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Terrain and vegetation of snow avalanche sites at Rogers Pass, British
Columbia
Schaerer, P. A.
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Chapter 5-3
TERRAIN AND VEGETATION OF SNOW AVALANCHE SITES AT ROGERS PASS, BRITISH COLUMBIA.
P. A . S c h a e r e r
The D i v i s i o n of Building R e s e a r c h of t h e National
R e s e a r c h Council of C a n a d a is engaged i n a s t u d y of snow a v a l a n c h e s a t R o g e r s P a s s w h e r e t h e T r a n s C a n a d a Highway c r o s s e s t h e
S e l k i r k Mountain r a n g e i n B r i t i s h Columbia. T h e l o c a t i o n was c h o s e n b e c a u s e n u m e r o u s a v a l a n c h e s r e a c h the v a l l e y e v e r y w i n t e r and the c o n t r o l of t h e m is a n i m p o r t a n t p a r t of t h e m a i n t e n a n c e of
/
the highway. i-_I -2 T h e t e r r a i n a n d t h e c l i m a t e a t R o g e r s P a s s a r e t y p i c a l f o r t h e mountains of t h e s o u t h e r n i n t e r i o r of B r i t i s h Columbia. The e l e v a t i o n of t h e n a r r o w v a l l e y s is between 900 and 1320 m (2950 and 4350 f t ) a n d the m o u n t a i n s r i s e t o 3000 m (9850 f t ) above s e a l e v e l . 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 R o g e r s P a s s , a t e l e v a t i o n 1320 m (4350 f t ) , is 1 . 30 m ( 5 1 i n ) ; 0.80 m (32 i n ) of i t f a l l s i n t h e f o r m of snow. A v a l a n c h e s s t a r t t o s l i d e a t e l e v a t i o n s of between 1500 a n d 2 300 m (5000 a n d 7500 f t ) a n d r u n out on t h e a l l u v i a l f a n s and t a l u s s l o p e s i n the v a l l e y s . M o s t a v a l a n c h e s i t e a r e a s a r e between 50,000 and 200,000 m 2 (540,000 a n d 2 , 1 5 0 , 0 0 0 f t 2 ) . P R O B L E M T w o of t h e o b j e c t i v e s of t h e s t u d y a r e t o o b t a i n i n f o r m a t i o n to f a c i l i t a t e i d e n t i f i c a t i o n of a v a l a n c h e s i t e s and t o e s t i m a t e t h e magnitude and the f r e q u e n c y of o c c u r r e n c e ofa v a l a n c h e s f r o m f e a t u r e s of t e r r a i n a n d vegetation. T h e f o r m a t i o n of a v a l a n c h e s is the r e s u l t of t e r r a i n f a c t o r s s u c h a s s l o p e a n d r u g g e d n e s s a n d is a l s o influenced by t h e w e a t h e r ( e . g.
,
snowfall, wind, t e m p e r a t u r e ) . T h e v a r i o u s p a r a m e t e r s a r e i n t e r r e l a t e d a n d i t is often difficult t o i s o l a t e t h e i r influence.T h e p r e s e n t s t u d i e s a r e c o n c e r n e d with a v a l a n c h e s l a r g e enough to be s i g n i f i c a n t t o t r a f f i c , t o s t r u c t u r e s i n t h e i r p a t h , a n d t o hydrology, geomorphology, and f o r e s t r y o p e r a t i o n s . L a r g e a v a l a n c h e s develop when t h e t e r r a i n a n d the snow condition i n t h e t r a c k below t h e s t a r t i n g point p e r m i t a l a r g e a m o u n t of snow t o be r e m o v e d . T h e r e a r e m a n y o t h e r s m a l l a v a l a n c h e s t h a t a r e
s i g n i f i c a n t only to a v a l a n c h e h a z a r d f o r e c a s t i n g and skiing s a f e t y .
T Y P E S O F AVALANCHE TERRAIN
The t h r e e following c h a r a c t e r i s t i c t y p e s of t e r r a i n w h e r e a v a l a n c h e s develop can be found a t R o g e r s P a s s and in o t h e r a r e a s of s o u t h e r n B r i t i s h Columbia.
