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UBC Theses and Dissertations

Scale models of gravel bed rivers Parent, Alain Paul 1988

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SCALE MODELS OF GRAVEL BED RIVERS By ALAIN PAUL PARENT B . S c , U n i v e r s i t y de M o n t r e a l , 1985 THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES D e p a r t m e n t o f Geog raphy We a c c e p t t h i s t h e s i s as c o n f o r m i n g t o t h e r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA S e p t e m b e r 1988 (c) A la in Pau l P a r e n t , 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geography The University of British Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 September 1, 1988 The combined studies of flow processes in a ful l size gravel-bed r iver and i ts assoc iated smal l-scale model have demonstrated the applicability of the similitude concepts as well as the potential of scale models for quantitative research in f l u v i a l geomor phology. This sca l ing i n v e s t i g a t i o n considered the comparison of velocity profi les, shear s t ress measurements, flow structure and surface bed material samples. An extension of the s t r i c t comparison on a detailed level of f ie ld and laboratory processes in a specific case consisted in the proposition of a "generic model" framework according to which laboratory systems are viewed as part of the same family as f ie ld ones if some crucial conditions are sat is f ied. The poo l - r i f f le sequence, used as a vehicle fo r the demonstration herein, is presented as a poorly Known but ubiquitous river phenomenon which would benef it from such research methodology. The laboratory study performed herein revealed the need for an appraisal of the variabi l i ty of shear s t ress est imates in intermediate re la t i ve roughness f lows such as f ie ld size gravel-bed rivers. Knowledge of elements of the history of the prototype (field) river was also demonstrated to be important fo r the appraisal of model performance. T a b l e o f c o n t e n t s A b s t r a c t i i T a b l e o f c o n t e n t s i i i L i s t o f t a b l e s v i i L i s t o f f i g u r e s v i i i L i s t o f s y m b o l s x v A k n o w l e d g m e n t s x v i i 1 .0 I n t r o d u c t i o n 1 2 . 0 S c a l e m o d e l s o f a l l u v i a l r i v e r s 8 2 . 1 G e n e r a l i t i e s 8 2 . 2 S i m i I i t u d e 9 2 . 3 D i m e n s i o n a l a n a l y s i s a n d t h e d e v e l o p m e n t o f s c a l i n g i v 2 . 4 F r o u d i a n s c a l i n g l a w s a n d n a t u r a l c h a n n e l b e h a v i o u r 2 . 4 . 1 N a t u r a l s t r e a m b e h a v i o u r o v e r a r a n g e o f s c a l e s ( h y d r a u l i c g e o m e t r y r e s u l t s ) 18 2 . 4 . 2 D e c o n s t r a i n e d h y d r a u l i c g e o m e t r y p l o t 21 2 . 5 C r i t e r i a f o r a c l o s e r t e s t o f t h e r e p r e s e n t a t i v e n e s s o f s c a l e m o d e I s 2 . 5 . 1 T h e r e p r o d u c t i o n o f f r i c t i o n a l c h a r a c t e r i s t i c s o r o f r o u g h n e s s l e n g t h 24 2 . 5 . 2 T h e r e p r o d u c t i o n o f s e d i m e n t e n t r a i n m e n t c o n d i -t i o n s a n d e x c e s s s t r e s s 3 0 2 . 5 . 3 R e p r o d u c t i o n o f f l o w p a t t e r n s a n d b e d s h e a r s t r e s s d i s t r i b u t i o n 32 2 . 6 G e n e r i c m o d e l l i n g c o n c e p t a n d g e o m o r p h o 1 o g i c a 1 r e s e a r c h 3 3 3 . 0 F i e l d s t u d y 36 3 . 1 S i t e s e l e c t i o n , l o c a t i o n a n d d e s c r i p t i o n 36 3 . 2 B e d m a t e r i a l s a m p l i n g 42 3 . 3 M e a s u r e m e n t s e t - u p , i n s t r u m e n t s a n d e s t i m a t i o n o f s h e a r s t r e s s 4 5 3 . 4 R e s u l t s f r o m f l o w m e a s u r e m e n t s 52 3 . 5 P r o b l e m s r a i s e d by f i e l d m e a s u r e m e n t s 68 4 . 0 L a b o r a t o r y s t u d y and c o n f o r m a t i o n 7 5 4 . 1 F l u m e d e s c r i p t i o n , i n s t r u m e n t a t i o n a n d h y d r o d y n a m i c m e a s u r e m e n t p r o c e d u r e s 7 5 4 . 2 S c a l e m o d e l l i n g o f B l a n e y C r e e K : s t r a t e g y 4 . 2 . 1 G e n e r a l c o n s i d e r a t i o n s 78 4 . 2 . 2 B e d m a t e r i a l s c a l i n g 79 4 . 2 . 3 W o r k i n g a s s u m p t i o n s r e g a r d i n g f i e l d c o n d i t i o n s . 84 4 . 2 . 4 I n i t i a l e x p e r i e n c e w i t h a n d a d j u s t m e n t s t o t h e mode 1 88 4 . 3 L a b o r a t o r y r e s u l t s a n d t h e i r c o n f o r m a t i o n t o f i e l d 4 . 3 . 1 B e d m o r p h o l o g y c o n f o r m a t i o n 92 4 . 3 . 2 F l o w m e a s u r e m e n t c o n f o r m a t i o n 9 7 4 . 3 . 3 S u r f a c e b e d m a t e r i a l c h a r a c t e r i s t i c s a n d c o n f o r -m a t i o n 109 4 . 4 E x t e n d e d o b s e r v a t i o n s on s i t e ' s c h a r a c t e r i s t i c s f r o m f i e l d a n d l a b o r a t o r y r e s u l t s 1 1 7 5 . 0 C o n c l u s i o n s 1 2 3 R e f e r e n c e s 1 2 7 A p p e n d i x : Bed c o n f i g u r a t i o n s i n low m o b i l i t y r i v e r s . 1 39 A . 1 G e n e r a l c o n s i d e r a t i o n s 139 A . 2 B e d f o r m c l a s s i f i c a t i o n a n d c o n d i t i o n s o f p o o l - r i f f l e o c c u r r e n c e 1 4 2 A . 3 F l o w p r o c e s s e s , o r i g i n a n d e v o l u t i o n o f t h e p o o l -r i f f l e p h e n o m e n o n 1 4 7 A . 4 P o o l - r i f f l e s e q u e n c e s : p e r s p e c t i v e s a n d p r o s p e c t s . . 151 L i s t o f t a b ! e s v i i iT a b l e 1. G r a i n s i z e d i s t r i b u t i o n p e r c e n t i l e d i a m e t e r s ( i n mm) f o r s e l e c t e d b e d m a t e r i a l s a m p l e s , B l a n e y C r e e k 46 T a b l e 2. Mean h y d r a u l i c p a r a m e t e r s o f N o v e m b e r 20 a n d 23 , 1 9 8 6 h y d r o l o g i c a l e v e n t s , B l a n e y C r e e k 5 5 T a b l e 3 . Mean h y d r a u l i c a n d g e o m e t r i c p a r a m e t e r s f r o m r u n 14 a n d c o m p a r i s o n w i t h t a r g e t v a l u e s a s f o u n d f r o m F r o u d e s i m i l a r i t y p r i n c i p l e s 98 T a b l e 4 . H y d r a u l i c a n d g e o m e t r i c p a r a m e t e r s f o r r u n 15 1 22 v i i i L i s t o f f i g u r e s F i g u r e 1. G r a i n s i z e v e r s u s d i s c h a r g e p l o t f o r s e l e c t e d s e t o f d a t a 2 3 F i g u r e 2 . D e c o n s t r a i n e d h y d r a u l i c g e o m e t r y p l o t 2 3 F i g u r e 3 . R e s i s t a n c e d i a g r a m 28 F i g u r e 4 . T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a R e s e a r c h F o r e s t : l o c a t i o n i n s e t a n d B l a n e y C r e e K f i e l d s i t e l o c a t i o n 38 F i g u r e 5 . T o p o g r a p h i c map o f B l a n e y C r e e k f i e l d s i t e 39 F i g u r e 6 . V i e w s o f B l a n e y C r e e k f i e l d s i t e 4 0 F i g u r e 7 . S i z e d i s t r i b u t i o n s o f b e d m a t e r i a l s a m p l e s a t d o w n s t r e a m r i f f l e s i t e , B l a n e y C r e e k 44 ix F i g u r e 8 . C o m p a r i s o n o f p o o l e d s i z e d i s t r i b u t i o n s f o r g r i d s a m p l e s f r o m d o w n s t r e a m r i f f l e a n d w i t h i n r e a c h p o o l a n d r i f f l e s i t e s , B l a n e y C r e e k 44 F i g u r e 9 . O t t l a b o r a t o r y c u r r e n t m e t e r r o d -m o u n t e d a r r a y 48 F i g u r e 1 0 . T e m p o r a l c h a n g e s i n w a t e r l e v e l , N o v e m b e r 2 0 a n d 2 3 , 1 9 8 6 e v e n t s 54 F i g u r e 1 1 . D i m e n s i o n l e s s v e l o c i t y p r o f i l e s , r i f f l e a r e a , N o v e m b e r 2 3 , 1 9 8 6 5 7 F i g u r e 1 2 . D i m e n s i o n l e s s v e l o c i t y p r o f i l e s , p o o l a r e a , N o v e m b e r 2 3 , 1 9 8 6 58 F i g u r e 1 3 . D i m e n s i o n l e s s v e l o c i t y p r o f i l e s , p o o l a r e a ( l o w e r s t a g e ) , N o v e m b e r 2 3 , 1 9 8 6 58 F i g u r e 1 4 . L o c a l v a r i a b i l i t y i n v e l o c i t y p r o f i l e s , s t a t i o n L, N o v e m b e r 2 3 , 1 9 8 6 , 60 X F i g u r e 1 5 . L o c a l v a r i a b i l i t y i n v e l o c i t y p r o f i l e s , s t a t i o n P, N o v e m b e r 2 3 , 1 9 8 6 60 F i g u r e 1 6 . S c a n s a t d o w n s t r e a m e n d o f p o o l , s t a t i o n V , N o v e m b e r 2 3 , 1 9 8 6 62 F i g u r e 1 7 . S c a n s a t d o w n s t r e a m e n d o f r i f f l e , s t a t i o n W, N o v e m b e r 2 3 , 1 9 8 6 62 F i g u r e 1 8 . N e a r - b e d i n t e r n a l s h e a r e s t i m a t e s a t v e l o c i t y s a m p l i n g s t a t i o n s , B l a n e y C r e e k , N o v e m b e r 2 3 , 1 9 8 6 64 F i g u r e 1 9 . N e a r - b e d i n t e r n a l s h e a r e s t i m a t e s a t v e l o c i t y s a m p l i n g s t a t i o n s , B l a n e y C r e e k , N o v e m b e r 2 0 , 1 9 8 6 65 F i g u r e 2 0 . N e a r - b o t t o m a n d n e a r - s u r f a c e f l o w l i n e s , B l a n e y c r e e k , N o v e m b e r 2 0 , 1 9 8 6 66 F i g u r e 2 1 . N e a r - b o t t o m a n d n e a r - s u r f a c e f l o w l i n e s , B l a n e y c r e e k , N o v e m b e r 2 3 , 1 9 8 6 67 xi F i g u r e 2 2 . N e a r - b o t t o m a n d n e a r - s u r f a c e f l o w l i n e s , B l a n e y c r e e k , N o v e m b e r 2 3 , 1 9 8 6 ( s t a f f g a u g e a t 4 0 cm) 69 F i g u r e 2 3 . L a t e r a l b e d m a t e r i a l s o r t i n g i n p r o x i m a l p o o l , B l a n e y C r e e k 7 3 F i g u r e 2 4 . N o v e m b e r 2 3 , 1 9 8 6 h y d r o l o g i c a l e v e n t , B l a n e y C r e e k 7 3 F i g u r e 2 5 . G e n e r a l v i e w o f t h e w a t e r r e c i r c u l a t i n g f l u m e a t t h e D e p a r t m e n t o f G e o g r a p h y , T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a 76 F i g u r e 2 6 . C o m p a r i s o n o f l a b o r a t o r y s e d i m e n t m i x t u r e w i t h t h e c o r r e c t e d a n d n o n -c o r r e c t e d s c a l e d s i z e d i s t r i b u t i o n s o f t h e t r u n c a t e d v o l u m e t r i c s a m p l e o f B l a n e y C r e e k 81 F i g u r e 2 7 . C r i t i c a l s h e a r v e l o c i t y - b a s e d g r a p h i c a l c o r r e c t i o n p r o c e d u r e t o t h e t r u n c a t e d v o l u m e t r i c s a m p l e o f B l a n e y C r e e k 8 3 xi F i g u r e 2 8 . L e g o - b u i l t m o d e l o f B l a n e y C r e e k b e d r o c k o u t c r o p 87 F i g u r e 2 9 . T o p o g r a p h i c a l map o f r u n 13 f i n a l b e d c o n f i g u r a t i o n 93 F i g u r e 3 0 . T o p o g r a p h i c a l map o f r u n 14 f i n a l b e d c o n f i g u r a t i o n 94 F i g u r e 3 1 . I n i t i a l a n d f i n a l c o n d i t i o n s , r u n 14 96 F i g u r e 3 2 . D i m e n s i o n l e s s v e l o c i t y p r o f i l e s , i n p o o l , r u n 14 1 0 0 F i g u r e 3 3 . D i m e n s i o n l e s s v e l o c i t y p r o f i l e s , o v e r r i f f l e , r u n 14 1 0 0 F i g u r e 3 4 . D i m e n s i o n l e s s v e l o c i t y p r o f i l e s , d o w n s t r e a m p a r t o f r i f f l e , r u n 14 101 F i g u r e 3 5 . D i m e n s i o n l e s s v e l o c i t y p r o f i l e s , U p s t r e a m p a r t o f r i f f l e , r u n 14 101 F i g u r e 3 6 . D i m e n s i o n l e s s v e l o c i t y p r o f i l e s , d o w n s t r e a m p a r t o f p o o l , r u n 14 102 xi i i F i g u r e 3 7 . D i m e n s i o n 1 e s s v e l o c i t y p r o f i l e s , u p s t r e a m p a r t o f p o o l , r u n 14 102 F i g u r e 3 8 . L o c a l s h e a r s t r e s s v a r i a b i l i t y t e s t , d i m e n s i o n 1 e s s v e l o c i t y p r o f i l e s , r u n 14 104 F i g u r e 3 9 . N e a r - b e d i n t e r n a l s h e a r e s t i m a t e s a t v e l o c i t y s a m p l i n g s t a t i o n s , r u n 14 1 0 6 F i g u r e 4 0 . N e a r - b o t t o m a n d n e a r - s u r f a c e f l o w l i n e s , r u n 14 1 1 0 F i g u r e 4 1 . S i z e d i s t r i b u t i o n o f s u r f a c e b e d m a t e r i a l s a m p l e s , r u n 14 112 F i g u r e 4 2 . G r a i n s i z e d i s t r i b u t i o n , r u n 1 4 , i n i t i a l a n d f i n a l s u r f a c e m a t e r i a l s 1 1 3 F i g u r e 4 3 . G r a i n s i z e d i s t r i b u t i o n c o m p a r i s o n , r u n 14 a n d f i e l d ( s c a l e d ) s u r f a c e s a m p l e s 1 15 F i g u r e 4 4 . D e v i a t i o n f r o m o n e - m i n u t e a v e r a g e v e l o c i t y , m a x i m u m a n d m i n i m u m v a l u e s i n p o o l 1 1 8 xi v F i g u r e 4 5 . D e v i a t i o n f r o m o n e - m i n u t e a v e r a g e v e l o c i t y , m a x i m u m a n d m i n i m u m v a l u e s o v e r r i f f l e 118 XV L i s t o f s y m b o l s b H y d r a u l i c g e o m e t r y ( d e p t h / d i s c h a r g e ) exponent C(j D r a g c o e f f i c i e n t d Mean c h a n n e l d e p t h Dj G ra in s i ze d i a m e t e r ( s u b s c r i p t r e f e r s t o p e r c e n t i l e s o f t h e s i ze d i s t r i b u t i o n ) f H y d r a u l i c g e o m e t r y ( w i d t h / d i s c h a r g e ) exponent f f F r i c t i o n f a c t o r F s S h i e l d s ' p a r a m e t e r F^ D rag f o r c e Fg G r a v i t a t i o n a l f o r c e Fj I n e r t i a f o r c e F v V i s c o u s f o r c e g G r a v i t a t i o n a l a c c e l e r a t i o n k s E q u i v a l e n t s a n d r o u g h n e s s L L e n g t h s c a l e Q C h a n n e l d i s c h a r g e R H y d r a u l i c r a d i u s Re F low Reyno l d s number Rp P a r t i c l e R e y n o l d s n u m b e r s S u b m e r g e d s p e c i f i c w e i g h t o f s ed imen t E n e r g y g r a d i e n t . Mean f l o w v e l o c i t y A v e r a g e v e l o c i t y f l u c t u a t i o n s P a r t i c l e s e t t l i n g v e l o c i t y S h e a r v e l o c i t y Mean c h a n n e l w i d t h L o c a l f l o w d e p t h F u n c t i o n o f S p e c i f i c w e i g h t Von Ka rman c o n s t a n t S c a l e r a t i o ( p r o t o t y p e / m o d e l ) o f c h a r a c t e r i s t i c p a r a m e t e r x D y n a m i c v i s c o s i t y K i n e m a t i c v i s c o s i t y D i m e n s i o n l e s s n u m b e r D e n s i t y Bounda r y s h e a r s t r e s s S h i e l d s ' c r i t i c a l s h e a r s t r e s s A c k n o w l e d g e m e n t s It is a g r e a t p l e a s u r e t o e x p r e s s my m o s t s i n c e r e r e c o g n i t i o n t o my s u p e r v i s o r , Dr M i chae l C h u r c h , f o r h i s t h o u g h t f u l g u i d a n c e and h is i n t e l l e c t u a l c o n t r i b u t i o n wh i c h g r e a t l y h e l p e d t h e p r o p e r c o m p l e t i o n o f t h i s r e s e a r c h p r o j e c t . Dr Chu r ch p r o v i d ed t h e f r a m e w o r k t o na r row my v a s t i n i t i a l i n t e n t i o n s t o a p r o j e c t o f a s i ze a p p r o p r i a t e t o t h e Ma s t e r ' s l e v e l . I would a l so l ike t o thank Dr Douw G. S t e y n f o r his c o m m e n t s and his s u p p o r t w i t h i n t r u m e n t a t i o n . I am a l so g r a t e f u l t o Dr Olav S laymaker , t o Dr Edward Hickin f r o m t h e D e p a r t m e n t o f Geography a t Simon F r a s e r Un i v e r s i t y and t o Dr M ichae l C. Quick f r o m t h e D e p a r t m e n t o f C iv i l eng inee r i ng o f t h e U n i v e r s i t y o f B r i t i s h Columbia who kindly l oaned t h e i r c u r r e n t m e t e r s f o r t h e f i e l d mea su r emen t s . The help o f A l a s t a i r Ross and Michel L apo i n t e dur ing t h e f i e l d work ( t h a t means under ve r y n a s t y cond i t i on s ) was ve r y much a p p r e c i a t e d . M iche l has a l s o b e en a m o s t i n t e r e s t i n g c o l l e a g u e t o e x c h a n g e w i t h in Mo l i e r e ' s l a nguage . In t h e l a b o r a t o r y and in g e n e r a l , t h e he lp o f and t h e i n t e r a c t i o n s w i t h J o h n W o l c o t t w e r e a l s o a much a p p r e c i a t e d s t i m u l u s . D i s c u s s i o n s w i t h my c o l l e a g u e D a v i d M c L e a n w e r e a l w a y s l e s s o n s w i t h i n t e l l e c t u a l f a l l o u t s . I am a l s o g r a t e f u l t o t h e p eop l e a t t h e U.B.C. R e s e a r c h F o r e s t who p r o v i d e d equ i pmen t and s u p p o r t f o r t h e f i e l d wo r k . F inal ly t h e help o f Sy l v i e La londe in t h e f i e l d and w i t h some o f t h e f i g u r e s is g r a t e f u l l y a c k n o w l e d g e d . F inanc ia l s u p p o r t t o t h e a u t h o r was p r o v i d ed v ia a g r a n t and f e l l o w s h i p f r o m t h e N a t u r a l S c i e n c e s and E n g i n e e r i n g R e s e a r c h Counc i l o f C a n a d a . xv i i i The Sands o f Time were e r oded by The R iver o f C o n s t a n t Change 1.0 I n t r o d u c t i o n S i n c e t h e i n t r o d u c t i o n o f q u a n t i t a t i v e m e t h o d s i n t o g e o m o r p h o l o g y , t h e d om i n an t , f u n c t i o n a l a p p r o a c h t o l a n d f o r m s t u d y ( C h o r l e y , 1978) has y i e l d e d i n c r e a s i n g i n s i g h t i n t o t h e b e h a v i o u r o f g e o m o r p h o l o g i c a l s y s t e m s . In f l u v i a l g e o m o r p h o l o g y , t h e f i r s t m i l e s t o n e o f t h o s e e a r l y q u a n t i t a t i v e d e v e l o p m e n t s is p e r h a p s t h e c l a s s i c a l t e x t b o o k o f L e o p o l d and o t h e r s (1964) . A m o n g s t t h e main c o n t r i b u t i o n s o f f u n c t i o n a l s t u d i e s t o g e o m o r p h o l o g y , t h e r e c o g n i t i o n o f s o m e f a i r l y c o n s i s t e n t s c a l e r e l a t i o n s h i p s i n v o l v i n g m o r p h o l o g i c a l a n d / o r h y d r o d y n a m i c p a r a m e t e r s o f s t r e a m channe l s has been o f u t m o s t in te res t . Emp i r i c a l m e t h o d s wh i ch f i r s t l e a d t o t h o s e g e n e r a l i z a t i o n s r e g a r d i n g a l l u v i a l c h a n n e l b e h a v i o u r a l s o r a i s e d s o m e c r u c i a l q u e s t i o n s a b o u t t h e m e c h a n i s m s wh i c h w e r e i n v o l v e d . In c o n s e q u e n c e , t h e main ly e m p i r i c a l f u n c t i o n a l a p p r o a c h g r a d u a l l y g a v e way t o p r o c e s s - o r i e n t e d s t u d i e s wh ich w e r e a imed a t d e v e l o p i n g l aws o f s t r e a m b e h a v i o u r . H o w e v e r , t h e r e a r e c o n s i d e r a b l e d i f f i c u l t i e s as we l l as much work a head b e f o r e su ch laws c an be d e v e l o p e d . Th i s s i t u a t i o n r e s u l t s f r o m t h e immense c o m p l e x i t y and t h e s i ze o f t h e f l u v i a l s y s t e m , t h e e p i s o d i c a n d i r r e g u l a r o c c u r e n c e o f g e o m o r p h o l o g i c a l l y 2 e f f e c t i v e e v e n t s , t h e t e m p o r a l u n s t e a d i n e s s o f t h e i r f o r c i n g f u n c t i o n s , and f r o m t h e d i f f i c u l t i e s t o o b t a i n r e p r e s e n t a t i v e q u a n t i t a t i v e m e a s u r e m e n t s o f t h e p a r a m e t e r s wh ich g o v e r n t h e r e s p o n s e o f t h e f l u v i a l s y s t e m . Consequen t l y , t h e change in t h e s cope o f r e s e a r c h in f l u v i a l g e o m o r p h o l o g y w a s n e c e s s a r i l y a c c o m p a n i e d by i n c r e a s e d c o n s i d e r a t i o n t o g e o m o r p h o l o g i c a l s u b j e c t s m o r e r e s t r i c t e d in s p a c e and t i m e ( t h e " r e a l i s t i c a p p r o a c h " o f C h o r l e y , 1978 ) . T h o s e s m a l l e r , mo re m a n a g e a b l e s y s t e m s a p p e a r e d s u s c e p t i b l e t o b r i n g i n s i g h t t o some p a r t i c u l a r l y r e l e v a n t p r o b l e m s c o n c e r n i n g t h e b e h a v i o u r o f t h e f l u v i a l s y s t e m . C o n c u r r e n t l y , a n o t h e r f a c t r ap i d l y e m e r g e d : t h e m e a s u r e m e n t and s u b s e q u e n t t r e a t m e n t o f a v e r a g e h y d r o d y n a m i c v a l u e s c o u l d n o t f u l f i l l t h e o b j e c t i v e o f a c o m p l e t e p h y s i c a l u n d e r s t a n d i n g o f nume rou s f e a t u r e s o f t h e f l u v i a l s y s t e m . A v e r a g e - v a l u e f o r m u l a t i o n s in r i v e r s t u d i e s h a v e b e e n h e l p f u l in many p r o b l e m s (main ly o f p r a c t i c a l n a t u r e ) up t o a p o i n t : f o r ' i n s t a n c e , C h u r c h (1985) ha s r e v i e w e d t h e u s e f u l n e s s a nd l i m i t a t i o n s o f a v e r a g e - v a l u e f o r m u l a t i o n s f o r t h e e s t i m a t i o n o f s e d i m e n t t r a n s p o r t in g r a v e l - b e d r i v e r s . A l so t h e m a t h e m a t i c a l f o r m u l a t i o n s , which have been so popu la r s ince t h e eme rgen ce o f d i g i t a l c o m p u t e r s (and wh ich a r e , i n c i d e n t a l l y , m o s t l y b a s e d on a v e r a g e h y d r o d y n a m i c q u a n t i t i e s ) , c ou l d n o t b r i n g r e a l p r o g r e s s t o t h e s u b j e c t o f f l u v i a l g e o m o r p h o l o g y . A c k n o w l e d g i n g t h i s s i t u a t i o n and t h e f a c t t h a t w a t e r f l o w can be d e s c r i b e d by i t s mean o r f l u c t u a t i n g c o m p o n e n t s , i t h a s b e c o m e i n c r e a s i n g l y e v i d e n t t h a t d e t a i l e d o b s e r v a t i o n s , in s p a c e and t ime , o f t h e f l o w dynam i c s and s t r u c t u r e o v e r p a r t i c u l a r c o n f i g u r a t i o n s o f i n t e r e s t , c o u l d i m p r o v e ou r p h y s i c a l u n d e r s t a n d i n g o f t h e i n t e r a c t i o n s a m o n g s t r i v e r h y d r o d y n a m i c s , s e d i m e n t t r a n s p o r t p h e n o m e n a a n d t h e g e o m e t r i c a l c o n f i g u r a t i o n o f s t r e a m b o u n d a r i e s . H o w e v e r , d e t a i l e d f l o w m e a s u r e m e n t s a r e d i f f i c u l t and s o m e t i m e s i m p r a c t i c a l t o p e r f o r m in f u l l - s c a l e s i t u a t i o n s : t h i s is due t o i n h e r e n t c h a r a c t e r i s t i c s o f g e o m o r p h o l o g i c a l s y s t e m s m e n t i o n e d a b o v e . Hence , an a l t e r n a t i v e t o f i e l d s t u d i e s has b e e n t o w o r k u n d e r e x p e r i m e n t a l c o n d i t i o n s . P h y s i c a l ( o r s ca l e ) mode ls o f r i v e r s ba sed on t h e t h e o r y o f s im i l a r i t y have b e en e x t e n s i v e l y u s ed in r i v e r e n g i n e e r i n g g e n e r a l l y in o r d e r t o s o l v e some s p e c i f i c p r o b l e m s r e l a t e d t o r i v e r m a n a g e m e n t . In g e o m o r p h o l o g y , p h y s i c a l mode l s gene ra l l y have been used f r o m a mo r e d e s c r i p t i v e p e r s p e c t i v e ( f o r i n s t a n c e , c ons i de r t h e work r e p o r t e d by Schumm and o t h e r s , 1987) a l t hough some a u t h o r s have c o n s i d e r e d t h e p r i n c i p l e s o f p h y s i c a l mode l l i n g , as p r e s c r i b e d by t h e t h e o r y o f s i m i l a r i t y , t o s t u d y u n d e r c o n t r o l l e d c o n d i t i o n s s o m e a s p e c t s o f r i v e r b e h a v i o u r ( S o u t h a r d and o t h e r s , 1 9 8 0 , 1 9 8 4 ) . The f o l l o w i n g q u o t a t i o n s s ummar i z e we l l t h e v i e w p o i n t o f m o s t g e o m o r p h o l o g i s t s r e g a r d i n g p h y s i c a l m o d e l s : " S c a l e mode l l i n g is e s s e n t i a l l y an e m p i r i c a l e x e r c i s e whose va lue is l im i t ed in a geomorph i c c o n t e x t by t h e e x t e n d e d s p a t i a l s c a l e s common t o g e o m o r p h o l o g i c a l p r o b l e m s . D e s p i t e t h e use o f v a r i o u s m o d e l l i n g s t r a t e g i e s , g e o m o r p h o l o g y r ema i n s e s s e n t i a l l y a f i e l d s c i e n c e . " ( K n i g h t o n , 1984, p.6) " A l t h o u g h h a r dwa r e mode ls have been e x t e n s i v e l y u s ed f o r d e s c r i p t i v e and p r e d i c t i v e p u r p o s e s , t h e y a r e i n h e r e n t l y i n c a p a b l e o f p r o v i d i n g t h e l a s t l ink in t h e cha in o f s c i e n t i f i c i n v e s t i g a t i o n , t h e t h e o r y o r t h e law." (Mos ley and Z i m p f e r , 1978, p .450) The b a s e s f o r su ch s t a t e m e n t s , wh ich may ho l d f o r many g e o m o r p h o l o g i c a l s i t u a t i o n s , w e r e h o w e v e r n o t f o r m a l l y e x p o s e d . In a r e c e n t book , Schumm and o t h e r s (1987) r e m a r k on t h e v a r i o u s a d v a n t a g e s o f p h y s i c a l mode l s b u t a l s o c o n s i d e r s c a l e mode l l i n g as t o o i n e x a c t a s c i e n c e a t p r e s e n t t o p r o v i d e r e l i a b l e q u a n t i t a t i v e r e s u l t s wh ich c an be app l i ed t o n a t u r a l s y s t e m s . The s t a t e m e n t s o f Schumm and o t h e r s a r e t o o g ene r a l and s u s c e p t i b l e t o r e s t r a i n q u a n t i t a t i v e f l u v i a l g e o m o r p h o l o g y : s c a l e mode l l i n g , wh ich is s u p p o r t e d by f o r m a l r e a s o n i n g known as s i m i l a r i t y t h e o r y , c an be u s ed in some c i r c u m s t a n c e s as a t o o l f o r r e s e a r c h . The r ange o f c ond i t i o n s f o r which t h i s is p o s s i b l e m u s t be e s t a b l i s h e d . C o n s e q u e n t l y , t h e h y p o t h e s i s t o be s t u d i e d h e r e i n c an be f o r m u l a t e d as f o l l o w s . U n d i s t o r t e d s c a l e m o d e l s o f a l l u v i a l r i v e r s c o n s t r u c t e d on s i m i l a r i t y p r i nc i p l e s can be used f o r t h e c o l l e c t i o n a n d a n a l y s i s o f q u a n t i t a t i v e h y d r o d y n a m i c m e a s u r e m e n t s t h a t wou ld l e a d t o a b e t t e r u n d e r s t a n d i n g o f m e c h a n i c a l p r o c e s s e s in r i v e r s a n d e v e n t u a l l y t o t h e d e v e l o p m e n t o f l aws or t h e o r i e s o f a l l u v i a l r i v e r b e h a v i o u r . O t h e r w i s e f o r m u l a t e d , i f t h e t h e o r e t i c a l f r a m e w o r k p r o v i d e d by t h e c o n c e p t o f s i m i l a r i t y be r e s p e c t e d , phys i ca l mode l s can be u s ed n o t s o l e l y as d e s c r i p t i v e t o o l s b u t a l s o in o r d e r t o s y s t e m a t i c a l l y c o l l e c t a nd a n a l y z e f u n d a m e n t a l o b s e r v a t i o n s a b o u t r i v e r d ynam i c s . Once a l a b o r a t o r y s i t u a t i o n is shown t o be s im i l a r t o t h e f i e l d , g e n e r a l i z a t i o n o f t h e o b s e r v a t i o n s c o l l e c t e d in t h e mode l is a l l owed . A n a t u r a l e x t e n s i o n o f t h e p r i n c i p a l h y p o t h e s i s o f t h e c u r r e n t r e s e a r c h , w h i c h f o c u s e s on a p a r t i c u l a r f i e l d - l a b o r a t o r y c o m p a r i s o n , is t h e p r o p o s a l o f a " g e n e r i c " m o d e l l i n g s t r a t e g y f o r g e o m o r p h o l o g i c a l r e s e a r c h u n d e r c o n t r o l l e d c o n d i t i o n s . I f t h e mode l l i n g e x e r c i s e t u r n s o u t t o be a p p r o p r i a t e f o r r e s e a r c h and h e n c e c o m p a t i b l e w i t h t h e p r o c e s s e s o b s e r v e d in t h e f i e l d , p h y s i c a l m o d e l l i n g c a n be c o n s i d e r e d t o be a p p l i c a b l e t o a r a n g e o f r i v e r s a nd c o n s e q u e n t l y t o a more gene r a l s i t u a t i o n . The c o n c e p t o f t h e g e n e r i c mode l is f u r t h e r d i s c u s s e d t h r o u g h t h i s t h e s i s . In t h e p e r s p e c t i v e o f t h e a b o v e a r g u m e n t s , t h i s t h e s i s is main ly c o n c e r n e d w i t h t h e p r a c t i c a l v e r i f i c a t i o n o f s i m i l a r i t y and h e n c e w i t h t h e p o s s i b i l i t y o f wo r k i n g u n d e r c o n t r o l l e d c o n d i t i o n s f o r r i v e r s . T h i s e x e r c i s e is p e r f o r m e d f o r a p a r t i c u l a r g e o m o r p h o l o g i c a l p h enomenon wh ich s e ems t o d e s e r v e a t t e n t i o n . The c h o i c e o f t h e phenomenon o f i n t e r e s t is a l so gu i ded by t h e s t a t e o f t h e s y s t e m which p r e v a i l s under , f o r m a t i v e c o n d i t i o n s . Hence wo rk in low t r a n s p o r t i n t e n s i t y r i v e r s r e p r e s e n t s a • f i r s t p r a c t i c a l a s p e c t o f s e l e c t i o n . In t h e f i e l d , s u ch a s t a t e is m o s t o f t e n m e t in sma l l t o i n t e r m e d i a t e s i ze g r a v e l - b e d r i v e r s . C o n s i d e r i n g t h e p h y s i c a l l i m i t a t i o n s in t h e s c a l i n g o f l e n g t h s c a l e s o f n a t u r a l r i v e r s ( e s p e c i a l l y t h e b ed m a t e r i a l s i z e d i s t r i b u t i o n f o r s a n d and f i n e r g r a i n e d f u l l - s c a l e r i v e r s ) , s m a l l t o i n t e r m e d i a t e . s i z e , r e l a t i v e l y c o a r s e b e d d e d r i v e r s a l s o r e p r e s e n t a p r a c t i c a l a l t e r n a t i v e f o r t h e c u r r e n t r e s e a r c h s t r a t e g y . An i m m e n s e a d v a n t a g e o f w o r k i n g u n d e r e x p e r i m e n t a l c o n d i t i o n s is t h a t t h e e x p e r i m e n t a l i s t has t h e f r e e d o m t o man i pu l a t e t h e s y s t e m and t h e r e b y t o c h a r a c t e r i z e i t s r e s pon s e w i t h r e g a r d t o t h e c o n t r o l l i n g p a r a m e t e r s . A l so t h e t i m e s c a l e o f o b s e r v a t i o n is c o n s i d e r a b l y r e d u c e d . In a d d i t i o n , f o r t h e c a s e o f low t r a n s p o r t i n t e n s i t y r i v e r s , only t h e h i ghe s t f l o w s wh ich t r a n s p o r t a s i g n i f i c a n t amoun t o f s e d i m e n t n e ed t o be m o d e l e d . Th i s r e s u l t s in t h e s i m p l i f i c a t i o n o f t h e h y d r o g r a p h o f a p a r t i c u l a r s i t u a t i o n and f u r t h e r c o n t r a c t s t h e t i m e s c a l e o f o p e r a t i o n . The p r i n c i p l e s o f t h e t h e o r y o f s i m i l a r i t y , wh ich b r i n g t h e t h e o r e t i c a l s u p p o r t t o work unde r c o n t r o l l e d , s m a l l - s c a l e e x p e r i m e n t a l c o n d i t i o n s , a r e f u l l y d e s c r i b e d in c h a p t e r 2. The m o s t f u n d a m e n t a l c h a r a c t e r i s t i c f e a t u r e o f low t r a n s p o r t i n t e n s i t y r i v e r s c o n s i s t s o f t h e p o o l - r i f f l e s e q u e n c e . 