A case history of forecasting frazil ice

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A case history of forecasting frazil ice

Williams, G. P.

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Ser

TH1

N21r2

no.

591

c. 2

! BLDG

I

NATIONAL RESEARCH COUNCIL O F CANADA

CONSEIL NATIONAL D E RECHERCHES DU CANADA

A CASE HISTORY O F FORECASTING F R A Z I L I C E

by

Go P. W i l l i a m s

R e p r i n t e d f r o m

PROCEEDINGS

Banff S y m p o s i a on T h e R o l e of Snow

and

I c e i n Hydrology

S e p t e m b e r 1972

Vol. 2, p. 1212

-

1 2 2 3

SEARCH COUNCIL

R e s e a r c h P a p e r No.

2x1

, , ,.

_.

' -.~----..-I. -.-. '. ..--....

.-".".

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of t h e

D i v i s i o n of Building R e s e a r c h

P r i c e 2 5 c e n t s

OTTAWA

T h i s p u b l i c a t i o n i s being d i s t r i b u t e d b y t h e Division of Building R e s e a r c h of 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 of Canada. I t should not b e r e p r o d u c e d i n whole o r in p a r t without p e r m i s s i o n of t h e o r i g i n a l p u b l i s h e r . T h e Di- v i s i o n would b e g l a d to b e of a s s i s t a n c e i n obtaining s u c h p e r m i s s i o n . P u b l i c a t i o n s of t h e D i v i s i o n m a y b e o b t a i n e d by m a i l - ing t h e a p p r o p r i a t e r e m i t t a n c e ( a Bank, E x p r e s s , o r P o s t Office M o n e y O r d e r , o r a c h e q u e , m a d e p a y a b l e t o t h e R e c e i v e r G e n e r a l of Canada, c r e d i t NRC) t o 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 of Canada, Ottawa. K I A 0 R 6 . S t a m p s a r e n o t a c c e p t a b l e . A l i s t of a l l p u b l i c a t i o n s of t h e D i v i s i o n i s a v a i l a b l e a n d m a y b e obtained f r o m the P u b l i c a t i o n s S e c t i o n , D i v i s i o n of Building 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 of C a n a d a , Ottawa. KIA OR

6.

A case history of forecasting frazil ice

D i v i s i o n o f B u i l d i n g Research,

BationaZ Research CounciZ o f Canada, Ottawa, Ontario

ABSTRACT: Twenty y e a r s ' r c c o r d s o f b l o c k a g e of i n t a k e r a c k s by f r a z i l i c e a t power p l a n t s l o c a t e d on t h e Ottawa R i v e r were examined t o d e f i n e t h e w e a t h e r c o n d i t i o n s and h e a t l o s s e s u n d e r which f r a z i l o c c u r r e d . blarginal i c c b l o c k a g e took p l a c e when t h e r a t e of h c a t l o s s a v e r a g e d about 30-40 c a l / s q cm/hr f o r s e v e r a l h o u r s p r c c e d i n g t h e time when f r a z i l was r e p o r t e d . I c e b l o c k a g e s u f f i c i e n t t o s h u t down t h e g e n e r a t o r s o c c u r r e d when t h e r a t e o f h e a t l o s s was g r e a t e r t h a n a b o u t 40 c a l / s q cm/hr f o r 12 h o u r s o r more. i?lmost a l l f r a z i l problems o c c u r r e d d u r i n g w e s t e r l y o r n o r t h r ~ e s t e r l y f l o w s o f r e l a - t i v e l y d r y a i r , with wind speeds a v e r a g i n g a t l e a s t 2.25 t o 4 . 5 m/s

(5-10 mph) .

RESUbE: On a examinf l e s r e l c v c s , s u r une pEriode d e v i n g t a n s , de llencombrement d e s g r i l l e s d e rise d ' e a u p a r l e f r a z i l d a n s d e s c e n t r a l c s a l i m e n t E e s p a r l a r i v i s r e O u t a o u a i s , a f i n d e d g t e r m i n e r l e s c o n d i t i o n s atmosphEriques c t l a p e r t e d e c h a l e u r a s s o c i 6 e s

