Variation of ground snow loads in British Columbia

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Variation of ground snow loads in British Columbia

Schaerer, P. A.

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VARIATION OF G R O U N D SNOW LOADS IN BRITISH COLUMBIA A c c o r d i n g t o t h e N a t i o n a l B u i l d i n g Code o f C a n a d a , 1965, ( 1 ) d e s i g n s n o w - l o a d s f o r r o o f s m u s t b e d e t e r m i n e d by m u l t i p l y i n g a g r o u n d l o a d w i t h a p p r o p r i a t e snow l o a d f a c t o r s . The g r o u n d l o a d v a r i e s w i t h l o c a t i o n a n d t h e v a r i a t i o n i s p a r t i c u l a r l y g r e a t i n t h e moun- t a i n s w h i c h c o v e r m o s t o f t h e P r o v i n c e o f B r i t i s h C o l u m b i a . The p r i n c i p a l f a c t o r s t h a t i n - f l u e n c e t h e a c c u m u l a t i o n o f snow on t h e g r o u n d a r e g e o g r a p h i c a l l o c a t i o n , e l e v a t i o n , a s p e c t a n d f o r e s t c o v e r . C l i m a t i c r e g i o n s , f o r t h e s o u t h e r n p a r t o f B r i t i s h C o l u m b i a a s d e f i n e d i n t h e " B r i t i s h C o l u m b i a A t l a s o f R e s o u r c e s , ~ ( 2 ) a r e shown o n F i g u r e 1 . M a j o r s t o r m p a t h s , moun- t a i n r a n g e s , a n d l a r g e b o d i e s o f w a t e r i n f l u e n c e t h e c l i m a t e o f a n a r e a , a n d c o n s e q u e n t l y t h e amount o f s n o w f a l l a n d m e l t i n g o f snow. S u p e r i m p o s e d on g e n e r a l c l i m a t i c r e g i o n s a r e l o c a l v a r i a t i o n s o f c l i m a t e a s s o c i a t e d w i t h i n d i v i d u a l m o u n t a i n s a n d v a l l e y s . I t i s w e l l known t h a t t h e a m o u n t o f snow o n t h e g r o u n d u s u a l l y i n c r e a s e s w i t h e l e v a t i o n . Lower a i r t e m p e r a t u r e , w h i c h c a u s e s m o r e p r e c i p i t a t i o n t o f a l l a s snow i n s t e a d o f r a i n , a n d w h i c h d e l a y s m e l t i n g ,

is

t h e p r i n c i p a l r e a s o n f o r t h e i n c r e a s e i n snow d e p t h w i t h e l e v a t i o n . The a s p e c t a n d t r e e c o v e r d e t e r m i n e w h e t h e r o r n o t t h e w i n d i n f l u e n c e s t h e d e p o s i - t i o n o f snow a n d t h e d e g r e e t o w h i c h s o l a r r a d i a t i o n p r o d u c e s m e l t i n g b e t w e e n s n o w f a l l s . The e f f e c t s o f a s p e c t a n d f o r e s t c o v a r h a v e b e e n s u m m a r i z e d by Neiman ( 3 ) .

S u p p l e m e n t No. 1 t o t h e N a t i o n a l B u i l d i n g Code 1 9 6 5 ( 4 ) recommends g r o u n d l o a d s f o r a number o f s t a t i o n s . T h e s e l o a d s w e r e c a l c u l a t e d from c l i m a t i c r e c o r d s a n d r e f l e c t o n l y t h e c l i m a t i c v a r i a t i o n s a t t h e p l a c e s w h e r e w e a t h e r o b s e r v a t i o n s w e r e made. I n m o u n t a i n r e g i o n s t h e L o a d s a r e a p p l i c a b l e m a i n l y t o L o c a t i o n s i n t h e v a l l e y s . L i t t l e i s known how t h e snow l o a d s c h a n g e f o r L o c a t i o n s a t h i g h e r e l e v a t i o n s . The N a t i o n a l R e s e a r c h C o u n c i l i n i t i a t e d a n o b s e r v a t i o n p r o g r a m i n 1 9 6 6 w i t h t h e p u r p o s e o f o b t a i n i n g n u m e r i c a l v a l u e s a b o u t t h e i n c r e a s e o f t h e g r o u n d l o a d w i t h e l e v a t i o n .