Cliff
T h e i m p o r t a n t c h a r a c t e r i s t i c h e r e i s one o r m o r e s t e e p o u t c r o p s of bedrock in a s l o p e c o v e r e d with loose r o c k , g r a s s , o r s h r u b ( F i g u r e 1 ) . Snow s t a r t s t o s l i d e on o r just below the s t e e p p a r t s of the t e r r a i n and then s e t s m o r e snow into motion on the slope below. The r o c k o u t c r o p s have s u r f a c e slope angles g r e a t e r than 4 Z 0 and i n o r d e r f o r a v a l a n c h e s to grow t o a l a r g e s i z e the s l o p e i m m e d i a t e l y below the s t a r t i n g point m u s t have a n incline of 2 8 " o r m o r e . Cliff-type t e r r a i n can u s u a l l y be r e c o g n i z e d by i t s l a c k of vegetation and can be identified on black and white a i r photographs a s h o r i z o n t a l bands i n a light tone.
Gullies
Gully-type s i t e s c o n s i s t of a V - s h a p e d m a i n gully with n u m e r o u s b r a n c h e s of s t e e p - s i d e d g u l l i e s ( F i g u r e 2 ) . T h e r e i s u s u a l l y no w e l l defined avalanche s t a r t i n g zone. A v a l a n c h e s m a y s t a r t i n a n y one of the g u l l i e s a n d , when descending into the m a i n channel, s e t into motion the u n s t a b l e snow on the s t e e p gully s i d e s .
The v a r i o u s s i d e gullies often p r o d u c e a v a l a n c h e s independently and a t d i f f e r e n t t i m e s . S e v e r a l a v a l a n c h e s m a y be o b s e r v e d a t s h o r t i n t e r v a l s when w e a t h e r conditions a r e f a v o r a b l e f o r t h e i r f o r m a t i o n .
L e e Slopes
T h i s type of t e r r a i n can be found on the l e e w a r d side of mountain r i d g e s w h e r e l a r g e a m o u n t s of w i n d - t r a n s p o r t e d snow a r e deposited ( F i g u r e 3 ) . T h e slope n e e d not have a n y s t e e p s e c t i o n s , s u c h a s c l i f f s , but i n o r d e r t o p r o d u c e l a r g e a v a l a n c h e s i t would a p p e a r t h a t the incline should be m o r e than 2 8 O . A v a l a n c h e s u s u a l l y f r a c t u r e c l o s e t o the r i d g e i n the convex p a r t of the slope just above t h e d e e p e s t a c c u m u l a t i o n of snow.
The
6 7
a v a l a n c h e s i t e s i n v e s t i g a t e d a t R o g e r s P a s s could be c a t e g o r i z e d a s follows: 36 cliff s i t e s ; 1 5 gully s i t e s ; 9 l e e s l o p eF i g u r e 1
-
Cliff type s i t e . Avalanches s t a r t to slide a t
the two s h a r p d r o p s of slope.
F i g u r e 2
-
Gully type avalanche s i t e .
F i g u r e 3
-
L e e s l o p e s i t e . The d i r e c t i o n of the prevailing wind i s f r o m the right-hand s i d e ; a v a l a n c h e s s t a r t t o s l i d e on the s l o p e s in the shade. of gully and l e e slope s i t e s .The cliff a n d l e e s l o p e a v a l a n c h e s a r e e i t h e r funnelled into a channel o r m a y continue t o s l i d e o v e r a n open s l o p e . The gully type i s by i t s n a t u r e a s s o c i a t e d with a channelled t r a c k .
INFLUENCE O F TERRAIN ON THE FREQUENCY O F AVALANCHE OCCURRENCE
The n u m b e r of t i m e s that a v a l a n c h e s will o c c u r i s d e t e r m i n e d m o r e by c l i m a t e (i. e .