8ecause o f i t s u b i q u i t y in s u c h r i v e r s , t h e p o o l - r i f f l e s e q u e n c e r e p r e s e n t s a n a t u r a l veh i c l e f o r t h e c u r r e n t r e s e a r c h . As t h e p o o l - r i f f l e s e q u e n c e is t h e r i v e r c o n f i g u r a t i o n in wh i c h m e a s u r e m e n t s wi l l be p e r f o r m e d , a b r i e f s t a t e m e n t on t h e s u b j e c t is in o r d e r . Th i s m a t e r i a l , c o n t a i n e d in an append i x , d e m o n s t r a t e s our l i m i t e d u n d e r s t a n d i n g r e g a r d i n g t h e m o r p h o l o g y and s e d i m e n t o l o g y o f t h e p o o l - r i f f l e s e q u e n c e as we l l as t h e l a ck o f k n o w l e d g e o f t h e f l o w p r o c e s s e s wh ich e n t a i l i t s f o r m a t i o n and m a i n t e n a n c e . In t h i s p r o j e c t , h y d r o d y n a m i c m e a s u r e m e n t s we re p e r f o r m e d in a f i e l d p o o l - r i f f l e s i t u a t i o n , B l a n e y c r e e k ( d e s c r i b e d in c h a p t e r 3), and in a c o r r e s p o n d i n g phys i ca l mode l ( d e s c r i b ed in c h a p t e r 4). The c o m p a r i s o n o f h y d r o d y n a m i c m e a s u r e m e n t s f r o m t h e l a b o r a t o r y and f i e l d e x e r c i s e s , t h a t is t h e t e s t o f t h e c u r r e n t r e s e a r c h h y p o t h e s i s , is a l s o p r e s e n t e d in c h a p t e r 4. B o t h c h a p t e r s a l s o f e a t u r e some e x t e n d e d d i s cuss ion abou t t h e o b s e r v e d p r o c e s s e s and t h e i r i m p l i c a t i o n s r e g a r d i n g t h e c r e e k c o n d i t i o n . / 8 2.0 Scale models of alluvial rivers 2.1 G e n e r a l i t i e s Th i s c h a p t e r p r e s e n t s t h e o r e t i c a l and e m p i r i c a l s u p p o r t f o r h y d r o d y n a m i c m o d e l l i n g a n d i n t r o d u c e s t h e c r i t e r i a wh i c h d e t e r m i n e t h e r a n g e o f s i t u a t i o n s f o r wh ich u n d i s t o r t e d phys i ca l mode l s a r e a u s e f u l a l t e r n a t i v e f o r r e s e a r c h p r a c t i c e . H i s t o r i c a l l y , s c a l e m o d e l l i n g t e c h n i q u e s w e r e a l m o s t e n t i r e l y d e v e l o p e d in e n g i n e e r i n g . A b o u t a c e n t u r y ago , t h e s e t e c h n i q u e s w e r e c o n s i d e r e d as an i n t r a n s m i s s i b l e " a r t " and w e r e l a r g e l y b a s e d on t h e m o d e l e r ' s e x p e r i e n c e . However , s c a l e mode l s t u d i e s h a v e g a i n ed t h e o r e t i c a l b a s e s and w i dened in s c o p e o f a p p l i c a t i o n s i n c e t h e i n t r o d u c t i o n o f d i m e n s i o n a l a n a l y s i s in t h e f i e l d o f f l u i d mechan i cs a t t h e beg inn ing o f t h e t w e n t i e t h c e n t u r y (Ray l e i gh , 1915 ; B u c k i n g h a m , 1915). The c omp l e x i t y o f t h e boundary c ond i t i on s comb ined w i t h t h e d i f f i c u l t i e s t o f o r m u l a t e f l o w and s e d i m e n t t r a n s p o r t p henomena s t i l l c o n s t r a i n t h e u s e o f m a t h e m a t i c a l m o d e l s f o r t h e p r e d i c t i o n o r s t u d y o f c h a n n e l p r o c e s s e s . S c a l e m o d e l s in f l u v i a l g e o m o r p h o l o g y as we l l as in e n g i n e e r i n g a r e u s e f u l f o r t h e s t udy or p r e d i c t i o n o f channe l f l ow , and o f a d j u s t m e n t s in t h e c a s e o f a m o v a b l e b ed m o d e l . H o w e v e r , f o r t h e phys i ca l mode l i n f o r m a t i o n t o be v a l i d , c e r t a i n l a w s and l i m i t a t i o n s i n h e r e n t t o t h e p h y s i c a l c h a r a c t e r i s t i c s o f t h e open c h a n n e l f l ow phenomenon mus t be r e s p e c t e d . 2 . 2 S i m i l i t u d e S im i l i t u de is an i m p o r t a n t c o n c e p t in c o n t e m p o r a r y f l u i d m e c h a n i c s . S i m i l i t u d e a n a l y s i s and a s s o c i a t e d s i m i l i t u d e t h e o r y c o n c e n t r a t e p r ima r i l y on t h e d e v e l o p m e n t o f h y d r o d y n a m i c mode l s ca l i ng laws . The c o n c e p t o f and r a t i o n a l e f o r s ca l e mode ls in e n g i n e e r i n g a r e made c l e a r in t h e f o l l o w i n g q u o t a t i o n . " A mode l o f some phys i ca l s y s t e m ( t h e p r o t o t y p e ) may be t h o u g h t o f as a n o t h e r phys i ca l s y s t e m , norma l l y b u t n o t n e c e s s a r i l y , o f s m a l l e r s i z e , which r e p r o -duces t h e phys i ca l phenomenon in such a way t h a t measurements made in t h e model can be used , by t h e a p p l i c a t i o n o f c o r r e c t s c a l i n g f a c t o r s , t o p r e d i c t a c c u r a t e l y t h e phenomena t o be e x p e c t e d in t h e p r o -to t ype . " ( S h a r p , 1981, p .29) A c c o r d i n g l y , t w o p h y s i c a l s y s t e m s o f d i f f e r e n t s i z e w h i c h a c c u r a t e l y r e p r o d u c e t h e p h y s i c a l p henomena o f e a c h o t h e r a r e sa i d t o be " s i m i l a r " . P h y s i c a l l y s im i l a r s y s t e m s c an be d e s c r i b e d by a s e t o f a n a l y t i c a l e q u a t i o n s w h i c h i n t e r r e l a t e t h e i r c h a r a c t e r i s t i c p a r a m e t e r s . The c h a r a c t e r i s t i c p a r a m e t e r s o f a p h y s i c a l s y s t e m c o n s i s t o f t h e i n d e p e n d e n t q u a n t i t i e s wh i ch a r e r e qu i r e d f o r a c o m p l e t e d e f i n i t i o n o f t h e p h y s i c a l p h e n o m e n o n ( Y a l i n , 1971a) . A d i s t i n c t i o n can a l so be made b e t w e e n t h e p a r a m e t e r s and t h e v a r i a b l e s o f a s y s t e m . The a c t u a l v a r i a b l e s o f a p h y s i c a l p henomenon a r e d i m e n s i o n l e s s and a r e c o m p o s e d o f c h a r a c t e r i s t i c p a r a m e t e r s . S i n ce t h r e e d imens i on s a r e g e n e r a l l y u sed t o d e s c r i b e open c h a n n e l p h y s i c s ( i .e . l e n g t h , t i m e and mas s ) , t h e r e a r e t h r e e c o n d i t i o n s o f s i m i l a r i t y . G e o m e t r i c s i m i l a r i t y r e q u i r e s t h a t t h e l i n ea r p r o p o r t i o n s a r e K e p t b e t w e e n t h e mode l and t h e p r o t o t y p e and h e n c e t h a t t h e y have t h e same shape. K inemat i c s i m i l a r i t y r e q u i r e s t h a t t h e s h a p e o f s t r e a m l i n e s a t any p a r t i c u l a r t i m e is p r e s e r v e d . F i n a l l y d y n a m i c s i m i l a r i t y n e c e s s i t a t e s t h a t t h e r a t i o s o f c o r r e s p o n d i n g f o r c e s in t h e mode l and t h e f u l l s c a l e s y s t e m a re p r e s e r v e d . H o w e v e r , one c an c a l l f o r a m o r e r i g o r o u s d e f i n i t i o n o f s i m i l a r i t y . No va k and C a b e l K a (1981) i n t r o d u c e t h e m o r e r e s t r i c t i v e c o n c e p t o f m e c h a n i c a l s i m i l a r i t y . Two s y s t e m s a r e r e c o g n i z e d as m e c h a n i c a l l y s i m i l a r i f t h e y a r e g e o m e t r i c a l l y s im i l a r and i f , f o r p r o p o r t i o n a l m a s s e s a t h omo l ogou s p o i n t s , t h e p a t h s d e s c r i b e d in p r o p o r t i o n a l t i m e s a r e a l s o g e o m e t r i c a l l y s im i l a r . A c c o r d i n g t o s u c h a d e f i n i t i o n , m e c h a n i c a l s i m i l a r i t y a l w a y s i n c l u d e s g e o m e t r i c , K i n e m a t i c a nd d y n a m i c s i m i l a r i t y . M e a n w h i l e , d y n a m i c s i m i l a r i t y i n c l u d e s k i n e m a t i c b u t n o t n e c e s s a r i l y g e o m e t r i c s i m i l a r i t y . T h e l a t t e r c i r c u m s t a n c e r e p r e s e n t s t h e c a s e o f a d i s t o r t e d mode l . M e c h a n i c a l s i m i l a r i t y r e q u i r e s t h a t f o r c e s a r e b e i n g p r o p o r t i o n a l l y s c a l e d f r o m one s y s t e m t o a n o t h e r . Hence , r a t i o s o f a c t i n g f o r c e s may r e p r e s e n t a d e q u a t e d i m e n s i o n l e s s v a r i a b l e s t o deve l op bas i c mode l laws. F o r c e s in a f l u i d s y s t e m can be c h a r a c t e r i z e d a s : f o r c e s e x t e r n a l t o t h e f l u i d ( g r a v i t a t i o n a l and p r e s s u r e d i f f e r e n c e f o r c e s ) , f o r c e s r e l a t e d t o t h e phy s i c a l p r o p e r t i e s o f t h e f l u i d ( v i s c o u s f o r c e s ) and f o r c e s r e s u l t i n g f r o m t h e f l u i d ' s m o t i o n ( d r a g f o r c e s ) . A n o t h e r , f i c t i t i o u s , f o r c e m i gh t a l s o be c o n s i d e r e d ; t h e h y p o t h e t i c a l i n e r t i a f o r c e . I n e r t i a f o r c e is d e s c r i b e d as be i ng equal t o t h e r e s u l t a n t o f a l l t h e f o r c e s a c t i n g in a p a r t i c u l a r s y s t e m (i.e. o f t h e same magn i tude as t h e r e s u l t a n t ) bu t a c t i n g in t h e o p p o s i t e d i r e c t i o n . Th i s f o r c e t hu s d e s c r i b e s t h e s t a t e o f t h e p h y s i c a l s y s t e m . It is h y p o t h e t i c a l s i n c e i t is n o t d i r e c t l y m e a s u r a b l e . B e c a u s e any f l u i d s y s t e m e x p e r i e n c e s i n e r t i a l f o r c e s , t h e r a t i o o f t h e i n e r t i a l f o r c e t o a c t i n g f o r c e s c o n s t i t u t e s t h e b a s i s o f s im i l i t u d e a n a l y s i s . A c c o r d i n g t o N e w t o n ' s s e c o n d law o f m e c h a n i c s , i n e r t i a f o r c e , Fj, c a n be e x p r e s s e d as (Kobus, 1980) : F j r p l 3 = p i 2 y 2 = p Q 2 / l 2 ( 2 . 1 ) 1 w h e r e p i 3 is p r o p o r t i o n a l t o t h e m a s s , p is t h e m a s s d e n s i t y o f t h e f l u i d and 1 is a c h a r a c t e r i s t i c l e n g t h o f t h e s y s t e m , V is t h e v e l o c i t y o f t h e f l u i d and Q is t h e f l u i d d i s c h a r g e . Fo r t h e sake o f t h i s p a r t i a l ana lys i s , we wil l c on s i d e r t h e t h r e e p r i n c i p a l f o r c e s in o p e n c h a n n e l s y s t e m s . T h e g r a v i t a t i o n a l f o r c e , F g , o f a f l u i d p a r t i c l e is g i v en by i t s ' w e i g h t , i . e . : F g = p l 3 g ( 2 . 2 ) w h e r e g is t h e g r a v i t a t i o n a l a c c e l e r a t i o n . The v i s c o u s f o r c e F v may be d e r i v e d f r o m t h e equa t i o n o f shea r s t r e s s f o r a v i s c o u s f l u i d T = u. d v / d y i.e.: F v / l 2 = u.v/1 a n d F v = u-Vl = u.Q/1 ( 2 . 3 ) w h e r e u. is t h e c o e f f i c i e n t o f d y n a m i c v i s c o s i t y . F i n a l l y t h e r e s u l t a n t d r a g f o r c e s F^ can be e x p r e s s e d as f o l l o w s F d = p l 2 v2 C d (2.4) w h e r e is t h e d r a g c o e f f i c i e n t . S i m i l i t u d e a n a l y s i s a p p l i e s t h e p r i n c i p l e s t h e o r y by c r e a t i n g r a t i o s o f f o r c e s and , t h u s , h omogeneou s e q u a t i o n . If t h e d e p e n d e n t v a r i a b l e is r e p r e s e n t e d by t h e d r ag f o r c e s , one can w r i t e o f s i m i l a r i t y a d imens i ona l l y F d = * < F . ' F g ' F v ) o r in a nond imens i ona l f o r m : ( 2 . 5 ) * ( i i r j r g r y wh ich a l s o is e q u i v a l e n t t o : ( 2 . 6 ) * ( PQ V ) o r ( V 1 ) ( 2 . 7 ) P j 2 V 2 ) 5 g y j ( g l ) 0 . 5 y w h e r e v = u , /p i s k n o w n a s t h e c o e f f i c i e n t o f k i n e m a t i c v i s c o s i t y . The l a t t e r e x p r e s s i o n ( 2 . 7 ) c o n t a i n s t w o f u n d a m e n t a l q u a n t i t i e s on t h e r i g h t , name ly t h e F r o u d e and Reyno l d s n u m b e r s . T h o s e t w o d i m e n s i o n l e s s p a r a m e t e r s c an be u s ed t o d e v e l o p s c a l i n g r e l a t i o n s h i p s wh i ch d e f i n e h y d r o d y n a m i c m o d e l l i n g . H o w e v e r , t h e t r e a t m e n t o f h y d r o d y n a m i c s c a l i n g c a n n o t be s a t i s f a c t o r i l y c o m p l e t e d a t t h i s p o i n t , w h i c h r e p r e s e n t s an i n h e r e n t l i m i t a t i o n o f t h e a p p r o a c h . S i m i l i t u d e ana l y s i s p r o c e e d s on ly v ia f o r c e r a t i o s . As p o i n t e d o u t by Ya l in (1971a), d imen s i ona l a n a l y s i s c o n s i s t s o f a m o r e f l e x i b l e and g e n e r a l m e t h o d wh ich a l s o d o e s n o t r e q u i r e any p a r t i c u l a r i n f o r m a t i o n a b o u t t h e f o r m u l a t i o n o f t h e p h y s i c a l p h enomenon . Th i s m e t h o d wi l l be c o n s i d e r e d in t h e f o l l ow i ng s e c t i o n . 2.3 D imens i ona l a na l y s i s and t h e d e v e l o p m e n t o f s c a l i n g  c r i t e r i a D imens i ona l a n a l y s i s is a m e t h o d o f p a r t i a l a n a l y s i s , i.e. a m e t h o d wh i ch is u s ed in t h e c h a r a c t e r i z a t i o n o f a p h y s i c a l s y s t e m when i t has become t o o comp lex t o be ana l y zed by we l l f o r m u l a t e d m e t h o d s (Sharp , 1981). The m e t h o d , which is b a sed on t h e p r i n c i p l e o f d imen s i o na l h o m o g e n e i t y , p r o v i d e s no a n a l y t i c a l e xp l ana t i o n o f t h e phys i ca l s y s t e m o f i n t e r e s t bu t r e p r e s e n t s an e x c e l l e n t s t a r t f o r e x p e r i m e n t a l i n v e s t i g a t i o n s . S i n c e o p e n c h a n n e l m e c h a n i c s a r e r a t h e r c o m p l e x , t h e y a r e p a r t i c u l a r l y s u s c e p t i b l e t o be s t u d i e d via such a m e t h o d . I n d e e d , in c o n t r a s t w i t h s i m i l i t u d e a n a l y s i s , w h i c h r e q u i r e s t h a t t h e r e s e a r c h e r d e t e r m i n e s t h e r e l a t i o n s a m o n g s t some p a r a m e t e r s o f a s y s t e m , d imen s i o na l a na l y s i s n e c e s s i t a t e s on ly t h a t one d e t e r m i n e s t h e c h a r a c t e r i s t i c p a r a m e t e r s o f t h e s y s t e m ( Y a l i n , 1 9 71a ) . T h e a p p l i c a t i o n o f s i m p l e a n d w e l l - f o r m u l a t e d r u l e s wi l l t h e n d e t e r m i n e p o s s i b l e d i m e n s i o n l e s s c o m b i n a t i o n s f r o m t h e d imens i ona l q u a n t i t i e s and , a t t h e same t i m e , t h e c r i t e r i a f o r m e c h a n i c a l s i m i l a r i t y . Two m e t h o d s o f d i m e n s i o n a l a n a l y s i s w e r e i n t r o d u c e d i n t o t h e f i e l d o f h y d r a u l i c s e a r l y t h i s c e n t u r y by R a y l e i g h and by B u c k i n g h a m ( o p . c i t , ) . The l a t t e r is p e r h a p s t h e b e s t known and wi l l be r e v i e w e d b r i e f l y . Buck i ngham (1915) p r e s e n t s t h e f u l l d edu c t i o n o f his m e t h o d as we l l as s e v e r a l o f i t s a p p l i c a t i o n s t o t h e f i e l d o f f l u i d m e c h a n i c s . B r i e f l y , h i s m e t h o d s h o w s t h a t t h e n u m b e r o f v a r i a b l e s in a c o n s i s t e n t , n o n d i m e n s i o n a l a n d f u n c t i o n a l equa t i on is r e l a t e d t o t h e number n o f c h a r a c t e r i s t i c p a r a m e t e r s o f t h e phys i ca l phenomenon o f i n t e r e s t and t o t h e number m o f d imens i on s i n v o l v e d . Buck i ngham s h o w e d t h a t t h e r e s u l t o f a c o n s i s t e n t a n a l y s i s w o u l d c o n t a i n ( n - m ) d i m e n s i o n l e s s v a r i a b l e s wh ich he c a l l e d i r - t e r m s ( h e n c e , h i s d e d u c t i v e m e t h o d ha s b e c o m e known as t h e i r - t h e o r e m ) . In t h e s i m p l e s t f o r m o f t h e m e t h o d , i r - t e r m s c an be d e r i v e d i n d e p e n d e n t l y o f e a c h o t h e r . The m e t h o d a l s o n e c e s s i t a t e s t h e s e l e c t i o n o f s o m e b a s i c q u a n t i t i e s ( Y a l i n , 1971). Those p a r a m e t e r s , which shou ld be equal in number t o t h e n u m b e r o f d i m e n s i o n s and wh i c h s h o u l d c o n t a i n a l l t h e f u n d a m e n t a l d imens i on s o f t h e p henomenon o f i n t e r e s t , c an be c omb i n ed , one a t a t i m e , w i t h e a c h o t h e r p a r a m e t e r wh i ch d e s c r i b e s t h e p h e n o m e n o n in o r d e r t o c r e a t e d i m e n s i o n l e s s v a r i a b l e s . If we c on s i d e r t h e c a s e o f an i n compres s i b l e s t e a d y un i f o rm f l o w o v e r a g r a n u l a r b e d u n d e r low s e d i m e n t t r a n s p o r t c o n d i t i o n s , we c an d e d u c e t h e f o l l o w i n g l i s t o f c h a r a c t e r i s t i c p a r a m e t e r s (we wi l l i g n o r e s u r f a c e t e n s i o n e f f e c t s wh ich shou ld be i r r e l e v a n t in t h e p r e s e n t a n a l y s i s ) : (1) P h y s i c a l p a r a m e t e r s o f t h e f l u i d p : w a t e r d e n s i t y u» : w a t e r c o e f f i c i e n t o f dynamic v i s c o s i t y (2) P a r a m e t e r s o f t h e f l o w V : mean v e l o c i t y o f t h e f l o w S : ene rgy g r a d i e n t (3) P a r a m e t e r s which d e s c r i b e t h e g e o m e t r y ( i.e t h e channe l ) where t h e phenomenon o c c u r s w : channe l w id th d : channe l mean d e p t h (4) P a r a m e t e r s wh i ch d e s c r i b e t h e c h a r a c t e r i s t i c s o f t h e b o u n -d a r y m a t e r i a l s D : a c h a r a c t e r i s t i c g r a i n s i ze d i a m e t e r . ( 5 ) The gene r a l c o n s t a n t g, t h e a c c e l a r a t i o n due t o g r a v i t y . We sha l l a s sume f o r now t h a t t h e c h a r a c t e r i s t i c d i a m e t e r a c c o u n t s f o r t h e e f f e c t s o f t h e boundary ma t e r i a l on t h e g ene r a l f l o w s i t u a t i o n . G e n e r a l l y , t h e med i an d i a m e t e r o f t h e g r a i n s i z e d i s t r i b u t i o n , D 5 0 1 is e m p l o y e d . T h e s e l e c t i o n o f p, V and d a s t h e b a s i c q u a n t i t i e s n e c e s s a r y f o r t h e a p p l i c a t i o n o f d i m e n s i o n a l a n a l y s i s l e a d s t o t h e f o l l o w i n g d i m e n s i o n l e s s e x p r e s s i o n : Vd w TT i ; i = 1 , 5 = { ( 9 d ) 0 . 5 , S } ( 2 . 8 ) whe r e v is t h e c o e f f i c i e n t o f K i n e m a t i c v i s c o s i t y , as in e q u a t i o n ( 2 . 7 ) . i r l a n d i r 2 r e p r e s e n t t h e w e l l - k n o w n F r o u d e and Re yno l d s numbe r s wh i ch w e r e a l s o o b t a i n e d f r o m s i m i l i t u d e a n a l y s i s a b o v e . i r 3 a n d i r 4 c o n s i s t o f t h e w i d t h t o d e p t h r a t i o and o f a r e l a t i v e r o u g h n e s s p a r a m e t e r r e s p e c t i v e l y . i r 5 i s s i m p l y t h e e n e r g y g r a d i e n t w h i c h a l r e a d y r e p r e s e n t s a d i m e n s i o n l e s s v a r i a b l e by i t s e l f . I f X x s t a n d s f o r t h e r a t i o p r o t o t y p e / m o d e l o f any c h a r a c t e r i s t i c p a r a m e t e r x o f t h e a b o v e l i s t o f d i m e n s i o n l e s s v a r i a b l e s , t h e n one c an w r i t e , f o r s i m i l a r i t y r e q u i r e m e n t s : X U 2 x„x g * d 1 . 0 1 . 0 X v X<j = 1 . 0 ( 2 . 9 ) kw * d = 1 . 0 X d» c a s e o f a = 1 . 0 a n d X w g e o m e t r i c a l l y a r e a l l e q u i v a l e n t f o r t h e s im i l a r , u n d i s t o r t e d mode l and c an be s u b s t i t u t e d by a u n i q u e l e n g t h s c a l e X ) . X g i s f i x e d on e a r t h and X v is a l s o f i x e d i f one w o r k s w i t h t h e same f l u i d a t a b o u t t h e same t e m p e r a t u r e . Cons ide r i r l a n d i r 2 ( a l l o t h e r i r c a r r y i n g r e d u n d a n t i n f o r m a t i o n ) p l u s t h e f o l l o w i n g s i t u a t i o n : X g = 1 a n d X v = 1 ( 2 . 1 0 ) A c c o r d i n g t o t h e l a t t e r c o n d i t i o n s , t h e r e a l i z a t i o n o f a sma l l e r s c a l e , me chan i c a l l y s im i l a r mode l f o r any g i v en p r o t o t y p e wou ld appea r i m p o s s i b l e s i n c e F r o u d e s c a l i n g law r e s u l t s f o r t h e v e l o c i t y s c a l e ( X y 2 = X j ) a r e i n c o m p a t i b l e w i t h t h o s e f r o m R e y n o l d s ' s c a l i n g l aw ( X y = 1 / X | ) . T h e r e f o r e , a s t r i c t l y m e c h a n i c a l l y s i m i l a r s m a l l s c a l e m o d e l w o u l d n o t b e p o s s i b l e t o r e a l i z e u n d e r s u c h c o n d i t i o n s . H o w e v e r , as we s ha l l d i s c u s s l a t e r , t h e r e l a x a t i o n o f one o f t h e s e t w o s c a l i n g c r i t e r i a is a d m i s s i b l e i f s o m e l i m i t i n g c o n d i t i o n s a r e r e s p e c t e d . The F roude law is c o n s i d e r e d p r edom inan t s ince , in b o t h t h e mode l and t h e p r o t o t y p e , w a t e r f l o w is p r ima r i l y g o v e r n e d by g r a v i t a t i o n a l f o r c e s . If we a c c e p t t h a t t h e a c t i o n o f g r a v i t y is ove rwhe lm ing o v e r t h a t o f o t h e r f o r c e s and i f we a l so inc lude g e o m e t r i c s i m i l a r i t y as a b a s i c r e q u i r e m e n t f o r m e c h a n i c a l s i m i l a r i t y , i . e : X w = X d = X D = X S = 1 ( 2 . H ) we c an f u r t h e r d e r i v e f r o m TT1 t h e F r o u d e s c a l i n g v e l o c i t y scale Xy = ( X , )°-5 By s u b s t i t u t i o n i n t o o r ( 2 . 1 2 ) man i p u l a t i o n o f t h e l a t t e r s c a l i n g law, we can a l so o b t a i n a s e r i e s o f s c a l e r a t i o s f o r o t h e r k i nemat i c and dynamic v a r i a b l e s o f t h e phys i ca l s y s t e m , f o r i n s t an c e : X Q = ( X , ) 2 . 5 X t = ( X , ) ° - 5 X F = X p X , 3 X T 0 = X , ( 2 . 1 3 ) w h e r e X-t s t a n d s f o r t h e t i m e s c a l e , Xp f o r t h e f o r c e s c a l e a n d X x o f o r t h e s h e a r s t r e s s s c a l e . Th i s s e t o f s c a l i n g l aws d e f i n e s w h a t is r e f e r e d t o as a F r o u d i a n m o d e l . N o t e t h a t t h e same d i m e n s i o n l e s s v a r i a b l e s o r s c a l i n g l a w s c o u l d h a v e b e e n o b t a i n e d f r o m t h e s i m i l i t u d e ana l y s i s p r e s e n t e d in s e c t i o n 2.2: t h i s c o n f i r m s t h e i r p h y s i c a l s i g n i f i c a n c e . 2.4 F r o u d i a n s c a l i n g l aws a n d n a t u r a l r i v e r c h a n n e l s 2.4.1 Na t u r a l s t r e a m behav iou r o v e r a r ange o f s c a l e s ( h y d r a u l i c g e o m e t r y r e s u l t s ) It is l e g i t i m a t e t o ask w h a t is t h e s i g n i f i c a n c e o f F r o u d i a n s c a l i n g l aws w i t h r e s p e c t t o t h e b e h a v i o u r o f n a t u r a l c h a n n e l s . O t h e r w i s e s t a t e d , t h e q u e s t i o n " a r e n a t u r a l s t r e a m s F roude mode ls o f each o t h e r ? " may be a s ked . The i ssue is o f m a j o r i m p o r t a n c e r e g a r d i n g t h e a p p r o p r i a t e n e s s o f u s i n g t h e o r e t i c a l l y d e r i v e d s c a l i n g r e l a t i o n s h i p s t o m o d e l n a t u r a l r i v e r b e h a v i o u r a t a sma l l e r s c a l e . D o w n s t r e a m h y d r a u l i c g e o m e t r y s t u d i e s , wh ich a r e nume rou s in f l u v i a l g e o m o r p h o l o g y , a s w e l l as c o n c e p t u a l l y e q u i v a l e n t r e g ime c a n a l s s t u d i e s in e n g i n e e r i n g , p r o v i d e t h e ba s i c s o u r c e o f i n f o r m a t i o n f o r t h e p r e s e n t c o n s i d e r a t i o n s . T h o s e emp i r i c a l l y o b t a i n e d r e l a t i o n s a r e c u s t o m a r i l y p r e s e n t e d in t h e f o r m o f power f u n c t i o n s , such as: y j = a Q f x ( 2 . 1 4 ) w h e r e y-, r e p r e s e n t any p a r a m e t e r p a r t o f t h e f l o w c on t i n u i t y equa t i on (Q = Vwd), a is a c o n s t a n t o f p r o p o r t i o n and x an e x p o n e n t . Q f is r e f e r r e d t o as f o r m a t i v e d i s c h a r g e . The n e c e s s i t y f o r t h e l a t t e r d i s c ha r ge p a r a m e t e r is a r e s u l t o f t h e u n s t e a d y c h a r a c t e r o f n a t u r a l r i v e r f l o w s . Most o f t e n , t h e b a n K f u l l d i s c h a r g e ha s b e e n u s e d as a s u r r o g a t e t o t h e " f o r m a t i v e " d i s c h a r g e a l t h o u g h i t s s i g n i f i c a n c e in t e r m s o f r i v e r m o r p h o l o g y has n o t y e t b e en d e m o n s t r a t e d on a un i ve r sa l b a s i s ( K n i g h t o n , 1 9 8 4 ) . T h i s r e f e r e n c e d i s c h a r g e i s n e v e r t h e l e s s c o n s i d e r e d s u f f i c i e n t f o r t h e p r e s e n t a n a l y s i s . M o r e o v e r , f o r t h e a c t u a l c o n s i d e r a t i o n s , t h e w i d t h / d i s c h a r g e ( e x p o n e n t b) as we l l as t h e d e p t h / d i s c h a r g e ( e x p o n e n t f ) r e l a t i o n s wi l l be o f m o s t i n t e r e s t , s i n c e t h e t h i r d r e l a t i o n is t h e n f i x e d . D o w n s t r e a m h y d r a u l i c g e o m e t r y r e s u l t s a r e s u m m a r i z e d in v a r i o u s p a p e r s . In an u n p u b l i s h e d m a n u s c r i p t , C h u r c h (1980 ) r e p o r t s b e x p o n e n t s b e t w e e n 0.5 and 0 .57 and f e x p o n e n t s b e t w e e n 0 . 32 and 0 . 47 ( e x c e p t one o f 0 . 59 ) . T h e s e hyd rau l i c g e o m e t r y r e l a t i o n s common l y show h igh p r o p o r t i o n s o f e x p l a i n e d v a r i a n c e . The r e s u l t s a b o u t t h e c h a n n e l w i d t h and d e p t h r e l a t i o n s h i p e x t e n d w i t h d i s c h a r g e o v e r n ine o r d e r s o f m a g n i t u d e ( F e r g u s o n , 1986 ) . T h e f o r e g o i n g r a n g e s o f e x p o n e n t s c o n s i d e r e d s o m e e x p e r i m e n t a l c h a n n e l s as w e l l as s o m e i r r i g a t i o n c a n a l a nd g r a v e l b e d r i v e r d a t a , a l l f o r s u b c r i t i c a l f l o w ( i .e . F r o u d e numbe r < 1.0), and m o s t l y N o r t h A m e r i c a n . C l a s s i c a l r e g i m e s t u d i e s ( e . g . I ng i i s , 1949 ; L a c e y , 1958 ; S i m o n s and A l b e r t s o n , 1960 j B l ench , 1969 ; e t c . ) t yp i c a l l y s u g g e s t b and f v a l u e s o f 0 . 5 0 and 0 . 33 r e s p e c t i v e l y f o r m o s t l y Indian c a n a l s . Hey and Tho rne (1986) a lso r e p o r t b e x p o n e n t s o f 0 .50 and f e x p o n e n t s o f 0 . 3 5 f r o m B r i t i s h g r a v e l b ed r i v e r s . F ina l ly , Ming (1983) r e p o r t s b e x p o n e n t s b e t w e e n 0.39 and 0 .55 (mos t l y near 0 . 4 8 - 0 . 5 0 ) and f e x p o n e n t s b e t w e e n 0 .30 and 0.43 ( a v e r age o f 0 . 36 ) f r o m Ch i n e s e and Russ i an r i v e r s t u d i e s . Hence , a c c o r d i n g t o w o r l d w i d e s t u d i e s f r o m a v a r i e t y o f e n v i r o n m e n t s , t h e r e l a t i v e c o n s i s t e n c y o f t h e b and f e x p o n e n t s is r ema r kab l e and i t may be s u g g e s t e d t h a t t h e s e bea r a v e r a g e va lues o f abou t 0 .50 and 0 .35 r e s p e c t i v e l y . However , f o r a ve r y w ide r a n g e o f s c a l e s , i n c l u d i n g t h e w o r l d ' s l a r g e s t r i v e r sys tems , i t appea rs t h a t a b e x p o n e n t o f abou t 0 .55 may be more a p p r o p r i a t e ( K e l l e r h a l s and C h u r c h , in p r e s s ) . L e t us now r e c o n s i d e r one r e s u l t , which was p r e s e n t e d in o u r s i m i l i t u d e a n a l y s i s a b o v e , in l i g h t o f t h e l i k e l y p r e d o m i n a n c e o f t h e F r o u d e n u m b e r c r i t e r i o n f o r m e c h a n i c a l s i m i l a r i t y o f o p e n c h a n n e l s . In e q u a t i o n ( 2 . 7 ) , t h e F r o u d e number was a l so g iven in t e r m s o f d i s c ha r ge . This f o r m o f t h e F r o u d e n u m b e r c a n be r e f o r m u l a t e d t o s e t t h e r e q u i r e d p r o p o r t i o n a l i t y f o r F r o u d e s i m i l a r i t y b e t w e e n t h e d i s c h a r g e and t h e g e o m e t r i c a l p a r a m e t e r s o f t h e channe l , i.e: Q 0 . 40 g 0 . 2 0 ( 2 . 1 5 ) If we c o n s i d e r L as t h e l e n g t h s c a l e , e q u a t i o n (2.15) shows t h a t , f o r F roude s imi lar channe l s , b and f e x p o n e n t s shou ld b o t h be equal t o 0.40. C lear ly t h i s does n o t seem t o be t h e c a se f o r a l l u v i a l c h a n n e l s a c c o r d i n g t o t h e a f o r e m e n t i o n e d e m p i r i c a l r e s u l t s o f h y d r a u l i c g e o m e t r y s t u d i e s . The r e a s on s f o r such a s i t u a t i o n w a r r a n t f u r t h e r c o n s i d e r a t i o n . 2.4.2 D e c o n s t r a i n e d h y d r a u l i c g e o m e t r y p l o t The a p p r o a c h t o c l a r i f y t h e a b o v e c o n c e r n s r e g a r d i n g t h e behav iou r o f n a t u r a l channe l s has been t o p roduce t h e equ i v a l en t o f a d o w n s t r e a m hydrau l i c g e o m e t r y p l o t in which t h e c o n s t r a i n t s r e l a t e d t o t h e r oughnes s l e n g t h s ca l e (or gra in s ize s ca l e as a s u r r o g a t e ) wh i ch a ppea r in n a t u r a l d r a i n a g e ba s i n s wou ld be c o n t r o l l e d . I t is w e l l d o c u m e n t e d in many g e o m o r p h o l o g i c a l s t u d i e s t h a t t h e r i v e r bed med ian g ra in s ize d i ame t e r d imin ishes f r o m t h e h e a d w a t e r s o f a d r a i n a g e b a s i n t o i t s o u t l e t . V a r i a t i o n in t h e r o u g h n e s s l e n g t h is l e s s we l l d o c u m e n t e d b u t e v e n i f s t e e p e r b e d f o r m s t e n d t o d e v e l o p in o r d i n a r y f i n e r g r a i n e d , m o s t d o w n s t r e a m r e a c h e s , F r o u d i a n p r o p o r t i o n s do n o t s e em t o be k e p t , as d e m o n s t r a t e d by t h e e m p i r i c a l r e s u l t s p r e s e n t e d a b o v e . T h e r e f o r e , n a t u r a l g r a v e l b ed r i v e r c h a n n e l d a t a ( e x t r a c t e d f r o m C h u r c h and Rood , 1983) w e r e p l o t t e d on one ( b a n k f u l l ) d i s c h a r g e t o w i d t h and d e p t h g r a p h t o g e t h e r w i t h s o m e e x p e r i m e n t a l d a t a f r o m Wolman and B r u s h (1961). D a t a w e r e s c r e e n e d t o e n s u r e t h a t f l o w cond i t i o n s f r o m t h o s e e x p e r i m e n t a l c h a n n e l s w e r e r e p r e s e n t a t i v e o f f i e l d s i t u a t i o n s , i . e . s u b c r i t i c a l a nd f u l l y h y d r a u l i c a l l y r o u g h ( r e f . s e c t i o n 2.5.1 below). The d a t a were s e l e c t e d in such a way t h a t a p l o t o f t h e g r a i n s i ze (a c o n t r o l l i n g l e n g t h s c a l e ) and d i s c h a r g e w o u l d a p p r o x i m a t e l y f o l l o w t h e F r o u d i a n s i m i l a r i t y c r i t e r i o n a s c o n t a i n ed in equa t i on (2.15): f i g u r e 1 p r e s e n t s t h e d a t a and t h e r e l a t i on sh i p f i t t e d by eye which has an e x p o n e n t o f 0 .34 . F igure 2 shows t h e d a t a po i n t s and t h e r e l a t i o n sh i p s f i t t e d by eye b e t w e e n c h a n n e l l e n g t h s c a l e s ( w i d t h and d e p t h ) and d i s c h a r g e . The r e s u l t s o f t h i s e x e r c i s e a r e m o s t i n t e r e s t i n g . T h e b a n d f e x p o n e n t s o f t h e r e l a t i o n s h i p b e t w e e n w i d t h / d i s c h a r g e and d e p t h / d i s c h a r g e a r e r e s p e c t i v e l y 0 . 