Method o f O b s e r v a t i o n

O b s e r v a t i o n s i t e s w e r e s e l e c t e d o n a number o f r n o u n t q i n s i n S o u t h e r n B r i t i s h C o l u m b i a , p r e f e r a b l y n e a r c e n t r e s o f p o p u l a t i o n w h e r e t h e r e was n e e d f o r i n f o r n l a t i o n on snow Loads. A most i m p o r t a n t r e q u i r e m e n t f o r t h e s e l e c t i o n o f s i t e s was a c c e s s i b i l i t y i n w i n t e r , e i t h e r by r o a d o r s k i tow. The l o c a t i o n s w h e r e o b s e r v a t i o n s w e r e made a r e shown o n F i g u r e 1 .

The v a r i a b l e s o f a s p e c t a n d v e g e t a t i o n w e r e m a i n t a i n e d c o n s t a n t a n d c h o s e n s o a s t o e n s u r e a l a r g e snow a c c u m u l a t i o n . T h i s was a c h i e v e d by making o b s e r v a t i o n s i n o p e n i n g s o f L a r g e f o r e s t s o n m o u n t a i n s i d e s w i t h u n i f o r m , w e s t t o n o r t h t o e a s t a s p e c t . The o p e n i n g s h a d w i d t h s o f a b o u t two t r e e h e i g h t s . Between 6 a n d 1 2 o b s e r v a t i o n s i t e s w e r e e s t a b l i s h ~ d on e a c h m o u n t a i n w i t h d i f f e r e n c e s i n e l e v a t i o n o f 3 0 0 f t t o 4 0 0 f t b e t w e e n them. I t was n o t a l w a y s p o s s i b l e t o f i n d i d e a l s i t e s a n d o f t e n i n f l u e n c e s o f w i n d a n d s u n c o u l d n o t b e e l i m i n a t e d . Some s i t e s h a d t o b e c h a n g e d a f t e r t h ~ f i r s t o r s e c o n d w i n t e r i n o r d e r t o ob- t a i n t h e b e s t p o s s i b l e c o n d i t i o n s .

The amount o f snow on t h e g r o u n d was m e a s u r e d w i t h a F e d e r a l s n o w s a m p l e r ; t h r e e s a m p l e s w e r e o b t a i n e d a t e a c h s i t e . M e a s u r i n g a snow c o u r s e o f t e n p o i n t s a t e a c h e l e v a - t i o n would h a v e b e e n m o r e a c c u r a t e , b u t t i m e d i d n o t p e r m i t t h i s number o f o b s e r v a t i o n s . The c o l l e c t i o n o f o b s e r v a t i o n s f r o m a s many s i t e s a s p o s s i b l e was c o n s i d e r e d t o b e more v a l u a b l e t h a n g r e a t e r a c c u r a c y . B e c a u s e t h e s i t e s w e r e c a r e f u l l y s e l e c t e d , t h e s p r e a d

I/

G e o t e c h n i c a l S e c t i o n , D i v i s i o n o f B u i l d i n g R e s e a r c h , N a t i o n a l R e s e a r c h C o u n c i l o f C a n a d a , O t t a w a .