,
f r e q u e n c y and magnitude of s n o w f a l l s , t e m p e r a t u r e s that r e m a i n c l o s e t o O°C, and d i r e c t i o n and s p e e d of wind) than by t e r r a i n c h a r a c t e r i s t i c s . Conclusions about how m u c h t e r r a i n influences avalanche o c c u r r e n c e can only be d r a w n i n conjunction with the c l i m a t e .T h e s l o p e incline a p p e a r s t o be the m o s t i m p o r t a n t p a r a m e t e r r e l e v a n t t o the f r e q u e n c y of o c c u r r e n c e of a v a l a n c h e s . O t h e r p a r a m e t e r s a r e the s h a p e of the t r a c k , i t s r u g g e d n e s s , vegetation c o v e r , a n d e x p o s u r e t o wind and s u n .
The a v a l a n c h e s c l o s e t o the highway a t R o g e r s P a s s have been r e c o r d e d s i n c e 1953. In F i g u r e 4 the a v e r a g e n u m b e r of m e d i u m and l a r g e s i z e a v a l a n c h e s a r e plotted a g a i n s t the a v e r a g e
-
"
-I0, 2 w > 4 c 0 . 1 20 3 0 4 0 44 9: M E A N I N C L I N E O F A V A L A N C H E T R A C K , DEGREES C H A N N E L E D T R A C K - 0 O P E N T R A C K-
- - - - - F i g u r e 4-
Relation between f r e q u e n c y of o c c u r r e n c e of a v a l a n c h e s and incline of t e r r a i n .slope angle of t h e avalanche t r a c k f o r the cliff-type and lee s l o p e - type s i t e s . Gully s i t e s could not be included in the a n a l y s i s because, a t m o s t of t h e m , a v a l a n c h e s w e r e c o n t r o l l e d by a r t i l l e r y . U s e of a r t i l l e r y p r o d u c e s s m a l l e r , m o r e f r e q u e n t a v a l a n c h e s than would o c c u r u n d e r n a t u r a l conditions. The incline was m e a s u r e d o v e r t h e full length of the t r a c k between the u s u a l s t a r t i n g zone and the beginning of the outrun zone, and the a n a l y s i s included only those a v a l a n c h e s that w e r e l a r g e enough to r u n o v e r the full length of t h e t r a c k .
The g r a p h shows that in o r d e r t o produce l a r g e a v a l a n c h e s a s t e e p e r incline i s n e c e s s a r y on open s l o p e s than in channels s u c h a s v a l l e y s , gullies and t r o u g h s . One i m p o r t a n t r e a s o n f o r the difference i s t h a t channels tend to have many s m a l l a v a l a n c h e s and the snow deposited by them f o r m s a smooth sliding s u r f a c e . Loup and Lovie (1967) a l s o r e p o r t a r e m a r k a b l e difference in the f r e q u e n c y of avalanches between gullies and open s l o p e s .
E x p o s u r e t o wind and s u n , v a r i a t i o n s of the incline, ruggedness of the t e r r a i n and ground c o v e r a r e r e s p o n s i b l e f o r the s c a t t e r of the points in F i g u r e 4. I t i s not y e t p o s s i b l e t o analyze the influence of t e r r a i n c h a r a c t e r i s t i c s . The method of slope angle d i s t r i b u t i o n , r e c o m m e n d e d by S t r a h l e r (1956), a p p e a r e d t o be p r o m i s i n g . The s u r f a c e slope a n g l e s w e r e d e t e r m i n e d with the
a i d of topographic m a p s of s c a l e 1:5000 a n d contour i n t e r v a l s 7 . 5 m ( 2 5 f t ) f o r 40 t o 100 r a n d o m points in 34 avalanche s i t e s . I t was not p o s s i b l e , h o w e v e r , t o e s t a b l i s h a r e l a t i o n between the f r e q u e n c y a n d s i z e of the a v a l a n c h e a n d the v a r i a n c e of the slope a n g l e s . Avalanche a c t i v i t y a p p e a r e d t o be u n r e l a t e d t o t h e m a x i - m u m s l o p e o r t o the p e r c e n t a g e of t e r r a i n s t e e p e r than a t h r e s h o l d a n g l e , e . g . , 42" which i s the m a x i m u m a n g l e of r e p o s e of t a l u s . I t was only the a v e r a g e s l o p e angle of t h e t r a c k t h a t c o r r e l a t e d with the f r e q u e n c y of t h e a v a l a n c h e s .