2

l a f o r - mation du f r a z i l . L'encombrement m a r g i n a l dC

2

_{l a g l a c e s e p r o d u i t} l o r s q u e l e s p e r t e s d e c h a l e u r a t t e i g n e n t en moyenne 30-40 c a l / c m 2 / h d u r a n t p l u s i e u r s h e u r e s a v a n t que l l o n ne s i g n a l e l e f r a z i l . Les encombrements c a u s a n t l ' a r r E t d e s g F n E r a t r i c e s s e p r o d u i s e n t l o r s q u e l a p e r t e d e c h a l e u r d 6 p a s s e e n v i r o n 40 c a l / c m 2 / h d u r a n t 12 h c u r e s a u moins. Presque t o u t c s l e s d i f f i c u l t 6 s a ~ ~ p a r a i s s e n t p a r v e n t r e l a t i v e m e n t s e c , de l ' o u e s t ou du n o r d - o u e s t , e t de 2.25 - 4 . 5 m/s (5-10 m i l l e s

2

l l h e u r c ) au nioins, en moyenne. INTRODUCTION

Hydro power p l a n t s l o c a t e d on t h e Ottawa R i v e r between t h e c i t i e s of Ottawa, O n t a r i o , and I l u l l , Qucbcc, have been p l a g u e d w i t h ri.vcr i c c problems s i n c c t h e i r c o n s t r u c t i o n l a t e i n t h e 1 9 t h c e n t u r y . Somc of t h e e a r l i e s t i . n v e s t i g a t i o n s of f r a z i l i n Canada have heen made a t t h e s c p l a n t s [ I ] . The i c e problems e n c o u n t e r e d a r e t h o s e co~nmon f o r r u n - o f - t h e - r i v e r hydro p l a n t s w i t h l i m i t e d f o r e b a y a r e a s . P r a z i l i c e f o r m a t i o n i n t h e r a p i d s u p s t r e a m from t h e dams ( F i g . 1)

i s o f t c n of s u f f i c i e n t q u a n t i t y t o b l o c k t h e i n t a k e r a c k s a t t h e powerhouses. O t h c r forms of i c e ( b o r d e r o r s h o r e i c e , f r a z i l s l u s h ) moving downstream d u r i n g an i c e r u n a l s o c o n t r i b u t e t o t h e b l o c k a g e . Thesc problcms o c c u r d u r i n g e a r l y w i n t e r and s p r i n g b r e a k - u p when t h e i c c c o v e r i s u n s t a b l e b u t n o t u s u a l l y d u r i n g t h e mid-winter p c r i o d when s t a b l e i c c c o v e r s t h e f o r e b a y a r e a .

Through c x p c r i e n c c , p l a n t o p e r a t o r s have d e v e l o p e d a keen a p p r e - c i a t i o n o f t h e w c a t h e r and flow conditions u n d e r which f r a z i l and o t h e r forms of i c e w i l l b l o c k t h e i n t a k e r a c k s . T h i s e x p e r i e n c e o f t e n e n a b l e s them t o anticipate i c e problcms and p r c p a r e nien and c q u i p ~ n c n t f o r t h c arduous t a s k o f rcmoving i c e from t h e r a c k s . E a r l y

investigators 111 nttem1)ted t o r e l a t e rveather c o n d i t i o n s t o f r a z i l o c c u r r e n c e b u t w i t h o u t a p p a r e n t s u c c e s s .

An e x c e l l e n t r e c o r d o f t i m e and d a t e o f i c e b l o c k a g e and t h e g e n e r a l t y p e o f i c e c a u s i n g t h e problem h a s been k e p t by t h e o p e r - a t o r s f o r many y e a r s . T h i s r e c o r d , a l o n g w i t 1 1 t h e w e a t h e r d a t a main- t a i n e d a t Uplands a i r p o r t ( F i g . 1) about 12 k i l o m e t r e s ( 7 . 5 m i l e s ) from t h e powerhouses, p r o v i d e d an e x c e l l e n t o p p o r t u n i t y t o a s s e s s t h e problem o f f o r e c a s t i n g f r a z i l . The r e s u l t s o f t h i s s t u d y a r e r e c o r d e d i n t h i s p a p e r . I-lopefully t h i s c a s e h i s t o r y will b e o f v a l u e i n d e v e l o p i n g methods o f f o r e c a s t i n g f r a z i l a t o t h e r s i t e s .

