e e s . T h e y a r e e x p e n s i v e , b u t g u a r a n t e e a h i g h d e g r e e o f f u t u r e s a f e t y . I t r n a y
be pointed out that defence by reforestation can only be applied in rupture
z o n e s t h a t a r e l o w e r t h a n t h e n a t u r a l t r e e l i n e a n d w h e r e t h e r e i s e n o u g h t o p s o i l t o s u p p o r t t h e i r g r o w t h .
Explo s ive s
T h e a c t i o n o f g r a v i t y c a u s e s s n o w t o c r e e p a n d s t r e s s e s t o d e v e l o p i n t h e s n o w c o v e r . W h e n t h e t e n s i l e a n d t h e s h e a r s t r e s s e s attain a critical value any additional Ioad or shock frorn an explosion rnay b e s u f f i c i e n t t o c a u s e a f a i l u r e t h a t w i t l p r o d u c e o n e o f t h e f o l l o w i n g :
a ) T h e s n o w s t r u c t u r e c o l l a p s e s a n d t h e s n o w s t a r t s t o s l i d e , picking up rnore snow as it advances until a large avalanche r e s u l t s .
b ) T h e s n o w s t a r t s t o s l i d e , b u t o n l y i n a s r n a l l a r e a , s o t h a t t h e a v a l a n c h e i s a r n i n o r o n e , t r a v e l l i n g a s h o r t d i s t a n c e .
_ 1 7 _ c ) I h e s n o w s t r u c t u r e c o l l a p s e s , b u t t h e f r i c t i o n b e t w e e n t h e c o l l a p s e d s n o w a n d t h e u n d e r l y i n g s n o w o r b e t w e e n t h e s n o w a n d t h e g r o u n d i s t o o h i g h t o a l l o w t h e s n o w t o s l i d e . T h i s d i s t u r b e d s n o w w i l l g a i n s t r e n g t h w i t h t i r n e a n d a s t a b l e s n o w c o v e r r e s u l t . C r a c k s w h i c h f o r r n w h e n t h e s n o w s t r u c t u r e c o l l a p s e s a r e o f t e n v i s i b l e o n t h e s u r f a c e . O c c a s i o n a l l y t h e a d d i t i o n a l s t r e s s e s p r o d u c e d b y a n e x p l o s i o n a r e n o t s u f f i c i e n t t o c a u s e t h e s n o w t o f a i l a n d n o t h i n g h a p p e n s .
Explosives rnay be applied for avalanche defence by two d i f f e r e n t r n e t h o d s :
( l ) T h e y a r e d e t o n a t e d i n t h e r u p t u r e z o n e w h e n t h e h a z a r d i s s t i l l I o w . l f t h e s t r u c t u r e f a i l s , p o s s i b i l i t i e s ( b ) o r ( c ) w i l l o c c u r ; t h e r e s u l t i n g a v a l a n c h e s w i l l b e s r n a l l a n d n o t d a n g e r o u s . T h e explosions and the release of rninor avalanches are continued as l o n g a s a h a z a r d e x i s t s ( " . g . d u r i n g a s n o w s t o r r n ) . l / [ i t h this technique, no rnajor avalanches are allowed to forrn.
( Z l E x p l o s i v e s a r e d e t o n a t e d o n l y w h e n t h e r e i s a p o s s i b i l i t y t h a t d a n g e r o u s a v a l a n c h e s w i l l o c c u r . T h e e x p l o s i o n r n a y r e s u l t i n a n y o f t h e t h r e e p o s s i b i l i t i e s ( a ) , ( b ) o r ( c ) . T h e d a n g e r o u s a r e a i s c l o s e d t o t r a f f i c , and if a rnajor avalanche results the snow is irnrnediately rernoved. I f t h e e x p l o s i o n c a u s e s o n l y a s t r u c t u r a l c o l l a p s e o f t h e s n o w c o v e r b u t n o a v a l a n c h e ( p o s s i b i l i t y ( c ) ) , t h e s i t e i s c o n s i d e r e d s a f e a g a i n . I t i s , h o w e v e r , d i f f i c u l t t o r e c o g n i z e w h e t h e r s u c h a s t a b i l i z a t i o n h a s taken place unless the snow cover in the rupture zone is studied. Vllhen n o a v a l a n c h e i s r e l e a s e d o r n o a p p a r e n t s t a b i l i z a t i o n c r e a t e d , t h e
explosion rnerely proves that the snow is rnore stable than had been p r e d i c t e d .
T h e f i r s t r n e t h o d r e q u i r e s a g r e a t q u a n t i t y o f e x p l o s i v e s ; t h e s e c o n d i s r n o r e e c o n o r n i c a l b u t d o e s n o t p r o v i d e s u c h a h i g h d e g r e e o f s a f e t y . W i t h t h e s e c o n d r n e t h o d t h e r e i s d a n g e r t h a t r n a j o r a v a l a n c h e s w h i c h w o u l d n o t s l i d e n a t u r a l l y r n a y b e r e l e a s e d , o r t h a t t h e s h o c k o f the explosion rnay release avalanches that take an unexpected path.