F r o m o b s e r v a t i o n s m a d e o v e r a t w e l v e - y e a r p e r i o d i t was found t h a t a v a l a n c h e s l o s e s p e e d on a n open snow s u r f a c e when the incline i s l e s s than 28". They begin t o d e p o s i t t h e i r snow when the t a l u s s l o p e s and a l l u v i a l f a n s of the outrun zones a r e f l a t t e r than t h i s c r i t i c a l angle. Depending on the s p e e d a t t a i n e d in the t r a c k , a n d t h e i r m a s s , a v a l a n c h e s m a y run m u c h f u r t h e r and s o m e t i m e s c l i m b the opposite s i d e of the valley.
T R E E GROWTH AS AN INDICATOR O F AVALANCHE FREQUENCY
A v a l a n c h e s have a s i g n i f i c a n t d e s t r u c t i v e effect on f o r e s t s . T h e i r t r a c k s can e a s i l y be r e c o g n i z e d in t h e mountain a r e a of southern B r i t i s h Columbia by the s c a r s on heavily f o r e s t e d s l o p e s . T h e dominant s p e c i e s of t r e e s a t R o g e r s P a s s a r e E n g e l m a n n S p r u c e ( P i c e a e n g e l m a n n i ) , W e s t e r n Hemlock ( T s u g a h e t e r o p h y l l a ) , and W e s t e r n Red C e d a r (Thuja p l i c a t a ) . Open a r e a s in the f o r e s t a r e c o v e r e d m a i n l y with Mountain A l d e r (Alnus
tenuifolia). T h e u p p e r t r e e l i n e i s g e n e r a l l y a t the 2000 m (6600 f t ) elevation.
T h e a g e of t h e t r e e s in the outrun zones of a v a l a n c h e s i s s o m e t i m e s a s s u m e d t o be e q u a l t o t h e n u m b e r of y e a r s t h a t h a v e p a s s e d s i n c e the l a s t l a r g e a v a l a n c h e . T h i s i s t r u e only f o r s i t e s w h e r e a v a l a n c h e s happen s o r a r e l y t h a t when one d o e s o c c u r l a r g e a r e a s of m a t u r e t i m b e r a r e d e s t r o y e d . A t s u c h s i t e s s e e d l i n g s begin t o grow i n the c l e a r e d a r e a a y e a r l a t e r a n d a young f o r e s t with n u m e r o u s t r e e s of e q u a l a g e then a p p e a r s . At s i t e s with m o r e f r e q u e n t a v a l a n c h e s the t r e e s a r e d e s t r o y e d w h e n e v e r t h e y have r e a c h e d a c e r t a i n c r i t i c a l a g e a n d height; the r e s u l t h e r e i s a growth of s m a l l t r e e s with v a r i a b l e a g e .
A s t u d y of t h e height a n d a g e of t r e e s in a v a l a n c h e outrun zones was m a d e a t R o g e r s P a s s a n d o t h e r a r e a s . Such o b s e r v a t i o n s m u s t be i n t e r p r e t e d with caution, however, because not only a v a l a n c h e s but a l s o s o i l conditions, c l i m a t e , and e x p o s u r e influence t h e growth of t r e e s . T r e e s m u s t be s e l e c t e d t h a t a r e fully exposed t o a v a l a n c h e s and not p r o t e c t e d by boulders o r a
ridge of t e r r a i n . It should a l s o be mentioned h e r e that avalanches usually s l i d e o v e r the snow s u r f a c e i n the outrun zone and r e m o v e v e r y little of the p r e v i o u s l y deposited snow.
The m o s t i n t e r e s t i n g conclusion of the t r e e study was that i t i s the height r a t h e r than the age of the t r e e s that d e t e r m i n e s how i t r e a c t s i n a v a l a n c h e s ; the r e a c t i o n v a r i e s between the s p e c i e s . Height and age of t r e e s a r e r e l a t e d however, e . g . , the s p r u c e t r e e s a t R o g e r s P a s s grow a t a n a v e r a g e r a t e of 20 c m ( 8 i n ) p e r y e a r , and the hemlocks a t about 8 c m ( 3 i n ) .