The s t u d y i s concerned o n l y w i t h t h o s e c a s e s where t h e o p e r a t o r s d e f i n i t e l y r e p o r t e d f r a z i l c o l l e c t i n g on t h e i n t a k e r a c k s . F r a z i l was i d e n t i f i e d by i t s a b i l i t y t o a d h e r e t o t h e r a c k s . Because i t f r e e z e s t o t h e r a c k s i t i s g e n e r a l l y more d i f f i c u l t t o remove t h a n o t h e r forms o f i c e .

A l l a v a i l a b l e i n f o r m a t i o n on w e a t h e r , w a t e r flow and i c e c o n d i - t i o n s was o b t a i n e d f o r each f r a z i l o c c u r r e n c e r e p o r t e d a t t h e power p l a n t s f o r t h e p e r i o d from December 1951 t o A p r i l 1971. Records p r i o r t o 1951 were n o t i n c l u d e d because t h e i n f o r m a t i o n a v a i l a b l e on w e a t h e r and i c e c o n d i t i o n s was i n c o m p l e t e . During t h e p e r i o d a n a l y z e d , f r a z i l was r e p o r t e d f o r t y - t w o t i m e s d u r i n g t h e month of December, once d u r i n g t h e month o f F e b r u a r y , and f i f t t z e n t i m e s d u r i n g t h e months o f March and A p r i l .

L i t t l e o r no 5 n f o r m a t i o n was a v a i l a b l e on t h e a r e a o f open w a t e r a v a i l a b l e f o r f r a z i l p r o d u c t i o n o r i c e movement f a c t o r s needed f o r a complete a n a l y s i s o f t h e problem. An i n d i c a t i o n o f t h e s e v e r i t y o f f r a z i l p r o d u c t i o n was o b t a i n e d from t h e number of c a s e s where b l o c k - age was s u f f i c i e n t t o cause l o a d r e d u c t i o n o r complete p l a n t s h u t - down.

WEATHER !3URING FRAZI L OCCURRENCE

Weather d a t a were o b t a i n e d f o r t h e 12-hour p e r i o d p r i o r t o e a c h r e p o r t e d f r a z i l o c c u r r e n c e . iVeather-condition d a t a b e f o r e t h i s 12-hour p e r i o d were examined and found t o b e u s u a l l y o f s e c o n d a r y i m p o r t a n c e f o r f r a z i l f o r m a t i o n a t t h e s i t c : . Hourly o b s e r v a t i o n s o f wind s p e e d and d i r e c t i o n , a i r and dew p o i n t t e m p e r a t u r e s were

o b t a i n e d from m e t e o r o l o g i c a l r e c o r d s . The amount o f s n o w f a l l and t h e d e g r e e o f c l o u d i n e s s d u r i n g t h e p e r i o d s were a l s o n o t e d b u t n o t used t o any e x t e n t i n t h e a n a l y s i s .

F i g u r e 2 g i v e s i n f o r m a t i o n on wind d i r e c t i o n , wind s p e e d , and a i r t e m p e r a t u r e f o r t h e f i f t y - e i g h t p e r i o d s a n a l y z e d . Wind d i r e c t i o n was p r e d o m i n a n t l y w e s t e r l y o r n o r t l ~ \ ~ e s t e r l y d u r i n g f r a z i l o c c u r - r e n c e s . The e a r l y i n v e s t i g a t o r s [ l ] a l s o n o t e d t h a t " f r a z i l a t t a c k s were i n v a r i a b l y accompanied by a n o r t h w e s t wind". The 12-hour a v e r a g e wind s p e e d p r i o r t o a f r a z i l o c c u r r e n c e was a l m o s t i n v a r i - a b l y g r e a t e r t h a n 5 mph (2.25 m / s ) . The one c a s e r e p o r t e d wherein t h e a v e r a g e wind s p e e d was l e s s t h a n 5 mph had a 12-hour a v e r a g e a i r t e m p e r a t u r e o f -8°F (-22OC) and an a v e r a g e dew p o i n t t e m p e r a t u r e o f -21°F (-29°C)

.

The a v e r a g e wind s p e e d was almost i n v a r i a b l y g r e a t e r t h a n 10 mph ( 4 . 5 m/s) f o r a v e r a g e a i r t e m p e r a t u r e s g r e a t e r t h a n +14OF (-10°C).