b e t w e e n t h e t h r e e samples p r o v a d t o be n o t more t h a n 1 i n . w a t e r e q u i v a l e n t , w h i c h was 3 t o 6 p e r c e n t o f t h e mean. A t each s i t e o n l y t h e maximulo w a t e r e q u i v a l e n t o f t h e snow p e r w i n t e r was c o n s i d e r a d f o r t h e s t u d y . The measurements were made a t i n t e r v a l s o f a b o u t t w o weeks a t t h e t i m e when t h e Load o f t h e snow o n t h e g r o u i d w o u l d be e x p e c t e d t o b e maximum. The h i 3 h e s t L o s d was r e a c h e d i n J a n u a r y o r e a r l y F e b r u a r y i n t h e v a l l e y s ; a t h i g h e l e v a - t i o n s , ( 3 5 0 0 f t above sea L e v 3 1 a t t h e c o a s t a n d 6000 f t above sea L e v e l i n t h e i n t e r i o r ) , t h e snow l o a d was maximum i n May,

R e s u l t s

The maximum snow L o s d s o n t h e ground, w h i c h may b e e x p r e s s e d i n i n c h e s w a t e r - e q u i v a l e n t o r i n p o u n d s p 9 r s q u a r e f o o t , were p l o t t e d a g a i n s t e l e v a t i o n . An example i s p r e s e n t e d i n F i g u r e 2 f o r M o u n t R e v e l s t o k e . I n t h i s example, t h e o b s e r v a t i o n a t 3900 f e e t d e v i a t e s s i g n i f i c a n t l y f r o m t h e c u r v e drawn t h r o u g h t h e p o i n t s . T h i s i n d i c a t e s t h a t t h e o b s e r v a t i o n was made a t a p a r t i c u l a r l y s h e l t e r e d s i t e . The r e l a t i o n s h i p b e t w e o n e L e v a t i o n a n d g r o u n d l o a d c a n b e a p p r o x i m a t e d w i t h t h e e q u a t i o n : where: L

=

l o a d o r w a t e r e q u i v a l e n t a t t h e e l e v a t i o n , H, abovs sea l e v e l , a, b, c

=

c o e f f i c i e n t s t h a t v a r y w i t h g e o g r a p h i c a l l o c a t i o n a n d y e a r . No a t t e m p t h a s b e e n made t o f i t c u r v s s b y regression a n a l y s i s , b e c a u s e t h e o b s e r v a t i o n s were n o t homogeneous. The c o s f f i c i e n t ~ o f t h o s q u a t i o n w i l l b e d e t e r m i n e d i n t h e f u t u r e when e r r o r s due t o i n f l u 3 n c e s o f a s p e c t arid v ~ g e t a t i o n h a v e bee2 m i n i m i z e d t h r o u g h i m -

p r o v e d s s l e c t i o n o f s i t e s , and a l a r g e nuinher o f o b s a r v a t i o n s a r e a v a i l a b l e .

The o b s e r v a t i o n s o f t h r e e y e a r s , 1 9 5 7 t o 1969, h a v e i n d i c a t e d t h a t w i t h i n t h 3 same c l i m a t i c r e g i o n , t h s y e a r l y g r o x n d l o a d

-

e l e v a t i o n c u r v e s h a v a a s i m i l a r shape, b u t t h e o r i g i n s o f t h s . c u r v a s may d i f f e r , The r e l a t i o n i s more c u r v e d i n a wet m i l d c l i m a t e w i t h h e a v y s n o w f a l l s t h a n i n a d r y c o l d c l i m a t e . F i g u r e 3 shows o b s e r v a t i o n s t y p i c a l f o r sev- e r a l c l i m a t i c r e g i o n s . Ttie arnount o f p r e c i p i t a t i o n i s h i g h . a t t h e c o a s t (Seymour M o u n t a i n ) a n d d e c r e a s e s e a s t w a r d t h r o u g h t h e wet a n d dsy s o u t h e a s t i n t e r i o r t o t h e d r y Rocky Moun- t a i n s . The w e t t e r t h e c l i m a t e , t h e s t r o n g e r . i s t h e i n c r e a s e o f s n o w l o a d w i t h e l e v a t i o n . M o s t n o t e w o r t h y a r o t h e o b s e r v a t i o n s f r o m Seymour m o u n t a i n , w h i c h a r e a l m o s t i d e n t i c a l w i t h t h o s e o b t a i n e d a t ~ U J J o t h e r s t a t i o n s n o a r Vancouver. The r e a s o n f o r t h e p r o n o u n c a d change i n t h e r a t e o f i n c r e a s e a t t h e 2 0 3 0 - f t e l e v a t i o n i s t h a t m o s t s t o r m s n e a r Vancouver d e p o s i t snow above t h i s e l a v z t i o r ~ , and p r o d u c e r a i n a t t h e l o w s r e l e v a t i o n .