Engelmann Spruce i s found m o s t frequently in avalanche s i t e s . I t s chance f o r s u r v i v a l i n avalanches i s s m a l l when i t i s t a l l e r than 3 . 5 m ( 1 1 . 5 f t ) . L a r g e r t r e e s a r e usually broken o r uprooted. S p r u c e t r e e s s h o r t e r than 3. 5 m (11. 5 f t ) a r e flexible
enough to bend under the i m p a c t of the avalanche, and t r e e s s h o r t e r than about 1 . 5 m ( 5 ft) a r e buried i n the n a t u r a l snow c o v e r and a r e well protected.
W e s t e r n Hemlock that a r e exposed t o a v a l a n c h e s a r e u s u a l l y broken n e a r the snow s u r f a c e , 1.2 to 2.0 m (4 to 6 . 5 f t )
above ground. When the t r e e t r u n k does not b r e a k the t r e e s a r e s t r i p p e d of t h e i r needles and b r a n c h e s and then decline.
W e s t e r n C e d a r was found unsuitable a s a n i n d i c a t o r of avalanche activity, because of a g r e a t v a r i a t i o n in the r a t e of growth and s t r e n g t h . Some c e d a r s a r e o b s e r v e d to s u r v i v e avalanches y e a r a f t e r y e a r and grow 8 m (26 f t ) high and o t h e r s , with equal e x p o s u r e , b r e a k when they a r e only 3 m (10 ft) high.
B e c a u s e s p r u c e s u r v i v e s a v a l a n c h e s b e t t e r and grows m o r e rapidly than h e m l o c k , f o r e s t s a t s i t e s where avalanches have not o c c u r r e d f o r 30 to 50 y e a r s contain mainly l a r g e s p r u c e t r e e s with only a few s m a l l hemlock t r e e s . A s t a n d of mixed s p r u c e and hemlock of equal height and a g e s would indicate that no avalanches had r e a c h e d i t s location f o r 100 o r p e r h a p s m o r e y e a r s .
The s t u d i e s on t r e e s i n avalanche s i t e s enabled the following c a t e g o r i e s t o be established:
( a ) Alder and willow b r u s h (no coniferous t r e e s higher than 1 . 5 m ( 5 f t ) . L a r g e avalanches o c c u r frequently, usually once p e r y e a r .
( b ) F e w l a r g e t r e e s , no branches on the s i d e facing the avalanche below 8 m (26 ft) above ground, few s m a l l t r e e s . Avalanches o c c u r once e v e r y one to t h r e e y e a r s . They produce mainly windblast and deposit snow not d e e p e r than 1.0 m ( 3 . 3 ft).
( c ) No l a r g e t r e e s a n d no d e a d wood f r o m l a r g e t r e e s ; s p r u c e t r e e s 3 . 5 t o 4 . 5 m ( 1 1 . 5 t o 1 5 f t ) high a n d h e m l o c k 1 . 5 t o 2 . 0 m ( 5 t o 6 . 5 f t ) high, d a m a g e d b r a n c h e s a n d b a r k . L a r g e a v a l a n c h e s o c c u r about once e v e r y t h r e e t o t e n y e a r s a n d d e p o s i t d e e p snow.
(a)
B r o k e n l a r g e t r e e s a n d d e n s e g r o w t h of s m a l l t r e e s . L a r g e a v a l a n c h e s have o c c u r r e d not m o r e f r e q u e n t l y than once i n t e n y e a r s a n d probably not even once in t h i r t y y e a r s .CONCLUSION
Avalanche s i t e s c a n be identified f r o m f e a t u r e s of t e r r a i n a n d vegetation. T h e r e a r e n u m e r o u s p a r a m e t e r s that d e t e r m i n e the f r e q u e n c y of o c c u r r e n c e and the s i z e of a v a l a n c h e s . An i d e a of the a v e r a g e f r e q u e n c y of a v a l a n c h e s can be obtained f r o m the a v e r a g e incline of the a v a l a n c h e t r a c k a n d the type a n d height of t r e e s in the outrun zone. G e n e r a l c l a s s i f i c a t i o n s could be e s t a b l i s h e d f o r the R o g e r s P a s s a r e a a n d i t i s expected that they could be extended t o o t h e r a r e a s in W e s t e r n Canada.