Dew p o i n t t e m p e r a t u r e s were r e l a t i v e l y low, p a r t i c u l a r l y d u r i n g t h e s p r i n g o c c u r r e n c e s o f f r a z i l . The a v e r a g e d i f f e r e n c e between a i r and dew p o i n t t e m p e r a t u r e s f o r t h e 12-hour p e r i o d s was 1 3 . 7 " F (7.6"C) f o r t h e s i x t e e n s p r i n g o c c u r r e n c e s ; 7.2"F (4.0°C) f o r t h e f o r t y - t w o e a r l y w i n t e r o c c u r r e n c e s . The h i g h e s t 12-hour a v e r a g e dew p o i n t t e m p e r a t u r e r e c o r d e d p r i o r t o a f r a z i l o c c u r r e n c e was 15°F (-9.4'C). The h i g h e s t r e c o r d e d a v e r a g e a i r t e m p e r a t u r e was

24OF ( - 4 . S ° C ) .

C l e a r o r p a r t i a l l y c l e a r sky c o n d i t i o n s p r e v a i l e d f o r t h e major- i t y o f t h e f r a z i l o c c u r r e n c e s . L i g h t snow f l u r r i e s o c c u r r e d d u r i n g s e v e r a l o f t h e 12-hour p e r i o d s , b u t s n o w f a l l s g r e a t e r t h a n ' h a l f an i n c h \\rere r e p o r t e d o n l y t w i c e .

SURFACE HEAT LOSSES D U R I N G FRAZIL OCCURRENCES

Weather d a t a from t h e 12-hour p e r i o d s were used t o c a l c u l a t e t h e a v e r a g e r a t e o f s u r f a c e h e a t l o s s from open w a t e r . The r a t e o f h e a t l o s s n o t o n l y d e t e r m i n e s t h e r a t e a t which r i v e r w a t e r c o o l s p r i o r t o f r a z i l f o r m a t i o n , b u t a l s o t h e s u b s e q u e n t r a t e o f f r a z i l p r o d u c t i o n . S u r f a c e h e a t l o s s e s were o b t a i n e d by c a l c u l a t i n g t h e d i f f e r e n t components o f a t m o s p h e r i c h e a t exchange: n e t s h o r t - w a v e r a d i a t i o n , n e t long-wave r a d i a t i o n , e v a p o r a t i o n , and c o n v e c t i o n . Values f o r short-wave r a d i a t i o n were o b t a i n e d from t h e N a t i o n a l Research Council m e t e o r o l o g i c a l s t a t i o n ( F i g . 1 ) and a l b e d o v a l u e s f o r d i f f e r e n t a l t i t u d e s o f t h e sun from G e i g e r [ 2 ] . Long-wave r a d i a t i o n , c o n v e c t i o n , and e v a p o r a t i o n l o s s e s were c a l c u l a t e d u s i n g t h e formulae proposed by Michel [ 3 ] . D e t a i l s o f and l i m i t a t i o n s t o c a l c u l a t i n g t h e s e t e r m s from w e a t h e r r e c o r d s have b e e n w e l l reviewed i n t h e l i t e r a t u r e [ 4 , 5 J . Slichel shows t h a t h i s proposed formulae f o r e v a p o r a t i o n and t o t a l h e a t l o s s e s compare r e a s o n a b l y w e l l w i t h formulae proposed by v a r i o u s o t h e r a u t h o r s . F i g u r e 3 shows t h e f r e q u e n c y d i s t r i b u t i o n s o b t a i n e d f o r t h e d i f f e r e n t components o f s u r f a c e h e a t b a l a n c e f o r t h e 12-hour p e r i o d s p r i o r t o t h e r e p o r t e d f r a z i l o c c u r r e n c e s . The h e a t g a i n e d from s h o r t - w a v e r a d i a t i o n d u r i n g t h e s e p e r i o d s i s n o t shown on t h i s diagram b e c a u s e i t was a v e r y s m a l l p e r c e n t a g e o f t h e t o t a l c a l - c u l a t e d h e a t l o s s . A s f r a z i l was r e p o r t e d m o s t l y a t n i g h t o r i n t h e e a r l y morning, t h e amount o f s o l a r r a d i a t i o n o c c u r r i n g d u r i n g t h e p r e c e d i n g 12-hours was n o t g r e a t . The c o n t r i b u t i o n o f n e t long- wave r a d i a t i o n t o t o t a l h e a t l o s s , r a n g i n g from 7 t o 15 c a l / s q cm/hr, was o f t e n o f s e c o n d a r y importance compared t o t h e o t h e r components. I t was i m p o r t a n t , however, f o r t h o s e c a s e s w i t h e x t r e m e l y low a i r t e m p e r a t u r e s , c l e a r s k i e s , and l i g h t winds. I n t h e s e s i t u a t i o n s of r e l a t i v e l y low r a t e o f t o t a l h e a t l o s s , long-wave r a d i a t i o n c o n t r i b u t e d 30-40 p e r c e n t o f t h e t o t a l h e a t l o s s . The c o n t r i b u t i o n o f t h e e v a p o r a t i v e term r a n g i n g from 6 t o 30 c a l / s q cm/hr, was e s p e - c i a l l y s i g n i f i c a n t d u r i n g s p r i n g f r a z i l o c c u r r e n c e s when dew p o i n t t e ~ n p e r a t u r e s were much l e s s t h a n a i r t e m p e r a t u r e s . Convection was u s u a l l y t h e most i m p o r t a n t h e a t l o s s p r o c e s s , p a r t i c u l a r l y d u r i n g p e r i o d s o f h i g h wind and r e l a t i v e l y low a i r t e m p e r a t u r e s .