4 2 5 and 0 .417. It t h e r e f o r e a p p e a r s t h a t when t h e c o n s t r a i n t s o f t h e n a t u r a l e n v i r o n m e n t s a r e w i t h d r a w n , a l l u v i a l c h a n n e l s n e a r l y c o n f o r m t o t h e r e q u i r e m e n t s o f F r o u d i a n s im i l a r i t y p r e s c r i b e d by e q u a t i o n (2.15). Th i s o c c u r r e d e v e n t h o u g h t h e c o n t r o l on t h e g r a i n s i ze s c a l e in t e r m s o f t h e F r o u d e r e q u i r e m e n t s f o r s i m i l a r i t y was n o t e x a c t ( s e e f i g u r e 1). It is t h u s p r o p o s e d t h a t f i g u r e 2 p r e s e n t s c l e a r e v i d e n c e t h a t n a t u r a l channe l s do f o l l o w t h e p r o p o r t i o n s p r e s c r i b e d by F r o u d i a n s i m i l a r i t y l aws unde r f a v o r a b l e c o n d i t i o n s . The l a t t e r a r e n o t me t in n a t u r a l s y s t e m s due t o t h e c o n s t r a i n t i m p o s e d upon by t h e b o u n d a r y m a t e r i a l as an u n d e r l y i n g f a c t o r a f f e c t i n g r e s i s t a n c e t o f l o w , whence c h a n n e l d i m e n s i o n . In t h e c o n t e x t o f t h e p r e s e n t r e s e a r c h p r o j e c t , t h e m o s t s i g n i f i c a n t impac t o f t h e r e s u l t s shown on f i g u r e 2 c o n s i s t s in t h e v a l i d a t i o n o f F r o u d e s c a l i n g l aws f o r t h e s t u d y o f some n a t u r a l r i v e r s a n d o f t h e i r p r o c e s s e s in s m a l l s c a l e e x p e r i m e n t s . In f a c t , f i g u r e 2 ea s i l y s pan s a r a n g e b e t w e e n t y p i c a l g r a v e l - b e d r i v e r p r o t o t y p e s and f e a s i b l e c o n t r o l l e d mode l s . A p p r o p r i a t e m o d e l s t u d i e s c a n on l y be r e a l i z e d i f c e r t a i n c o n d i t i o n s , wh ich wi l l be t h e s u b j e c t o f t h e n e x t Figuro 1. Grain alza voraua dlicharg* plot for tho toloctod tot of data 100 E 0.01 0.1 H 0.0001 100 10 H 0.1 0.01 H 0.001 0.0001 0.01 1 100 Bankfull Olaehargo ( ema ) Flgur* 2. Doconatralnod hydraulie goomotry plot 0.01 1 100 Bankfull Dlicharg* ( ema ). s e c t i o n , a r e r e s p e c t e d . 2.5 C r i t e r i a f o r a c l o s e r t e s t o f t h e r e p r e s e n t a t i v e n e s s o f s c a l e m o d e l s 2.5.1 The r e p r o d u c t i o n o f f r i c t i o n a l c h a r a c t e r i s t i c s o r o f r o u g h n e s s l e n g t h The b a l a n c e b e t w e e n t h e p r o p e l l i n g and r e s i s t i n g f o r c e s a l o n g t h e b o u n d a r y o f a c h a n n e l r e f l e c t s i t s f r i c t i o n a l c h a r a c t e r i s t i c s . The p r e s e r v a t i o n o f t h e s e f o r c e s is e s s e n t i a l in t h e c o n t e x t o f m e c h a n i c a l s i m i l a r i t y . F o r r i v e r s wh ich have a r e l a t i v e l y low bed s e d i m e n t t r a n s p o r t i n t e n s i t y , a r i g i d b ed a s sumpt i on is admiss ib le f o r t h e pu rpose o f t h e ana lys i s o f t h e f l o w r e s i s t a n c e p r o b l e m . C h a n n e l f l o w r e s i s t a n c e r e p r e s e n t s a v e r y c o m p l e x , c o m p o s i t e phenomenon ( see a r e v i e w p a p e r by B a t h u r s t , 1982). Common s i m p l i f i c a t i o n s o f t h e p r o b l e m i n c l ude t h e a s sump t i o n o f s t e a d y un i f o rm f l o w , t h e use o f a v e r a g e g e o m e t r i c and hydrau l i c p a r a m e t e r s , t h e l a t t e r o f wh i c h a r e i d e a l i z e d as u n i f o r m l y d i s t r i b u t e d , and t h e use o f a r e p r e s e n t a t i v e d i a m e t e r o f t h e g r a i n s i ze d i s t r i b u t i o n . In g e n e r a l t e r m s , i f we c o n s i d e r t h e p r o p u l s i v e g r a v i t a t i o n a l f o r c e f o r any g i v en c r o s s - s e c t i o n t o be r e s i s t e d o v e r t h e c h a n n e l p e r i m e t e r by a s h e a r f o r c e ( r e p r e s e n t e d p e r un i t a r e a by t h e s h e a r s t r e s s TQ ) , and i f we r e a r r a n g e , we g e t t h e we l l - known un i f o rm f l o w f o r m u l a t i o n ( H e n d e r s o n , 1966) T 0 = y R S ( 2 . 1 6 ) where R is t h e hydrau l i c rad ius and S t h e ene rgy s lope o f t h e f l o w (in m o s t p r a c t i c a l c a s e s , i d e n t i c a l w i t h t h e w a t e r s u r f a c e slope). L e t us c o n s i d e r t h e r e s u l t s o f a d imens i ona l a na l y s i s o f t h e r e s i s t a n c e p r o b l e m f o r t w o - d i m e n s i o n a l f l o w . C o n s i d e r t h e f o l l o w i n g s e t o f c h a r a c t e r i s t i c p a r a m e t e r s f o r f l o w in a rough c h a n n e l , i.e. u, p, D, R and V. Any m e c h a n i c a l q u a n t i t y r e l a t e d t o t h e f l o w p r o c e s s can be e x p r e s s e d as a f u n c t i o n o f t h e s e f i v e c h a r a c t e r i s t i c p a r a m e t e r s ( Y a l i n , 1971a) . I f we c o n s i d e r t h e a v e r a g e s h e a r s t r e s s TQ a c t i n g on t h e b ounda r y , wh ich i n c o r p o r a t e s t h e d r i v i n g g r a d i e n t S o f t h e s y s t e m , we c an g e t , u s i n g t h e ir t h e o r e m , t h e f o l l o w i n g d i m e n s i o n l e s s g r o u p i n g ; TO VR f { ( 2 . 1 7 ) p V 2 v R L e t us cons i de r f o r t h e moment t h e s i gn i f i c an ce o f t h e t e r m on t h e l e f t s i de o f e q u a t i o n (2.17). S u b s t i t u t i o n o f t h i s t e r m , in t h e f o r m o f t h e l e f t member o f e q u a t i o n ( 2 . 17 ) , i .e . T 0 = f f p V 2 ( 2 . 1 8 ) (whe r e f f is a c o e f f i c i e n t t e r m e d t h e f r i c t i o n f a c t o r ) , i n t o e q u a t i o n (2 .16) g i v e s f f = g R S / V 2 ( 2 . 1 9 ) E q u a t i o n (2.19) is e q u i v a l e n t in f o r m t o t h e D a r c y - W e i s b a c h formula f f = 8 g R S V2~ 8 V»2 V2 ( 2 . 2 0 ) w h e r e V* = ( g R S ) 0 - 5 = ( T 0 / p ) 0 - 5 i s k n o w n as t h e s h e a r v e l o c i t y . The D a r c y - W e i s b a c h f o r m u l a i n c l u de s a d i m e n s i o n l e s s v a r i a b l e , a r e s i s t a n c e c o e f f i c i e n t , wh i c h a c c o u n t s f o r t h e b a l a n c e b e t w e e n r e t a r d i n g and p rope l l i ng f o r c e s . S h e a r d i s t r i b u t i o n o v e r an a l l u v i a l b o u n d a r y is n e v e r u n i f o r m . T h e r e f o r e , f o r d e t a i l e d h y d r o d y n a m i c m e a s u r e m e n t s as i n t e n d e d in t h i s t h e s i s , a l o c a l e x p r e s s i o n f o r s h e a r s t r e s s e s t i m a t i o n is n e e d e d . The s h e a r v e l o c i t y i n t r o d u c e d a b o v e a l s o a p p e a r s in t h e d e v e l o p m e n t o f t h e c l a s s i c a l b o u n d a r y l a y e r t h e o r y . Ove r r o u g h b o u n d a r i e s , t h e v e l o c i t y p r o f i l e c a n be e x p r e s s e d a s : T 0 ( V 2 - V 1 ) -- ( 2 . 2 1 ) p ( 2 . 3 K ) l o g 1 0 ( y 2 / y 1 ) whe r e tc is known as t h e Von K a r m a n c o n s t a n t . E q u a t i o n (2.21) p r o v i d e s an e s t i m a t e v i a m e a s u r e m e n t o f t h e l o c a l v e l o c i t y p r o f i l e o f t h e i n t e r n a l f l u i d s h e a r a t s ome d i s t a n c e a b o v e t h e b e d . E q u a t i o n (2.21) a l l ows a c l o s e r t e s t o f t h e s p a t i a l d i s t r i b u t i o n s o f t h e s h e a r f o r c e in t h e mode l and t h e p r o t o t y p e : v e l o c i t y p r o f i l e s r e p r e s e n t t h e p r i n c i p a l mea su r emen t s t o do so . F r om i t s d i m e n s i o n l e s s n a t u r e , t h e D a r c y - W e i s b a c h f r i c t i o n f a c t o r o f e q u a t i o n (2 . 20 ) c a n be u s ed t o f o r m u l a t e t h e r e q u i r e m e n t s f o r t h e p r e s e r v a t i o n o f m e c h a n i c a l s i m i l a r i t y w i t h r e g a r d t o g e n e r a l f r i c t i o n a l c o n s i d e r a t i o n s . It . c a n be r e e x p r e s s e d as f o l l o w s : 9R f f = c o n s t a n t S ( 2 . 2 2 ) i.e i t c an d e c o m p o s e d i n t o t h e F r o u d e number and t h e ene rgy g r a d i e n t . E q u a t i o n (2 .22) i n f o r m s t h a t t h e r e q u i r e m e n t s f o r t h e p r e s e r v a t i o n o f r e s i s t i n g f o r c e s wou ld be e n s u r e d by a F roude m o d e l . H o w e v e r , t h e p r e s e r v a t i o n o f t h e f l o w r e s i s t a n c e c o n d i t i o n mu s t a l s o c o n s i d e r some s c a l e e f f e c t s wh ich wi l l be t h e o b j e c t o f t h e d i s cuss ion be low. In t h e a b o v e d imens i ona l ana l y s i s o f t h e r e s i s t a n c e p rob lem which l ed t o e q u a t i o n (2.17), we h a v e shown t h a t t h e f r i c t i o n f a c t o r f f mus t be a f u n c t i o n o f t h e Reyno lds number and t h e r e l a t i v e r o u g h n e s s o f t h e b o u n d a r y . N u m e r o u s e x p e r i m e n t a l s t u d i e s in f l u i d m e c h a n i c s wh ich c o n s i d e r e d p ipe f l o w s i t u a t i o n s h a ve r e v e a l e d t h e n a t u r e o f t h e r e l a t i o n s h i p s b e t w e e n t h e f r i c t i o n f a c t o r , t h e Reyno l d s number and t h e r e l a t i v e b o u n d a r y r o u g h n e s s . T h e s e r e l a t i o n sh i p s can be r e p r e s e n t e d by a p l o t o f f f v e r s u s t h e Re yno l d s numbe r f o r v a r i o u s r e l a t i v e r o u g h n e s s v a l u e s , su ch as f i g u r e 3 ( m o d i f i e d f r o m Rouse , 1959 by Kobus, 1980). The a b o v e a n a l y s i s and t h e r e s u l t i n g s o - c a l l e d r e s i s t a n c e d i a g r am as f i g u r e 3 w e r e f i r s t d e ve l o ped f r o m pipe e x p e r i m e n t s b u t i t is p o s s i b l e t o u se t h e m f o r open c h a n n e l s i f t h e h y d r a u l i c r a d i u s is u s ed and i f k s , t h e e q u i v a l e n t r o u g h n e s s o f t h e b ounda r y m a t e r i a l s , is p r o p e r l y ' d e f i n e d ( M i d d l e t o n and S o u t h a r d , 1984). The f o u r r e g i o n s wh i ch c an be r e c o g n i z e d on t h e r e s i s t a n c e d i a g r am ( f i g u r e 3) c o r r e s p o n d t o d i s t i n c t f l o w c o n d i t i o n s i . e . l a m i n a r , t u r b u l e n t h y d r a u l i c a l l y s m o o t h , t r a n s i t i o n a l and t u r b u l e n t h y d r a u l i c a l l y r o u g h . In t h e p r e s e n t r e s e a r c h c o n t e x t , p r o t o t y p e c o n d i t i o n s , l i k e m o s t r i v e r s i t u a t i o n s , b e l o n g t o t h e h y d r a u l i c a l l y r o u g h d o m a i n . It is VR Re = 4 v Re f f 0 - 5 = (2 g S ) 0 - 5 v F i g u r e 3 . R e s i s t a n c e d i a g r a m M o d i f i e d f r o m R o u s e ( 1 9 5 9 ) a n d K o b u s ( 1 9 8 0 ) ' ro CO e s s e n t i a l t o p r e s e r v e t h e s e c o n d i t i o n s in t h e s c a l e mode l in o r d e r t o e n s u r e m e c h a n i c a l s i m i l a r i t y . As we n o t e d e a r l i e r , s t r i c t m e c h a n i c a l s i m i l a r i t y can be s a t i s f i e d on ly i f t h e mode l and t h e p r o t o t y p e a r e t h e same. This is t h e r e s u l t o f t h e c o n t r a d i c t i o n b e t w e e n t h e Reyno lds and F r o u d e s c a l i n g l aws . We a l s o n o t e d t h a t g r a v i t a t i o n a l f o r c e s w e r e p r e d o m i n a n t . C o n s e q u e n t l y , we m u s t s e e k f o r t h e p o s s i b i l i t y o f some c o m p r o m i s e r e g a r d i n g t h e r e p r o d u c t i o n o f v i s c o u s f o r c e s in o r d e r t o r e a l i z e a s c a l e m o d e l . F i g u r e 3 r e v e a l s t h a t , u nde r t h e hyd rau l i c a l l y r ough f l o w reg ime, f o r any r e l a t i v e r o u g h n e s s v a l u e , t h e f r i c t i o n f a c t o r is i n d e p e n d e n t o f t h e f l o w Re yno l d s numbe r . Th i s s i g n i f i e s t h a t , under t h i s f l o w r e g i m e , v i s c o u s f o r c e s . a r e n o t r e l e v a n t . It is t h e r e f o r e p o s s i b l e t o o b t a i n a p p r o x i m a t e m e c h a n i c a l s i m i l a r i t y p r o v i d e d t h a t t h e r o u g h n e s s l e n g t h is p r e s e r v e d ( t h a t shou ld be p r o v i d ed by t h e g e o m e t r i c a l s c a l i n g o f t h e b e d m a t e r i a l s i z e d i s t r i b u t i o n ) and t h a t t h e l im i t i n g c o n d i t i o n r e p r e s e n t e d by t h e dashed l ine on f i g u r e 3 is r e s p e c t e d . T he l i m i t b e t w e e n r e s p e c t i v e l y h y d r a u l i c a l l y r o u g h a n d t r a n s i t i o n a l r e g i o n s on f i g u r e 3 c an be a p p r o x i m a t e d by t h e f o l l o w i n g f o r m u l a (Rouse , 1959) : 4 Re ( f f ) 0 - 5 k s / 4 R > 200 (2.23) If t h e l a t t e r l im i t i n g c o n d i t i o n o r c r i t e r i o n is r e s p e c t e d , t h e f r i c t i o n f a c t o r is p r e s e r v e d . N e v e r t h e l e s s , t h e s i m i l a r i t y is s a i d t o be a p p r o x i m a t e s i n c e t h e Reyno l d s number is s omehow s a c r i f i e d . This howeve r does n o t r e p r e s e n t a ma jo r d rawback i f t h e phenomena t o be l ooked a t in t h e mode l do no t s t r o n g l y depend on smal l s c a l e t u r b u l e n c e , such as is t h e c a s e f o r t h e v e l o c i t y p r o f i l e ( M i d d l e t o n and S o u t h a r d , 1984) o r f o r l a r g e s c a l e e d d i e s a nd s e c o n d a r y f l o w s ( Y a l i n , 1982) . When t h e Reyno lds number is i n c r e a s ed f r o m a mode l t o a p r o t o t y p e , t h e t u r b u l e n t ene rgy c a s c ade (or t h e t r a n s f e r o f t u r bu l e n c e f r o m t h e l a r g e r t o t h e s m a l l e r s c a l e s ) is l e n g t h e n e d and p r o p o r t i o n a l l y s m a l l e r s c a l e e d d i e s (Ya l i n , 1982) d e v e l o p whi le t h e s t r u c t u r e o f t h e t u r b u l e n c e a t l a r g e r s c a l e s is n o t much changed . This c o n v e n i e n t p h enomenon f r o m t h e p e r s p e c t i v e o f p h y s i c a l mode l l i ng is r e f e r r e d t o as t h e Reyno l d s numbe r s i m i l a r i t y (M idd le ton and S o u t h a r d , 1 9 8 4 ) . 2.5.2 R e p r o d u c t i o n o f s e d i m e n t e n t r a i n m e n t c o n d i t i o n s and e x c e s s s t r e s s The m a t t e r o f r e p r o d u c i n g t h e s e d i m e n t e n t r a i n m e n t c o n d i t i o n s is s t r o n g l y r e l a t e d t o t h a t o f f r i c t i o n a l c h a r a c t e r i s t i c s as p r e s e n t e d a b o v e . It is h o w e v e r a p p r o a c h e d f r o m a d i f f e r e n t p e r s p e c t i v e w h i c h f u r t h e r q u a l i f i e s t h e l im i t i n g c o n d i t i o n s w h i c h f r a m e p h y s i c a l m o d e l l i n g . In t h e c a s e o f t h e r e p r o d u c t i o n o f e n t r a i n m e n t c ond i t i o n s one is c o n c e r n e d a b o u t t h e f l o w cond i t i o n s a round each p a r t i c l e on t h e s t r e a m b e d . The r e p r o d u c t i o n o f e n t r a i n m e n t c ond i t i o n s m u s t i n v o l v e a c o r r e c t a l b e i t n o t n e c e s s a r i l y e x a c t s c a l i n g o f t h e p r e s s u r e f i e l d a r o u n d any p a r t i c l e on t h e s t r e a m b e d , a phenomenon wh i ch is d e t e r m i n e d by t h e comb ina t i on o f d r ag and l i f t f o r c e s . in h is c l a s s i c s t u d y , Sh i e l d s (1936) p r e s e n t e d t h e r e s u l t s o f a p h y s i c a l a n a l y s i s o f t h e s e d i m e n t t r a n s p o r t p r ob l em . Two d i m e n s i o n l e s s v a r i a b l e s w e r e p r o p o s e d in o r d e r t o d e s c r i b e t h e s e d i m e n t e n t r a i n m e n t c o n d i t i o n s i.e. a p a r t i c l e Re yno l d s number v. k s Rp = ( 3 . 2 4 ) v p l u s a v a r i a b l e w h i c h e x p r e s s e d t h e b a l a n c e b e t w e e n t h e p r o p e l l i n g f o r c e and t h e we igh t o f a p a r t i c l e , i.e. R S F s = ( 2 . 2 5 ) (8 -1 ) D w h e r e s r e p r e s e n t s t h e s u b m e r g e d s p e c i f i c w e i g h t o f s e d i m e n t . F r om low t o high p a r t i c l e Reyno lds number , as in t h e c a s e o f a v e r a g e f l o w a n a l y s i s ( s e c t i o n 2 .5 .1) , t h e f l o w c o n d i t i o n s a r o u n d a g r a i n on t h e s t r e a m bed a r e s u c c e s s i v e l y l am ina r , s m o o t h t u r b u l e n t , t r a n s i t i o n a l o r f u l l y r o u g h . T h e s e f o u r f l o w s t a t e s c an be r e c o g n i z e d in t h e Sh ie lds d iagram, o f which t h e l a t e s t v e r s i o n p r e s e n t e d by Ya l in and Ka r ahan (1979a) wi l l be c o n s i d e r e d h e r e i n . H y d r a u l i c a l l y r o u g h c o n d i t i o n s a r e f o u n d in t h e f i e l d s i t u a t i o n and h e n c e a r e d e s i r e d in t h e m o d e l . R e c e n t Sh i e l d s d i a g r ams r e v e a l t h a t t h e p a r a m e t e r o f e q u a t i o n (2 . 25 ) un ique ly , d e t e r m i n e s t h e e n t r a i n m e n t c o n d i t i o n s f o r p a r t i c l e R e y n o l d s n u m b e r a b o v e 7 0 ( Y a l i n , 1971a) . S t a r t i n g w i t h t h e l a t t e r c r i t e r i o n and a f t e r s o m e m a n i p u l a t i o n , f o r an u n d i s t o r t e d , g e o m e t r i c a l l y s im i l a r mode l , t h e f o l l o w i n g e x p r e s s i o n c an d e f i n e t h e l imit (Yal in, 1971a) f o r t h e s ca l i ng f a c t o r o f t h e mode l ; 7 0 X L " 1 = [ j O . 6 7 ( 2 . 2 6 ) ( V „ ' K S » / v ) w h e r e ' d e n o t e s p r o t o t y p e v a l u e s . A L is t h e l e n g t h r a t i o o f t h e p r o t o t y p e t o t h e mode l . The p r e s e r v a t i o n o f t h e l a t t e r l im i t i n g c o n d i t i o n o r c r i t e r i o n t o g e t h e r w i t h t h e use o f F r o u d i a n s c a l i n g l aws e n s u r e s t h a t t h e m o b i l i t y n u m b e r i .e. T g / T j ( w h e r e T C i s t h e c r i t i c a l s h e a r s t r e s s f o r s e d i m e n t e n t r a i n m e n t as f o u n d f r o m t h e S h i e l d s d i ag r am) is p r e s e r v e d in t h e mode l . The p r e s e r v a t i o n o f t h e mob i l i t y number a p p e a r s t o be o f impo r t a n c e t o t h e a p p r o p r i a t e r e p r o d u c t i o n o f b e d f o r m g e o m e t r y and c o n s e q u e n t l y o f f o r m r o u g h n e s s ( Y a l i n a nd K a r a h a n , 1 9 7 9 b ) . T he l a t t e r i s s u e is m o s t l y i m p o r t a n t f o r s and b ed r i v e r s , as d i s c u s s e d by Ya l i n (1982). 2.5.3 Rep r oduc t i o n o f f l o w p a t t e r n s and bed shea r s t r e s s d i s t r i b u t i o n A c o r o l l a r y o f m e c h a n i c a l s i m i l a r i t y in open c h a n n e l s is t h a t a t homologous po i n t s and homo logous t imes b e t w e e n t h e mode l a nd t h e p r o t o t y p e t h e f l o w l i n e s a r e g e o m e t r i c a l l y s i m i l a r ( k i n e m a t i c s i m i l a r i t y ) and t h e f o r c e s a c t i n g on t h e b e d a r e a p p r o p r i a t e l y s c a l e d ( d y n a m i c a l s i m i l a r i t y ) . H o w e v e r , i t is v e r y r a r e l y v e r i f i e d i f t h e mode l c o n f o r m s t o t h e p r o t o t y p e in t h o s e r e s p e c t s . In t h e e v e n t u a l i t y o f mode l s t u d i e s o f r i v e r p r o c e s s e s , t h e i s s ue is o f u t m o s t i m p o r t a n c e i f g e n e r a l i z a t i o n o f t h e r e s u l t s is f o r e s e e n . The p r o t o t y p e h y d r o d y n a m i c m e a s u r e m e n t s c o n s i s t o f t h e b a s i c i n f o r m a t i o n f o r t h e c o m p a r i s o n w i t h t h e mode l s i t u a t i o n : t h e s e p r i n c i p a l m e a s u r e m e n t s a r e t h e v e l o c i t y m e a s u r e m e n t s and t h e a s s o c i a t e d s h e a r e s t i m a t e s o b t a i n e d v ia e q u a t i o n (2.21) and i t s d i s t r i b u t i o n . To e f f e c t a c l o s e r t e s t on p o s s i b l e e f f e c t s o f t h e a p p r o x i m a t e m e c h a n i c a l s i m i l a r i t y p r o v i d e d by F r o u d i a n m e t h o d s o f s c a l i n g , t h e f l o w s t r u c t u r e mus t a l so be c o n s i d e r e d . These mea su r emen t s we r e p e r f o r m e d f o r t h e f i e l d and t h e mode l s i t u a t i o n . 2.6 Gene r i c mode l c o n c e p t a nd g e o m o r p h o l o g i c a l r e s e a r c h B o t h s e c t i o n s 2.5.1 and 2.5.2 p r e s e n t e d e x p l i c i t l i m i t i n g c o n d i t i o n s o r c r i t e r i a w h i c h r e s p e c t i v e l y c o n s t r a i n e d t h e a b s o l u t e s i ze o f t h e p r o t o t y p e and t h e a b s o l u t e s i ze o f t h e s e d i m e n t s p r e s e n t on i t s b e d . In f a c t , i t is e f f e c t i v e l y impos s i b l e t o p e r f o r m l a b o r a t o r y i n v e s t i g a t i o n wh ich c o u l d l e ad t o s c i e n t i f i c g e n e r a l i z a t i o n f o r m o s t p r o t o t y p e s i t u a t i o n s o f f i n e b e d , l a r g e s c a l e r i v e r s . In t h e l a t t e r c a s e s , i t is impo s s i b l e t o s a t i s f y in o r d i n a r y c i r c u m s t a n c e s e q u a t i o n ( 2 . 2 3 ) and (2 .26) b e c a u s e o f t h e l a r g e s c a l e d i f f e r e n c e b e t w e e n t h e c u s t o m a r y models- and p r o t o t y p e s . H o w e v e r , in t h e e v e n t u a l i t y t h a t t h e s c a l i n g r a t i o is r a t h e r sma l l , i t may be p o s s i b l e t o mode l s and b ed r i v e r p r o c e s s e s by us ing warmer w a t e r in t h e mode l and e ven t o m e e t t h e s t r i c t m e c h a n i c a l s i m i l a r i t y r e q u i r e m e n t s . Th i s s t r a t e g y w i l l p r o d u c e a d e c r e a s e in w a t e r v i s c o s i t y and p r o v i d e a d d i t i o n a l a l b e i t l i m i t e d f r e e d o m t o t h e t h e e x e r c i s e . S o u t h a r d , Boguchwa l and Romea (1980) d e m o n s t r a t e d t h e s u c c e s s o f t h e l a t t e r m e t h o d by a c o m p a r i s o n o f r i p p l e g e o m e t r i c a l p a r a m e t e r s f r o m f i e l d a n d l a b o r a t o r y s t u d i e s . C o m p l e t e m e c h a n i c a l s i m i l a r i t y c a n e v e n t u a l l y be o b t a i n e d i f t h e s c a l i n g r a t i o is sma l l e r t h a n or equal t o abou t 2.5 by t a k i ng a d v a n t a g e o f t h e change in w a t e r v i s c o s i t y f r o m 10 t o 90 d e g r e e s Ce l c ius . In t h e c a s e o f smal l and medium s c a l e g r a v e l bed r i v e r s , i t is p o s s i b l e t o c o n s i d e r r e d u c i n g t h e s i ze o f t h e bed sed imen t wh i l e p r e s e r v i n g t h e f u l l y r o u g h f l o w c o n d i t i o n s a t t h e t h r e s h o l d o f t r a n s p o r t . F o r a s e r i e s o f s i t u a t i o n s , t h e f o r c e s wh ich a c t in n a t u r a l s t r e a m s c an a p p a r e n t l y be a p p r o p r i a t e l y r e p r o d u c e d when some l im i t i n g c o n d i t i o n s a r e r e s p e c t e d ( s e c t i o n 2.5.1). In s u c h c i r c u m s t a n c e s , s c i e n t i f i c i n v e s t i g a t i o n s in t h e l a b o r a t o r y wou ld be l e g i t i m a t e . An e x t e n s i o n o f t h i s c u r r e n t h y p o t h e s i s l e a d s us t o t h e i n t r o d u c t i o n o f t h e c o n c e p t o f a g e n e r i c m o d e l . In an e t y m o l o g i c a l s e n s e , " g e n e r i c " s t a n d s f o r an e l e m e n t wh ich is p a r t o f a who le , a f am i l y . By e x t e n s i o n , a gene r i c mode l is one wh ich is r e p r e s e n t a t i v e o f a f a m i l y in wh ich i nd i v i dua l s s h a r e s o m e t h i n g in c ommon . If a l a b o r a t o r y c h a n n e l r e s p e c t s t h e l im i t i n g c o n d i t i o n s wh i ch w e r e e x p o s e d in t h i s c h a p t e r , i t c an t h u s be p a r t o f a f am i l y o f s i t u a t i o n s and can be used t o i n v e s t i g a t e p h y s i c a l p r o c e s s e s in a q u a n t i t a t i v e m a n n e r . A g e n e r i c m o d e l l i n g f r a m e w o r k w o u l d n o t n e c e s s i t a t e t h e r e p r o d u c t i o n o f some e x a c t b o u n d a r y cond i t i on s , in c o n t r a s t w i th t h e n o rma l e n g i n e e r i n g a p p r o a c h t o mode l s . The c o n c e p t o f g e n e r i c mode l l i n g was a p p r o a c h e d by Hooke (1968) in a d i s c u s s i o n o f B r uun ' s (1966) p a p e r . Hooke a r g u e d t h a t g e o m o r p h o l o g i s t s a r e s e l d o m i n t e r e s t e d in t h e s t udy o f a p a r t i c u l a r s y s t e m b u t in g e n e r a l p r i n c i p l e s wh ich app ly t o a p o p u l a t i o n o f s y s t e m s . He t h e r e f o r e p r o p o s e d a g e n e r a l l a b o r a t o r y p r o c e d u r e r e f e r e d t o as " s i m i l a r i t y o f p r o c e s s " . The b a s i c r e q u i r e m e n t s and p r o p e r t i e s o f " s i m i l a r i t y o f p r o c e s s " a re t h a t (1) t h e g r o s s s c a l i n g r e l a t i o n s h i p s be met ; (2) t h e mode l r e p r o d u c e s some m o r p h o l o g i c c h a r a c t e r i s t i c s o f t h e p r o t o t y p e ; and (3) t h e p r o c e s s e s wh ich p r o d u c e d t h e mode l ' s c h a r a c t e r i s t i c s can l o g i c a l l y be a s s umed t o h a v e t h e same e f f e c t in t h e p r o t o t y p e . Hooke f u r t h e r s t a t e d : " In t h i s t y p e o f s t ud y t h e l a b o r a t o r y s y s t e m s a re t r e a t e d as smal l s y s t e m s in t h e i r own r i g h t , n o t as s c a l e mode l s o f p r o t o t y p e s . " ( H o o k e , 1968, p . 392 ) Hooke 's s t a t e m e n t goes beyond Schumm e t al (1987) v iews on t h e use o f s c a l e mode l s d e s c r i b e d in t h e i n t r o d u c t i o n . However t h e r e q u i r e m e n t s o f H o o k e ' s " s i m i l a r i t y o f p r o c e s s " i n t r o d u c e d a b o v e do n o t e n c o m p a s s t h e c r i t e r i a f o r m e c h a n i c a l s i m i l a r i t y wh ich w e r e a n a l y t i c a l l y d e v e l o p e d in t h i s c h a p t e r and wh ich p r o v i d e e x p l i c i t l y l i m i t i n g c o n d i t i o n s f o r s c a l i n g o p e r a t i o n s . T h e g e n e r i c mode l c o n c e p t i n c l u d e s in a d d i t i o n t h e r e c o g n i t i o n o f s o m e p r o c e s s - m a g n i t u d e r e l a t i o n n o t f o r m a l l y r e c o g n i z e d in Hooke ' s s i m i l a r i t y o f p r o c e s s . The v a l i d i t y o f t h e g e n e r i c m o d e l f r a m e w o r k w i l l be c o n f i r m e d only when i t has been d e m o n s t r a t e d t h a t t h e p r o c e s s e s can be t h e same in t h e mode l and t h e p r o t o t y p e . Such a t e s t a p p e a r s t o be n e c e s s a r y in o r d e r t o e n s u r e t h a t l a b o r a t o r y r e s u l t s wi l l n o t be r e c e i v e d s c e p t i c a l l y in t h e g e o m o r p h o l o g i c a l c o m m u n i t y . T h e r e f o r e , a l o g i c a l f i r s t s t e p b e f o r e f u n d a m e n t a l r e s e a r c h is b e i n g u n d e r t a k e n u n d e r t h e t e n e t o f g e n e r i c mode l l i n g is t o v e r i f y i f w h e t h e r o r n o t t h e t h e o r e t i c a l b a c k g r o u n d app l i e s in a p a r t i c u l a r s i t u a t i o n . 