The d e c r e a s e i n t h e r a t e o f chan3e o f snow l o a d f r o m West t o E a s t , o r wet t o d r y , i s a l s o n o t i c e a b l e o n a s m a l l e r s c a l e a s shown i n F i g u r e 4. The d i s t a n c e b e t w e e n t h e s e t h r a e l o c a t i o n s i s a b o u t 33 m i l u s .

The g e n e r a l shape o f t h e c u r v s does n o t change f r o m y e a r t o y e a r . I t r e m a i n s s t r o n g l y c u r v e d i n wet, m i l d c l i m a t e a n d c l o s e t o l i n e a r i n d r y , c o o l r e g i o n s . The s t e e p - n e s s o f t h e c u r v a changes a l i t t l e f r o m one y e a r t o t h e n e x t , a n d a l s o i t s o r i g i n .

S i m i l a r r e s u l t s h a v e b e e n r e p o r t e d b y o t h e r s . A l i n e a r c o r r e l a t i o n o f snow-water e q u i v a l e n t w i t h e l e v a t i o r , uraa f o u n d by G o l d i n g ( 5 ) o n t h e e a s t e r n s l o p e o f t h e Rocky M o u n t a i n s i n A l b e r t a and b y L a r s t k a ( 6 ) i n Wyoming. B o t h a r e a s h a v a a d r y c l i m a t e . A n d e r s o n and West ( 7 ) r e p o r t e d f o r t h e C e n t r a l S i e r r a Nevada a n d P a c k e r ( 8 ) f o r N o r t h e r n I d a h o , r e l a t i o n s t h a t were n u n - l i n e a r w i t h a g r e a t e r r a t e o f i n c r e a s e o f s n o w l o a d a t h i g h e l e v a t i o n s . B o t h a r e a s h a v a h o s v y s n o w f a l l s . A s e c o n d d e g r e e f u n c t i o n b e t w e e n snow l o a d and e l e v a t i o n was a l s o d e t e r m i n e d f o r S w i t z e r l a n d by Z i n g g ( 9 ) a n d i s s p e c i f i e d t h e r e f o r t h e d e s i g n o f b u i l d i n g s . The c l i m a t e i n t h e S w i s s m o u n t a i n s i s s i m i l a r t o t h e c l i m a t e o f S o u t h e a s t I n t e r i o r B r i t i s h Columbia. C o n c l u s i o n

F I G U R E I

C L I M A T E R E G I O N S A N D SNOW O B S E R V A T I O N S T A T I O N S I N SOUTHERN B R I T I S H CO\v!i!,A

L I I I I I I t

1000 2000 3000 4000 5000 6000 E L E V A T I O N I N FEET

F I G U R I 2 M A X I M U M SNOWLOADS M O U N T REVELSTOKE 1969

I I I I I I _I 1000 2000 3000 4000 5000 6000 7000 E L E V A T I O N I N FEET

.

F I G U R E 3 M A X I M U M S H O W L O A D S I N I969 ",,I..

,

I I I I I I I 1000 ZOO0 3000 4000 5000 E L E V A T I O N I N FEET

F I G U R E 4 M A X I M U M SNOW L O A D S I N WEST KOOTENAY 1969

1, 4 1

,,

.