The r a t e s o f s u r f a c e h e a t l o s s g i v e an i n d i c a t i o n o f t h e s e v e r - i t y o f t h e f r a z i l problem. S i x t y p e r c e n t o f t h e c a s e s o c c u r r e d when t h e t o t a l s u r f a c e h e a t l o s s a v e r a g e d 31-45 c a l / s q cm/hr. The lower l i m i t d e f i n e s t h e minimum r a t e o f h e a t l o s s r e q u i r e d b e f o r e f r a z i l p r o d u c t i o ~ ~ \<as s u f f i c i e n t t o a f f e c t t h e i n t a k e r a c k s . I f t h e r a t e o f h e a t l o s s was below a c r i t i c a l l e v e l o f a b o u t 30 c a l / s q cm/hr, s e r i o u s f r a z i l a c c u m u l a t i o n d i d n o t o c c u r a t t h e i n t a k e s . The u p p e r l i m i t , above a b o u t 50 c a l / s q cm/hr, was a s s o c i a t e d w i t h t h e c a s e s where f r a z i l p r o d u c t i o n was s e v e r e enough t o c a u s e p a r t i a l o r complete shutdown o f t h e p l a n t s .

The f o r e g o i n g a s s e s s m e n t of w e a t h e r c o n d i t i o n s and r a t e o f h e a t l o s s d u r i n g f r a z i l o c c u r r e n c e s d e f i n e s t h e r a t h e r broad limits w i t h i n which f r a z i l problems o c c u r r e d a t t h i s s i t e . Two examples a r e p r e s e n t e d i n t h e n e x t s e c t i o n of t h e p a p e r t o i l l u s t r a t e how

t h i s i n f o r m a t i o n might be used i n p r a c t i c a l f o r e c a s t i n g .

COI\IPARISON OF WEATi-IER CONDITIONS, HEAT LOSS AND WATER TEblPERATURE A i r t e m p e r a t u r e s , dew p o i n t t e m p e r a t u r e s , h e a t l o s s e s , and w a t e r t e m p e r a t u r e s were p l o t t e d , u s i n g 4-hour a v e r a g e s , f o r t h e e a r l y w i n t e r p e r i o d , 5-25 December, 1969, and f o r t h e s p r i n g p e r i o d , 3-10 A p r i l , 1971 ( F i g s . 4 and 5 ) . Wind d i r e c t i o n and wind s p e e d a r e a l s o shown on t h e s e p l o t s . F r a z i l was r e p o r t e d t h r e e t i m e s d u r i n g t h e e a r l y w i n t e r p e r i o d and once d u r i n g t h e s p r i n g p e r i o d . These p e r i o d s were chosen f o r d e t a i l e d a n a l y s i s b e c a u s e a c c u r a t e w a t e r t e m p e r a t u r e r e c o r d s were a v a i l a b l e .