3.0 F i e l d s t u d y 3.1 S i t e s e l e c t i o n , l o c a t i o n and d e s c r i p t i o n The n a t u r e o f t h i s r e s e a r c h p r o j e c t p r e d e t e r m i n e d a number o f c r i t e r i a f o r f i e l d s i t e s e l e c t i o n . The s ize o f t h e s t r e a m and o f i t s bed m a t e r i a l we r e i m p o r t a n t f o r an e v e n t u a l s ca l i ng e x e r c i s e in a 0 .5 m e t r e w ide f l u m e . T h e r e f o r e , t h e s t r e a m n e e d e d t o be sma l l - ( a b o u t 5 t o 8 m e t r e s in w i d t h ) w i t h w e l l - d e v e l o p e d a l l u v i a l p oo l and r i f f l e f o r m s and a l s o n e e d e d t o bea r r e l a t i v e l y c o a r s e bed m a t e r i a l , mo s t l y in t h e g r a v e l t o c obb l e r ange . The f o r m e r r e s t r i c t i o n r e g a r d i n g t h e s i ze o f t h e s t r e a m f o l l o w s f r o m o p e r a t i o n a l a n d i n s t r u m e n t a l l i m i t a t i o n s . The l a t t e r r e s t r i c t i o n was i m p o r t a n t t o e n s u r e t h a t t h e beer m a t e r i a l c o u l d be s c a l e d in t h e l a b o r a t o r y t o manageab le s i zes ( i .e . f i n e s a n d s and c o a r s e r ) . B e c a u s e o f t h e r e s t r i c t i o n s e t by l a b o r a t o r y mode l l i n g , t h e s e l e c t e d s t r e a m r e a c h wou ld a l s o n e e d t o be n e a r l y s t r a i g h t . F i n a l c r i t e r i a w e r e t h e a c c e s s i b i l i t y o f t h e s i t e f r o m t h e campus o f t h e Un i v e r s i t y o f B r i t i s h C o l u m b i a in V a n c o u v e r , a s w e l l a s i t s r e l a t i v e s e c l u s i o n , f o r s a f e t y r e a s o n s . A sma l l s t r e a m in t h e l o w e r C o a s t Mou t a i n s in t h e Un i v e r s i t y o f B r i t i s h Columbia Reseach F o r e s t , a bou t 5 0 Km e a s t o f Vancouve r , was f o u n d t o b e s t m e e t t h e above r e qu i r emen t s . A 25 m e t r e s long r e a c h o f B laney c r e e k , u p s t r e a m o f B laney lake ( s ee f i g u r e 4) was s e l e c t e d . The r e a c h , l o c a t e d d o w n s t r e a m f r o m a b e d r o c k ' o u t c r o p has an a v e r a g e w id th o f 7 m, is nea r l y s t r a i g h t a n d b e a r s w e l l - d e v e l o p e d a l l u v i a l p o o l - r i f f l e f e a t u r e s . The p r e p a r a t i o n o f t h e f i e l d s i t e i n v o l v e d p run i ng o f o v e r h a n g i n g v e g e t a t i o n , mapp ing , b e d m a t e r i a l s amp l i ng and i n s t a l l a t i o n o f b r i d g e s and r e f e r e n c e c a b l e s a c r o s s t h e c r e e k f o r t h e p u r p o s e o f f a c i l i t a t i n g v e l o c i t y m e a s u r e m e n t s . C o n v e n t i o n a l s u r v e y m e t h o d s w e r e u s e d t o p r o d u c e a t o p o g r a p h i c a l map ( f i g u r e 5) f r o m abou t 2 0 0 po i n t s . The map shows t h e p r i n c i p a l c o n f i g u r a t i o n a l e l e m e n t s o f t h e r e a c h which wi l l be d e s c r i b e d be low. These can be f u r t h e r a p p r e c i a t e d f r o m f i g u r e 6. A t r a n s v e r s e l og c o n t r o l s t h e d o w n s t r e a m bed l e v e l o f t h e s t ud y r e a c h . The r i g h t bank o f t h e c r e e k is comp lex in f o r m , gene ra l l y v e r t i c a l and unde r c u t as we l l as p a r t l y made o f and r e i n f o r c e d by o r g a n i c d e b r i s . Immed i a t e l y d o w n s t r e a m o f t h e o u t c r o p t h e main f l o w impinges on t h e l e f t r e i n f o r c e d bank. The r e , a l a r ge t r e e s tump ( t h e a r e a was l ogged in 1959) ho lds c o a r s e m a t e r i a l b e t w e e n i t s r o o t s . Abou t 3-4 m e t r e s be low th i s f e a t u r e t h e l e f t bank t u r n s a l l u v i a l a nd c o n s t i t u t e s t h e e x t e n s i o n o f t h e r i v e r b e d l o c a l l y t o p p e d w i t h some o v e r b a n k s a nd d e p o s i t s . The b a n k f u l l l e v e l i s n o t p a r t i c u l a r l y w e l l d e f i n e d a l ong t h e c r eek , due t o t h e c omp l e x i t y o f t h e banks , b u t a f e w i n d i c a t o r s ( c u t b a n k s , o v e r b a n k f i n e d e p o s i t s , l im i t o f v e g e t a t i o n ) s t i l l a l low i t s d e t e r m i n a t i o n . Six i m p o r t a n t f e a t u r e s (numbered f r o m 1 t o 6 on f i g u r e 5) can be r e c ogn i z ed on t h e r i v e r bed ( r e f e r t o f i g u r e s 5 and 6). C o n t o u r ( e l e v a t i o n i n cm ) B a n k f u l l c h a n n e l l i m i t L a r g e o r g a n i c d e b r i s B a n k u n d e r m i n i n g l i m i t S u r f a c e b e d m a t e r i a l g r i d s a m p l e s Bedrock B e d u n i t s F i g u r e 6. V i e w s o f B l a n e y C r e e K f i e l d s i t e ( a ) L o o k i n g u p s t r e a m , f r o m t h e t r a n s v e r s e l o g . I n t h e f o r e g r o u n d , r i f f l e a n d d i s t a l p o o l . N o t e t h e c h a r a c t e r o f t h e o v e r h a n g i n g r i g h t b anK . T r i p o d o n b e d r o c K o u t c r o p i n t h e b a c k g r o u n d i n d i c a t e s f r o m w h e r e t h e n e x t p h o t o g r a p h was s h o t . ( b ) L o o k i n g d o w n s t r e a m , f r o m t h e b e d r o c k o u t c r o p . I n t h e f o r e g r o u n d , p r o x i m a 1 p o o l ( n o t e c o a r s e b e d m a t e r i a l ) , c r o s s - o v e r a r e a a n d f i n e r s e d i m e n t a c c u m u l a t i o n b a r . I n t h e b a c k g r o u n d , man s t a n d s on t r a n s v e r s e l o g w h i c h i s t h e d o w n s t r e a m l i m i t o f t h e m a p p e d r e a c h 41 A t t h e u p s t r e a m l imi t o f t h e s t ud y r e a c h , (1) a i m r e l i e f b ed r o c k o u t c r o p is f o u n d t r a n s v e r s e t o t h e c r e e k . Al l o f t h e w a t e r a t low f l o w s t a g e is d i s c h a r g ed on i t s l ower l e f t s ide . A t h igh s t a g e s t h e o u t c r o p is e n t i r e l y c o v e r e d by w a t e r a l t hough mos t o f t h e w a t e r s t i l l f l o w s on i t s l e f t . The upper r i g h t s i de o f t h e o u t c r o p is a c t u a l l y t o p p e d by v e g e t a t e d g r a v e l l y d e p o s i t s ( n o t s h o w n on t h e map) wh i c h f u r t h e r e n c o u r a g e s m o s t o f t h e w a t e r t o f l o w v i a t h e l e f t s i d e . Immediate ly be low t h e o u t c r o p , on t h e l e f t s ide o f t h e c r e e k , a poo l ( t h e p rox ima l poo l - 2) has d e ve l o ped : i t s or ig in is t o be r e l a t e d t o t h e s t u m p t o w a r d s wh i ch t h e main t h r e a d o f u p s t r e a m f l o w is o r i e n t e d . The p r o x i m a l p oo l b e d is e i t h e r b ed r o c k or c o v e r e d by l a r ge bou l de r s (some o f which show up on t h e map) espec i a l l y a t i t s d o w n s t r e a m end . On t h e l e f t s ide and d o w n s t r e a m end o f t h e p r o x i m a l p o o l an a c c u m u l a t i o n o f c o a r s e m a t e r i a l r e i n f o r c e d w i t h some o r g a n i c d e b r i s p r oduce s a mound wh i ch is e a s i l y d i s c e r n a b l e on t h e t o p o g r a p h i c map (3 ) . O p p o s i t e t h e p r o x i m a l p o o l n e a r t h e r i g h t b ank a s m a l l a c cumu l a t i on bar (4) has d e ve l o ped on t h e edge o f t h e b ed r o c k ou t c r op , j u s t above t h e a r e a where t h e f l o w c r o s s e s o v e r t o t h e s e c o n d (o r d i s t a l ) p oo l a t low f l o w s t a g e s . The m a t e r i a l o f t h i s ba r and a d j a c e n t c r o s s - o v e r a r e a is c l e a r l y f i n e r t h a n t h a t o f t h e (5) r i f f l e and (6) d i s t a l poo l . F i na l l y , t h e r i f f l e and t h e d i s t a l poo l r e p r e s e n t t h e t w o f e a t u r e s o f m o s t i n t e r e s t h e r e i n . The r i f f l e which c o n s t i t u t e s t h e e x t e n s i o n o f t h e p r o x i m a l p oo l s h oa l i n g is o f d i a gona l n a t u r e and has w e l l - d e f i n e d c r e s t and s l i p f a c e which a r e abou t 15 m e t r e s l o n g . The d i s t a l p oo l l o c a t e d a l o ng and subpa ra l l e l 42 t o t h e r i g h t bank g radua l l y widens t o w a r d s i t s d o w n s t r e a m end wh ich is f i x e d by t h e t r a n s v e r s e l o g . The f l o w mea su r emen t s were t a k e n o v e r t h e s e f e a t u r e s . 3 .2 B e d m a t e r i a l s a m p l i n g To a v o i d d i s r u p t i n g t h e s t a b i l i t y o f t h e b ed f e a t u r e s w i th in t h e s e l e c t e d s t ud y r e a c h , i t was n e c e s s a r y t o sample t h e b ed m a t e r i a l on t h e n e x t d o w n s t r e a m r i f f l e . A c o m p l e t e bed m a t e r i a l s amp l i ng ( i . e . v o l u m e t r i c , s u r f a c e l a y e r , a r e a l a nd s u r f a c e g r i d ) was p e r f o r m e d a t t h a t l o c a t i o n , f o l l o w i n g t h e p r o c e d u r e s d e s c r i b e d in C h u r c h e t a l (1987) . M o r e o v e r s u r f a c e g r i d samples we re t a k e n on t h e r i f f l e and in t h e d i s t a l poo l w i t h i n t h e s t u d y r e a c h . The l a t t e r samp les would a s c e r t a i n by c o m p a r i n g g r i d s u r f a c e s a m p l e s i f t h e c o m p l e t e s a m p l i n g p r o g r a m m e a t t h e d o w n s t r e a m l o c a t i o n c o u l d be c o n s i d e r e d r e p r e s e n t a t i v e o f t h e s i t e ' s c o n d i t i o n s . A 1 X 1 m gr id sample was used t o sample 25 s t one s on t h e s u r f a c e o f t h e r i f f l e s i t e d o w n s t r e a m o f t h e s t u d y r e a c h . A f t e r t h i s samp le was c o m p l e t e d and t h e s t o n e s s a ved t o be inc luded in t h e n e x t a r e a l sample, t h e m a t e r i a l w i th in t h e 1 X 1 m a r e a was p a i n t e d and l a t e r c o l l e c t e d . A f t e r w a r d s t h e d e p t h o f t h e l a r g e s t s t o n e which d e t e r m i n e s t h e s u r f a c e l aye r d e p t h was n o t e d and t h i s s amp l e a c c o r d i n g l y e x c a v a t e d . F i na l l y , a v o l u m e t r i c (bulk) sample was t a k e n a f t e r t h e s u r f a c e l aye r had b e e n c l e a r e d . No b e d d i n g c h a n g e was e n c o u n t e r e d d u r i n g e x c a v a t i o n . The s u b s u r f a c e m a t e r i a l , m o s t l y g r a n i t e , was n o t e d t o be o x i d i z e d a l b e i t n o t d e c a y e d . In t h e l a b o r a t o r y , samples w e r e o v e n - d r i e d and s i e v e d us i ng c o n v e n t i o n a l p r o c e d u r e s . To e n a b l e c o m p a r i s o n w i t h o t h e r s a m p l e s w h i c h a r e d i m e n s i o n a l l y e q u i v a l e n t , t h e a r e a l s a m p l e s i z e d i s t r i b u t i o n was a d j u s t e d f o l l o w i n g t h e m e t h o d p r o p o s e d by K e l l e r h a l s and B r a y (1971). F i g u r e 7 p r e s e n t s t h e r e s u l t s o f t h e s i e v e a na l y s i s o f t h e s e d i m e n t s amp l e s f r o m t h e d o w n s t r e a m r i f f l e l o c a t i o n . The d e f i c i e n c y in c o a r s e m a t e r i a l a t t h e s u r f a c e w i t h r e s p e c t t o t h e s u b s u r f a c e m a t e r a l is e spec i a l l y e v i d e n t i f o ne c o n s i d e r s t h e g r i d s a m p l e . T he s u r f a c e l a y e r a nd v o l u m e t r i c s a m p l e s show s i m i l a r d i s t r i b u t i o n s . In t h e s t udy r e a c h , f i v e (on t h e r i f f l e ) and f o u r (in t h e d i s t a l poo l ) 1 X 1 m g r i d samp les o f 25 c l a s t s ea ch we r e t a k e n . The samp l i ng s t a t i o n s a r e i n d i c a t e d on f i g u r e 5. No s p a t i a l t r e n d in s i z e d i s t r i b u t i o n c h a r a c t e r i s t i c s c o u l d be d e t e c t e d on e i t h e r f e a t u r e . H o w e v e r , one a d d i t i o n a l s amp l e t a k e n nea r t h e c r o s s - o v e r a r e a s h o w e d a c l e a r l y f i n e r s i z e d i s t r i b u t i o n t h a n any o t h e r s amp l e . F i g u r e 8 c o m p a r e s t h e s i z e d i s t r i b u t i o n o f w i t h i n r e a c h poo l and r i f f l e b e d s u r f a c e m a t e r i a l s amp l e s as wel l as t h a t o f t h e r i f f l e s i t e d o w n s t r e a m o f t h e s t u d y r e a c h . The c o m p a r i s o n o f t h e s ize d i s t r i b u t i o n s w i th in t h e s t udy r e a c h r e v e a l t h a t r i f f l e m a t e r i a l is s l i g h t l y c o a r s e r in g ene r a l t h a n t h e poo l bu t t h a t median d i a m e t e r o f b o t h s ize d i s t r i b u t i o n a r e a l m o s t i d e n t i c a l . The s i ze d i s t r i b u t i o n o f t h e g r i d s a m p l e f r o m t h e r i f f l e d o w n s t r e a m o f t h e s t u d y r e a c h is n o t much d i f f e r e n t t h a n b o t h p r e v i o u s s a m p l e s , e s p e c i a l l y in t h e c o b b l e r a n g e . Th i s v a l i d a t e s c o n s i d e r a t i o n o f t h e v o l u m e t r i c s amp l e f r o m t h e d o w n s t r e a m a r e a as r e p r e s e n t a t i v e o f t h e s t ud y r e a c h s u b s u r f a c e m a t e r i a l . T h e r e f o r e , i t was d e c i d e d t o u se t h e 44 Figure 7. Size distribution! of bod material samples at downstroam rlfflo slto, Blanoy Crook Slovo slzo (in phi) Figure 8. Comparison of pooled slzo distribution* for grid ssmpios from downstroam riffle and within reach pool and riffle sites, Blsney Creek -9.0 -7.0 -8.0 -3.0 Sieve size (In phi) d o w n s t r e a m bulk s amp l e and t h e w i t h i n r e a c h poo l and r i f f l e g r i d . s amp l e s f o r s c a l e mode l l i n g c o n s i d e r a t i o n s . S o m e g r a i n s i ze d i s t r i b u t i o n p a r a m e t e r s f o r t h e s e s amp l e s a r e g i v en in tab le 1. The n o m o g r a p h s o f C h u r c h e t a l (1987) s u g g e s t t h a t a l l s amp l e s w e r e a c t u a l l y t o o sma l l f o r c o n f i d e n t c h a r a c t e r i z a t i o n ( i .e . t h e l a r g e s t s t o n e s hou l d r e p r e s e n t 0.1 V. o f t h e sample w e i g h t ) o f t h e b e d m a t e r i a l . The r e p r e s e n t a t i v e n e s s o f t h e v o l ume t r i c sample is an i m p o r t a n t i ssue in t h e c o n t e x t o f s c a l e mode l l i n g o f b e d m a t e r i a l . The l a r g e s t c l a s t o f t h e s i z e d i s t r i b u t i o n o f t h e bulk s amp l e ( a b o u t 128 mm) r e p r e s e n t e d abou t 4 p e r c e n t o f t h e t o t a l sample we igh t ( a c co rd i ng t o Church e t a l , 1987), a s i t u a t i o n which was j u dged t o be r e a s onab l e ( i t is in f a c t common in many f i e l d mea su r emen t p rog rammes ) a l b e i t n o t i d e a l . 3.3 M e a s u r e m e n t a r r a n g e m e n t s , i n s t r u m e n t s a nd e s t i m a t i o n o f  s h e a r s t r e s s In o r d e r t o p e r f o r m m e a s u r e m e n t s s a f e l y a t h igh f l o w s , t h r e e t r a n s v e r s e 60 cm wide, 7 m long aluminum b r i dges we re la id a c r o s s t h e c r e e k a t abou t s ix m e t r e i n t e r v a l s . They we re l a n d e d on t h e t o p o f t h e r i g h t bank and moun t ed on p i l l a r s along t h e l e f t bank. A f o u r t h b r i dge 40 cm wide, 7 m long was moved a round on t h e t h r e e f i x e d b r i dge s and o n t o one land ing on a p iece o f wood deb r i s on t h e r i g h t bank t o al low mea su r emen t s a t i n t e r m e d i a t e l o c a t i o n s and in t h e c r o s s - o v e r a r e a . A s e t o f n ine r e f e r e n c e l e v e l l e d c ab l e s was i n s t a l l e d a t 46 T a b l e 1 G r a i n s i z e d i s t r i b u t i o n p e r c e n t i l e d i a m e t e r s ( i n mm ) f o r s e l e c t e d b e d m a t e r i a l s a m p l e s , B l a n e y c r e e k Samp 1e G r i d ( d o w n s t r e a r n s i t e ) V o 1 u m e t r i c ( B u l k ) Gr i d ( s t u d y r e a c h ) P o o l R i f f l e P e r c e n t i 1 e s D1 6 D35 D50 D65 D84 D90 2 6 . 9 4 1 . 6 51 . 3 6 1 . 8 99 . 0 1 1 2 . 2 3 . 1 1 1 . 3 2 5 . 1 4 3 . 4 9 1 . 8 9 7 . 7 1 5 . 4 3 0 . 7 4 7 . 8 5 9 . 7 8 3 . 9 9 7 . 7 2 1 . 0 34 . 8 49 . 9 6 5 . 8 1 1 9 . 4 1 4 8 . 1 about 2.5 m above t h e l o w e s t po in t on t h e bed . F rom t h e cab l e n e t w o r k , w a t e r s u r f a c e and b ed e l e v a t i o n s c o u l d be d e t e r m i n e d f o r e a c h v e l o c i t y s a m p l i n g s t a t i o n . T h e l a t t e r e l e v a t i o n s c o u l d be c o m p a r e d w i t h t o p o g r a p h i c a l map e l e v a t i o n s wh i le t h e f o r m e r were u s e f u l t o e s t i m a t e t h e w a t e r s u r f a c e s lope dur ing t h e measu r emen t p rog ramme. A s t a f f gauge was i n t a l l e d in t h e r e a c h t o m o n i t o r t h e c h a n g e s in w a t e r l e v e l d u r i n g t h e m e a s u r e m e n t p e r i o d s . The v e l o c i t y p r o f i l e s w e r e m e a s u r e d w i t h s i x p r o p e l l o r c u r r e n t - m e t e r s (C1 and C2 O t t l a b o r a t o r y c u r r e n t m e t e r s , f i v e wi th 30 mm d i a m e t e r / 1 0 cm p i t c h - maximum v e l o c i t y o f 2 m/s and one w i th 50 mm d i ame t e r / 2 5 cm p i t ch - maximum v e l o c i t y o f 4 m/s) c lamped t o a 3 m long, 20 mm d i a m e t e r r o d (see f i g u r e 9) . Ana log s igna l s we r e t r a n s m i t t e d v ia 15 m c ab l e s t o s ix channe l s on a CR5 p u l s e c o u n t e r d a t a - l o g g e r ( C a m p b e l l S c i e n t i f i c I n s t r u m e n t s ) wh ich was i n s t a l l e d on t h e r i g h t b a n k . T h e s uppo r t i n g r o d had a smal l 10.5 X 6 cm p l a t e a t i t s base t h a t e n su r ed i t would r e s t nea r l y on t h e t o p o f bed p a r t i c l e s . A minimum d i s t a n c e o f t w o t imes t h e p r ope l l o r d i ame t e r f r o m t h e b o u n d a r y and b e t w e e n m e t e r s a p p e a r e d s u f f i c i e n t a c c o r d i n g t o l a b o r a t o r y t e s t s on w a l l p r o x i m i t y e f f e c t s a nd m u t u a l i n t e r a c t i o n s b e t w e e n p r o p e l l e r m e t e r s p e r f o r m e d by Ben i n i (1962) . T h e r e f o r e , t h e f i r s t m e t e r ' s a x i s was s e t a t s ix c e n t i m e t e r s f r o m t h e p l a t e and t hu s abou t s ix c e n t i m e t e r s on a v e r a g e a b o v e t h e t o p o f b e d p a r t i c l e s , wh ich c o r r e s p o n d s r ough l y t o s u r f a c e Dgo/2 ( s e e O90 v a l u e s on t a b l e 2 ) . This was assumed t o be s u f f i c i e n t t o avo id t h e m e t e r be ing in t h e wake o f p r o t r u d i n g b e d p a r t i c l e s . The n e x t t w o m e t e r s ' 48 F i g u r e 9. O t t l a b o r a t o r y c u r r e n t m e t e r r o d - m o u n t e d a r r a y axes were s e t a t 12 and 18 cm r e s p e c t i v e l y f r o m t h e p l a t e . The f i x e d d i s t a n c e b e t w e e n n e a r - b e d c u r r e n t m e t e r s w o u l d a l s o e n s u r e a c c u r a t e d e t e r m i n a t i o n o f l o c a l i n t e r n a l s h e a r . F i n a l l y , t h e t h r e e o t h e r m e t e r s w e r e a p p r o x i m a t e l y l o g a r i t h m i c a l l y s p a c e d t h r o u g h t h e r ema i n i n g w a t e r d e p t h . The upper m e t e r was usual ly s e t a bou t t w o d i a m e t e r s be low t h e w a t e r s u r f a c e . In s h a l l o w e s t f l o w s , some o f t h e t h r e e upper m e t e r s could e i t he r be ou t o f t h e wa t e r or s e p a r a t e d by 6 cm. V e l o c i t y s a m p l i n g s t a t i o n s w e r e d e t e r m i n e d p a r t l y on morpho log i ca l g rounds bu t a l so t o o b t a i n a c o m p l e t e c o v e r a g e o f t h e r i f f l e and d i s t a l poo l . In o r d e r t o o b s e r v e t h e s t r u c t u r e o f t h e f l ow , 5 0 cm f l a g s we re a t t a c h e d t o t h e b o t t o m and near s u r f a c e m e t e r c l amps . The i r o r i e n t a t i o n was measu red f r o m t h e b r i d g e s w i t h a c o m p a s s . It was b e l i e v e d t h a t t h e f o r e g o i n g m e a s u r e m e n t s w e r e a c c u r a t e w i t h i n 2 - 5 d e g r e e s . The f l o w l i n e i n f o r m a t i o n t h r o u g h t h e d e p t h was used t o a l ign t h e s ix O t t m e t e r s w i t h t h e l o c a l f l o w d i r e c t i o n . In a c t u a l i t y p r o p e l l o r c u r r e n t m e t e r s w i l l g i v e a c c u r a t e r e a d i n g s w h e n t h e y a r e o r i e n t e d w i t h i n 15 d e g r e e s f r o m t h e m e a n c u r r e n t a x i a l d i r e c t i o n (A.S.M.E., 1971). H o w e v e r in t h e d e e p e s t , f a s t e s t November 23 f l o w s , t h e n e a r - b e d f l a g was n o t e v e r y w h e r e v i s ib le and n e a r - b e d a l i g n m e n t had t o be j u d g e d f r o m t h a t o f n e i g h b o u r i n g s t a t i o n s . P r o p e i l o r c u r r e n t m e t e r s g e n e r a l l y o v e r e s t i m a t e a x i a l v e l o c i t y p u l s a t i o n s and u n d e r e s t i m a t e t r a n s v e r s e c o m p o n e n t s ( J e p s o n , 1967) . H o w e v e r , i f t h e a v e r a g e a m p l i t u d e o f v e l o c i t y f l u c t u a t i o n o r t u r b u l e n c e i n t e n s i t y is l e s s t h a n 0.2 t imes t h e mean v e l o c i t y , O t t m e t e r e r r o r is l e s s t h a n 1 V. (Kulin and C o m p t o n , 1975) . E x p e r i m e n t a l m e a s u r e m e n t s f o r h i gh r e l a t i v e r o u g h n e s s f l o w s su ch as B laney C r eek (Nowel l and Chu r ch , 1979) s u g g e s t v a l u e s o f t u r b u l e n c e i n t e n s i t y V ' / V * ( V b e i n g t h e a v e r a g e v e l o c i t y f l u c t u a t i o n s ) in t h e o r d e r o f 2.0 f o r t h e l o w e s t t h i r d o f t h e w a t e r d e p t h . Us i ng t h e l a t t e r f i g u r e , e q u a t i o n (2.21) and mean f r i c t i o n f a c t o r v a l u e s o f m e a s u r e d e v e n t s ( s e e t a b l e 3 and d e s c r i p t i o n be l ow ) , t h e t u r b u l e n c e i n t e n s i t y nea r t h e bed o f t h e c r e e k would app r o x ima t e l y a v e r a g e 0.3 t o 0 .35 V. A c co rd i ng t o Kulin and Comp ton ' s (1975) ru le o f t humb , O t t m e t e r e r r o r s due t o v e l o c i t y f l u c t u a t i o n s unde r B laney C reek c ond i t i o n s cou ld e x c e e d 1 V. near t h e bed , bu t n o t in a d r a s t i c a l f a s h i o n . The l a t t e r c o n d i t i o n s w e r e c o n s i d e r e d r e l a t i v e l y c o n s e r v a t i v e and i t wa s a s s u m e d t h a t a x i a l o r l a t e r a l e f f e c t s w e r e r e l a t i v e l y i n s i g n i f i c a n t . The t i m e i n t e r v a l o f v e l o c i t y p r o f i l e m e a s u r e m e n t s was s e l e c t e d t o be one m inu te . It was assumed t h a t a one minute t i m e i n t e r v a l w o u l d be s u f f i c i e n t t o o b t a i n r e p r e s e n t a t i v e v e l o c i t y p r o f i l e s , a s i t u a t i o n wh i ch a p p e a r e d n o t u n r e a s o n a b l e f o r a 7 m wide c r e e k . In any c a s e , i t was d i f f i c u l t t o ' c o n s i d e r l o n g e r t i m e i n t e r v a l s due t o t h e man i p u l a t i o n s and p r o c e d u r e s a s s o c i a t e d w i t h e a c h m e a s u r e m e n t . T h e s e c o u l d c on sume a t l e a s t 5 m i n u t e s a n d s i n c e B l a n e y c r e e k was h y p o t h e s i z e d t o b e a r a f l a s h y f l o o d r e g i m e , i t was r e a s o n e d t h a t l o n g e r t i m e i n t e r v a l s f o r v e l o c i t y measu r emen t would have r e s u l t e d in t h e end in a more r e s t r i c t e d s p a t i a l c o v e r a g e o f t h e c r e e k . This c o n c e r n was bo rne ou t in t h e s ub sequen t high f l o w m e a s u r e m e n t s . Due t o s ome t e c h n i c a l d i f f i c u l t i e s , m o s t o f t e n 4 c u r r e n t m e t e r s w e r e a c t u a l l y in w o r k i n g c o n d i t i o n a t v a r i o u s t i m e s du r i ng f i e l d v e l o c i t y m e a s u r e m e n t s . F o r t u n a t e l y t h e t w o m o s t i m p o r t a n t n e a r - b e d m e t e r s w e r e o p e r a t i o n a l a t a l l t i m e s . The f l o w s p e e d and o r i e n t a t i o n i n f o r m a t i o n ga i ned can be used t o e x p l o r e t h e h y d r o d y n a m i c s o f t h e r i f f l e a nd d i s t a l p o o l . V e l o c i t y p r o f i l e d a t a w e r e c o l l e c t e d in o r d e r t o a l l ow bed s h e a r s t r e s s e s t i m a t i o n s f o r s e v e r a l l o c a t i o n s a c r o s s t h e s t udy s i t e . F o r t h e s e pu rpose s t h e l oga r i t hm i c law o f t h e wal l f o r h y d r a u l i c a l l y r o u g h f l o w ( e q u a t i o n 2.21) was a s s u m e d t o be a p p r o p r i a t e . In a c t u a l i t y , t h e law o f t h e wa l l p r o v i d e s an e s t i m a t i o n o f t h e i n t e r n a l f l u i d s h e a r a t some d i s t a n c e above the bed. T h e r e is r e l a t i v e l y a b u n d a n t e x p e r i m e n t a l e v i d e n c e t h a t t h e l aw a p p l i e s u n d e r c o n t r o l l e d f i e l d a n d l a b o r a t o r y c o n d i t i o n s , a l b e i t o f t e n on ly a p p r o x i m a t e l y t h r o u g h t h e who l e w a t e r d e p t h . Some a u t h o r s h a v e o b s e r v e d v e l o c i t y p r o f i l e s t o be k i n ked and h a v e p r o p o s e d t h a t t h e law o f t h e wa l l is a pp l i c a b l e in t h e l o w e s t 1 5 - 2 0 X o f t h e w a t e r d e p t h on l y . H o w e v e r , t h e l a t t e r i ssue depends on t h e r e l a t i v e r oughnes s as we l l as on t h e r o u g h n e s s e l e m e n t s p a c i n g (Nowe l l and C h u r c h , 1979) a t t h e so l id boundary o f t h e f l o w . Mos t o f our p r o f i l e s s u g g e s t e d t h a t t h e v e l o c i t y d i s t r i b u t i o n was l o g a r i t h m i c n e a r t h e b e d . The use o f e q u a t i o n (2.21) in t h i s a r e a a ppea r e d j u s t i f i e d t o e s t i m a t e b ed s h e a r . H o w e v e r , b e c a u s e o f t h e l i m i t e d n u m b e r o f p o i n t s , i t c o u l d n o t be s a t i s f a c t o r i l y d e t e r m i n e d a t which a v e r a g e r e l a t i v e he i gh t above t h e bed t h e t h e law o f t h e wa l l f a i l s . U n f o r t u n a t e l y , due t o some u n c e r t a i n t i e s a b o u t t h e e x a c t l o c a t i o n o f t h e a c t u a l o r i dea l i z ed (in t h e c a s e o f a g ranu la r s u r f a c e ) b ed l e v e l , ou r v e l o c i t y p r o f i l e d a t a c ou l d n o t a l l ow n ea r b ed s h e a r s t r e s s e s t i m a t i o n us i ng t h e c o n c e p t o f t h e d i s p l a c e m e n t h e i g h t ( J a c k s o n , 1981) . In p r a c t i c e , t h i s p a r a m e t e r is f o u n d by l i n e a r i z i n g n o n - l i n e a r v e l o c i t y p r o f i l e s b u t our p r o f i l e s had t o o f e w po i n t s t o do so . The phys i ca l s i g n i f i c a n c e and h e n c e f o r m u l a t i o n o f t h e d i s p l a c e m e n t h e i g h t a r e n o t w e l l - k n o w n (M i dd l e t o n and S o u t h a r d , 1984). A l t h o u g h some a u t h o r s r e p o r t on t h e r e l a t i v e c o n s i s t e n c y o f t h e d i s p l a c e m e n t h e i g h t w i t h r e s p e c t t o t h e r oughnes s he i gh t under c o n t r o l l e d c o n d i t i o n s ( J a c k s o n , 1981; B a y a z i t , 1 9 8 2 ) , t h e m a r k e d nonun i f o r m i t y o f t h e f l o w cond i t i o n s a t t h e s t u d y s i t e wou ld a p p e a r t o p r e c l u d e s u c h g e n e r a l i z a t i o n s . Due t o t h e above c o n c e r n s a bou t t h e e x a c t l o c a t i o n o f t h e c u r r e n t m e t e r s w i t h r e s p e c t t o an i dea l i z ed bed l e v e l , and t o t h e f o r m o f e q u a t i o n (2.21), we wi l l c o n s i d e r s h e a r e s t i m a t e s h e r e i n a s a c t u a l l y d e s c r i b i n g t h e l o c a l m o m e n t u m t r a n s f e r t h r o u g h t h e f l u i d . T h o s e i n t e r n a l s h e a r e s t i m a t e s t a k e n r e l a t i v e l y c l o s e t o t h e bed wil l be assumed t o be compa rab l e t o t h e t h e a c t u a l f o r c e a c t i n g pe r un i t a r e a o f bed . 3.4 R e s u l t s f r o m f l o w m e a s u r e m e n t s Two h y d r o l o g i c a l e v e n t s wi l l be c o n s i d e r e d . The f i r s t one w h i c h o c c u r r e d on N o v e m b e r 2 0 , 1986 c o r r e s p o n d s t o a p p r o x i m a t e l y h a l f - b a n k f u l l c o n d i t i o n s . A s e c o n d e v e n t on November 23 , 1986 dur ing which t h e b a n k f u l l l e v e l ( abou t 63 cm on t h e s t a f f gauge ) was r e a c h e d wi l l be o f m o s t i n t e r e s t . T w e n t y - o n e s t a t i o n s we re samp led dur ing b o t h e v e n t s . F igure 10 shows t h e v a r i a t i o n in w a t e r s t a g e du r i ng t h e p e r i o d o f m e a s u r e m e n t , w i t h l a b e l s f o r v e l o c i t y s a m p l i n g s t a t i o n s . November 20 measurements were t a k e n a f e w hour s a f t e r t h e w a t e r s t a g e a p p r o a c h e d b a n k f u l l s t a g e . T h e s t a g e g r a d u a l l y d im in i shed by a b o u t t w o c e n t i m e t e r s du r i n g t h e m e a s u r e m e n t pe r i od and a l l t h e measu red p r o f i l e s may t hu s be c ompa r ed . In c o n t r a s t , November 23 mea su r emen t s we r e t a k e n a t t h e peak and du r i ng t h e r e c e s s i o n o f a f l o o d . Con sequen t l y , t h e d a t a f r o m s t a t i o n s A t o K and L t o U wi l l be c o n s i d e r e d in t w o s e p a r a t e g roups . As a c on sequence o f t h e un s t e ad i n e s s o f t h e f l o w , near b a n k f u l l m e a s u r e m e n t s w e r e l i m i t e d in numbe r b u t p r o v i d e d a r e a s o n a b l e s p a t i a l c o v e r a g e o f t h e r e a c h . The a v e r a g e h y d r a u l i c p a r a m e t e r s o f b o t h e v e n t s a r e s umma r i z e d in t a b l e 2. T h e s e v a l u e s ( e x c e p t t h e s l o pe ) we re o b t a i n e d f r o m one c r o s s - s e c t i o n a l e s t i m a t e ( s t a t i o n s D t o H on N o v e m b e r 23 and e t o k on N o v e m b e r 20 ) , d i s c h a r g e be i ng e s t i m a t e d by t h e v e l o c i t y - a r e a m e t h o d . N o v e m b e r 2 3 d a t a c o r r e s p o n d t o n e a r b a n k f u l l s t a g e and w i l l be p a r t i c u l a r l y u s e f u l f o r m o d e l l i n g c o n s i d e r a t i o n s d e v e l o p e d in t h e n e x t c h a p t e r . The w a t e r s u r f a c e s l o p e c o u l d be e s t i m a t e d f r o m 5 r e l i a b l e w a t e r e l e v a t i o n p a i r s a l o ng t h e c r e e k on November 23 ( a t near b a n k f u l l s t a g e ) and 18 pa i r s on November 20 . D e t a i l e d h y d r o d y n a m i c m e a s u r e m e n t s wil l now be examined by c o n s i d e r i n g t h e shape and l o ca l and t e m p o r a l va r i ab i l i t y o f t h e v e l o c i t y p r o f i l e s as we l l as t h e s h e a r and f l o w p a t t e r n s t h r o u g h t h e s t u d i e d r e a c h . L e t us f i r s t c o n s i d e r w h a t i n d i c a t e s t h e shape o f t h e 54 Figure 10. Temporal change* In watar laval Nevambar 20 and 23, 1886 event* • ' i i i i i I 1 1 r— 40 80 120 160 200 240 Time, In minute* T a b l e 2 Mean h y d r a u l i c p a r a m e t e r s o f N o v e m b e r 2 0 a n d 2 3 , 1 9 8 6 h y d r o l o g i c a l e v e n t s , B l a n e y c r e e k N o v e m b e r 2 0 N o v e m b e r 2 3 D i s c h a r g e ( m 3 s _ 1 ) 2 . 62 5 . 0 0 M e a n v e l o c i t y ( m s " 1 ) 1 . 1 1 1 . 3 0 Mean d e p t h ( m ) 0 . 34 0 . 5 5 Mean w a t e r s u r f a c e s l o p e 0 . 0 0 9 5 0 . 0 1 0 2 H y d r a u l i e r a d i u s ( m ) 0 . 3 1 0 . 4 7 5 F r o u d e n u m b e r 0 . 6 1 0 . 5 6 R e y n o l d s n u m b e r 2 4 0 0 0 0 4 7 0 0 0 0 F r i c t i o n f a c t o r 0 . 1 9 0 . 