,

q u a d r a t i c e q u a t i o n whose p a r a m e t e r s depend on L o c a t i o n and year. When o b s e r v a t i o n s a r e a v a i l a b l e f o r many y e a r s i t s h o u l d b e p o s s i b l e t o c a l c u l a t e maximum snowloads t h a t c o u l d b e e x p e c t e d t o o c c u r once i n 30 years. The c o r r e l a t i o n between e l e v a t i o n and t h e 30-year maximum snowload can p r o b a b l y b e w r i t t e n i n t h e form o f a second degree e q u a t i o n w i t h t h e c o e f f i c i e n t s dependent on c l i m a t e . The 30-year maximum snow l o a d s can b e e x p e c t e d t o i n c r e a s e a t a g r e a t e r r a t e a t h i g h e l e v a t i o n i n wet c l i m a t e , s i m i l a r t o t h e r e l a t i o n s e s t a b l i s h e d f o r i n d i v i d u a l years; t h e i n c r e a s e i n snow l o a d w i t h e l e v a t i o n i s a l m o s t l i n e a r i n d r y c l i m a t e s , such a s t h a t o f t h e Rocky Mountains.

T h i s paper i s a c o n t r i b u t i o n o f t h e D i v i s i o n o f B u i l d i n g Research, N a t i o n a l Research C o u n c i l o f Canada, and i s p u b l i s h e d w i t h t h e a p p r o v a l o f t h e D i r e c t o r o f t h e D i v i s i o n .

R e f e r e n c e s

N a t i o n a l B u i l d i n g Code o f Canada, 1965. N a t i o n a l Research Council, o f Canada, A s s o c i a t e Committee on t h e N a t i o n a l B u i l d i n g Code, Ottawa, 1965. NRC 8305. B r i t i s h Columbia A t l a s o f Resources. B r i t i s h Columbia N a t u r a l Resources Conference, V i c t o r i a , B. C. 1956.

Meiman, J. R. Snow a c c u m u l a t i o n r e l a t e d t o e l e v a t i o n , aspect, and f o r e s t canopy. P r e s e n t e d a t t h e N a t i o n a l Workshop Seminar, Snow H y d r o l o g y , sponsored by t h e Canadian N a t i o n a l Committee I n t e r n a t i o n a l H y d r o l o g i c a l Decade; U n i v e r s i t y o f Mew Brunswick, F e b r u a r y 28 and 29, 1968.

C l i m a t i c I n f o r m a t i o n f o r B u i l d i n g D e s i g n i n Canada. Supplement No. 1 t o t h e N a t i o n a l B u i l d i n g Code of Canada, 1965, N a t i o n a l Research C o u n c i l of Canada, A s s o c i a t e Committee on t h e N a t i o n a l B u i l d i n g Code, Ottawa, 1965. NRC 8329.

G o l d i n g , D. L. Snow measurement on Marmot Creek e x p e r i m e n t a l watershed. P r e s e n t e d a t t h e N a t i o n a l Workshop Seminar, Snow H y d r o l o g y sponsored by t h e Canadian

N a t i o n a l Committee I n t e r n a t i o n a l H y d r o l o g i c a l Decade; U n i v e r s i t y o f New Brunswick, F e b r u a r y 28 and 29, 1968.

Garstka, W. U. I n t e r p r e t a t i o n o f snow surveys. T r a n s a c t i o n s , American Geophysical Union, Vol. 30, No. 3, June 1949, p. 412-420.

Anderson, H. W. and A. J. West. Snow a c c u m u l a t i o n and m e l t i n r e l a t i o n t o t e r r a i n i n wet and d r y years. Proceedings, 3 3 r d Annual M e e t i n g Western Snow Conference, Colorado Springs, 1965, p. 73-82.

Packer, P. E. E l e v a t i o n , a s p e c t , and c o v e r e f f e c t s on maximum snow p a c k w a t e r c o n t e n t i n a Western w h i t e p i n e f o r e s t . F o r e s t Science, Vol. 8, 1962, p. 225-235. Zingg, Th. Maximale S c h n e e l a s t e n i n d e r Schweiz (Maximum Snow Loads i n ~ w i t z e r l a n d ) S c h w e i z e r i s c h e Bauzeitung, VoL. 86, NO. 31, August 1968, p. 555-557.