Water t e m p e r a t u r e was c o n t i n u o u s l y r e c o r d e d t o an a c c u r a c y o f about 20.02"C w i t h a s e n s o r l o c a t e d 2 f e e t o r s o below t h e s u r f a c e a t t h e e n t r a n c e o f one o f t h e power i n t a k e s t r u c t u r e s . The s e n s o r was s h i e l d e d from d i r e c t s o l a r r a d i a t i o n by a c o v e r t h a t e x t e n d e d o v e r t h e i n t a k e s t r u c t u r e .

The c o r r e l a t i o n between r a t e o f h e a t l o s s , w e a t h e r c o n d i t i o n s , and s e v e r i t y o f f r a z i l b l o c k a g e i s e v i d e n t from t h e s e p l o t s . Blar- g i n a l i c e b l o c k a g e o c c u r r e d when t h e r a t e o f h e a t l o s s a v e r a g e d about 30-40 c a l / s q cm/hr f o r s e v e r a l h o u r s d u r i n g and p r e c e d i n g t h e o c c u r - r e n c e of f r a z i l (5-6 December). S e v e r e i c e b l o c k a g e , s u f f i c i e n t t o s h u t down t h e g e n e r a t o r s , o c c u r r e d when t h e r a t e o f h e a t l o s s was g r e a t e r t h a n about 40 c a l / s q cm/llr f o r 12-hours o r more (22-23 December). A l l f r a z i l problems o c c u r r e d d u r i n g w e s t e r l y o r n o r t h - w e s t e r l y flows o f r e l a t i v e l y d r y a i r , w i t h wind s p e e d s a v e r a g i n g a t

l e a s t 5-10 mph ( 2 . 2 5 - 4 . 5 m/s).

The u s e f u l n e s s o f an a c c u r a t e w a t e r t e m p e r a t u r e r e c o r d i n f r a z i l f o r e c a s t i n g i s i l l u s t r a t e d by t h e s e two examples. During t h e p e r i o d from 8-12 December, when w a t e r t e m p e r a t u r e s were above 32.4 ( 0 . 2 2 ' ~ ) , s e v e r a l h o u r s o f c o o l i n g would have been r e q u i r e d t o lower t h e

t e m p e r a t u r e t o 32OF (O°C) i f w e a t h e r c o n d i t i o n s conducive t o f r a z i l f o r m a t i o n h a d o c c u r r e d . In c o n t r a s t , d u r i n g t h e p e r i o d 17-19 December when w a t e r t e m p e r a t u r e s were c l o s e t o t h e f r e e z i n g p o i n t , a s h o r t e r c o o l i n g p e r i o d would have been s u f f i c i e n t . During t h e s p r i n g p e r i o d , w a t e r t e m p e r a t u r e s were c l o s e t o 32OF (O°C) f o r t h e e n t i r e p e r i o d . F r a z i l p r o d u c t i o n would have o c c u r r e d r a p i d l y d u r i n g t h e n i g h t a t t h i s p e r i o d i f w e a t h e r c o n d i t i o n s had f a v o u r e d i t .

The h e a t g a i n e d from s h o r t - w a v e r a d i a t i o n i s an i m p o r t a n t f a c t o r d e t e r m i n i n g w a t e r t e m p e r a t u r e f l u c t u a t i o n . During t h e s p r i n g p e r i o d , h e a t l o s s e s a t n i g h t a r e more t h a n o f f s e t by h e a t g a i n e d from s o l a r r a d i a t i o n d u r i n g t h e day and w a t e r t e m p e r a t u r e s f o l l o w a marked d i u r n a l c y c l e . During t h e w i n t e r p e r i o d , h e a t g a i n e d from s o l a r r a d i a t i o n i s u s u a l l y n o t s u f f i c i e n t t o o f f s e t h e a t l o s s e s d u r i n g t h e d a y , b u t i t i s s u f f i c i e n t t o a f f e c t w a t e r t e m p e r a t u r e and t h e r a t e o f h e a t l o s s d u r i n g t h e mid-day p e r i o d . I t i s n o t s u r - p r i s i n g , t h e r e f o r e , t h a t f r a z i l problems a r e n o t u s u a l l y r e p o r t e d d u r i n g t h e days t h a t h a v e a good d e a l of incoming s o l a r r a d i a t i o n .