2 2 5 v e l o c i t y p r o f i l e s t h r o u g h t h e s t u d y a r e a in t e r m s o f phys i ca l p r o c e s s e s . A p a r t f r o m a f e w s t a t i o n s l o c a t e d c l o s e t o t h e r i g h t bank which bea r more comp lex f o r m s , m o s t o f t h e November 23 p r o f i l e s we re k inked. C l o se t o t h e bed , a wake l aye r was f o u n d , t h e c h a r a c t e r i s t i c s o f wh ich a r e m o s t l y d e t e r m i n e d by t h e b o u n d a r y m a t e r i a l s and t h e i r a r r a n g e m e n t . A b o v e i t , an o u t e r l a y e r c o u l d be r e c o g n i z e d , t h e c h a r a c t e r i s t i c s o f wh ich a r e r a t h e r d e t e r m i n e d by t h e c h a n n e l c o n f i g u r a t i o n . B e i n g d e p e n d e n t on l o c a l f a c t o r s wake l a ye r c h a r a c t e r i s t i c s we r e more va r i ab l e t h a n t h o s e o f t h e o u t e r l a ye r , as f o u n d by Nowel l and C h u r c h ( 1 9 7 9 ) . It can be shown f r o m equa t i on (2.21) t h a t t h e s l ope o f t h e v e l o c i t y p r o f i l e i s p r o p o r t i o n a l t o t h e i n t e r n a l s h e a r i n t e n s i t y . F i g u r e s 11, 12 a n d 13 r e s p e c t i v e l y s h o w d i m e n s i o n l e s s v e l o c i t y p r o f i l e s f o r t h e r i f f l e , t h e p o o l - g r o u p 1 ( a t h igh s t a g e ) and t h e p o o l - g r o u p 2 (dur ing r e c e s s i o n ) . In g e n e r a l , t h e s e f i g u r e s i n d i c a t e t h a t t h e h i g h e s t v e l o c i t y g r a d i e n t s in t h e wake l aye r we re measu r ed o v e r t h e r i f f l e a r e a . One c an a l s o n o t e t h a t in a m a j o r i t y o f s i t u a t i o n s i n t e r n a l shear t h r ough t h e o u t e r l aye r was g r e a t e r t h a n t h a t o f t h e wake l a y e r f o r p oo l p r o f i l e s : in c o n t r a s t , t h e i n t e r n a l s h e a r is m o r e s im i l a r t h r o u g h o u t t h e who l e d e p t h o v e r t h e r i f f l e e ven t h o u g h t h e p r o f i l e s a t t h e l a t t e r l o c a t i o n s t i l l appear k inked . November 20 v e l o c i t y p r o f i l e s (no t shown here in ) we re more r egu l a r t h r o u g h t h e poo l and o f t e n appea r ed t o compr i se only a wake l a y e r , e s p e c i a l l y o v e r t h e r i f f l e , as t h e f l o w d e p t h was r e d u c e d . In a c t u a l i t y some s t a t i o n s o v e r t h e r i f f l e we re t o o sha l l ow t o a l l ow s a t i s f a c t o r y v e l o c i t y p r o f i l e s m e a s u r e m e n t s . Figure 11. Dimensionless velocity profiles Riffle area, Novambar 23, U 8 8 1.1 1.3 1.8 Point / Height averaged velocity • * Height averaged velocity w i t approximated by the velocity at 0.2 and 0.8 the water depth 58 Figure 12. Dlmenelonleea velocity profile* Pool area, Novombor 23 198* 0.01-| 1 1 1 1 1 1 1 1 1 1 1 1 1 1-0.1 0.3 0.8 0.7 0.8 1.1 1.3 1.8 Point / Height avoragod velocity Figure 13. Dlmenalonleaa velocity prof Ilea Pool areaOower atage),November 23 1888 i 1 1 1 1 r 0.7 0.8 1.1 1.3 1.8 Point / Height averaged velocity During t h e November 23 e v e n t , no e v i dence was n o t e d o f j e t t i n g (Rood, 1980) o f t h e f l o w f r o m t h e r i f f l e i n t o t h e d i s t a l poo l . In g e n e r a l , t h e v e l o c i t y p r o f i l e s in t h e poo l we re comp lex bu t ( e x c e p t c l o s e t o t h e r i g h t bank) bea r t h e wake shedd ing shape c h a r a c t e r i s t i c o f h igh r e l a t i v e r o u g h n e s s s t r e a m s , as c a n be seen f r o m f i g u r e s 12 and 13. L e t us now a d d r e s s t h e i ssue o f t h e l o ca l v a r i ab i l i t y o f t h e v e l o c i t y p r o f i l e s . Dur ing t h e N o v e m b e r 23 e v e n t , some s i m p l e t e s t s w e r e d o n e in o r d e r t o d e t e r m i n e t h e r e p r e s e n t a t i v e n e s s o f one l o c a l v e l o c i t y p r o f i l e m e a s u r e m e n t . A t s t a t i o n s L (poo l near r i g h t bank) and P (near r i f f l e c r e s t ) , t w o a d d i t i o n a l o n e - m i n u t e p r o f i l e s a t a d i s t a n c e o f app r o x ima t e l y 10 cm f r o m t h e s t a t i o n we re c o l l e c t e d . The se two s t a t i o n s c an be l o c a t e d on f i g u r e 18. T h e s e c o n s i d e r a t i o n s a b o u t t h e l o c a l d i v e r g e n c e o f t h e v e l o c i t y p r o f i l e s a r e r e l e v a n t t o t h e s p a t i a l v a r i a t i o n o f t h e i n t e r n a l s h e a r and f u r t h e r q u a l i f y how t h e s e h y d r o d y n a m i c e s t i m a t e s m i g h t be i n t e r p r e t e d . F o r s t a t i o n L, o n e p r o f i l e s h o w s a c l e a r d i v e r g e n c e in t h e o u t e r l a y e r ( f i g u r e 14). Th i s m i gh t be a t t r i b u t a b l e t o t h e p r ox im i t y o f t h e banks and in p a r t i c u l a r o f a hang ing l og ( s e e f i g u r e 5 o r 6) wh ich may d i s t o r t t h e n e a r - s u r f a c e l o c a l f l o w f i e l d . Wake l a y e r g r a d i e n t s w e r e s u b s t a n t i a l l y s i m i l a r b u t t h e i r i n t e n s i t y w a s s o m e w h a t d i f f e r e n t . I n t e r n a l s h e a r e s t i m a t e s f r o m t h e b o t t o m o f t h e p r o f i l e s d i f f e r e d a t t h e m o s t by 20 X. S t a t i o n P showed some c o n s i s t e n c y in t h e o u t e r l a y e r g r a d i e n t b e t w e e n p r o f i l e s a l t h o u g h t h e wake l a y e r s w e r e m a r k e d l y d i f f e r e n t ( f i g u r e 15). I n t e r n a l s h e a r e s t i m a t e s in t h i s c a s e d i f f e r e d by up t o 3.6 60 Flgura 14. Local variability In velocity proflloa Station L, November 23, 1986 > 0.2 -0.01 0.8 o.s 0.2 -0.1 0.08 0.08 -0.02 -0.01 Maan point valoclty, in m/a Flguro 18. Local variability In valoclty profllaa Station P, Novombor 23, 1986 i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 r -0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Moan point velocity, in m/a t i m e s , wh i ch is c o n s i d e r a b l e . S t a t i o n P is a l s o l o c a t e d nea r t h e r i f f l e c r e s t , wh i ch may c o r r e s p o n d t o a m o s t s p a t i a l l y v a r i a b l e a r e a in t e r m s o f v e l o c i t y d i s t r i b u t i o n . It was u n f o r t u n a t e l y impo s s i b l e t o s t u d y t h i s i s s ue f u r t h e r in t h e f ie ld . Some simple t e s t s were made a t t h e end o f t h e November 23 p e r i o d on t h e r e p l i c a b i l i t y o f a v e l o c i t y p r o f i l e in o r d e r t o i n v e s t i g a t e p o t e n t i a l s o u r c e s o f v a r i a b i l i t y f r o m a t e m p o r a l v i e w p o i n t . T e n o r n ine s u c e s s i v e m e a s u r e m e n t s were t a k e n while t h e w a t e r l e v e l was n e a r l y c o n s t a n t a t S t a t i o n V and W r e s p e c t i v e l y ( s t a t i o n s shown on f i g u r e 18), t h e f o r m e r l o c a t e d a t t h e d o w n s t r e a m end o f t h e , d i s t a l poo l and t h e l a t t e r o v e r t h e d i s t a l p a r t o f t h e r i f f l e . S t a t i o n V p r o f i l e was l o c a t e d in t h e i n t e n s e shea r i ng a r ea a long t h e r i g h t bank ( s ee f u r t h e r d e s c r i p t i o n b e l o w ) . The v e l o c i t y p r o f i l e s w e r e i n d e ed r a t h e r comp lex ( f i g u r e 16) and t h i s may be a t t r i b u t a b l e t o t h e l o ca l p r o t u s i o n o f t h e bank as we l l as t o t h e i n t en s e mixing and p e r h a p s t o j e t t i n g e f f e c t s . In any c a s e , t h e v a r i a b i l i t y in t h e o n e - m i n u t e a v e r a g e p r o f i l e s f o r t h i s p o o l l o c a t i o n is i n t e r e s t i n g , e s p e c i a l l y n e a r t h e b o u n d a r y . O u t e r l a y e r behav iou r is s een t o be more c o n s i s t e n t . S t a t i o n W ( f i g u r e 17) on t h e r i f f l e shows k inked p r o f i l e s f r o m which wake and o u t e r l a y e r s c an be r e c o g n i z e d . The a v e r a g e p r o f i l e s f o r t h e l a t t e r s t a t i o n a r e m o r e c o n s i s t e n t t h a n f o r t h e poo l l o c a t i o n e v e n nea r t h e bed . T h e r e f o r e , a t l e a s t f o r some poo l l o c a t i o n s , t h e t e m p o r a l v a r i a t i o n in t h e f o r m o f t h e v e l o c i t y p r o f i l e s may f u r t h e r d i s c o u r a g e s t r o n g g e n e r a l i z a t i o n f r o m a un ique p r o f i l e f o r one p a r t i c u l a r s t a t i o n . Th i s c o n s i d e r a t i o n , c omb i n ed w i t h 100 80 •o SO > o S 20 10 8 8 -100 Figure 16. Sean* at downaiream and of pool Station V, November 23, 1886 1.3 1.8 1.7 Mean point velocity, in m/a Figure 17. Scana at downstream end of riffle Station W.November 23, 1886 0.8 0.7 0.9 i r 1.3 1.8 Mean point velocity, In m/a t h e l o c a l v a r i a b i l i t y o f s o m e p r o f i l e s , was i m p o r t a n t t o c o n s i d e r b e f o r e any i n t e r p r e t a t i o n o f t h e s h e a r e s t i m a t e s can be p r o d u c e d . F igure 18 shows t h e d i s t r i b u t i o n o f t h e va lues o f i n t e r n a l shea r e s t i m a t e d v ia equa t i on (2.21) f o r November 23 . Fo r t h o s e s t a t i o n s wh ich w e r e r e p l i c a t e d , a v e r a g e v a l u e s a r e c o n s i d e r e d . A c c o r d i n g t o 11 c o m p a r a b l e e s t i m a t e s , a v e r a g e i n t e r n a l shea r a t high f l o w s t a g e was l a r g e r on t h e r i f f l e (82 Pa) t h a n in t h e poo l (44 Pa ) . Us ing t h e u n i f o r m f l o w f o r m u l a t i o n ( e q u a t i o n 2.16), t h e s h e a r s t r e s s is e s t i m a t e d t o be 4 7 . 5 Pa , i.e. s o m e w h a t l o w e r t h a n t h e a v e r a g e o f t h e 11 n e a r b a n k f u l l s t a t i o n s : h o w e v e r , t h e s e s t a t i o n s d id n o t c o v e r s u f f i c i e n t bed a r e a t o a r g u e on a s t r i c t c o m p a r i s o n . A t l owe r s t a g e on November 23 , only one va lue was measu r ed o v e r t h e r i f f l e (212 Pa) while t h e a v e r a g e o f s e v e n shea r e s t i m a t e s in t h e poo l was 25 .8 Pa . D e s p i t e l o c a l v a r i a b i l i t y , N o v e m b e r 23 m e a s u r e m e n t s t e n d t o s u g g e s t t h a t t h e h i g he s t shea r va l ues ( 335 , 212 and 136 Pa) were r e c o r d e d o v e r t h e r i f f l e o r nea r t h e r i f f l e c r e s t as wel l as in t h e m o s t u p s t r e a m p a r t o f t h e s t u d y r e a c h . A c c o r d i n g t o t h e m e a s u r e d s t a t i o n s a v e r a g e shea r a t t h e l ower d i s c ha r ge o f November 20 was 22.2 Pa on t h e r i f f l e and 21.8 pa in t h e poo l , i.e. t h e shea r appea r ed t o be s imi lar on t h e a v e r a g e , a l t h o u g h q u i t e v a r i a b l e w i t h i n e a c h b e d u n i t ( f i g u r e 19). T h e s e s h e a r e s t i m a t e s a r e s o m e w h a t s m a l l e r t h a n t h e un i f o rm f l o w app rox ima t i on which amoun t s t o 28.9 Pa . Once aga i n , h i ghe s t s h ea r s we re measu r ed in t h e upper p a r t o f t h e s t u d y r e a c h . F i gu res 20 and 21 show t h e f l ow l i n e s f o r t h e November 23 N e a r - b e d F i g u r e 1 8 . i n t e r n a l s h e a r e s t i m a t e s a t B l a n e y C r e e k , N o v e m b e r v e l o c i t y s a m p l i n g s t a t i o n s 2 3 , 1 9 8 6 9.5 B a n k f u l l c r o s s - s e c t i o n B a n k f u l l c h a n n e l l i m i t s R i f f l e c r e s t L T e s t s t a t i o n 0.5 0-6 336. 45-\36. 4.1 16.6 73. 1.6 •46.5 PK 34.6 25.6 7.2 16.8 7.4 51-3 9-9 W V N o t e : s h e a r u n i t s a r e P a s c a l s 1 m en cn F i g u r e 2 1 . N e a r - b o t t o m a n d n e a r - s u r f a c e f l o w l i n e s , B l a n e y C r e e k , N o v e m b e r 2 0 , 1 9 8 6 . N o t e : F l o w l i n e s a r e p r o p o r t i o n a l t o v e l o c i t y . 1 cm r e p r e s e n t s 1 m s - ^ ^ B a n k f u l l c h a n n e l l i m i t s R i f f l e c r e s t — • N e a r - s u r f a c e f l o w l i n e s — " N e a r - b e d f l o w l i n e s 1 m cn ( a t high s t a g e only) and November 20 e v e n t s r e s p e c t i v e l y . The s t r u c t u r e o f t h e s e c o n d a r y f l o w s in t h e d i s t a l poo l cou ld be r e l a t e d t o t h e p r e s s u r e d i s t r i b u t i o n a r i s i n g f r o m s u r f a c e f l o w imp i ngemen t e v e n i f a t a low ang l e ( G a l l i n a t t i , 1984). The f l o w s t r u c t u r e is p a r t i c u l a r l y we l l d e v e l o p e d and w a r r a n t s some q u a l i t a t i v e c o m m e n t s . T he c o m p a r i s o n o f f i g u r e s 2 0 and 21 r e v e a l s t h a t p a r t i c u l a r l y t h e s u r f a c e f l o w t e n d s t o be more a l igned w i th t h e banks a t h i ghe r d i s c h a r g e . F i g u r e 22 p r e s e n t s t h e r e s u l t s o f f l o w s t r u c t u r e o b s e r v a t i o n s made a t t h e end o f November 23 e v e n t when t h e f l o w s t a g e was a l m o s t t h a t o f t h e e v e n t o f N o v e m b e r 20 ( w a t e r l e v e l on s t a f f gauge was 40 cm). These o b s e r v a t i o n s w e r e c o l l e c t e d f r o m t h e b r i d g e s w i t h 3 m l ong p l a s t i c f l a g s and a compass . The wide d i v e r g e n c e b e t w e e n t h e s u r f a c e a n d b o t t o m f l o w l i n e s is in e v e r y c a s e e v i d e n t . D i v e r g e n c e is g r e a t e r a t low f l o w s t h a n a t h igher s t a g e s . In t h e d i s t a l p o o l t h e b o t t o m f l ow l i n e s t e n d t o be pa r a l l e l w i t h t h e r i f f l e f a c e . S u r f a c e f l o w is ob l ique t o t h e r i f f l e ax i s in t h e u p s t r e a m p a r t o f t h e d i s t a l poo l bu t subpa ra l l e l t o i t in i t s d o w n s t r e a m e n d . Th i s we l l d e v e l o p e d c i r c u l a t i o n may h a v e some c o n s e q u e n c e s f o r s e d i m e n t e v a c u a t i o n f r o m t h e p o o l . It a l s o wi l l be u s e f u l t o i n v e s t i g a t e t h e f i e l d - f l u m e c o m f o r m a t i o n in t h e f o l l o w i n g c h a p t e r . 3.5 P r o b l e m s r a i s e d by t h e f i e l d m e a s u r e m e n t s The t o p i c s in t h i s s e c t i o n c o v e r t h e r e v e r s a l h y p o t h e s i s ( d e s c r i b e d in t h e a p p e n d i x ) p l u s o b s e r v a t i o n s on b e d l o a d cr. t r a n s p o r t and t r a n s p o r t c o n d i t i o n s as we l l as f l o w s t r u c t u r e du r i ng Novembe r 23 e v e n t . If one be l i e v e s . t h e above i n t e r n a l shea r va lues and i f one m a k e s a b s t r a c t i o n o f t h e i r l i m i t a t i o n s due t o r e s t r i c t e d s p a t i a l c o v e r a g e ( p a r t i c u l a r l y o v e r t h e r i f f l e ) , i t wou l d a ppea r t h a t t h e i n t e r n a l s h e a r i n c r e a s e s more o v e r t h e r i f f l e t h a n in t h e p o o l a s d i s c h a r g e i n c r e a s e s . In t h e s ame p e r s p e c t i v e , i t is p e r h a p s w o r t h w h i l e t o m e n t i o n t h a t t h e ana lys i s o f t h e v e l o c i t y p a t t e r n s f o r t h e l o w e s t c u r r e n t m e t e r s a l s o s u g g e s t e d t h a t n e a r - b e d v e l o c i t i e s w e r e h i ghe r o v e r t h e r i f f l e a t n e a r b a n k f u l l c o n d i t i o n s . T h e s e m e a s u r e m e n t s a p p e a r c o n t r a r y t o t h e v e l o c i t y o r s h e a r s t r e s s r e v e r s a l h y p o t h e s e s d e s c r i b e d in t h e append i x . A l t hough t h e bed t o pog r aphy t h r o u g h t h e r e a c h is a l so c o n t r o l l e d by t h e o b s t r u c t i o n s t o t h e f l o w , t h e a p p a r e n t l y l o w e r s h e a r in t h e d i s t a l p o o l r a i s e s unavo i d ab l y t h e i s s ue o f i t s m a i n t e n a n c e . If in B l aney c r e e k sed imen t t r a n s p o r t o f some c a l i b r e and i n t e n s i t y o c c u r s a t nea r b ank f u l l , how can t h e poo l t h e n n o t f i l l ? The s p a t i a l c o v e r a g e o f t h e f i e l d m e a s u r e m e n t s is n o t s u f f i c i e n t l y dense t o s e t t l e t h e l a t t e r i s s u e . H o w e v e r , some o t h e r , more g ene r a l c o n s i d e r a t i o n s abou t t h e c o n d i t i o n s in B laney C r eek on November 23 a r e w o r t h d i s cuss i ng . On N o v e m b e r 2 3 , on ly t h r e e b e d l o a d t r a n s p o r t s e d i m e n t s a m p l e s f r o m t h e l o w e r b r i d g e w e r e t a k e n a t b a n k f u l l c o n d i t i o n s , u s i ng a 7 6 mm open i ng H e l l e y - S m i t h s a m p l e r . T h o s e v e r y r e s t r i c t e d , one m i nu t e s amp l e s i n d i c a t e d t h a t p a r t i c l e s a t l e a s t o f medium g r a v e l s i ze (10 mm) we re t r a n s p o r t e d and t h a t m o s t t r a n s p o r t s eemed t o o c cu r a long t h e base o f t h e r i f f l e f a c e . It c o u l d n o t be d i r e c t l y d e t e r m i n e d i f l a r g e r m a t e r i a l w a s a c t u a l l y m o b i l e . A l t h o u g h t h e r e i s no d e f i n i t i v e i n d i c a t i o n t h a t t h e d i s c h a r g e we m e a s u r e d , a l b e i t n e a r b an k f u l l , was o f f o r m a t i v e c h a r a c t e r , t h e a p p a r e n t t r e n d in t h e s h e a r s t r e s s is un l i ke l y t o c h a n g e much un t i l t h e c r eek goes o v e r b a n k . L o w e r s t r e s s e s in t h e d i s t a l poo l , in t h e f a c e o f t h e e x i s t e n c e o f t h e p o o l , would e i t h e r i n d i c a t e low t r a n s p o r t i n t e n s i t y o r p r e f e r e n t i a l r o u t e s f o r s e d i m e n t o f v a r i o u s c a l i b r e . T h e l a t t e r p o s s i b i l i t y w o u l d be r e l a t e d t o t h e l a t e r a l s o r t i n g o f s e d i m e n t d o w n s t r e a m f r o m t h e f l o w o b s t r u c t i o n in t h e p r o x i m a l p o o l . G a l l i n a t t i ' s ( 1 9 8 4 ) q u a l i t a t i v e m o d e l o f t h e f l o w f i e l d a r o u n d s t r e a m b a n k o b s t r u c t i o n s , d e s c r i b e d in t h e append i x , may p r o v i d e t h e b a s i c mechan i sm f o r l a t e r a l s o r t i n g t o o c cu r . This p r o c e s s cou l d n o t be v e r i f i e d d i r e c t l y du r i ng t h e h y d r o l o g i c a l e v e n t s bu t was t o be e x a m i n e d in t h e f l u m e e x p e r i m e n t s . N e v e r t h e l e s s , t h e f a c t s t h a t f i n e r s ed imen t s a r e f o u n d nea r t h e r i g h t bank be low t h e o u t c r o p and t h a t v e r y c o a r s e m a t e r i a l s a c c u m u l a t e d i m m e d i a t e l y d o w n s t r e a m o f t h e o b t r u c t i o n s u g g e s t t h a t such a p r o c e s s may be ' a c t i v e . L a t e r a l s o r t i n g in t h e p r o x i m a l p oo l c a n f u r t h e r be a p p r e c i a t e d f r o m f i g u r e 2 3 . The t r a n s p o r t c o n d i t i o n s on N o v e m b e r 23 c an be f u r t h e r c h a r a c t e r i z e d by c o n s i d e r i n g t h e r a t i o o f t h e a v e r a g e d i m e n s i o n l e s s s h e a r s t r e s s t o t h e d i m e n s i o n l e s s c r i t i c a l s h e a r s t r e s s n e c e s s a r y t o e n t r a i n some c h a r a c t e r i s t i c b ed p a r t i c l e s . Us ing A n d r e w s ' (1983) r e s u l t s , wh i ch w e r e d e v e l o p e d f r o m a s e r i e s o f o b s e r v a t i o n s on v a r i o u s s i ze p a r t i c l e s t r a n s p o r t e d in s i n g l e t h r e a d , g r a v e l b e d r i v e r s , t h e c r i t i c a l d i m e n s i o n l e s s s h e a r s t r e s s n e c e s s a r y t o e n t r a i n t h e D 5 0 o f s u r f a c e m a t e r i a l s f o r B l aney c r e e k wou ld be 0 . 0 4 6 . Us ing t h e D 5 0 o f t h e s u r f a c e and t h e h y d r a u l i c r a d i u s , t h e d i m e n s i o n l e s s shea r s t r e s s f o r t h e b ank f u l l f l o w is c a l c u l a t e d t o be 0 . 0 57 , a t y p i c a l v a l ue f o r sma l l g r a v e l b e d s t r e a m s w i t h r e i n f o r c e d bank s ( And r ews , 1984). T h e r e f o r e t h e c r i t i c a l c o n d i t i o n s f o r s u r f a c e m a t e r i a l e n t r a i n m e n t wou ld h a v e b e e n e x c e e d e d by 27'/. (mob i l i t y numbe r o f 1.27) f o r November 23 b ank f u l l f l o w . This e x c e e d a n c e is low c o m p a r e d t o m o s t a c t i v e g r a v e l b ed r i v e r s w i t h a l l u v i a l p o o l - r i f f l e b e d f o r m s which on t h e a v e r a g e bea r an e x c e e d a n c e o f a bou t 100 7. (mob i l i t y number o f 2.0) a t b a n k f u l l d i s cha rge ( t h i s was e s t i m a t e d f r o m t h e d a t a o f Andrews , 1984). S imp le c a l c u l a t i o n s , u s i n g t h e D / S 9 0 / S a s a c r i t e r i o n , r e v e a l t h a t an i n c r e a s e o f more t h a n 50 7. in d i s c ha r ge or s hea r c o u l d mob i l i z e t h e b e d in g e n e r a l . A l e s s e r i n c r e a s e wou ld p robab l y i n i t i a t e l o ca l movemen t on t h e bed s ince t h e shea r is n o t u n i f o r m l y d i s t r i b u t e d o v e r t h e b e d . N e v e r t h e l e s s , s u ch a r i s e in f l o w wou ld c a u s e o v e r b a n k f l o w wh ich wou ld i n c r e a s e f l o w r e s i s t a n c e and c o n s e q u e n t l y l o w e r TQ ( H i r s c h and A b r a h a m s , 1981) . I t m i g h t t h e r e f o r e be p r o p o s e d t h a t B l a n e y c r e e k e x p e r i e n c e s l ow m o b i l i t y c o n d i t i o n s a t n e a r b a n k f u l l s t a g e , a l though t h e c r e e k is n o t paved in t h e s en se o f Bray and Chu r ch ( 1 980 ) . T h e o b s e r v a t i o n t h a t t h e s u b s u r f a c e m a t e r i a l is o x i d i z e d b u t n o t d e c a y e d t e n d s t o s u p p o r t su ch a s s e r t i o n s . P e r h a p s low t r a n s p o r t i n t e n s i t y c an be r e c o n c i l e d w i t h t h e c o n t i n u e d e x i s t e n c e o f t h e d i s t a l poo l in t h e e v e n t u a l i t y t h a t t h e l a t t e r r e p r e s e n t s a r e l i c t f e a t u r e . W h e t h e r t h i s is F i g u r e 2 3 . L a t e r a l b e d m a t e r i a l s o r t i n g i n p r o x i m a l p o o l , B l a n e y C r e e k . On t h e l e f t e d g e o f t h e p h o t o g r a p h b e g i n s t h e s m a l l a c c u m u l a t i o n b a r . F i g u r e 2 4 . N o v e m b e r 2 3 , 1 9 8 6 h y d r o l o g i c a l e v e n t , B l a n e y C r e e k . S t a f f g a u g e i s a p p r o x i m a t e l y a t 45 cm ( i . e . 18 cm b e l o w b a n k f u l l ) . Y e t o n e c a n c l e a r l y n o t i c e by t h e w a t e r s u r f a c e a p p e a r a n c e t h e e x i s t e n c e o f a s h e a r l i n e a l o n g t h e r i g h t b a n k . r e a s onab l e o r n o t wi l l be d i s c u s s ed in c h a p t e r 4. We may a l so e x p l o r e some c h a r a c t e r i s t i c s o f t h e f l o w on a g ene r a l l e v e l . It was n o t e d dur ing t h e f i e l d mea su r emen t s t h a t an ' e d d y ' o r s h e a r l ine su ch as d e s c r i b e d by L i s l e (1986) d e v e l o p e d in t h e d i s t a l poo l a t some d i s t a n c e f r o m t h e r i g h t banK. F i g u r e 24 s hows t h e Novembe r 23 e v e n t dur ing r e c ed i n g f l o w . A s h e a r l ine is c l e a r l y v i s i b l e a l o ng t h e r i g h t bank . Ou t s i de t h e r ange o f t h e p h o t o g r a p h t h e shea r l ine e x t e n d e d t o n e a r t h e c r o s s - o v e r a r e a . The same f e a t u r e o f t h e f l o w s t r u c t u r e is p r e s e n t a t h igher f l o w s b u t beg ins only a t some d i s t a n c e i n t o t h e p o o l . I t s u p s t r e a m e x t e n t a p p e a r s t o be r e l a t e d t o t h e c h ang i n g o r i e n t a t i o n o f t h e s t r e a m l i n e s w i t h f l o w s t a g e , as we d e s c r i b e d above . This f l o w f e a t u r e s u g g e s t e d t h a t t h e a p p r o a c h i n g f l o w was ob l i que l y t o w a r d t h e r e l a t i v e l y r e i n f o r c e d and c ohe s i v e r i g h t bank and t hu s t h a t t h e o r i g in o f t h e d i s t a l poo l cou ld be s imi lar t o t h a t o f t h e p rox ima l , i.e. o b s t r u c t i o n c o n t r o l l e d , 75 4.0 L a b o r a t o r y s t u d y and c o n f o r m a t i o n 4.1 F l u m e d e s c r i p t i o n , i n s t r u m e n t a t i o n a n d h y d r o d y n a m i c m e a s u r e m e n t p r o c e d u r e s The f l ume used in t h i s s t ud y is l o c a t e d in t h e Geography D e p a r t m e n t a t t h e U n i v e r s i t y o f B r i t i s h Co l umb i a . The t i l t i n g r e c i r c u l a t i n g f l u m e w i t h t r a n s p a r e n t a c r y l i c wa l l s ( s e e f i g u r e 25 ) is s u p p o r t e d on a s t e e l t r u s s . A l eve l l i ng j a c k near i t s d o w n s t r e a m end a l l ows bed s l ope t o be a d j u s t e d up t o abou t 0 .02 . The a p p a r a t u s is a bou t six" m e t r e s long and has a bed w i d t h o f 47 cm . A l a r g e r e s e r v o i r a t i t s d o w n s t r e a m end p r e v e n t s d r a w d o w n o r b a c k i n g up du r i ng pump ing . F r o m t h e b o t t o m o f t h e r e s e r v o i r t h e w a t e r f l o w s t h r o u g h a t i l t i n g 15.25 cm d i a m e t e r p l a s t i c r e t u r n p ipe t o a v a r i a b l e s p e e d impe l l e r pump d r i v en by a 1.0 h o r s epowe r m o t o r s i t u a t e d a t i t s ' u p s t r e a m e n d . W a t e r is pumped up t o t h e f l ume i n l e t where i t f l o w s t h r o u g h a h o n e y c o m b mesh b e f o r e e n t e r i n g t h e f l u m e b e d . S e d i m e n t in t r a n s p o r t f a l l s i n t o a d o w n s t r e a m r e s e r v o i r w h e r e only g ra ins sma l l e r t h a n 2 mm can e n t e r t h e r e t u r n pipe and be r e c i r c u l a t e d . P a r t i c l e s c o a r s e r t h a n 2 mm cou ld n o t s a f e l y be r e c i r c u l a t e d t h r o u g h t h e pump. T h e f l u m e h a d no i n s t a l l e d d e v i c e f o r d i s c h a r g e 76 F i g u r e 2 5 . G e n e r a l v i e w o f t h e w a t e r r e c i r c u l a t i n g f l u m e a t t h e D e p a r t m e n t o f G e o g r a p h y , T h e U n i v e r s i t y o f B r i t i s h C o l u m b i a r e g u l a t i o n . D i s c h a r g e was t h u s e s t i m a t e d v ia t h e v e l o c i t y - a r e a me t hod f r o m one d o c u m e n t e d c r o s s s e c t i o n (7 p r o f i l e s , each w i th 5 t o 7 o n e - m i n u t e i n t e r v a l m e a s u r e m e n t s ) a t some d i s t a n c e f r o m t h e f l ume e n t r y . Because o f t h i s d rawback , t h e d i s c ha r ge cou ld be s e t only app r ox ima t e l y f o r e a ch r un . A w a t e r l e v e l r e c o r d e r was s e t t o en su re t h a t d i s cha rge s t a y e d c o n s t a n t dur ing runs . V e l o c i t i e s in t h e f l u m e w e r e m e a s u r e d us ing a TSI 1239W r u g g e d i z e d h o t f i l m p r o b e w i t h 1.5 mm h e m i s p h e r i c a l t i p f i x e d on a 3.2 mm e lbow r o d . The ve r y s o f t w a t e r f r o m t h e munic ipa l w a t e r supp ly s u b s t a n t i a l l y r e d u c e d t h e usua l p r o b l e m o f p r o b e c o n t a m i n a t i o n d u r i n g m e a s u r e m e n t s . D u r i n g r u n s , w a t e r - w a s c o n s t a n t l y r e newed f r o m t h e t a p . The p r o b e had b e en c a l i b r a t e d in a r o t a t i n g drum dur ing au tumn 1986 by J ohn F. W o l c o t t . It was o p e r a t e d a t 19 d e g r e e s c e l c i u s o v e r h e a t r a t i o . Dur ing m e a s u r e m e n t s , t h e a n a l o g o u t p u t s igna l f r o m t h e p r obe was samp led a t 10 Hz, p r o c e s s e d by a TSI o p e r a t i n g modu l e and s e n t t o a CR21X ( C a m p b e l l S c i e n t i f i c I n s t r u m e n t ) d a t a l o g g e r . T he d a t a l o g g e r a l l o w e d t h e c a l c u l a t i o n o f t h e mean v o l t a g e as wel l as t h e e x t r a c t i o n o f t h e minimum and max imum va lues dur ing t h e one minute i n t e r v a l o f m e a s u r e m e n t . One m i nu t e is a v e r y s u f f i c i e n t i n t e r v a l t o c h a r a c t e r i z e t h e v e l o c i t y f i e l d . F r o u d i a n s i m i l a r i t y r e g a r d i n g f i e l d mea su r emen t t ime i n t e r v a l would r equ i r e 15 se conds in t h e f l u m e ( e q u a t i o n 2 .13) . A c c o r d i n g l y , f l u m e v e l o c i t y p r o f i l e s w e r e l i ke l y t o b e a r l e s s e r v a r i a b i l i t y . O n e - m i n u t e a v e r a g e as we l l as v o l t a g e v a l u e s w e r e c o n v e r t e d t o mean v e l o c i t y va lues a s sum ing t h a t t h e y f o l l o w e d a l o g - n o r m a l d i s t r i b u t i o n . A t each measu r emen t s t a t i o n , t h e p r obe ' s d i s t a n c e f r o m t h e w a t e r s u r f a c e and t h e w a t e r d e p t h we re measu red manual ly us ing a m i l l i m e t r e r u l e r . B e c a u s e o f i t s s ize and shape, t h e p robe cou ld ha rd l y be s e t c l o s e r t h a n 2 mm f r o m t h e bed . Fo r p r ope r o p e r a t i o n , t h e p robe mus t n o t be s e t t o o c l o s e t o t h e w a t e r - a i r i n t e r f a c e , i.e. a bou t 5 - 6 mm below t h e w a t e r s u r f a c e . Hence a l t h o u g h we c o u l d ea s i l y g e t more d e t a i l e d p r o f i l e s t h an under t h e f i e l d c ond i t i o n s we cou ld n o t g e t mea su r emen t s as c l o s e t o t h e w a t e r s u r f a c e , a l l p r o p o r t i o n s K e p t . F i na l l y , w a t e r s u r f a c e e l e v a t i o n s w e r e m e a s u r e d u s i ng a m i l l i m e t r e r u l e r s e t c a r e f u l l y a t t h e w a t e r s u r f a c e and r e a d f r o m a l e v e l . The same p r o c e d u r e was u s ed in o r d e r - t o c o n s t r u c t t o p o g r a p h i c a l maps o f t h e f i n a l b ed c o n f i g u r a t i o n o f f l u m e r u n s . N e a r - s u r f a c e a n d n e a r - b e d f l o w l i n e s w e r e d e t e r m i n e d w i t h r e s p e c t t o t h e f l ume wal ls us ing a f l a g and a p r o t r a c t o r moun t ed on t h e same r o d and r e a d f r o m above t h e f l ume . E r r o r s in t h e l a t t e r r ead i ngs we re c on s i d e r ed t o be o f t h e same magn i tude as in t h e f i e l d , i.e. 2 - 5 d e g r e e s . 4.2 S c a l e mode l l i n g o f B l aney C r e e K : s t r a t e g y 4 .2 .1 G e n e r a l c o n s i d e r a t i o n s As was a r g u e d in c h a p t e r 2, in o r d e r t o s t r i c t l y v e r i f y f i e l d t o l a b o r a t o r y c o n f o r m a t i o n on a h yd r odynam i c a l bas i s , t h e p r e s e n t mode l l i n g e x e r c i s e n e e d e d t o f o l l o w u n d i s t o r t e d F r o u d e mode l s c a l i n g l aws . The w i d t h o f t h e f l u m e d e t e r m i n e d t h e s c a l e r a t i o f r o m p r o t o t y p e t o m o d e l f o r n e a r b a n K f u l l c ond i t i on s , i.e. t h e f l ume w id th was 16 t imes sma l l e r t h a n t h a t o f t h e c r e e k . Ac co rd i ng l y t h e l e n g t h o f t h e s t udy r e a c h came up t o be a b o u t 1.6 m in t h e f l u m e . Such a s c a l e r a t i o s a t i s f i e s t h e r e q u i r e m e n t s o f e q u a t i o n ( 2 . 2 6 ) , u s i n g k s = 3 .5 Dgg f o r g r a v e l - b e d r i v e r s ( B r a y , 1982 ) . T h e o t h e r i m p o r t a n t l e n g t h s ca l e is t h a t o f t h e bed ma t e r i a l , which will be d i s c u s s e d in some d e t a i l . 4 .2.2 Bed m a t e r i a l s c a l i n g The m a t t e r o f b e d m a t e r i a l s p e c i f i c a t i o n f o r a mode l l i ng e x e r c i s e is n o t t r i v i a l ( r e c a l l s e c t i o n s 2.5.1 and 2 .5 .2 ) . In t h e c a s e o f m o s t s and b ed r i v e r s , i t is t h i s p r a c t i c a l c o n s i d e r a t i o n wh i ch p r e v e n t s in many s i t u a t i o n s t h e u t i l i s a t i o n o f an u n d i s t o r t e d m o d e l . The p a r a g r a p h s wh i ch f o l l o w d e s c r i b e t h e p r o c e d u r e s which we re t a k e n t o s c a l e t h e bed m a t e r i a l s i ze d i s t r i b u t i o n o f B l aney C r e e k in o r d e r t o b e s t m e e t mechan i ca l s i m i l a r i t y r e q u i r e m e n t s . A f i r s t s t e p in t h e s ca l i ng o f t h e v o l u m e t r i c bed m a t e r i a l s amp l e c o n s i s t e d o f t r u n c a t i n g i t a t t h e s u s p e n d e d - b e d m a t e r i a l l o a d l im i t u s i ng t h e f l o w c o n d i t i o n s o f N o v e m b e r 2 3 . The f o l l o w i n g s u s p e n s i o n c r i t e r i o n ( P a r k e r e t a l , ' 1 982 ) was used : V» > 1 . 0 ( 4 . 1 ) w h e r e V s is a p a r t i c l e s e t t l i n g v e l o c i t y as d e f i n e d by t h e R u b e y - W a t s o n law (D ingman, 1984). A c c o r d i n g l y , t h e uppe r l im i t f o r suspens ion f o r November 23 c ond i t i o n s was f o und t o be abou t 1.25 mm. This r e s u l t e d in t h e t r u n c a t i o n o f abou t 7.6 'A o f t h e s i z e d i s t r i b u t i o n . The t r u n c a t e d s i z e d i s t r i b u t i o n was t h e n F r o u d e - s c a l e d t o o b t a i n a t a r g e t , m o d e l s i z e d i s t r i b u t i o n . T r u n c a t i o n e l i m i n a t e d s o m e p a r t i c l e s i z e s f o r w h i c h s c a l e d p a r t i c l e Reyno l d s numbe r wou ld h a v e l i k e l y b e e n r e l a t i v e l y l ow ( a l l f i n e r t h a n v e r y f i n e sand in t h e model) and which would n o t h a ve a l l owed p r o p e r e n t r a i n m e n t c o n d i t i o n s t o be p r e s e r v e d , as d i s c u s s e d in s e c t i o n 2 . 5 . 