E x p l o s i v e s a r e e f f i c i e n t a g a i n s t a v a l a n c h e s c a u s e d b y s n o w f a l l , w i n d a n d u n s t a b l e s n o w c o v e r . A v a l a n c h e s c a u s e d b y h i g h t e r n p e r a t u r e s ( s p r i n g t h a w a v a l a n c h e s a n d s o r n e w e t s n o w d i r e c t - a c t i o n a v a l a n c h e s ) u s u a l l y c a n n o t b e c o n t r o l l e d b y e x p l o s i v e s . S p r i n g t h a w a v a l a n c h e s r n a y b e s u p p r e s s e d t o a c e r t a i n d e g r e e w h e n r n i n o r a v a l a n c h e s a r e r e l e a s e d d u r i n g e v e r y s n o w f a l l a n d t h e s n o w c o v e r i s n o t a l l o w e d t o b u i l d u p i n t h e r u p t u r e z o , , _ e .
T h e a d v a n t a g e s o f a v a l a n c h e c o n t r o l b y e x p l o s i v e s a r e l o w c o s t a n d f l e x i b i l i t y ; r t i s a d e f e n c e r n e t h o d t h a t c a n e a s i l y b e a d a p t e d t o n e w s i t u a t i o n s a n d n e w s i L e s . T h e e x p l o s i v e s m a y b e h a n d p l a n t e d c h a r g e s , p r e - p l a n t e d c h a r g e s o r s h e l l s f i r e d f r o r n a r t i l l e r y . H a n d p l a n t e d c h a r g e s A c h a r g e o f Z t o 5 I b o f d y n a r n i t e i s t o s s e d i n t o t h e z o n e w h e r e t h e a v a l a n c h e u s u a l l y f r a c t u r e s a n d i s d e t o n a t e d e i t h e r e l e c t r i c a l l y o r b y a p r e - l i t f u s e . T h i s m e t h o d r e q u i r e s t h a t t h e a v a l a n c h e r u p t u r e z o n e b e e a s i l y a c c e s s i b l e a t a n y t i r n e ( B ) . P r e - p l a n t e d c h a r g e s
Dynarnite charges are placed in the rupture zone of the avalanche in surnrner and are detonated electrically frorn a rernote point w h e n t h e a v a l a n c h e h a z a r d d i c t a t e s . T h e d i s t a n c e b e t w e e n c h a r g e s i s n o r m a l l y b e t w e e n 5 0 a n d 2 0 0 f t . D i f f e r e n t s e r i e s o f c h a r g e s a r e p l a n t e d s o t h a t t h e n u r n b e r o f t i r n e s a v a l a n c h e s c a n b e r e l e a s e d d u r i n g t h e w i n t e r i s l i m i t e d b y t h e n u r n b e r o f s e r i e s . T h i s r n a k e s i t i r n p o s s i b l e t o u s e f r e q u e n t e x p l o s i o n s a s t h e h a z a r d b u i l d s ( 9 ) . E x p e r i r n e n t s a t R o g e r s P a s s i n d i c a t e d t h a t p r e - p l a n t e d d y n a r n i t e a n d t h e a s s o c i a t e d w i r i n g i s o f t e n d i s t u r b e d b y anirnals. It would appear that this defence rnethod should only be used w h e n t h e e x p l o s i v e s a n d w i r e s a r e s p e c i a l l y p r o t e c t e d .
A r t i l l e r y f i r e
T h e e x p l o s i o n r e q u i r e d f o r t h e r e l e a s e o f a v a l a n c h e s r n a y b e p r o d u c e d b y a s h e l l f i r e d f r o r n a n a r t i l l e r y g u n , a r e c o i l l e s s gun, a rnortar o r a r o c k e t . A r t i l l e r y f i r e i s u s e d s u c c e s s f u l l y f o r a v a l a n c h e d e f e n c e i n a nurnber of countries (B). The cost of the weapon and the arnrnunition
e x c e e d s t h e c o s t o f h a n d p l a n t e d a n d p r e - p l a n t e d c h a r g e s , b u t t h e r n e t h o d r n a y b e r n o r e e c o n o r n i c a l t h a n d e f e n c e s t r u c t u r e s . C h o i c e o f a w e a p o n
d e p e n d s o n t h e d i s t a n c e f r o r n t h e p o s s i b l e f i r i n g p o s i t i o n t o t h e t r i g g e r z o r : ' e of the avalanches, the supRly of weapons, the supply and price of arnrnunition.
A t t h e r e q u e s t o f t h e N a t i o n a l R e s e a r c h C o u n c i l t h e C a n a d i a n A r r n y c o - o p e r a t e d i n s e l e c t i n g a s u i t a b l e w e a p o n f o r t h e a v a l a n c h e c o n t r o l a t R o g e r s P a s s . L t - C o l . C . H . L i t h g o w , C . O . , F i r s t B a t t a l i o n , T h e Q u e e n r s O w n R i f l e s o f C a n a d a , w a s a s s i g n e d t h e t a s k o f s t u d y i n g t h e p r o b l e r n a n d c o n d u c t i n g t r i a l s . H i s s t u d y , r n a d e i n N o v e r n b e r 1 9 5 7 , i n d i c a t e d t h e f o l l o w i n g :