A major f a c t o r i n f l u e n c i n g w a t e r t e m p e r a t u r e f l u c t u a t i o n s i s

t h e a r e a o f open w a t e r u p s t r e a m from t h e dams. I n t h e f a l l , when t h e i c e c o v e r i s f i r s t f o r m i n g , t h e a r e a o f open w a t e r i s much g r e a t e r t h a n i n t h e s p r i n g when much o f t h e r i v e r i s f r o z e n o v e r . T o t a l h e a t l o s t and g a i n e d from open w a t e r d u r i n g a d i u r n a l c y c l e i s c o n s e q u e n t l y much g r e a t e r d u r i n g t h e f a l l , l e a d i n g t o g r e a t e r d i u r n a l w a t e r t e m p e r a t u r e f l u c t u a t i o n . I n t h e s p r i n g , w a t e r f l o w i n g u n d e r t h e e x t e n s i v e s t r e t c h e s o f s h o r e - f a s t i c e would t e n d t o remain c l o s e t o t h e f r e e z i n g p o i n t .

T t e r a t e a t which w a t e r c o o l s when i t i s n e a r t h e f r e e z i n g p o i n t h a s been used a s an a i d i n f r a z i l f o r e c a s t i n g . Granbois [ 6 ] r e p o r t s t h a t f r a z i l o c c u r r e d o n l y vhen t h e r a t e o f t e m p e r a t u r e change was g r e a t e r t h a n O.Ol°C/hr between t e m p e r a t u r e s o f 0 . l 0 C and 0°C. The w a t e r t e m p e r a t u r e r e c o r d s shown on F i g u r e s 4 and 5 i n d i c a t e t h a t t h i s c o n c l u s i o n must be t r e a t e d w i t h c a u t i o n . The r a t e o f w a t e r t e m p e r a t u r e change d u r i n g t h e n i g h t o f 7-8 A p r i l , when f r a z i l was r e p o r t e d , was about O.O13"C/hr, which i s almost e x a c t l y t h e same r a t e o f c o o l i n g a s t h e p r e v i o u s n i g h t when f r a z i l \<as n o t r e p o r t e d . The r a t e o f c o o l i n g 10 h o u r s p r i o r t o t h e m a r g i n a l f r a z i l o c c u r r e n c e o f 5 December was about 0 . 0 2 2 " C / h r compared t o a b o u t 0 . O l 0 C / 1 l r r e c o r d e d p r i o r t o t h e more s e v e r e f r a z i l i c e r e p o r t e d f o r 19 December, 1969.

'rile f o r e c a s t i n g of f r a z i l would r e q u i r e i n f o r m a t i o n n o t o n l y on w e a t h e r and w a t e r t e m p e r a t u r e b u t a l s o on flow and i c e c o n d i t i o n s upstream from t h e s i t e . For example, l i m i t e d e v i d e n c e a v a i l a b l e f o r t h e Ottawa s i t e s u g g e s t e d t h a t s e v c r e f r a z i l o c c u r r e d i n t h e s p r i n g d u r i n g p e r i o d s o f h i g h flow on11 \+hen t h e a r c a o f open w a t e r s u b j e c t t o h e a t l o s s was g r e a t e s t . :\ knowledge o f upstream i c e movement i s e s p e c i a l l y n e c e s s a r y t o p r e d i c t whcn o t h e r forms o f i c e may com- b i n e w i t h f r a z i l t o produce b l o c k a g e s .

COYCLUSIONS

The r a t e o f h e a t l o s s from open w a t e r p r i o r t o f r a z i l o c c u r - r e n c e a t a s i t e g i v e s an i n d i c a t i o n o f t h e p o t c n t i a l s c v e r i t y o f t h e f r a z i l problem. F r a z i l was a problem a t t h e Otta\\ia s i t e o n l y when t h e r a t e o f t o t a l h e a t l o s s a v e r a g e d more t h a n 30 c a l / s q cm/hr o v e r t h e 12-hour p e r i o d p r i o r t o i t s o c c u r r e n c e . Convective llcat l o s s i s t h e most i m p o r t a n t f a c t o r d e t e r m i n i n g t o t a l h e a t l o s s c s , p a r t i c u l a r l y d u r i n g s e v e r e i c e problenls

.