2 . T h e t r u n c a t e d f i e l d s i z e d i s t r i b u t i o n h a s D ^ , D50 a n d 094 r e s p e c t i v e l y o f 5, 29 and 93 mm (see f i g u r e 26) . Ripples can o c cu r in t h e mode l i f a c e r t a i n p e r c e n t a g e o f t h e s i ze d i s t r i b u t i o n has a p a r t i c l e Re yno l d s numbe r s m a l l e r t h a n 5, i.e. t h e s i t u a t i o n f o r wh i ch l am ina r f l o w c o n d i t i o n s o c c u r in t h e wake o f t h e p a r t i c l e . J a e g g i (1986) c o m m e n t e d t h a t t h e s e c o n d i t i o n s c a n i n t e r f e r e w i t h b a r f o r m a t i o n in mode l s o f g r a v e l - b e d r i v e r s . He a l s o r e c o m m e n d e d t h a t a p a r t i c l e Re yno l d s numbe r o f 5 s h o u l d r e p r e s e n t a min imum c o n d i t i o n f o r g r a v e l - b e d r i v e r m o d e l s e d i m e n t s . A c c o r d i n g t o t h e a b o v e c o n s i d e r a t i o n s , a c o r r e c t i o n p r o c edu r e as p r o p o s e d in J aegg i (1986) can be app l ied in o r d e r t o improve t h e s i t u a t i o n . Th i s p r o c e d u r e is c o n c e r n e d w i t h t h e mode l s i z e d i s t r i b u t i o n p a r t i c l e s in t h e t r a n s i t i o n a l f l o w r e g i m e ( i . e . p a r t i c l e Reyno lds number f r o m 5 t o 7 0 ) . In f l o w mechan ics t e r m s , and a c c o r d i n g t o t h e shape o f b e d m a t e r i a l e n t r a i n m e n t c u r v e s in s e v e r a l s e d i m e n t t r a n s p o r t s t u d i e s , t h e c r i t i c a l s h e a r v e l o c i t y (or r e s i s t a n c e ) in t h e mode l would be t o o smal l f o r t h o s e f i e l d p a r t i c l e s w i t h f u l l y t u r b u l e n t w a k e wh i c h f a l l in t h e t r a n s i t i o n a l p a r t i c l e R e y n o l d s r a n g e w h e n s c a l e d . T h e 81 Figure 26. Comparison of laboratory aodlmont mixture with the corrected and non-eorrrected scaled slzs distributions of the truncsted volumetric sample of Blaney Creek Sieve size (In phi) c o r r e c t i o n p r o c e d u r e is g raph i ca l and i t is shown in f i g u r e 27 . It wi l l now be d e s c r i b e d in some d e t a i l . On f i g u r e 27 , t w o s c a l e s a r e p r e s e n t on t h e abc i s sa , i.e. t h a t o f t h e p r o t o t y p e and t h a t o f t h e F r o u d e - s c a l e d mode l gra in s i z e d i s t r i b u t i o n . A l s o , t h e c u r v e s o f c r i t i c a l s h e a r v e l o c i t i e s a t mode l d imens i on s f o r b o t h s i z e d i s t r i b u t i o n s a r e s h o w n . F o l l o w i n g J a e g g i ( 1986 ) , we a s s u m e d in o r d e r t o c a l c u l a t e t h e c r i t i c a l s h e a r v e l o c i t i e s t h a t t h e Sh i e l d s f a c t o r ( e q u a t i o n 2 .25) was c o n s t a n t a t 0 . 0 3 f o r 5<Rp<10, c h ang i n g l inear ly f o r 10<Rp<70 and c o n s t a n t a t 0 .05 f o r Rp>70. A t mode l f l o w c o n d i t i o n s , f o r g r a i n s l a r g e r t h a n a b o u t 2.3 mm in t h e mode l , t h e g r a i n r e s i s t a n c e o r e n t r a i n m e n t c o n d i t i o n s w i t h r e s p e c t t o t h e f i e l d a r e p r o p e r l y r e p r o d u c e d and t h e t w o v » c r c u r v e s o v e r l a p . The d i v e r g e n c e o f t h o s e c u r v e s f o r g r a i n s s m a l l e r t h a n 2 .3 mm i n d i c a t e s t h e d i f f e r e n c e in r e s i s t a n c e c o n d i t i o n s a r o u n d bed p a r t i c l e s in t h e mode l i f only F r o u d e s c a l i n g is a p p l i e d . S h i f t i n g o f t h e g r a i n s i z e d i s t r i b u t i o n c u r v e by t h e a m o u n t c o r r e s p o n d i n g t o t h e d i s c r e p a n c y b e t w e e n t h e t w o • v „ c r c u r v e s , as shown in f i g u r e 2 7 , r e s u l t s in a c o r r e c t i o n o f t h e s i ze d i s t r i b u t i o n . The f i n a l l y a d o p t e d c u r v e is s o m e w h a t c o a r s e r t h a n t h e s imp l e g e o m e t r i c a l l y s c a l e d t r u n c a t e d s i ze d i s t r i b u t i o n and is a imed t o g i v e a b e t t e r r e p r o d u c t i o n in t h e mode l o f e n t r a i n m e n t c o n d i t i o n s p r e s e n t in t h e p r o t o t y p e . F i gu r e 26 a l s o p r e s e n t s t h e f i n a l m i x t u r e u s ed in t h e mode l l i n g e x e r c i s e . A l so shown on f i g u r e 26 a r e t h e F r o u d e s c a l e d ( s ca l e r a t i o o f 17 had been used a t t h a t t ime) t r u n c a t e d and t h e c o r r e c t e d s i ze d i s t r i b u t i o n s f o r c o m p a r i s o n . It is s e e n t h a t t h e m i x t u r e a d o p t e d r e p r o d u c e s s u f f i c i e n t l y we l l t h e s c a l e d and c o r r e c t e d s i2e d i s t r i b u t i o n . T h e l a t t e r a c t u a l l y f a l l s b e t w e e n t h e t h e t w o t a r g e t s i ze d i s t r i b u t i o n s be low t h e D 5 0 . The m a t c h is p r e t t y g o o d in t h e v i c i n i t y o f t h e D 5 Q . A b o v e i t , t h e s e l e c t e d m i x t u r e d e p a r t s f r o m t h e d e s i r e d c u r v e , a c t u a l l y be i ng f i n e r t h a n d e s i r e d a b o v e t h e D Q Q . In any c a s e , f i e l d s a m p l e s u s u a l l y on l y r e p r e s e n t a p p r o x i m a t i o n s ( J a e g g i , 1986) so t h i s m i x t u r e is d e e m e d a c c e p t a b l e , d e s p i t e s ome d i f f e r e n c e s . F i n a l l y , i t s h o u l d be m e n t i o n e d t h a t t h e f i e l d and l a b o r a t o r y s ed imen t had t h e same s p e c i f i c g r a v i t y , i.e. j u s t a bou t 2 7 0 0 kg pe r cub ic m e t r e . 4 .2 .3 Wo rk i ng a s s u m p t i o n s r e g a r d i n g f i e l d c o n d i t i o n s S i n ce i t is a s s o c i a t e d w i t h a d i s c o n t i n u i t y in m o r p h o l o g i c (W i l l i ams , 1978 ) a n d h y d r o d y n a m i c p r o p e r t i e s , t h e b a n k f u l l s t a g e is o f t e n a s s u m e d t o b ea r some mo rphogene t i c s i g n i f i c a n ce ( K n i g h t o n , 1984) in a l l u v i a l r i v e r s . A l t h o u g h in a v a r i e t y o f r i v e r s t h e l o n g - t e r m m o r p h o l o g y is l i ke l y a d j u s t e d t o a r ange o f f o r m a t i v e d i s c h a r g e s , i t is p e r h a p s in t h o s e s t r e a m s f o r wh ich e x c e s s d i m e n s i o n l e s s s h e a r s t r e s s a t b a n k f u l l c o n d i t i o n s is sma l l t h a t t h e e q u a t i o n o f t h e d om i n an t a nd b a n k f u l l d i s c h a r g e is m o s t r e a s o n a b l e . A wo r k i n g a s s u m p t i o n in t h i s r e s e a r c h p r o j e c t was t o a c c e p t t h e l a t t e r s t a t e m e n t in o r d e r t o a t t e m p t t o d e v e l o p b ed m o r p h o l o g y f r o m an in i t ia l l y f l a t bed . P e r h a p s t h e g r e a t e s t r i s k a s s o c i a t e d w i t h t h i s a s s u m p t i o n shou ld be r e l a t e d t o t h e memory o f a p a r t i c u l a r l y high f l o o d or t o t h e i n h e r e n t h i s t o r i c a l c o m p o n e n t o f any e n v i r o n m e n t a l s y s t e m , as s t a t e d by G.G.S impson (1963) . T h e r e f o r e , in o r d e r t o p e r f o r m l a b o r a t o r y e x p e r i m e n t s f o c u s i n g on t h e s e l e c t e d p o o l - r i f f l e s equence hydrodynamics , we had t o c o l l e c t a s e t o f v e l o c i t y p r o f i l e s a t n ea r b a n k f u l l c o n d i t i o n s . The l a t t e r d e c i s i o n s w e r e a l s o a p p a r e n t l y s u p p o r t e d by s o m e h y d r o d y n a m i c m e a s u r e m e n t s a n d s e d i m e n t t r a n s p o r t c o n s i d e r a t i o n s i n t r o d u c e d in c h a p t e r 3 . T he f a c t t h a t t r a n s p o r t o f bed m a t e r i a l s ed imen t appea r ed r e l a t i v e l y low a t a s uppo sed l y n e a r f o r m a t i v e d i s c h a r g e s u g g e s t e d t h a t i t wa s poss i b l e t o run t h e f l ume w i t h o u t f e e d i n g sed imen t a t t h e i n l e t as mus t usual ly be done in t h e c a s e o f a more mobi le r i v e r . T h e r e f o r e , t h e a p p r o a c h was t o r un t h e f l ume un t i l t h e bed s t a b i l i z e d and t r a n s p o r t r a t e a p p r o a c h e d z e r o , a c o n d i t i o n which usual ly o c c u r r e d a f t e r a f e w hour s o f runn ing. When t h e s u c h c o n d i t i o n s w e r e a t t a i n e d v e l o c i t y m e a s u r e m e n t s w e r e p e r f o r m e d . The n e x t c o n s i d e r a t i o n s r e g a r d t h e d e t a i l s o f t h e c o n f i g u r a t i o n o f B l a n e y c r e e k f i e l d s i t e . E a r l y in t h e l a b o r a t o r y wo rk i t was h y p o t h e s i z e d t h a t t h e d e t a i l s in t h e c r e e k c o n f i g u r a t i o n w e r e n o t a l l i m p o r t a n t in d e t e r m i n i n g t h e c r e e k ' s bed t o p o g r a p h y and f l o w p r o c e s s e s . Hence, t h e p r e s e n c e o f t h e b e d r o c k o u t c r o p and b e d r o c k c o n t r o l f o r t h e p rox ima l poo l bed l e v e l as we l l as o f an o b s t r u c t i o n t o t h e f l o w and o f d o w n s t r e a m c o n t r o l p r o v i d e d by t h e t r a n s v e r s e log we r e j udged t o be m o s t r e l e v a n t t o t h e r e p r o d u c t i o n o f t h e f i e l d s i t u a t i o n . C o r r e s p o n d i n g l y one a p p a r e n t a d v a n t a g e o f B laney c r e e k s i t e was t h a t t h o s e w e l l - d e f i n e d b o u n d a r y c o n d i t i o n s wou l d a l l o w t h e c o m p a r i s o n o f o ngo i ng f l u v i a l p r o c e s s e s a t a l o ca l s c a l e . The c h a r a c t e r i s t i c s o f t h e c r e e k a l s o e n t a i l e d a s h i f t f r o m a pu r e l y g e n e r i c t o a mo r e s p e c i f i c mode l l i n g a p p r o a c h . The g e o m e t r i c a l s ca l i ng o f t h e u p s t r e a m and d o w n s t r e a m c o n t r o l s immed i a t e l y d e t e r m i n e d t h e l o c a l , impo sed b ed s l o p e : w h e n c e , t h e f l u m e s l o p e was a c c o r d i n g l y s e t t o z e r o . H o w e v e r , t h e f i n a l v a l ue o f t h e hydrau l i c g r a d i e n t wi l l a l so be a f f e c t e d by t h e r e s i s t a n c e o f t h e b o u n d a r y m a t e r i a l s w h i c h , in t u r n , d e p e n d s on a p p r o p r i a t e r e p r o d u c t i o n o f t h e f l o w c o n d i t i o n s , as r e p o r t e d in c h a p t e r 2. A n o t h e r i n i t i a l a s s u m p t i o n was t h a t bank r e s i s t a n c e d id n o t r e p r e s e n t an i m p o r t a n t e l e m e n t in t h e r e p r o d u c t i o n o f B l a n e y C r e e k h y d r o d y n a m i c s . On t h e one h a n d , t h i s was m o t i v a t e d by t h e f a c t t h a t bank r e s i s t a n c e is r a t h e r sma l l c o m p a r e d t o b ed r e s i s t a n c e f o r c h a n n e l s w i t h w i d t h t o d e p t h r a t i o a b o v e 10 t o 15 (Kn i gh t , 1981). On t h e o t h e r hand, i t a l s o r e p r e s e n t e d a w o r k i n g a s s u m p t i o n as i t a p p e a r e d e x c e e d i n g l y d i f f i c u l t t o mode l t h e r o u g h n e s s o f B l aney c r e e k b a n k s , e s p e c i a l l y c o n s i d e r i n g t h e u n d e r c u t a n d s i n u o u s c h a r a c t e r o f t h e r i g h t bank ( r e f e r t o f i g u r e s 5 and 6 ) -T h e r e f o r e , a t t h e beg inn ing o f l a b o r a t o r y work, t h e banks we re s m o o t h and s t r a i g h t . The f i r s t r un s w e r e p e r f o r m e d w i t h t h e m o s t s imp l e c o n f i g u r a t i o n d e s c r i b e d a b o v e . The b e d r o c k o u t c r o p was s c a l e d g e o m e t r i c a l l y a l t h o u g h t h e s t r a i g h t f l u m e wa l l c o n s t r a i n t i n t r o d u c e d sma l l l a t e r a l d i s t o r t i o n . F i g u r e 28 shows t h e mode l o u t c r o p , b u i l t u s i n g l e g o b l o c k s , a m a t e r i a l w h i c h . a l s o p r o v i d e d some n o t i o n a l r o u g h n e s s . C o m p a r i s o n w i t h f i g u r e 5 ( t h e f i e l d s i t e map) r e v e a l s t h e s i m p l i f i c a t i o n s o f i t s s hape . 87 L e g o - b u i l t m o d e l o f F i g u r e B l a n e y 28 . C r e e k b e d r o c k o u t c r o p F u r t h e r m o r e , t h e o b s t r u c t i o n t o t h e f l o w and t h e d i s t a n c e b e t w e e n t h e u p s t r e a m and d o w n s t r e a m c o n t r o l s we re a l so mode led g e o m e t r i c a l l y . The s c a l e r a t i o was 16 in a l l c a s e s . A n o t h e r i m p o r t a n t f e a t u r e o f t h e i n i t i a l c o n f i g u r a t i o n is t h e u p s t r e a m d e f l e c t i o n o f m o s t o f t h e u p s t r e a m incoming f l o w t o w a r d s t h e l e f t bank as o c c u r s in t h e f i e l d . Th i s was a l s o done by c o n s t r a i n i n g t h e u p s t r e a m c h a n n e l t o a b o u t 27 cm, c o n s i d e r i n g i t s f i e l d w i d th and t h e s ca l i ng r a t i o . This way, mo s t o f t h e i n com ing w a t e r f l o w e d v i a t h e l e f t s i de o f t h e o u t c r o p and imp inged a t a s im i l a r ang le on t h e o b s t r u c t i o n . F ina l ly , a t h i c k n e s s o f 10.3 cm o f bed m a t e r i a l f i l l e d t h e mode l ed s t udy r e a c h f l u s h t o t h e d o w n s t r e a m c o n t r o l l e v e l . The l e v e l o f t h e n o n - a l l u v i a l p r o x i m a l p o o l b e d was h o w e v e r c o n t r o l l e d by a p l a t e g l u e d w i t h g r a v e l s a nd s e t a t g e o m e t r i c a l l y s im i l a r h e i g h t . B e f o r e s t a r t i n g t h e pump t h e m a t e r i a l was c o m p a c t e d and l e v e l l e d . Then t h e pump was s t a r t e d and t h e d i s c h a r g e was g r a d u a l l y r a i s e d in o r d e r t o f i r s t s a t u r a t e t h e bed and t h e n t o a f l o w r a t e near o r s l i g h t l y be low t h e d e s i r e d o n e . C r o s s - s e c t i o n a l v e l o c i t y m e a s u r e m e n t s w e r e t h e n p e r f o r m e d t o d e t e r m i n e i f t h e f l o w r a t e needed t o be r e a d j u s t e d . 4 .2.4 In i t i a l e x p e r i e n c e w i t h and a d j u s t m e n t s t o t h e mode l Bed t o p o g r a p h y d e v e l o p e d v e r y r a p i d l y in t h e f i r s t f e w hours o f each run and i t cou ld be compa red w i th t h e t a r g e t f i e l d s i t e m o r p h o l o g y . In e a c h s u c c e s s i v e r u n , n e c e s s a r y r e a d j u s t m e n t s we r e made one a t a t ime in o r d e r t o d i s c r im i na t e t h e e f f e c t o f e a c h c o n f i g u r a t i o n a l e l e m e n t on b ed morpho logy . It was f o u n d t h a t s ome a d d i t i o n a l f a c t o r s n e e d e d t o be c o n s i d e r e d , amongs t t h e m t h e n e c e s s i t y o f no t i ona l r oughness on banKs (wh ich p r e v e n t e d l o c a l s c o u r a l o ng f l u m e s ides ) and t h e mode l l ing o f t h e mound f o u n d d o w n s t r e a m o f t h e o b s t r u c t i o n as wel l as o f some r e i n f o r c e m e n t on t h e upper r i g h t s ide o f t h e p r o x i m a l p oo l in o r d e r t o r e p r o d u c e i t s n o n - e r o d i b l e c h a r a c t e r ( r e f e r t o c h a p t e r 3, f i e l d s i t e d e s c r i p t i o n , and t o f i g u r e 5 ) . The mound a d j u s t e d t h e l o ca l amount o f w a t e r d i s cha rge a c r o s s t h e s t r e a m . Bank r oughnes s was s imu l a t ed us ing l ego b oa r d s and t h e mound a r e a and p r o x i m a l p o o l r e i n f o r c e m e n t by s m a l l c obb l e s . Even w i th t h o s e a d j u s t m e n t s i t was imposs ib le t o g row t h e m o s t i m p o r t a n t d i s t a l poo l e ven i f runs a l l l a s t e d o v e r 24 hours and e ven when t h e d i s cha rge was i n c r e a s ed by a f a c t o r o f 2 above t h e s c a l e d nea r b a n k f u l l va lue . A c c o r d i n g l y , t h e s inuous bank l i n e o f t h e r i g h t c r e e k bank was g e o m e t r i c a l l y s c a l e d us ing a luminum s h e e t on wh i ch some s a n d p a p e r was g l u ed t o p r o d u c e some n o t i o n a l r o u g h n e s s . I t s i n t r o d u c t i o n d id n o t r e s u l t in d i s t a l p o o l g r o w t h e i t h e r . A t t h i s p o i n t in t h e l a b o r a t o r y work t h e h ypo t he s i s eme r ged t h a t c u r r e n t c r e e k b e d m o r p h o l o g y c o u l d be i n h e r i t e d f r o m a d i f f e r e n t s t r e a m c o n f i g u r a t i o n . Some f i e l d e v i d e n c e s f o r a d i f f e r e n t s t r e a m c o n f i g u r a t i o n in t h e p a s t c an be o b s e r v e d abou t 10 m u p s t r e a m f r o m t h e b ed r o c k o u t c r o p . The r e a r e some s igns t h e r e o f an abandoned channe l which would have f l o w e d on t h e r i g h t s ide o f t h e o u t c r o p in t h e p a s t . This channe l is now b l o c k e d by an a c c u m u l a t i o n o f l a r g e o r g a n i c d e b r i s a nd s e d i m e n t s . The p e r t u r b e d s t a t e o f t h e low a r ea s a round t h e c r e e k ( a t t r i b u t e d t o t h e l o g g i n g o p e r a t i o n s n e a r l y t h r e e d e c a d e s ago) u n f o r t u n a t e l y p r e c l u d e s any d e t a i l e d f i e l d s t u d y o f p a s t c r e e k c o n f i g u r a t i o n . The eme rgen ce , o f t h e above p r opo s a l c on ce rn i ng t h e d i s t a l p oo l s t a t u s was f u r t h e r s t i m u l a t e d by t h e f a c t , a p p a r e n t f r o m f i e l d m e a s u r e m e n t s , t h a t t h e i n t e r n a l o r b ed s h e a r in t h e d i s t a l poo l a p p e a r e d t o be l o w e r t h a n o v e r t h e r i f f l e a t a s s u m e d f o r m a t i v e d i s c h a r g e , a s i t u a t i o n wh i c h a p p e a r e d c o n t r a d i c t o r y w i t h r e s p e c t t o poo l e x i s t e n c e (as d i s c u s s e d in s e c t i o n 3.5). One p o s s i b l e p a s t f l o w c o n f i g u r a t i o n c o u l d be t e s t e d w i t h i n t h e s t r a i g h t w a l l f l u m e by r e m o v i n g t h e d e f l e c t i o n o f t h e i n com ing f l o w made a b o v e t h e o u t c r o p and t h e r e b y f o r c i n g more w a t e r t o f l o w a long t h e r i g h t bank. A f t e r s e v e r a l h ou r s o f r unn i ng unde r t h e l a t t e r c o n f i g u r a t i o n , t h e c u r r e n t c r e e k c o n f i g u r a t i o n was r e i n s t a l l e d and t h e run f u r t h e r c o n t i n u e d . Th i s d i d n o t r e s u l t in a p p r o p r i a t e d i s t a l p o o l g r o w t h e i t h e r . A n o t h e r p a s t e v e n t w h i c h a f f e c t e d B l a n e y C r e e k d r a m a t i c a l l y was t h e i n t r o d u c t i o n o f t h e t r a n s v e r s e l o g wh i ch a c t e d as a new d o w n s t r e a m c o n t r o l . It is poss ib l e t h a t t h e a c t u a l l o w e s t b ed l e v e l in t h e d i s t a l poo l is r e l a t e d t o t h e a b s e n c e o f t h e d o w n s t r e a m l og a t s ome p o i n t in t h e p a s t . H o w e v e r , a l t e r i n g t h e c o n f i g u r a t i o n o f t h e c r e e k in t h e l a b o r a t o r y in o r d e r t o r e c o n s t i t u t e t h e a c t u a l b ed m o r p h o l o g y and h i s t o r y was d i f f i c u l t due t o s t r a i g h t f l ume wa l l s . It m u s t be u n d e r s t o o d h o w e v e r t h a t any s c e n a r i o wh ich e i t h e r p r opo se s t h a t t h e d i s t a l poo l is r e l i c t o r t h a t i t has n e v e r b e en a poo l in a m o r p h o g e n e t i c s e n s e i m p l i e s low t r a n s p o r t r a t e ( a t l e a s t o v e r t h e poo l ) o r t h e ma in t enance o f t h e c o m p e t e n c e in t h e a c t u a l poo l a r e a , e ven i f n o t f o r t h e l a r g e s t c l a s t s c a r r i e d by t h e s t r e a m ( r e f e r t o t h e l a t e r a l s o r t i n g h y p o t h e s i s in s e c t i o n 3 . 5 ) . F o r a l l t h e f o r e g o i n g f l ume runs , i t was o b s e r v e d t h a t t r a n s p o r t in wha t c o r r e s p o n d s t o t h e d i s t a l p o o l a r e a on ly o c c u r r e d du r i ng t h e p r o c e s s o f s u r f a c e c o a r s e n i n g e a r l y in t h e r u n . In c o n t r a s t , a c t i v e t r a n s p o r t o c c u r r e d f o r a p e r i o d o f s e v e r a l h ou r s o v e r t h e bu i l d i ng r i f f l e who se c r e s t , l e s s p r om inen t due t o t h e ab sen ce o f t h e d i s t a l p o o l , was n e a r l y s im i l a r t o t h e f i e l d . In c o n t r a s t t o t h e d i s t a l poo l , r i f f l e g r o w t h is a s s o c i a t e d t o t h e a c t u a l b ed c o n f i g u r a t i o n s i n c e i t c o n s i s t s o f t h e e x t e n s i o n o f t h e p r o x i m a l p o o l . In l i g h t o f t h o s e l a t t e r c o n s i d e r a t i o n s , t h e f i n a l a pp r oa ch t o t h e mode l l ing o f t h e B laney c r e e k s i t e was t o dig a t s c a l e t h e d i s t a l poo l be low t h e in i t i a l f l a t bed l e v e l and see i f i t would be p r e s e r v e d in t h e p r o c e s s o f a run . The a r r a n g e m e n t o f t h e e v e n t u a l r i f f l e w i t h a manua l l y e x c a v a t e d poo l was t h e n go ing t o be c ompa red w i t h t h e c r e e k morpho logy. . F o r t h e s e e x c a v a t e d p o o l r un s ( r un s 13 and 14 b e l ow ) , a l l c o n f i g u r a t i o n a l e l e m e n t s d e s c r i b e d a b o v e w e r e p r e s e r v e d and t h e u p s t r e a m d e f l e c t i o n o f t h e f l o w r e i n s t a l l e d . B e f o r e any commen t is made on t h e r e s u l t s o f t h e s e runs some g e n e r a l r e m a r k s a b o u t t h e r e l i c t p oo l h y p o t h e s i s , t h e f i e l d s i t e c h a r a c t e r i s t i c s and t h e aim o f t h i s r e s e a r c h p r o j e c t a ppea r t o be w a r r a n t e d . As p o i n t e d o u t by S impson (1963) , phys i ca l mode l s " a b s t r a c t wha t a r e be l i e ved t o be t h e e s s e n t i a l c o n f i g u r a t i o n a l s i m i l a r i t i e s o f h i s t o r i c a l e v e n t s " . H e n c e , d e s p i t e t h e man i p u l a t i o n done w i t h t h e c r e e k c o n f i g u r a t i o n in t h e f l ume , t h e s t a t u s o f t h e a c t u a l d i s t a l poo l c a n n o t be f u l l y a s c e r t a i n e d . F i na l l y , t h e s i t u a t i o n w i t h r e g a r d t o t h e d i s t a l p oo l is an i n t e r e s t i n g l e s s o n l e a r n e d f r o m t h e l a b o r a t o r y e x e r c i s e r a t h e r t h an a d rawback on t h e goa l s o f t h i s r e s e a r c h p r o j e c t . In a c t u a l i t y , t h e c o m p a r i s o n o f f i e l d and l a b o r a t o r y f l o w c h a r a c t e r i s t i c s w i l l c l a r i f y t h e d i s t a l p o o l s i t u a t i o n and wi l l s t i l l a l l o w t h e v e r i f i c a t i o n o f s i m i l a r i t y p r i n c i p l e s . 4.3 L a b o r a t o r y r e s u l t s and t h e i r c o n f o r m a t i o n t o f i e l d 4.3.1 Bed m o r p h o l o g y c o n f o r m a t i o n The r e s u l t s f r o m t w o r un s ( i .e . runs 13 and 14) w i th e x c a v a t e d d i s t a l p o o l w i l l be c o n s i d e r e d in t h i s p a r a g r a p h . Runs 13 and 14 l a s t e d a b o u t 100 and 2 0 0 hou r s r e s p e c t i v e l y . The f l o w dynam i c s and b e d m a t e r i a l c h a r a c t e r i s t i c s o f t h e l a t t e r run wi l l be f u r t h e r a n a l y z e d in t h e p a r a g r a p h s b e l ow . The t w o r u n s had s l i g h t l y d i f f e r e n t i n i t i a l c o n f i g u r a t i o n s and t h u s s l i g h t l y d i s t i n c t d e v e l o p m e n t s . The t o p o g r a p h i c a l map o f run 13 ( f i g u r e 29) is i n t r o d u c e d in o r d e r t o show t h a t a f i n a l bed c o n f i g u r a t i o n such as t h e one o f run 14 ( f i g u r e 30) can be a p p r o x i m a t e l y r e p l i c a t e d f o l l o w i n g t h e c u r r e n t l a b o r a t o r y p r o c e d u r e s . We n o t e f r o m f i g u r e s 29 and 30 t h e e x i s t e n c e o f t h e same gene r a l f e a t u r e s on t h e bed , which a r e in t u r n s imi lar t o t h e f e a t u r e s f o u n d in t h e f i e l d ( f i g u r e 5) . This s i t u a t i o n a p p e a r s t o s u p p o r t t h e a s s u m p t i o n r ega r d i ng t h e s i g n i f i c a n ce o f N o v e m b e r 23 d i s c h a r g e , t h e n o - f e e d p r o c e d u r e and t h e poo l F i g u r e 2 9 . T o p o g r a p h i c a l map o f r u n 13 f i n a l b e d c o n f i g u r a t i o n B a n k I i n e u>— C o u n t o u r ( e l e v a t i o n i n cm ) E x c a v a t e d a r e a 4. 4 4. 6 4. a 5. O 6. 2 5. 4 5. 6 5. a D i s t a n c e f r o m f l u m e i n l e t , i n m e t r e s CO F i g u r e 3 0 . T o p o g r a p h i c a l map o f r u n 14 f i n a l b e d c o n f i g u r a t i o n • B a n k 1 i n e •\.o— C o u n t o u r ( e l e v a t i o n i n cm E x c a v a t e d a r e a 4. 4 4. 6 ii. 8 6. 0 5. 2 5. 4 5. 6 5. fl D i s t a n c e f r o m f l u m e i n l e t , i n m e t r e s e x c a v a t i o n . Boundary c ond i t i o n s we re t h e same f o r each run , w i th t h e e x c e p t i o n o f t h e mound (3) a r e a , d o w n s t r e a m o f t h e s imu la ted o b s t r u c t i o n t o t h e f l o w . The l a t t e r was more p rom inen t in run 13 and th i s c au sed t h e t h e p rox ima l poo l (2) t o be more l inear and t h e c r o s s - o v e r t o be d i sp l a ced f u r t h e r d o w n s t r e a m . It a l so c a u s e d t h e e x c a v a t e d d i s t a l p oo l (6) t o sh r i n k . F i g u r e 31 s hows t h e i n i t i a l and f i n a l c o n f i g u r a t i o n s f o r run 14. Whereas t h e p rox ima l poo l in run 14 is pe rhaps wider t h a n i t shou ld be, t h e d i s t a l poo l had been p r e s e r v e d and t h e c r o s s - o v e r a r e a is a t an h o m o l o g o u s l o c a t i o n w i t h t h a t in t h e f i e l d . T h e c l o s ene s s in t o pog r aphy b e t w e e n f i e l d and run 14 t o pog r aphy was j u d g e d t o be s u f f i c i e n t c o n s i d e r i n g t h e u n a v o i d a b l e d e g r e e o f s i m p l i f i c a t i o n i n t r o d u c e d by t h e s c a l i n g o f t h e s i t e ' s v a r i o u s c o n f i g u r a t i o n a l e l e m e n t s . N o t e h o w e v e r t h a t t h e t o p o g r a p h y o f t h e sma l l a c c u m u l a t i o n b a r (4) is n o t t o be c o n s i d e r e d f o r s t r i c t compar i son w i t h t h e f i e l d s ince i t appea r s t o depend on u p s t r e a m supp ly o f s e d i m e n t and was s imply g r o w n h e r e i n by dumping s ed imen t in t h e a r e a . A lso t h e d i s t a l a r e a (i.e. be low 5 .55 m) o f t h e r i f f l e (5) was c r e a t e d by i n t r oduc i ng m a t e r i a l by h and . Th i s s u g g e s t s t h a t t h e l a t t e r f e a t u r e ' s g r o w t h may d e p e n d on u p s t r e a m s e d i m e n t supp l y , e v e n i f o f low i n t e n s i t y . In summary , be l ow t h e c r o s s - o v e r a r e a t h e main d i f f e r e n c e b e t w e e n t h e f i e l d and l a b o r a t o r y b e d t o p o g r a p h y o f r un 14 c o n s i s t s o f t h e shape o f t h e p rox ima l poo l (2) e x t e n s i o n and s hoa l i n g a r e a . The d i s c u s s i o n be l ow wi l l f u r t h e r c o n s i d e r t h e s i m i l a r i t y o f t h e s t r e a m h y d r o d y n a m i c s o v e r t h e r i f f l e and d i s t a l p oo l wh ich r e p r e s e n t t h e main t e s t o f t h e hydrodynamic s c a l i n g d o n e h e r e i n . 4 .3 .2 F low m e a s u r e m e n t c o n f o r m a t i o n Only t h e n e a r b a n k f u l l c o n d i t i o n s c a n be c o n s i d e r e d because t h e r e was i n s u f f i c i e n t f l o w d e p t h in t h e f l ume t o a l low v e l o c i t y p r o f i l e m e a s u r e m e n t s f o r a s c a l e d Novembe r 20 e v e n t . In t h e f i r s t i n s t a n c e , i t is e s s e n t i a l t o v e r i f y t h a t t h e g e n e r a l f l o w and e n t r a i n m e n t c o n d i t i o n s f o r t h e s c a l e d b a n k f u l l d i s c h a r g e r e p r o d u c e d t h o s e o f t h e f i e l d . The c r i t e r i o n f o r t h o s e c ond i t i o n s t o be me t was p r e s e n t e d in s e c t i o n 2.5.1. The g e o m e t r i c and h y d r a u l i c p a r a m e t e r s o f run 14 and November 23 d a t a a r e p r o v i d e d f o r an homologuous c r o s s - s e c t i o n in Tab le 3. A c co rd i ng t o t h e s e va lues and t h o s e f r o m bed m a t e r i a l sampl ing ( s e e be l ow ) , t h e c o n d i t i o n s s e t by e q u a t i o n (2 . 23 ) ( a c c e p t i n g k s = 3 . 5 ^90> u s i n g t h e s u r f a c e v a l u e s ) is m e t . T h e r e f o r e f l o w in t h e m o d e l was in t h e h y d r a u l i c a l l y r o u g h r e g i m e and r e s i s t i n g f o r c e s w e r e a c c o r d i n g l y a p p r o p r i a t e l y r e p r o d u c e d . In f a c t , t h e l a t t e r s i t u a t i o n is f u r t h e r s u p p o r t e d by t h e c l o s e c o r r e s p o n d a n c e o f t h e f r i c t i o n f a c t o r ( t a b l e 3) . L e t us c o n c e n t r a t e on t a b l e 3. The l a s t co lumn p r e s e n t s t h e r a t i o s w i th t a r g e t va lues , which a r e a lways w i th in 7 X o f u n i t y . The w a t e r s u r f a c e s l o p e e s t i m a t e wh ich was o b t a i n e d f r o m s e v e n l o n g i t u d i n a l e l e v a t i o n p a i r s a p p e a r s t o be e x a c t l y t h e same (in a c t u a l i t y t h e r isk o f e r r o r in t h e measu r emen t o f t h i s q u a n t i t y r e n d e r s t h i s e x a c t c o r r e s p o n d a n c e r a t h e r c o i n c i d e n t a l ) . The mode l may t h e n be sa id t o be v e r y mildly d y s f u n c t i o n a l b u t some u n c e r t a i n t i e s main ly r e l a t e d t o s l o p e T a b l e 3 Mean h y d r a u l i c a n d g e o m e t r i c p a r a m e t e r s f r o m r u n 14 a n d c o m p a r i s o n w i t h t a r g e t v a l u e s a s f o u n d f r o m F r o u d e s i m i l a r i t y p r i n c i p l e s D i s c h a r g e ( l s ~ 1 ) Run 14 v a l u e A 5 . 2 1 T a r g e t v a l u e f r o m F r o u d e 1 aw ( 1 / 1 6 ) B 4 . 8 8 R a t i o A / B 1 . 0 7 M e a n v e l o c i t y ( cm s - 1 ) 3 3 . 1 3 2 . 5 1 . 0 2 Mean w i d t h ( cm ) 4 3 . 0 4 3 . 8 0 . 9 8 M e a n d e p t h ( cm ) 3 . 6 6 3 . 4 3 1 . 0 7 H y d r a u l i e r a d i u s 3 . 1 3 ( cm ) 2 . 9 7 1 . 0 5 W a t e r s u r f a c e s l o p e 0 . 0 1 0 2 0 . 0 1 0 2 1 . 0 0 F r o u d e n u m b e r 0 . 5 5 0 . 5 6 0 . 9 8 R e y n o l d s n u m b e r 8 0 0 0 n / a n / a F r i c t i o n f a c t o r 0 . 2 2 9 0 . 2 2 5 1 . 0 2 m e a s u r e m e n t s do n o t a l l ow any f u r t h e r d e v e l o p m e n t o f t h e s e c o n s i d e r a t i o n s . The l a t t e r s t a t e m e n t is p r e f e r e d t o t h a t o f mi ld ly d i s t o r t e d mode l f r o m Dhamo tha ran and o t h e r s (1980) which has a g e o m e t r i c c o n n o t a t i o n . P o s s i b l e s o u r c e s wh i c h c o u l d a c c o u n t f o r t h i s mi ld d y s f u n c t i o n a r e p e r hap s t h e f a c t s t h a t s e d i m e n t s ( s c a l ed us ing a s ca l e r a t i o o f 17) and channe l w i d t h ( t h e l a t t e r due t o t h e i n t r o d u c t i o n o f t h e s inuous bank l a t e r in t h e p r o g r e s s o f t h e s t u d y ) a r e s l i g h t l y s m a l l e r t h a n t h e s c a l e r a t i o wou ld r e q u i r e . In a c t u a l i t y , t h e d i f f e r e n c e s m u s t m o s t l y a r i s e f r o m t h e s imp le f a c t t h a t d i s c h a r g e was s l i gh t l y h igher t h an t h e t a r g e t va lue dur ing t h e run . O t h e r i t ems l ike i gno rance o f s t r i c t s ca l i ng f o r bank r oughnes s did n o t seem t o make a s i g n i f i c a n t d i f f e r e n c e . We can now cons i de r t h e i ssue o f t h e shape o f t h e v e l o c i t y p r o f i l e s and t h e i r c o n f o r m a t i o n w i t h t h e f i e l d o n e s . It was poss i b l e t o measu re a much l a r g e r number o f p r o f i l e s under t h e s t e a d y f l o w c o n d i t i o n s o f t h e l a b o r a t o r y . F i g u r e s 32 and 33 show a l l t h e d i m e n s i o n l e s s v e l o c i t y p r o f i l e s in t h e d i s t a l poo l a nd o v e r t h e r i f f l e r e s p e c t i v e l y . T h e s e f i g u r e s c l e a r l y s u g g e s t t h a t t h e r e is m o r e v a r i a b i l i t y in t h e p o o l f l o w s t r u c t u r e t h a n o v e r t h e r i f f l e . Th i s is s o m e w h a t s im i l a r t o t h e s i t u a t i o n o b s e r v e d in t h e f i e l d . The l a r g e numbe r o f p r o f i l e s s amp l e d in t h e l a b o r a t o r y a l l ows f u r t h e r s u bd i v i s i o n s w i t h i n t h e s i t e i n t o u p p e r a nd lower a r e a s o f t h e r i f f l e and o f t h e poo l ( f i g u r e s 34, 35 , 36 and 37 ) . Two g ene r a l c ommen t s eme rge f r o m t h e l a t t e r s e r i e s o f f i g u r e s . F i r s t , t h e g r e a t e r v a r i a b i l i t y in t h e shape o f t h e p r o f i l e s in t h e p o o l , as men t i oned above , can even be b e t t e r 100 Figure 32. Dlmenalonleae velocity prof Ilea In pool, run 14 •o * ' e 1.00 0.80 0.80 -\ 0.20 H 0.10 0.08 0.08 H 0.02 0.01 0.10 1.00 0.80 0.80 H 0.20 H 0.10 0.08 0.08 -4 0.02 0.30 0.80 0.70 0.80 1.10 1.30 1.80 Point / Height averaged velocity Figure 33. Dlmenaionleaa velocity prof ilea over riffle, run 14 0.01 0.10 "i 1 1 1 1 1 1 r 0.30 0.80 0.70 0.80 1.10 1.30 1.80 Point / Height averaged velocity 101 J3 > 1.00 0.80 0.80 0.20 H 0.10 0.08 0.08 H 0.10 Figure 34. Dlmenalonleea velocity profHeo Downstream part of riff la, run 14 0.90 1.10 Point / Haight averaged velocity 1.00 0.80 H 0.80 0.20 0.10 0.08 0.08 Figure 36. Dimenelonlea* velocity prof ilea Upatream part of riffle, run 14 i r 0.10 0.30 0.S0 0.70 0.80 1.10 1.30 1.60 Point / Height averaged velocity 102 Figure 38. Dimensionless velocity profile* Downstream part of pool, run 14 e o 1.00 0.80 0.80 -0.20 -0.10 0.08 0.08 -0.10 0.90 1.10 1.30 1.80 Point / Height averaged veloeity Figure 37. Dlmenalonleea velocity profllea Upstream part of pool, run 14 1.00 0.80 0.80 -» o e e C 0.20 -0.10 0.08 0.08 -0.10 0.30 0.80 0.70 0.90 1.10 1.30 1.80 Point / Height averaged velocity a p p r e c i a t e d . S e c o n d , as in t h e f i e l d s i t u a t i o n t h e p r o f i l e s a r e l e s s Kinked o v e r t h e r i f f l e whe r ea s t h e shape o f t h e poo l p r o f i l e s ' is much m o r e c a p r i c i o u s . T h e s e l a t t e r o b s e r v a t i o n s may r e p r e s e n t a f i r s t i n d i c a t i o n f r o m c o m p a r a t i v e r i v e r h y d r o d y n a m i c s t h a t t h e d i s t a l p oo l may a c t u a l l y e f f e c t i v e l y be i n h e r i t e d . In a d d i t i o n , a n o t h e r s o u r c e o f v a r i a b i l i t y v a l i d f o r t h e d i s t a l poo l is t h e comp lex f l o w s t r u c t u r e engende r ed by t h e banks . S p e c i f i c o b s e r v a t i o n s f r o m f i g u r e s 34 t o 37 a r e (1) t h a t v e l o c i t y p r o f i l e s in t h e upper p a r t o f t h e r i f f l e a r e more v a r i a b l e in shape t h a n in t h e l o w e r p a r t ; (2) t h a t t h e s i t u a t i o n is c o n t r a r y in t h e p o o l . O b s e r v a t i o n (1) s hou l d be e x p e c t e d s i n c e t h e f l o w in t h e r i f f l e ' s uppe r p a r t a c t u a l l y c o n s t i t u t e s t h e e x t e n s i o n o f t h e p r o x i m a l p oo l wh i ch is n o t u n i f o r m in c h a r a c t e r . O b s e r v a t i o n (2) is p o s s i b l y r e l a t e d t o t h e e x t e n s i v e s h e a r i n g wh ich o c c u r s in t h e d o w n s t r e a m , d i s t a l p oo l a r e a ( f u r t h e r d i s c u s s e d b e l o w ) . J u s t as in c h a p t e r 3, l e t us now con s i d e r t h e va r i ab i l i t y o f t h e v e l o c i t y p r o f i l e m e a s u r e m e n t s . A t e s t o f l o c a l v a r i ab i l i t y in v e l o c i t y p r o f i l e s was made on a 4 X 4, f i v e mm s p a c e d gr id nea r t h e r i f f l e c r e s t a t 5.16 m a long t h e f l ume ( r e f e r t o f i g u r e 3 0 ) . F i g u r e 38 shows d i m e n s i o n l e s s v e l o c i t y p r o f i l e s f o r t h e s i x t e e n s t a t i o n s . It s hows t h a t v a r i a b i l i t y in t h e g r a d i e n t o f t h e v e l o c i t y p r o f i l e s o c c u r s f o r t h e wake l a y e r m o s t l y . I n t e r n a l n e a r - b o t t o m s h e a r f r o m t h o s e p r o f i l e s v a r i e d f r o m 0 .55 t o 9.71 Pa, w i t h a v e r a g e o f 2.76 and s t a n d a r d d e v i a t i o n o f 2 .54 Pa . T h e r e f o r e , on a l o c a l s c a l e , t h e i n t e r n a l s h e a r a p p e a r s t o v a r y cons i de rab l y , a t l e a s t o v e r t h e r i f f l e . The same t e s t was n o t p e r f o r m e d in t h e d i s t a l poo l . Flour* aa. Local ahaar atraaa variability taat Dimonalonloea valoclty proflloa, run 14 0.80 - r — . ° - 3 0 0-80 0.70 0.90 1.10 1.30 Point / Height averaged velocity 105 However , i t appea r s r e a s onab l e t o s u g g e s t t h a t t h e v a r i a t i o n in t h e poo l is a t l e a s t o f t h e same magn i tude s ince poo l p r o f i l e s a ppea r t o be m o r e v a r i a b l e . W i t h r e g a r d t o t h e l o c a l v a r i a b i l i t y o f i n t e r n a l s h e a r , t h e c l o s e s p a c i n g o f t h e 16 p r o f i l e m e a s u r e m e n t s a p p e a r s t o s u g g e s t t h e p o s s i b i l i t y o f i nd i v i dua l p a r t i c l e e f f e c t s on w a k e l a y e r g r a d i e n t , a l t h o u g h t h i s cou ld n o t be v e r i f i e d . F ina l ly one mus t cons i de r t h e f a c t t h a t t h e g r i d t e s t a l s o i n c l u d e s t e m p o r a l v a r i a b i l i t y e f f e c t s t h a t may be l o c a l l y i m p o r t a n t , as s u g g e s t e d by t h e f i e l d o b s e r v a t i o n s . H o w e v e r , t e m p o r a l v e l o c i t y p r o f i l e v a r i a b i l i t y c ou l d n o t be i n v e s t i g a t e d in t h e l a b o r a t o r y s ince we did n o t have a r o d - m o u n t e d a r r a y o f h o t - f i l m s . N e x t , c o n s i d e r t h e i n t e r n a l , n e a r - b e d s h e a r d i s t r i b u t i o n t h r o u g h t h e s t u d y a r e a . F i g u r e 39 p r e s e n t s a map o f t h e d i s t r i b u t i o n o f n e a r - b e d s h e a r e s t i m a t e s . The a v e r a g e shea r is 3.121 Pa o v e r t h e r i f f l e (26 s t a t i o n s ) and 2.636 Pa in t h e poo l (18 s t a t i o n s ) . T h e s e v a l u e s , when m u l t i p l i e d by t h e s c a l i n g r a t i o , g i v e 49 .94 and 42.18 Pa r e s p e c t i v e l y , i.e. n e a r l y t h e f i e l d u n i f o r m f l o w e s t i m a t e o f s h e a r s t r e s s ( s e c t i o n 3 . 4 ) . A l t hough we a ppa r en t l y g e t l a r g e r s t r e s s o v e r t h e r i f f l e in t h e l a b o r a t o r y , t h e a c t u a l d a t a s e t which c o v e r s a much g r e a t e r bed a r e a t h a n u n d e r t h e r e s t r i c t i v e c o n d i t i o n s o f t h e f i e l d i n d i c a t e s t h a t t h e a b s o l u t e d i f f e r e n c e may be s m a l l e r t h a n s u g g e s t e d by t h e f i e l d m e s u r e m e n t s . A c c o r d i n g l y , t h e l a b o r a t o r y r e s u l t s wou ld appea r t o s u g g e s t t h a t f i e l d s h e a r e s t i m a t e s a t n e a r b a n k f u l l c ond i t i on s a r e o v e r e s t i m a t e d f o r t h e r i f f l e and more a c c u r a t e f o r t h e poo l . However , t h e m o s t s i g n i f i c a n t po in t in t h e c o n t e x t o f t h i s F i g u r e 39 . Near -bed i n t e r n a l shear e s t i m a t e s a t v e l o c i t y s amp l i ng s t a t i o n s , run 14 Banks R i f f l e c r e s t Note : shear u n i t s a r e P a s c a l s . 4. 4 4. 6 4. a 5. o 6. 2 5. 4 D i s t a n c e f rom f lume i n l e t , in metres 107 r e s e a r c h p r o j e c t is t h a t t h e magn i tude o f n e a r - b e d f o r c e per un i t a r e a a p p e a r s t o be r e a s o n a b l y r e p r o d u c e d , s u p p o r t i n g t h e s i m i l a r i t y t h e o r y as we l l as t h e v a r i o u s d e c i s i o n s made a b o u t t h e bed m a t e r i a l s c a l i n g and t h e c o n f i g u r a t i o n in t h e c u r r e n t h y d r o d y n a m i c m o d e l l i n g e f f o r t . L o c a l v a r i a b i l i t y o f t h e s h e a r e s t i m a t e s d e s e r v e s f u r t h e r a t t e n t i o n b e f o r e any c o n c l u s i o n c an be a d d r e s s e d r ega r d i ng t h e e x i s t e n c e o f p a t t e r n s in s h e a r d i s t r i b u t i o n a t t h e b e d un i t s c a l e . D i s c u s s i o n is e a s i e r f r o m l a b o r a t o r y m e a s u r e m e n t s . F i g u r e 39 o f f e r s s u f f i c i e n t l y c l o s e l y s p a c e d shea r e s t i m a t e s t o c o n f i r m t h a t t h e l o c a l v a r i a b i l i t y c an be c o n s i d e r a b l e , as we a l s o f o u n d f r o m t h e g r i d t e s t d i s c u s s ed t w o pa r ag r aph s above . C o n s i d e r i n g t h i s r e a l i t y , i t may b e c o m e e x t r e m e l y d i f f i c u l t t o r e c o g n i z e any p a t t e r n o f shea r w i th in t h e l imit o f bed un i t s , such as a d v o c a t e d by t h e t e n e t o f a r e v e r s a l in b o t t o m t r a c t i v e f o r c e ( d e s c r i b e d in t h e a p p e n d i x ) . F o r i n s t a n c e , r u n 14 r e s u l t s do n o t s u p p o r t t h e f i e l d o b s e r v a t i o n t h a t h igher shea r t h r o u g h t h e sequence would be c o n c e n t r a t e d in t h e upper p a r t o f t h e r e a c h . The on ly g e n e r a l i z a t i o n s wh i ch may be a t t e m p t e d f r o m f i g u r e 39 c o n c e r n t h e upper p a r t o f t h e d i s t a l poo l and t h e d o w n s t r e a m a r e a o f t h e r i f f l e . In t h e f o r m e r r eg i on , t h e i n t e r n a l s h e a r a p p e a r s e f f e c t i v e l y t o be v e r y low and in t h e l a t t e r i t seems t o be r a t h e r un i f o rm compa r ed t o o t h e r a r e a s . It r ema i n s t h a t t h e l o c a l v a r i a b i l i t y o v e r s h a d o w s t h e s e g e n e r a l s t a t e m e n t s . T h i s s i t u a t i o n w a r r a n t s s o m e c o n s i d e r a t i o n o f t h e r a t i o n a l e beh i nd and t h e c h a r a c t e r i s t i c s o f s u c h v a r i a b i l i t y . P e r h a p s one s o u r c e o f v a r i a b i l i t y c o m e s f r o m t h e i n t r i n s i c c h a r a c t e r i s t i c s o f h igh r e l a t i v e r o u g h n e s s f l o w s . As s u g g e s t e d a b o v e , p e r h a p s i n d i v i d u a l p a r t i c l e s on t h e b e d do h a v e s i g n i f i c a n t e f f e c t on t h e v e l o c i t y p r o f i l e on a l o ca l bas i s and t h e r e b y r e d u c e t h e p h y s i c a l s i g n i f i c a n c e o f a s i n g l e l o c a l measu r emen t . This cou ld n o t be t e s t e d in t h i s p r o j e c t . It is h o w e v e r e x p e c t e d t h a t a spa t i a l l y a v e r a g e d shea r e s t i m a t e f r o m an i n t e n s i v e c o v e r a g e m u s t b e a r s ome s i g n i f i c a n c e w i t h i n g i v en b e d u n i t s . F o r i n s t a n c e , t h e bulK f l o w s h e a r e s t i m a t e f r o m e q u a t i o n (2.16) f o r run 14 is 3.13 Pa, which is e s s en t i a l l y t h e r i f f l e a r e a n e a r - b e d i n t e r n a l shea r a v e r a g e , where t h e f l o w is m o r e u n i f o r m t h a n in t h e d i s t a l p o o l . C l o s e c o r r e s p o n d a n c e b e t w e e n t h e l o c a l s p a t i a l l y a v e r a g e d and u n i f o r m f l o w m e t h o d s f o r s h e a r e s t i m a t i o n o v e r r e l a t i v e l y h i gh r e l a t i v e r o u g h n e s s f l o w s was a l so r e p o r t e d by E i n s t e i n and El Samni (1949). A b r i e f c o n s i d e r a t i o n o f r un 13 s h e a r e s t i m a t e s f u r t h e r c h a r a c t e r i z e s t h e v a r i a t i o n in a v e r a g e o r i n t e r n a l s h e a r . Run 13, d e s p i t e a s l i g h t l y d i f f e r e n t c o n f i g u r a t i o n , was p e r f o r m e d a t t h e same d i s c h a r g e as r un 14. A v e r a g e i n t e r n a l s h e a r c a l c u l a t e d f r o m v e l o c i t y p r o f i l e s was f o u n d t o be 3.26 Pa and 1.36 Pa f o r t h e r i f f l e and d i s t a l p oo l r e s p e c t i v e l y . T he d i f f e r e n c e comes f r o m t h e f a c t t h a t l a r ge l o ca l s h ea r s in t h e d i s t a l poo l as f o u n d in run 14 we r e n o t measu red f o r run 13 a l t h o u g h t h e s p a t i a l c o v e r a g e was compa rab l e . Because o f some m o r p h o l o g i c a l d i f f e r e n c e s t h e p a t t e r n s o f s h e a r w i t h i n t h e un i t s c a n n o t be c ompa r ed a t a d e t a i l e d l e v e l . T h e r e f o r e , i t is d i f f i c u l t t o c o n c l u d e w h e t h e r o r n o t t h e d i s t a l poo l and t h e r i f f l e s p a t i a l l y a v e r a g e d s h e a r s a r e d i f f e r e n t . N e v e r t h e l e s s , t w o c a s e s (i.e. t h e f i e l d and run 13 mea su r emen t s ) t e n d t o show 109 t h a t d i s t i n c t l y g r e a t e r a v e r a g e s h e a r t a k e s p l a c e o v e r t h e r i f f l e . F i na l l y , t h e f l o w s t r u c t u r e as r e v e a l e d by t h e f l o w l i n e s c an be c o m p a r e d . - F i gu r e 40 shows t h e f l o w l i n e s a t c h a n n e l f o r m i n g d i s c h a r g e . By c o m p a r i n g i t w i t h f i g u r e 21, one r e a l i z e s t h a t p a t t e r n s a r e s im i l a r . Th i s a p p e a r s t o v a l i d a t e t h e s i m p l i f i c a t i o n o f t h e c o n f i g u r a t i o n and t h e poo l e x c a v a t i o n a p p r o a c h in t h e s e n s e t h a t t h e m a c r o f l o w s t r u c t u r e is a d e q u a t e l y r e p r o d u c e d . I t s r e p r o d u c t i o n c o m b i n e d w i t h a p p r o p r i a t e s c a l i n g o f t h e i n t e r n a l s h e a r f u r t h e r c o n f i r m t h a t t h e f i e l d a nd f l u m e s i t u a t i o n s a r e h y d r o d y n a m i c a l l y s i m i l a r . H o w e v e r , a f i n a l t e s t o f t h e h y d r o d y n a m i c s i m i l a r i t y is i n t r o d u c e d in t h e n e x t s e c t i o n . 4 .3 .3 S u r f a c e b e d m a t e r i a l c h a r a c t e r i s t i c s and c o n f o r m a t i o n The c o m p a r i s o n o f t h e c h a r a c t e r i s t i c s o f t h e s u r f a c e m a t e r i a l s b e t w e e n t h e f i e l d and f l u m e s i t u a t i o n r e p r e s e n t s a f u r t h e r t e s t o f t h e s u c c e s s o f t h e mode l l i n g p r og r amme . It. s p e c i f i c a l l y a d d r e s s e s t h e r e p r o d u c t i o n o f e n t r a i n m e n t c ond i t i o n s in t h e mode l ( s e c t i o n 2.5.2) f o r i f t h e r e was e x c e s s s h e a r f o r c e f o r a c e r t a i n c l a s s ( t h e f i n e s t p a r t i c l e s ) o f bed s e d i m e n t , t h e c h a r a c t e r i s t i c s o f t h e s u r f a c e l a y e r and l i ke l y t h e f r e q u e n c y o f b e d m a t e r i a l t r a n s p o r t w o u l d n o t be dynam i ca l l y e q u i v a l e n t in t h e f i e l d and t h e mode l . S u r f a c e c o a r s e n i n g o f a w ide ly d i s t r i b u t e d m i x t u r e ha s b e e n o b s e r v e d in a v a r i e t y o f l a b o r a t o r y e x p e r i m e n t s by G. P a r k e r and a s s o c i a t e s ( e . g . P a r k e r e t a l , 1982). C o a r s e n i n g F i g u r e 40. N e a r - b o t t o m a n d n e a r - s u r f a c e f l o w l i n e s , r u n 14 • B a n k s R i f f 1 e c r e s t N e a r - s u r f a c e f l o w l i n e s ~ N e a r - b e d f l o w l i n e s 4 . 4 4 . 6 4. a 5 . 0 6 . 2 5 . 4 5.5 D i s t a n c e f r o m f l u m e i n l e t , i n m e t r e s I l l a p p e a r s n o t t o be r e l a t e d t o p r e f e r e n t i a l e n t r a i n m e n t o f some p a r t i c l e s i zes b u t r a t h e r a c t s t o r e g u l a t e t h e va r i ous s ed imen t s i z e s in t r a n s p o r t in p r o p o r t i o n t o t h e i r p r e s e n c e in t h e s u b s u r f a c e m a t e r i a l s , a p r o c e s s wh i ch has b e c o m e Known as equ i l i b r i um t r a n s p o r t . A r m o u r d e v e l o p m e n t u n d e r s t e a d y f l o w ( P a r k e r e t a l , 1982) was an i n d i c a t i o n t h a t c o a r s e n i n g c o n s t i t u t e s a mob i l e b e d p h e n o m e n o n . R e p r o d u c t i o n o f t h e a rmou r l a y e r c h a r a c t e r i s t i c s in t h e s c a l e mode l wou ld t h u s i n d i c a t e t h a t i n t e r a c t i o n s o f b e d p a r t i c l e s d u r i n g b e d f o r m a t i o n w e r e p r o p e r l y a c c o u n t e d f o r . A v i s u a l a p p r e c i a t i o n o f s u r f a c e c o a r s e n i n g wh i ch o c c u r r e d du r i n g r un 14 c an be g a i n ed f r o m f i g u r e 31. The s u r f a c e ma t e r i a l s we re samp led o v e r t h e r i f f l e us ing a 10 X 10 cm, 1 cm space gr id which was s e t a t t w o s ide by s ide l o c a t i o n s . Hence 2 0 0 s t o n e s w e r e c o l l e c t e d . In t h e poo l , 5 0 s t o n e s w e r e c o l l e c t e d a t t h e same spac ing b u t us ing a r u l e r d i s p o s e d r a n d o m l y . In b o t h c a s e s t h e s t o n e s w e r e c o l l e c t e d w i t h t w e e z e r s and t h e i r b - a x e s w e r e m e a s u r e d w i t h c a l i p e r s . The r e s u l t s f r o m t h e sampl ing e x e r c i s e a r e shown on f i g u r e 41 w h e r e t h e ' c o m b i n e d ' s amp l e c u r v e a c t u a l l y c omb i ne s t h e poo l and r i f f l e s t o n e s . It is s een t h a t t h e d i f f e r e n c e b e t w e e n t h e poo l and r i f f l e s amp l e s is sma l l , t h e r i f f l e b e i ng s l i g h t l y c o a r s e r t h a n t h e poo l , bu t n o t qu i t e in t h e same way as in t h e f ie ld . In g e n e r a l , t h e D50 o f t h e s u r f a c e in t h e l a b o r a t o r y a r e f r o m 3.41 t o 4.06 mm. S ca l ed up, t h i s is equ i va l en t t o 54.6 t o 65 mm, i.e. s l i g h t l y c o a r s e r t h a n f o r t h e f i e l d s i t u a t i o n ( r e f e r t o t a b l e 2 ) . F i g u r e 42 s h o w s t h e g r a i n s i z e 112 Figure 41. Size distribution* of surface bod material sample* run 14 -3.0 -1.0 1.0 3.0 Sieve size (In phi) 113 Figure 42. Grain alze distribution, run 14 Initial and final aurfaea materials 1.0 -, — Grain size (In phi) 114 d i s t r i b u t i o n o f t h e i n i t i a l m i x t u r e and t h a t o f t h e s u r f a c e m a t e r i a l a t t h e end o f t h e run . It can be eas i ly s een t h a t s u r f a c e c o a r s e n i n g i n v o l v e d t h e s e l e c t i v e e n t r a i n m e n t o f f i n e r p a r t i c l e s in t h i s n o - f e e d s i t u a t i o n . F o r t h e f i n a l s u r f a c e , t h e D i g . D 5 0 a n d D 9 0 a r e r e s p e c t i v e l y 6 . 15 , 1.89 and 1.16 t i m e s c o a r s e r t h a n f o r t h e i n i t i a l m i x t u r e . T h e s e r a t i o s compare wel l w i t h t h o s e f r o m f i e l d d a t a ( t a b l e 1). This c o r r e s p o n d a n c e s u g g e s t s t h a t s e d i m e n t t r a n s p o r t is p r o b a b l y q u i t e low in t h e f i e l d and s u p p o r t s t h e n o - f e e d p r o c e d u r e in t h e l a b o r a t o r y wo r k . F i g u r e 43 c o m p a r e s t h e v a r i o u s s amp l e s w i t h t h e i r s c a l e d f i e l d e q u i v a l e n t s . F l ume s u r f a c e a n d s c a l e d f i e l d s u r f a c e ( a c t u a l l y f r o m t h e d o w n s t r e a m samp l i ng s i t e ) compa re v e r y we l l o v e r a l l g ra in s i zes . However , a c ompa r i s on o f poo l as we l l as r i f f l e s amp l e s f u r t h e r r e v e a l s t h a t t h e s u r f a c e m a t e r i a l s we r e c o a r s e r in t h e f l u m e e s p e c i a l l y b e l o w t h e nso* C o a r s e r s i ze d i s t r i b u t i o n c h a r a c t e r i s t i c s f o r t h e f i n e s t p a r t o f f l u m e s i ze d i s t r i b u t i o n is p e r h a p s t o be e x p e c t e d b e c a u s e o f t h e t r u n c a t i o n and c o r r e c t i o n app l i e d t o t h e f l u m e m i x t u r e . The-s i z e d i s t r i b u t i o n s o f t h e p o o l s a m p l e s a r e s e e n t o be r e l a t i v e l y c l o s e a b o v e t h e D 5 0 w h e r e a s r i f f l e s e d i m e n t s s e em f i n e r t h a n w a n t e d a b o v e D 7 5 . It is a l so wo r t hwh i l e t o n o t e t h a t t h e r u n 13 s a m p l e s ( n o t s h o w n h e r e i n ) a l s o c o r r o b o r a t e t h e l a t t e r o b s e r v a t i o n s . The f i r s t t h i n g t o c o n s i d e r in o r d e r t o a s s e s s t h e f o r e g o i n g d i s c r e p a n c i e s is s a m p l i n g l i m i t a t i o n s . T he g r i d s u s ed t o s amp l e s u r f a c e s e d i m e n t s in t h e f i e l d and l a b o r a t o r y w e r e n o t g e o m e t r i c a l l y s im i l a r . More f u n d a m e n t a l l y s amp l e s a r e 115 Figure 43. Grain alia distribution comparison Run 14 snd field (seslod) surface aamples 1 -i • m-Graln alze In phi Flume surface — I Field surface (acaled) •O- Flume riffle —A— Field riffle (acaled) •X-- Flume pool Field pool (acaled) 116 a l w a y s a p p r o x i m a t i o n s t o a c e r t a i n d e g r e e . I n d e e d i t is w e l l - k n o w n t h a t t h e r e e x i s t s a p r a c t i c a l l o w e r l im i t t o which t h e s u r f a c e m a t e r i a l s may be s amp l e d ( C h u r c h e t a l , 1987) . Th i s r e a l i t y is more c r i t i c a l f o r t h e f i n e r f l ume sed imen t and may a l s o p a r t l y a c c o u n t f o r t h e f a c t t h a t some l a b o r a t o r y s amp l e s a p p e a r c o a r s e r . A n o t h e r e l e m e n t c o n s i s t s o f t h e f a c t t h a t t h e f i e l d v o l u m e t r i c s amp l e was t r u n c a t e d and f u r t h e r a d j u s t e d f o r mode l l ing pu rposes in o r d e r t o g e t c l o s e r t o f i e l d r e s i s t a n c e o r e n t r a i n m e n t c o n d i t i o n s in t h e l a b o r a t o r y . T h e p o s s i b i l i t y o f s ome f i n e r p a r t i c l e s in t h e l a b o r a t o r y a c t u a l l y e x p e r i e n c i n g l o w e r d i m e n s i o n l e s s c r i t i c a l s h e a r s t r e s s t h a n t h e c o r r e s pond i ng f i e l d ones would en cou r age t h e l ower p a r t o f t h e s i z e d i s t r i b u t i o n t o b e c o m e c o a r s e r . F i n a l l y , a d d i t i o n a l e f f e c t s may a r i s e f r o m t h e f a c t t h a t c o a r s e m a t e r i a l in t h e s e l e c t e d m i x t u r e was u n d e r r e p r e s e n t e d f r o m t h e s t a r t ( s e e f i g u r e 2 6 ) . P e r h a p s some o p e r a t i n g a s s u m p t i o n s m a d e f o r t h e t h e l a b o r a t o r y wo r k h a ve a s i g n i f i c a n t i n f l u e n c e as w e l l . T h e a s s u m p t i o n t h a t t h e b ed mo rpho l ogy is mainly d e t e r m i n e d v ia a s i ng l e o r r e s t r i c t e d r a n g e o f d i s c h a r g e may n o t h a v e a l l owed d e v e l o p m e n t o f e x a c t l y s i m i l a r s u r f a c e c h a r a c t e r i s t i c s . P e r h a p s t h e d e c i s i o n t o i gnore s ed imen t f e e d i n g t o t h e s y s t e m c o u l d h a ve b e e n d e t r i m e n t a l . H o w e v e r , t h e a r g u m e n t s p r e s e n t e d a b o v e r e g a r d i n g t h e s i m i l a r i t y o f t h e s i z e d i s t r i b u t i o n p e r c e n t i l e s b e t w e e n t h e f i e l d and l a b o r a t o r y seem t o c o n s t r a i n t h i s l a t t e r e v e n t u a l i t y . In summary , d e s p i t e s ome minor d i s c r e p a n c i e s , t h e a c t u a l s i ze d i s t r i b u t i o n c h a r a c t e r i s t i c s o f t h e f i e l d and f l ume can be 117 c o n s i d e r e d c l o s e . I t m u s t a l s o b e c o n s i d e r e d t h a t t h e f r i c t i o n a l c h a r a c t e r i s t i c s o f t h e s t r e a m which mos t l y depend on a b ed m a t e r i a l l e n g t h s c a l e w e r e a p p r o p r i a t e l y r e p r o d u c e d in t h e l a b o r a t o r y . It is t h e r e f o r e s u g g e s t e d t h a t t h e a c t u a l mode l r un r e p r o d u c e d s a t i s f a c t o r i l y t h e c h a r a c t e r i s t i c s o f t h e s u r f a c e b e d m a t e r i a l a nd t h a t d y s f u n c t i o n , i f any , was r e l a t i v e l y s m a l l . 4 .4 E x t e n d e d o b s e r v a t i o n s on s t u d y s i t e c h a r a c t e r i s t i c s  f r o m f i e l d a nd l a b o r a t o r y , T u r b u l e n c e c h a r a c t e r i s t i c s w e r e n o t s p e c i f i c a l l y m e a s u r e d w i t h i n t h e l im i t s o f t h i s p r o j e c t . H o w e v e r t h e minimum and maximum v e l o c i t y which we re r e c o r d e d a t e a ch po in t mea su r emen t in t h e f l u m e c an be l o o k e d upon as an index o f t u r b u l e n c e c h a r a c t e r i s t i c s . F i g u r e s 44 and 45 p r e s e n t a g r a p h o f t h e d e v i a t i o n f r o m a v e r a g e v e l o c i t y f o r t h e minimum and max imum v a l u e s r e c o r d e d w i t h i n t h e o n e - m i n u t e p e r i o d v e r s u s t h e r e l a t i v e h e i g h t , f o r t h e p o o l and r i f f l e u n i t r e s p e c t i v e l y . T h e s e f i g u r e s s u g g e s t t h r e e t h i n g s : (1) t h e minimum v e l o c i t y v e r s u s t h e r e l a t i v e h e i g h t b e a r a l i n e a r r e l a t i o n s h i p , (2) t h e max imum v e l o c i t y v e r s u s t h e r e l a t i v e h e i g h t b e a r a l o g a r i t h m i c r e l a t i o n s h i p , and (3) f o r b o t h t h e minimum and t h e max imum v e l o c i t y , t h e d e v i a t i o n s a r e g r e a t e r a t t h e r i f f l e s t a t i o n s . T h e l a t t e r p o i n t s u g g e s t g r e a t e r t u r b u l e n c e i n t e n s i t y in g e n e r a l o v e r t h e r i f f l e a r e a . In t h i s p a r t i c u l a r s t u d y , i t a p p e a r s t h a t t h o s e m a c r o f l o w s t r u c t u r e s w h i c h b r i n g t h e g r e a t e s t d e v i a t i o n f r o m t h e a v e r a g e v e l o c i t y a r e m o r e a c t i v e 118 > a Figure 44. Deviation* from one-mlnut* average velocity Maximum and minimum value* in pool 0.00 0.20 0.40 0.60 0.80 Relative height Figure 48. Devlatlone from one-minute average velocity Maximum and minimum valuea over riffle 200 190 180 170 160 180 140 130 120 110 100 90 80 70 60 80 40 30 20 • max + min • 0 l • • • • W + • • + 0.00 0.20 0.40 0.60 0.80 Relative height 119 o v e r t h e r i f f l e a r e a t h a n o v e r t h e d i s t a l p o o l . T h e s e a r e e x p o r t e d f r o m t h e e x t r e m e l y t u r b u l e n t p r o x i m a l p oo l i n t o t h e r i f f l e a r e a and d i s s i p a t e b e f o r e r e a c h i n g t h e d i s t a l p o o l . S i m i l a r m e a s u r e m e n t s f o r t h e c a s e o f a f u l l y a l l u v i a l p o o l - r i f f l e s e q u e n c e c o u l d h e l p c l a r i f y t h e q u e s t i o n o f m a c r o - t u r b u l e n c e t h r o u g h t h e s e q u e n c e , a s r a i s e d by R o o d (1980). It is w o r t h w h i l e t o d e s c r i b e b r i e f l y t h e r e s u l t s f r o m dye i n j e c t i o n s a t t h e end o f run 14 in t h e d i s t a l poo l nea r t h e r i g h t banK. F low p a t t e r n s o f f i g u r e 4 0 we re c o n f i r m e d . The e x i s t e n c e o f a r i f f l e s l ip f a c e v o r t e x cou ld be d e t e c t e d . More i n t e r e s t i n g l y t h e p r e s e n c e o f a s h e a r l i ne t h r o u g h t h e d i s t a l poo l , which was n o t e d dur ing t h e November 23 high f l o w e v e n t ( see f i g u r e 21), was a l so d e t e c t e d . I n j e c t i o n o f dye nea r t h e r i g h t banK above t h e d i s t a l poo l (near t h e c r o s s - o v e r ) r e v e a l e d t h a t t h e l o ca l w a t e r f l o w e d c l o s e t o t h e banK (w i th in a f e w c e n t i m e t e r s ) w i t h o u t any m i x i n g w i t h t h e ma i n f l o w . C o n v e r s e l y , d ye , when i n j e c t e d o v e r t h e r i f f l e , r e v e a l e d t h a t t h e main w a t e r body did n o t f l o w t o t h e r i g h t o f . t h e shear , l i ne , i.e. a l o n g t h e r i g h t b a n k . T h e s e o b s e r v a t i o n s h o l d w h e r e v e r t h e dye was i n j e c t e d and a t any d e p t h w i th in t h e d i s t a l p o o l . T h u s , a p p a r e n t l o n g i t u d i n a l s h e a r i n g f r o m t h e f i e l d s i t e c o u l d be r e p r o d u c e d and f u r t h e r o b s e r v e d in t h e l a b o r a t o r y . T h i s r e p r e s e n t s a f u r t h e r i n d i c a t i o n o f t h e s u c c e s s o f t h e s ca l i ng e x e r c i s e and o f t h e mode l l ing s t r a t e g y . I n j e c t i o n o f dye nea r t h e bed a t t h e po s i t i on o f t h e shea r l ine r e v e a l e d d i v e r g e n c e o f t h e c u r r e n t a t t h e b ed . A l t hough t h e s e phenomenon o c c u r r e d , i t would appear as t hough t h e y had 120 no m o r p h o g e n e t i c s i g n i f i c a n c e w i t h r e s p e c t t o t h e d i s t a l poo l s i n c e t h e l a t t e r was dug and s e e m s t o r e p r e s e n t a r e l i c t f e a t u r e , as we d i s cu s sed above . The mac ro f l o w s t r u c t u r e would t h e n me r e l y r e p r e s e n t an a d j u s t m e n t t o an imposed morpho logy f r o m t h e s t a r t . N e v e r t h e l e s s i t may help poo l p r e s e r v a t i o n by r o u t i n g some o f t h e e n t e r i n g sed imen t a long t h e r i f f l e f a c e and n o t a long t h e r i g h t bank. Dur ing u p s t r e a m poo l d e ve l opmen t , f o r ea ch s ing le run , t h e phenomenon o f l a t e r a l s o r t i n g d u r i n g t h e s c o u r i n g o f t h e p r o x i m a l p o o l c o u l d be o b s e r v e d . T h e c o a r s e s t p a r t i c l e s t r a v e l l e d on t h e s i de o f t h e o b s t r u c t i o n whi le f i n e s t m a t e r i a l m o v e d m o s t l y a l o n g t h e f a c e o f t h e d e v e l o p i n g r i f f l e . A c c o r d i n g l y , on ly t h e f i n e s t e n t r a i n e d m a t e r i a l s e n t e r e d t h e d i s t a l p oo l in wh i ch t h e s h e a r was l a r g e enough t o e v a c u a t e t h e m . Hence t h e p r e s e r v a t i o n o f t h e d i s t a l poo l would appea r t r u l y t o be l i n k ed t o t h e f l o w hyd rodynam i c s imposed by t h e c o n f i g u r a t i o n o f t h e c r e e k and t o t h e a s s o c i a t e d s e d i m e n t t r a n s p o r t r o u t e s . Now, c o n c e r n i n g t h e g e n e r a l s e d i m e n t t r a n s p o r t c o n d i t i o n s , i t may be a s ked i f b e d f o r m s would deve l op in B laney C r eek s t ud y s i t e i f t h e r e we r e no o b s t r u c t i o n s t o t h e f l o w (i.e. i f t h e c h a n n e l w e r e s t r a i g h t ) . The q u e s t i o n is e s p e c i a l l y v a l i d s i n c e t h e e x c e s s shea r a t high f l o w s is r a t h e r low, as d i s c u s s ed in c h a p t e r 3. To app roa ch t h i s i s sue , an add i t i ona l f l ume run was p e r f o r m e d w i t h t h e same sed imen t m i x t u r e (run 15) a f t e r t h e main body o f t h e r e s e a r c h had been accomp l i shed . The f l ume bed was s e t a t 0.01 ( t h e a c t u a l s lope va lue in t h e s t ud y r e a c h a t near b an k f u l l d i s cha rge ) . The f l o w r a t e was near t h a t o f run 14 ( s ee t a b l e 4 f o r c o m p l e t e i n f o r m a t i o n a b o u t h y d r a u l i c and g e o m e t r i c p a r a m e t e r s f o r run 15). The run l a s t e d t w o days and no b e d f o r m s d e v e l o p e d . The s l ope appea r ed t o s l i gh t l y i n c r ea se and t h e a v e r a g e d e p t h was lower t h a n in run 14. The a v e r a g e s h e a r f r o m s i x v e l o c i t y p r o f i l e s a nd v i a u n i f o r m f l o w f o r m u l a t i o n was 2.66 and 2.56 Pa r e s p e c t i v e l y . A l t hough a bed a r m o u r d e v e l o p e d ( n o t a n a l y z e d ) , no p l a n a r s e g r e g a t i o n o f s e d i m e n t o c c u r r e d . T h e r e f o r e , i t a p p e a r s t h a t , w i t h o u t t h e p r e s e n c e o f o b s t r u c t i o n s t o t h e f l o w , no u n d u l a t i o n w o u l d deve l op on t h e bed a t b an k f u l l f l o w . 