Long-wave r a d i a t i o n a11d c v a p o r a t i o n h e a t l o s s e s can b e , however, o f almost e q u a l i m p o r t a n c e i n somc s i t u a t i o n s . The h e a t g a i n e d fro111 s h o r t - w a v c r a d i a t i o n can bc a

major f a c t o r i n d e t e r m i n i n g t h e d u r a t i o n o f t h e f r a z i l problem, p a r t i c u l a r l y d u r i n g s p r i n g o c c u r r e n c e s .

F r a z i l o c c u r r e d under f a i r l y w e l l d e f i n e d w e a t h e r condi t i o n s a t t h i s s i t e . The dominant wind d i r e c t i o n was p r i n ~ a r i l y w e s t e r l y o r northwesterly. The a v e r a g e wind s p e e d d u r i n g t h e t w e l v e h o u r s p r i o r t o o c c u r r e n c e was 10 mph ( 4 . 5 m/s) o r g r e a t e r f o r a v e r a g e a i r temper- a t u r e s above + l J ° F (-10°C). Average wind s p e e d s were almost always g r e a t e r t h a n 5 mph ( 2 . 2 5 m/s) even a t e x t r e m e l y low t e m p e r a t u r e s . Dew p o i n t t e m p e r a t u r e s were r e l a t i v e l y low, p a r t i c u l a r l y d u r i n g s p r i n g o c c u r r e n c e s o f f r a z i l .

Although t h e r a t e o f h e a t l o s s and w e a t h e r c o n d i t i o n s undcr which f r a z i l w i l l o c c u r p r o b a b l y w i l l be d i f f e r e n t a t o t h e r s i t e s , t h e g e n e r a l r e q u i r e m e n t s f o r p r a c t i c a l f o r e c a s t s a r e t h e same. A c c u r a t e w a t e r t e m p e r a t u r e r e c o r d s , flow r e c o r d s , and i n f o r m a t i o n on upstream i c e c o n d i t i o n s , combined w i t h w e a t h e r f o r e c a s t s , a r e nceded. T h i s i n f o r m a t i o n s h o u l d be a v a i l a b l e on an h o u r l y b a s i s i f a few h o u r s advance warning o f i c e b l o c k a g e i s r e q u i r e d a t w a t e r i n t a k e s .

ACKNOWLEDGMENTS

The a u t h o r i s i n d e b t e d t o R . .i\rmour f o r h e l p i n a n a l y s i s , t o Mr. W . Knight f o r c o o p e r a t i o n i n i n s t a l l i n g t h e w a t e r t e m p c r a t u r e r e c o r d e r , and t o t h e E . B. Eddy Company f o r a c c e s s t o t h e power- house r e c o r d s .

T h i s p a p e r i s a c o n t r i b u t i o n from 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 .

REFERENCES

[ l ] MURPHY, J . (1909). The i c e q u e s t i o n - a s i t a f f e c t s Canadian w a t e r powers - w i t h s p e c i a l r e f e r e n c e t o f r a z i l and a n c h o r i c e T r a n s . Roy. S o c . C a n . , S e c t i o n 111, p p . 143-177.

[ 2 ] GEIGER, R . (1965). The c l i m a t e n e a r t h e ground. Harvard U n i v e r s i t y P r e s s ( t r a n s l a t e d from 4 t h e d i t i o n , F r i e d r i c h Vieweg & Sohn, Brunswick, Germany 1961).

[ 3 ] NICHEL, B. (1971). Winter regime o f r i v e r s and l a k e s . Cold Regions S c i e n c e and E n g i n e e r i n g Monograph I l l - B l a , Hanover, N . H .

[ 4 ] FREYSTEINSSON, S . (1970). C a l c u l a t i o n o f f r a z i l i c e p r o d u c t i o n . I n t . Assoc. H y d r a u l i c Research Symposium, I c e and i t s Action on H y d r a u l i c S t r u c t u r e s , R e y k j a v i k , I c e l a n d , S e c t i o n 2 . 1 . [ 5 ] FERGUSON, H . L . (1968). A p r e l i m i n a r y e s t i m a t e o f t h e i c e -

s e a s o n e n e r g y b a l a n c e f o r t h e N i a g a r a R i v e r . B u l l . I n t . Assoc. S c i . H y d r o l . , No. 3 , pp. 41-58.

[6] GRANBOIS, K . J . (1953). Combatting f r a z i l i c e i n h y d r o - e l e c t r i c s t a t i o n s . Power Apparatus and Systems, No. 5 , pp. 111-116.

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