122 T a b l e 4 H y d r a u l i c a n d g e o m e t r i c p a r a m e t e r s f o r r u n 15 D i s c h a r g e 5 . 1 0 ( 1 s - 1 ) M e a n v e l o c i t y 4 1 . 0 ( cm s _ 1 ) W i d t h 4 7 . 0 ( cm ) M e a n d e p t h 2 . 6 4 ( cm ) H y d r a u 1 i c r a d i u s 2 . 3 7 ( cm ) W a t e r 0 . 0 1 1 s u r f a c e s l o p e F r o u d e n u m b e r 0 . 8 1 R e y n o l d s n u m b e r 7 0 0 0 F r i c t i o n f a c t o r 0 . 1 3 123 5 .0 C o n c l u s i o n s The p r e s e n t s t u d y ha s c o n f i r m e d ou r h y p o t h e s i s t h a t u n d i s t o r t e d p h y s i c a l m o d e l l i n g b a s e d on t h e p r i n c i p l e s o f s im i l i t u d e r e p r e s e n t s a r e a s o n a b l e a l t e r n a t i v e t o f i e l d r e s e a r c h in g e o m o r p h o l o g y f o r t h e c a s e o f low t r a n s p o r t i n t e n s i t y and sma l l t o i n t e r m e d i a t e s i ze r i v e r s . A s p e c i f i c v e r i f i c a t i o n o f some d e t a i l e d h y d r o d y n a m i c q u a n t i t i e s f o r a s m a l l p o o l - r i f f l e , g r a v e l b ed s t r e a m (B laney C r e e k ) r e v e a l e d t h a t , d e s p i t e some u n c o n t r o l l a b l e v a r i a b i l i t y m o s t l y r e l a t e d t o t h e n a t u r e o f t h e e n v i r o n m e n t a nd t h e l i m i t a t i o n s o f t h e m e a s u r e m e n t s , f i e l d p r o c e s s e s c an be c o n v i n c i n g l y r e p r o d u c e d . Hence t h e c l a ims o f K n i g h t o n (1984) and Mos ley and Z i m p f e r (1978) as we l l as t h e v i e w p o i n t t a k e n by S c h u m m and o t h e r s ( 1 9 8 7 ) , i n t r o d u c e d in c h a p t e r 1, appear n o t t o be va l id f o r t h e geomorpho l og i c a l s y s t e m wh ich was s t u d i e d h e r e i n . C o n c u r r e n t l y , t h e s u b j e c t o f l o c a l v a r i a b i l i t y in t h e v e l o c i t y p r o f i l e s and s h e a r e s t i m a t e s o v e r a r i v e r b e d o f r e l a t i v e l y h igh r oughne s s was shown t o d e s e r v e some a t t e n t i o n in f u t u r e r e s e a r c h . Th i s i ssue mus t p robab l y be t a c k l e d b e f o r e any p r o c e s s o r i e n t e d s t u d i e s d e s i g n e d t o s t u d y s a l i e n t c h a r a c t e r i s t i c s ( n o t a b l y t e m p o r a l v a r i a b i l i t y o r i n t e r m i t t e n c y ) and e f f e c t s o f m a c r o f l o w s t r u c t u r e s in p o o l - r i f f l e s t r e a m s can be u n d e r t a k e n . The p r e s e n t r e s e a r c h a l so i n t e nded t o d e m o n s t r a t e a sca l i ng s t r a t e g y d e s i g n a t e d as " g e n e r i c " m o d e l l i n g f o r a p o o l - r i f f l e s e q u e n c e , gu i ded by a p a r t i c u l a r f i e l d p r o t o t y p e . In a " g e n e r i c mode l " t h e o b j e c t is t o r e p r o d u c e c e r t a i n key f e a t u r e s o f some f u l l s i z e p h e n o m e n o n , in t h i s c a s e , t h e p o o l - r i f f l e c o u p l e t . Th i s p a r t i c u l a r m o d e l o f B l a n e y C r e e k f a i l e d t o do t h i s s p o n t a n e o u s l y a n d t h i s r a i s e d t h e q u e s t i o n w h e t h e r t h e p o o l - r i f f l e s e q u e n c e in t h e p r o t o t y p e s t r e a m is r e l i c t . In t h e p r e s e n t s i t u a t i o n , p a r t o f t h e b o u n d a r y c o n d i t i o n s had t o be man ipu la ted : t h i s r e p r e s e n t e d a s h i f t f r o m a " g e n e r i c " t o a more " s p e c i f i c " m o d e l . A v e r y i m p o r t a n t l e s s o n f r o m t h e l a t t e r s i t u a t i o n is t h a t h i s t o r i c a l s p e c i f i c s ( t h e c o n f i g u r a t i o n a l a s p e c t s in S i m p s o n ' s (1963) sense) c a n n o t be c on s i d e r e d in a gene r i c mode l : i t appea r s t h a t s o m e p o o l - r i f f l e f e a t u r e s a r e o f t h i s t y p e . T h e s e d i s c o v e r i e s in f a c t r e p r e s e n t a f u n d a m e n t a l and c o m p r e h e n s i v e l e s s o n on t h e n a t u r e o f g e o m o r p h o l o g i c a l i n v e s t i g a t i o n s b u t do n o t i n v a l i d a t e t h e c o n c e p t o f " g e n e r i c " mode l l ing as a t o o l f o r r e s e a r c h on t h e p o o l - r i f f l e t o p i c . F r o m a n o t h e r p e r s p e c t i v e , t h e s e e l e m e n t s r e p r e s e n t a c o n t r i b u t i o n made by t h e l a b o r a t o r y e x p e r i m e n t t o t h e u n d e r s t a n d i n g o f t h e h i s t o r i c a l d e v e l o pmen t o f t h e r i v e r r e a c h o f i n t e r e s t . It appea r s t h a t o b s t r u c t i o n t o t h e main t h r e a d o f f l o w in B l aney C r e e k c o n t r o l s r i f f l e and poo l d e v e l o p m e n t a nd l o c a t i o n , a s f o u n d by G a l l i n a t t i ( 1 9 8 4 ) a nd L i s l e ( 1986 ) . T h e o b s e r v a t i o n s a l s o p r o v i d e d a d d i t i o n a l e l e m e n t s f o r t h e unde r s t and i ng o f t h e mechan isms and r e s pon s e o f v e r y low t r a n s p o r t i n t e n s i t y s t r e a m s . In t h e c a s e o f B laney C r e e k , i t a p p e a r s t h a t low t r a n s p o r t i n t e n s i t y c omb i n ed w i t h p r e f e r e n t i a l t r a n s p o r t r o u t e s e n c o u r a g e t h e p r e s e r v a t i o n o f t h e d i s t a l poo l . A bo ve a l l , t h e main c o n t r i b u t i o n o f t h i s r e s e a r c h p r o j e c t l i e s in t h e r e c o g n i t i o n t h a t mode l s t u d i e s a r e l e g i t i m a t e t o o l s f o r r e s e a r c h w i t h r e s p e c t t o f u l l - s c a l e g r a v e l - b e d r i v e r s i t u a t i o n s . This emerg ing f a c t can be embod ied i n t o a c o n c e p t o f g e n e r i c m o d e l l i n g a c c o r d i n g t o wh i c h q u a n t i t a t i v e o b s e r v a t i o n s done in a mode l s i t u a t i o n f o r which p r ope r c o n s i d e r a t i o n o f s c a l e e f f e c t s is made c an be gene ra l i z ed and used t o i n t e r p r e t r i v e r b e h a v i o u r . G e n e r i c m o d e l s in f l u v i a l g e o m o r p h o l o g y a r e r e s t r i c t e d t o s o m e f a m i l y o f f i e l d s i t u a t i o n s d e t e r m i n e d b a s i c a l l y by e c o n o m i c a l a nd p r a c t i c a l l i m i t a t i o n s o f l a b o r a t o r y f l umes t h e m s e l v e s a n d / o r by t h e n a t u r e o f p r o t o t y p e bed m a t e r i a l . In a " g e n e r i c " mode l , a p r o t o t y p e r e a c h may be used t o guide t h e mode l s c a l e s bu t t h e o b j e c t is n o t t o r e p r o d u c e e x a c t boundary de ta i l s o f t h e r e a c h . Such mode ls can in f a c t be sa id t o be p a r t o f t h e same f a m i l y as t h e f i e l d s i t u a t i o n . The impo r t a n c e o f t h e s e c o n c e p t u a l a r g u m e n t s l i e s in t h e f a c t t h a t m e a s u r e m e n t p r o g r a m m e in many f i e l d s i t u a t i o n s a r e i m p r a c t i c a l , i n d e e d i m p o s s i b l e , t o p e r f o r m . An i m m e d i a t e f a l l o u t o f s u ch an a s s e r t i o n is t h a t r a p i d p r o g r e s s on g r a v e l - b e d r i v e r p r o b l e m s p o t e n t i a l l y c o u l d be r e a l i z e d in a r e l a t i v e l y s h o r t p e r i o d o f t i m e , l a b o r a t o r y c o n d i t i o n s a l l ow i ng t h e m a n i p u l a t i o n by t h e e x p e r i m e n t a l i s t o f t h e f a c t o r s c o n t r o l l i n g r i v e r p r o c e s s e s . In p a r t i c u l a r , t h e p o o l - r i f f l e s e q u e n c e was me r e l y t h e v e h i c l e f o r a m o d e l - f i e l d c o n f o r m a t i o n e x e r c i s e h e r e i n . H o w e v e r , t h e nume rous p oo r l y known a s p e c t s o f p o o l - r i f f l e c h a r a c t e r i s t i c s c o u l d be e x am i ned in a t h o r o u g h e x p e r i m e n t a l p r o g r a m m e c omb i n ed w i t h f i e l d o b s e r v a t i o n s . 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United States Geological Survey Profes-sional Paper 2 7 1 . Wo lman, M.G. and B r u s h , L.M. , 1961, F a c t o r s c o n t r o l l i n g t h e s ize and shape o f s t r e a m channe l s in noncohes i ve c o a r s e s a n d s . United States Geological Survey Profesional Paper 2 8 2 G , p . 1 8 3 - 2 1 0 . Ya l i n , M.S., 1971a, Theory of Hydraulic Models. L o n d o n , M a c m i 11 a n . Ya l in , M.S., 1971b, On t h e f o r m a t i o n o f dunes and meande r s . Proceedings of the 14th International Congress of the International Association for Hydraulic Research, vo l . 3, paper C13, p. 1-8. Ya l i n , M.S., 1982, On t h e s i m i l a r i t y o f p h y s i c a l m o d e l s . in Hydraulic Modelling in Maritime E ngineering, London, Thomas T e l f o r d L t d , p. 1-14. 138 Ya l i n , M.S. and K a r ahan , E., 1979a , I n c e p t i o n o f s e d i m e n t t r a n s p o r t . Proceedings of the American Society of Civil Engineers, Journal of the Hydraulics Division, v o l . 105, p. 1 433 - 1443 . Ya l i n , M.S. and K a r ahan , E., 1979b , S t e e p n e s s o f s e d i m e n t a r y dunes . Proceedings of the American Society of Civil Engineers, Journal of the Hydraulics Division, v o l . 105, p. 3 8 1 - 3 9 2 . Yang , C.T., 1971, F o r m a t i o n o f p oo l s Resources Research, v o l . 7 , p. and r i f f l e s . 1 5 6 7 - 1 5 7 4 . Water Z e l l e r , J . , 1967, Meande r i ng c h a n n e l s in S w i t z e r l a n d . Inter-national Association of Scientific Hydrology Publication no. 75, p. 1 7 4 - 1 8 6 . Appendix : Bed c o n f i g u r a t i o n s in low mobi l i ty r i v e r s A.1 G e n e r a l c o n s i d e r a t i o n s A ub i q u i t o u s b ed c o n f i g u r a t i o n in l o w - m o b i l i t y , g r a v e l - b e d r i v e r s c o n s i s t s o f more o r l e s s r egu l a r undu la t i ons o f t h e bed , in t h e o r d e r o f s e v e r a l t i m e s t h e c h a n n e l w i d t h . T o p o g r a p h i c a l l y h igh a r e a s o f m o s t l y a g g r a d a t i o n a l n a t u r e a r e t e r m e d r i f f l e s whe r ea s low p o r t i o n s o f t h e bed which e x pe r i e n c e m o s t l y s c o u r i n g c o n d i t i o n s a t s o m e f o r m a t i v e d i s c h a r g e a r e known as p o o l s . In some c h a n n e l s , s t o r a g e b a r s a r e a l s o p r e s e n t . In a c t u a l i t y , t h e s e t h r e e f e a t u r e s , i .e t h e b a r - p o o l - r i f f l e , c a n be c o n s i d e r e d as a s i ng l e f u n d a m e n t a l un i t o f c h a n n e l o r g a n i z a t i o n w h i c h b e a r s m o r p h o l o g i c a l , h y d r o d y n a m i c a l and s e d i m e n t o l o g i c a l ( C h u r c h and J o n e s , 1982 ; F e r g u s o n and W e r r i t y , 1983 ; T h o m p s o n , 1986) as we l l as b i o l o g i c a l ( B e s c h t a and P l a t t s , 1986) s i g n i f i c a n c e . A l t h o u g h s e d i m e n t o l o g i c a l ( L e o p o l d e t a l , 1964) a nd h y d r o d y n a m i c (Wo lman, 1955) c r i t e r i a h a v e b e en i n v o k e d , b e d e l e v a t i o n r e c o r d s p r o v i d e t h e s i m p l e s t a nd m o s t n a t u r a l i n f o r m a t i o n t o d i s t i n g u i s h b e t w e e n p o o l s a n d r i f f l e s . R e g r e s s i o n ( R i c h a r d s , 1 976a , M i lne , 1982a) o r t h e m o r e s o p h i s t i c a t e d b e d f o r m d i f f e r e n c i n g (O 'Ne i l l and A b r a h a m s , 1984) t e c h n i q u e s have been a d v o c a t e d in o r d e r t o d i s c r im ina t e b e t w e e n poo l s and r i f f l e s f r o m a r e c o r d o f l ong i tud ina l bed e l e v a t i o n s . H o w e v e r , t h e s e p r o c e d u r e s i n e v i t a b l y i n t r o d u c e s o m e s i m p l i f i c a t i o n . In r e a l i t y , p o o l s a n d r i f f l e s d i s p l a y a v a r i e t y o f t r i d i m e n s i o n a l m o r p h o l o g i c a l e x p r e s s i o n s r e l a t e d t o t h e h y d r o d y n a m i c and t r a n s p o r t c o n d i t i o n s o f any g i v e n s i t u a t i o n . Bed e l e v a t i o n s e r i e s a r e n e v e r t h e l e s s u s e f u l t o s t ud y t h e s p a c i n g o f t h e p o o l - r i f f l e s e q u e n c e t o s e e i f any s i g n i f i c a n t s c a l e c o n s t r a i n s t h e i r b e h a v i o u r . S e v e r a l a u t h o r s h a v e empha s i z e d t h e r e l a t i v e c o n s i s t e n c y o f mean p o o l - t o - p o o l o r r i f f l e - t o - r i f f l e s p a c i n g ( L e o p o l d e t a l , 1964 ; K e l l e r , 1972, ; H a r v e y , 1975 ; R i c h a r d s , 1976a , 1976b ; K e l l e r and Me l ho rn , 1978 ; Mi lne, 1982a ; C h u r c h and J o n e s , 1982), be ing on t h e a v e r a g e 5 t o 7 t imes t h e channe l w i d t h . These s t u d i e s s u g g e s t an a p p a r e n t c o n s i s t e n c y o v e r a wide range o f s c a l e s o f t h i s i s o m e t r i c s c a l i n g o f p o o l - r i f f l e s e q u e n c e s w i t h c h a n n e l w i d t h , a p p a r e n t l y v a l i d e v e n f o r t h e c a s e o f b ed r o c k channe l s (Ke l l e r and M e l h o r n , 1 9 7 8 ) . H o w e v e r , c o n s i d e r a t i o n o f t h e s c a t t e r o f t h e s e p r o p o s e d i s o m e t r i c r e l a t i o n s h i p r e v e a l s t h a t s o m e o t h e r c o n t r o l s on s p a c i n g do e x i s t . Some ob v i o u s b u t l o c a l c o n t r o l s a r e t h e i n h o m o g e n e i t y o f f l o o d p l a i n m a t e r i a l s a n d p l a n f o r m i r r e g u l a r i t i e s (M i lne , 1 982a ) . T he l a t t e r h a v e a d e t e r m i n i n g e f f e c t on p o o l l o c a t i o n ( G a l l i n a t t i j 1984 ; L i s l e , 1986 ) . P o s s i b l e e f f e c t s o f t h e c h a r a c t e r i s t i c s o f b ounda r y m a t e r i a l s a nd s t r e a m a b i l i t y t o a r r a n g e t h e m v i a t r a n s p o r t i n t o p o o l - r i f f l e u n i t s h a v e n o t b een a d d r e s s e d y e t . The l a t t e r i s s ue wou ld a p p e a r t o be p a r t i c u l a r l y s u s c e p t i b l e t o be s t u d i e d in c o n t r o l l e d and p r o p e r l y s c a l e d l a b o r a t o r y c h a n n e l s . In s ome s i t u a t i o n s , ' p e r h a p s s p a c i n g v a r i a b i l i t y c a n be a c c o u n t e d f o r by t h e c o e x i s t e n c e o f d i f f e r e n t l a r g e - s c a l e b e d f o r m s a r i s i n g f r o m w a t e r s h e d c o n t r o l s ( C h u r c h and J o n e s , 1982) o r by t h e l ag b e f o r e b e d f o r m a d d i t i o n as a s t r e a m i n c r e a s e s i t s s i n u o s i t y in t h e p r o c e s s o f m e a n d e r d e v e l o p m e n t ( Ke l l e r and Me l ho rn , 1973 ; Mi lne, 1982a) . F o r s u ch c r e a t i o n o f a d d i t i o n a l b e d f o r m s t o o c cu r a long a r i v e r l e n g t h , Thompson (1986) p r o p o s e d an i n t e r - r i f f l e t h r e s h o l d d i s t a n c e o f 10 t i m e s t h e c h a n n e l w i d t h . A mo r e ba s i c i s s ue c o n s i s t s o f . the or ig in o f t h e s e bed u n d u l a t i o n s in g r a v e l - b e d r i v e r s . T h i s r e p r e s e n t s a r a t h e r f u n d a m e n t a l p r o b l e m o f f l u v i a l morpho logy which has o f t e n been l i n ked w i t h t h e m e a n d e r i n i t i a t i o n en i gma . Th i s r a p p r o c h e m e n t is b a s e d on t h e f a c t t h a t in many a l b e i t n o t a l l f i e l d s i t u a t i o n s , b a r o r p o o l l o c a t i o n s a l t e r n a t e f r o m one bank t o t h e o t h e r , i m i t a t i n g t h e m e a n d e r i n g p a t t e r n , e v e n in s t r a i g h t c h a n n e l s . L a b o r a t o r y s t u d i e s in s t r a i g h t c h a n n e l s h a v e a l s o s o m e t i m e s r e s u l t e d in t h e d e v e l o p m e n t o f a l t e r n a t i n g b a r s (Chang e t a l , 1971 ; S u kegawa , 1974 ; H.Ikeda, 1973 , 1983 ; J a e g g i , 1980,1983,1984 ; S. Ikeda, . 1984): t h e s e wou ld a p p e a r t o r e p r e s e n t t h e f i r s t s t a g e o f m e a n d e r d e v e l o p m e n t . M o r e o v e r , a l t e r n a t e b a r s do b e a r some a n a l o g i e s w i t h f i e l d p o o l - r i f f l e s e q u e n c e s on m o r p h o l o g i c a l and h y d r o d y n a m i c a l g r o u n d s . However , i t is y e t unc lea r wha t is t h e a c t u a l link b e t w e e n t h e p o o l - r i f f l e s e q u e n c e and t h e m e a n d e r i n g p h e n o m e n o n in a l l u v i a l r i v e r c h a n n e l s . S ome a u t h o r s who c o n s i d e r t h e p o o l - r i f f l e s e q u e n c e t o be p r e r e q u i s i t e f o r meande r d e v e l o pmen t 142 h a v e a c c o r d i n g l y p r o p o s e d c o n c e p t u a l mode l s o f a l l u v i a l r i v e r d e v e l o p m e n t ( e . g . K e l l e r , 1972 ; L e w i n , 1976 ; T h o m p s o n , 1986). The r a p p r o c h e m e n t and c a u s a l l ink b e t w e e n t h e s e t w o phenomena a r e a p p a r e n t l y f u r t h e r j u s t i f i e d by t h e r e l a t i v e c o n s i s t e n c y in t h e p r o p o r t i o n b e t w e e n h a l f meande r w a v e l e n g t h and i n t e r r i f f l e d i s t a n c e o v e r a wide r ange o f s c a l e s . However , t h e f a c t t h a t m e a n d e r i n g c h a n n e l s w i t h f i n e - g r a i n e d b e d m a t e r i a l do n o t show a p o o l - r i f f l e s e q u e n c e p r o p e r i n d i c a t e s t h a t b o t h phenomena may c o i n c i d e on ly unde r p a r t i c u l a r a l l uv i a l c i r c u m s t a n c e s . In a d d i t i o n , t h e m e a n d e r i n g p h e n o m e n o n o c c u r s unde r v a r i e d h y d r o d y n a m i c c o n d i t i o n s , i .e . m i c r o s t r e a m s o v e r h y d r o p h o b i c p l a t e s (Go r i c k y , 1973 ) , in g l a c i e r i c e , in o c e a n c u r r e n t s and in t h e a t m o s p h e r e (Leopo ld e t a l , 1964 ; Ze l l e r , 1967) . Meande r i n g t h u s a p p e a r s t o r e p r e s e n t a f l o w - i n d u c e d phenomenon f o r which d e v e l o pmen t may be t r i g g e r e d by a d i s t i n c t p e r t u r b i n g f a c t o r f o r e a c h o f t h e s e s i t u a t i o n s ( Pa r ke r , 1976). A. 2 B e d f o r m c l a s s i f i c a t i o n a n d c o n d i t i o n s o f p o o l - r i f f l e  o c c u r r e n c e B e d f o r m s in g e n e r a l a nd t h e p o o l - r i f f l e s e q u e n c e in p a r t i c u l a r a r e g e n e r a t e d by s h e a r i n g f l o w o v e r a g ranu l a r bed and d i sp l ay a wide v a r i e t y o f shape and s i ze r e l a t e d t o t h e c h a r a c t e r i s t i c s o f t h e e n v i r o n m e n t in wh i c h t h e y g r o w . A t t e m p t s t o u n i f y in a un i que c o n c e p t u a l m o d e l b e d c o n f i g u r a t i o n s o f v a r i o u s s c a l e s a r e r a r e in t h e l i t e r a t u r e . I ndeed t h e d o m i n a n t f l o w - r e g i m e c l a s s i f i c a t i o n s o f b e d f o r m s ( e . g . S imons and R i c h a r d s o n , 1961 ; S o u t h a r d , 1971) do n o t c o v e r t h e w ide r a n g e o f c o n d i t i o n s m e t in n a t u r a l s y s t e m s , be i ng m o s t l y b a s e d on s and f l u m e e x p e r i m e n t s . J a c k s o n (1975) p r o p o s e d a u n i f y i n g mode l o f b e d f o r m s b a s e d on c o n c e p t u a l a r g u m e n t s p i o n e r e d in Russ i an l i t e r a t u r e . Th i s mode l has t h e m e r i t o f a p p a r e n t l y be ing b a s e d on f l u i d - d ynam i c p r o c e s s e s ( no t e s t a b l i s h e d o r w e l l - f o r m u l a t e d in s o m e c a s e s ) a n d s p a t i o - t e m p o r a l p a r a m e t e r s . J a c k s o n r e c o g n i z e d t h r e e g r o u p s o f b e d c o n f i g u r a t i o n s , n a m e l y m i c r o f o r m s , m e s o f o r m s a n d m a c r o f o r m s , t h e l a t t e r wh ich a r e t h e r e a l m o f p o o l - r i f f l e s e q u e n c e s and c h a n n e l b a r s . L e e d e r (1983) a l s o o f f e r e d an a p p a r e n t l y g e n e t i c b e d f o r m c l a s s i f i c a t i o n b a s e d on b e d f o r m m e c h a n i c s wh ich i n t e g r a t e d l a r g e - s c a l e f e a t u r e s . E v i d e n c e s o f j u x t a p o s e d doma in s f o r meso and m a c r o f o r m s a r e p r o v i d e d by H . l keda ' s ( 1 9 7 3 , 1 9 7 5 ) e x p e r i m e n t a l a nd f i e l d r e s u l t s . The l im i t s o f o c c u r r e n c e o f e a c h t y p e o f b e d c o n f i g u r a t i o n c an p r e s u m a b l y be d e s c r i b e d v i a some h y d r o d y n a m i c p a r a m e t e r s . Some a u t h o r s h a v e t r i e d t o d e s c r i b e r e q u i r e m e n t s f o r va r i ous bar t y p e s t o deve l op in t e r m s o f t h e s t r e a m ene rgy g r a d i e n t (Kopa l i an i and Romash in , 1 9 7 0 ; C h u r c h a n d J one s , -1982; F l o r s h e i m , 1985 ) . H o w e v e r , t h o s e d e s c r i p t i o n s do n o t t a k e i n t o a c c o u n t t h e c h a r a c t e r i s t i c s o f t h e bed ma t e r i a l t o be e n t r a i n e d unde r a g i v en e n e r g y g r a d i e n t in a r e l a t i v e manne r a n d a r e u n l i k e l y t o b e g e n e r a l i z a b l e . P r e l i m i n a r y c l a s s i f i c a t i o n a t t e m p t s by t h e w r i t e r have t e n d t o s u g g e s t t h a t in f u l l y r o u g h s u b c r i t i c a l f l o w s t h e s u c c e s s i o n o f b e d f o r m s f r o m ba r f o r m s t o dunes is g o v e r n e d by t h e mob i l i t y number ( d e s c r i b e d in s e c t i o n 2 .5 .2 ) . P o o l - r i f f l e s e q u e n c e s as we l l as a l t e r n a t e b a r s ( o f s a n d - b e d l a b o r a t o r y e x p e r i m e n t s ) o c c u r when t h e mob i l i t y number is low (i .e. gene ra l l y be low 3). Dune beds a ppea r a t h ighe r r e l a t i v e s t r e s s e s f o r a g iven g ra in s i ze , and t h e i r s hape a l s o a p p e a r s t o be a f u n c t i o n o f t h e mob i l i t y p a r a m e t e r (Ya l in and Ka r ahan , 1979b) . The e x i s t e n c e o f a wide v a r i e t y in t h e m o r p h o l o g i c a l e x p r e s s i o n s o f b e d f o r m s in moun t a i n r i v e r s has a c t u a l l y b e e n l ong r e c o g n i z e d in t h e E a s t E u r o p e a n and Russ i an l i t e r a t u r e ( G e r g o v , 1983) . I n c r e a s i n g mob i l i t y numbe r is e x p e c t e d t o have an e f f e c t on p o o l - r i f f l e morpho logy a l t h ough t h i s has n o t been d o c u m e n t e d . F o r i n s t a n c e , an i n c r e a s e in mob i l i t y number c an be r e l a t e d t o an i n c r e a s e in s e d i m e n t t r a n s p o r t o r in t h e amoun t o f m a t e r i a l in t r a n s i t , a nd h e n c e t o a g r e a t e r i m p o r t a n c e o f t h e b a r c o m p o n e n t in t h e f u n d a m e n t a l p o o l - r i f f l e unit. The m o b i l i t y numbe r m a g n i t u d e has a l s o some i n f l u en c e on t h e m i g r a t i o n o f t h e s e q u e n c e . A l t e r n a t e b a r s in l a b o r a t o r y e x p e r i m e n t s and p o o l - r i f f l e s e q u e n c e s o f low s i n o u s i t y r i v e r s a r e s o m e t i m e s m i g r a t i n g (H.Ikeda, 1975 ; F e r g u s o n and W e r r i t y , 1983 -; J a e g g i , 1985) . The s p e e d o f m i g r a t i o n , wh i ch is v e r y v a r i a b l e £ u t r e l a t i v e l y sma l l t o n i l in m o s t g r a v e l - b e d r i v e r s , is u n d o u b t l y r e l a t e d t o t h e low e x c e s s s t r e s s a v a i l a b l e t o e n t r a i n t h e bed p a r t i c l e s . F a c t o r s which p r o m o t e r i f f l e o r ba r s t a b i l i t y e i t h e r o r i g i n a t e f r o m p l a n f o r m c o n t r o l ( K i n o s h i t a and Miwa, 1974 j L i s l e , 1986) o r f r o m s e d i m e n t o l o g i c a l a nd h y d r o d y n a m i c a l f a c t o r s ( C h u r c h and J o n e s , 1982). As we s ha l l f u r t h e r d i s c o v e r b e l ow , b a r mob i l i t y is l i ke l y r e l a t e d t o b a r shape. In a d d i t i o n , i t is i n t e r e s t i n g t o n o t e t h a t some c r i t e r i a •for a l t e r n a t e b a r o c c u r r e n c e ( S u k egawa , 1974 ; J a e g g i , 1984) a l s o i n c l u d e d t h e mob i l i t y n umbe r . H o w e v e r , s o m e c o n f u s i o n b e t w e e n b a r t y p e s (Shen , 1961 ; J a e g g i , 1983, 1984) a nd p o s s i b l e s c a l i n g o r r e g ime d i f f i c u l t i e s ( t h e f o r m e r r e l a t e d t o t h e f l o w c o n d i t i o n s in some l a b o r a t o r y channe l s and t h e l a t t e r t o t h e f i x e d w i d t h o f t h e channe l , r e f . Chang, 1985) in some e x p e r i m e n t s impede t h e use o f t h e s e c r i t e r i a f o r p o o l - r i f f l e s e q u e n c e o c c u r r e n c e . The same r e a s o n s c a u s e t h e r e s u l t s on l a b o r a t o r y b a r g e o m e t r y ( h e i g h t and s p a c i n g ) o f S . l keda (1984) t o be r e c e i v e d s c e p t i c a l l y . T h e l o w e r l i m i t f o r p o o l - r i f f l e o c c u r r e n c e w o u l d c o r r e s p o n d t o t h r e s h o l d c o n d i t i o n s . H o w e v e r , in c h a n n e l s in w h i c h p l a n f o r m i r r e g u l a r i t i e s o r s t r e a m s i d e o b s t r u c t i o n i n t e r s e c t t h e l o c a l f l o w , c r e a t i n g l a r g e s c a l e v o r t i c e s , p o o l - r i f f l e s e q u e n c e s c an p o s s i b l y d e v e l o p e v e n i f t h e f l o w a v e r a g e m o b i l i t y numbe r is s m a l l e r t h a n 1.0 ( i .e . t h e s o - c a l l e d f o r c e d b a r s o f K i n o s h i t a and Miwa, 1974) . In t h e c a s e o f p l a n f o r m c o n t r o l l e d p o o l and r i f f l e s e q u e n c e s , w h e t h e r o r n o t t h e f l o w a v e r a g e mob i l i t y numbe r is be low or above 1.0, poo l and r i f f l e l o c a t i o n and g e o m e t r y a r e mos t l y d e t e r m i n e d by t h e i n t e r s e c t i o n t o t h e f l o w in p l an ( G a l l i n a t t i , 1984 ; L i s l e , 1986). C l a s s i f i c a t i o n s o f b a r - p o o l - r i f f l e s e q u e n c e s in f u l l y a l l u v i a l m i l i eus a r e p r ima r i l y b a s e d on ba r f o r m . T h e r e is a c o n f u s i n g a r r a y o f t e r m s c o n c e r n i n g b a r c l a s s i f i c a t i o n ( S m i t h , 1978) . The l a t t e r s i t u a t i o n is l i k e l y a c o n s e q u e n c e o f t h e i r m o r p h o l o g i c a l d i v e r s i t y and t h e l a ck o f a g l o b a l a p p r o a c h t o t h e l a r g e - s c a l e b e d f o r m a s p e c t in f u l l - s c a l e r i v e r s . R i f f l e p l a n f o r m may v a r y f r o m t r a n s v e r s e , s o m e t i m e s w i t h med ia l b a r ( e . g . R i c h a r d s , 1976b) , t o d i a g o n a l o r a l t e r n a t e b a r s h a p e ( e . g . F e r g u s o n and W e r r i t y , 1983) . ( N o t e t h a t t h e t r a n s v e r s e s t e p - p o o l s y s t e m s o f moun t a i n s t r e a m s ( W h i t t a K e r and J a e g g i , 1 982 ) o c c u r u n d e r d i f f e r e n t h y d r o d y n a m i c c o n d i t i o n s and r e p r e s e n t a n o t h e r doma in o f b ed f o r m a t i o n . ) C h u r c h and J o n e s (1982) d e s c r i b e d a g r a d a t i o n in r i f f l e and ba r morpho logy which is p o s s i b l y r e l a t e d t o t h e m o b i l i t y n u m b e r ; a t g r e a t e r mob i l i t y , t h e b a r c o m p o n e n t f i l l s t h e c h a n n e l and t h e r i f f l e r e p r e s e n t s m e r e l y i t s l e ad i ng e d g e . Th i s p r o p o s a l r ema i n s t o b e v e r i f i e d . C h u r c h and J o n e s (1982) a l s o a t t e m p t e d a c l a s s i f i c a t i o n f o r a l l u v i a l g r a v e l - b e d c h a n n e l s m a c r o f o r m s . In a d d i t i o n t o m o r p h o l o g i c a l c r i t e r i a , t h i s c l a s s i f i c a t i o n d i s t i n g u i s h e s b e t w e e n t h e p r ima r i l y r e s i s t a n c e f u n c t i o n o f r i f f l e s and t h e sed imen t s t o r a g e r o l e o f b a r s . In t h e d i s cuss i on o f t h e l a t t e r p a p e r , P. A s h m o r e p r o p o s e s a c l a s s i f i c a t i o n s c h e m e s im i l a r t o t h e i r s wh ich a l s o i n c l u d e s b a r c o m p l e x e s and r e m n a n t s , a r e f i n e m e n t p a r t i c u l a r l y r e l e v a n t f o r t h e c a s e o f m u l t i t h r e a d r i v e r s c h a n n e l s . T h e s e a t t e m p t s h o w e v e r r ema i n d e s c r i p t i v e and h e n c e n o t b a s e d on h y d r o d y n a m i c q u a n t i t i e s . In p a r t l y a l l u v i a l m i l i e u s , m a c r o f o r m c l a s s i f i c a t i o n c a n e i t h e r be b a s e d on p o o l t y p e f o r o b s t r u c t i o n - c o n t r o l l e d p o o l - r i f f l e s e q u e n c e s ( e . g . B e s c h t a and P l a t t s , 1986) o r b a r t y p e in t h e c a s e o f bedrock, canyons (e .g . Bake r , 1984). The e m e r g e n c e o f a c o m p l e t e c l a s s i f i c a t i o n f o r l a r g e - s c a l e b e d f o r m s is h igh ly d e s i r a b l e in o r d e r t o p r o v i d e a u n i v e r s a l f r a m e o f r e f e r e n c e wh ich wi l l e l i m i n a t e t h e c on f u s i o n s due t o u n r e g u l a t e d n o m e n c l a t u r e in t h e l i t e r a t u r e . A.3 F low p r o c e s s e s . o r i g i n and e v o l u t i o n o f t h e p o o l - r i f f l e  p h e n o m e n o n S t u d i e s o f f l o w p r o c e s s e s in p o o l - r i f f l e u n i t s a t f o r m a t i v e f l o w s a r e r a r e . V e l o c i t y m e a s u r e m e n t s p e r f o r m e d t h r o u g h a s e l e c t e d s e q u e n c e w e r e r e p o r t e d by K e l l e r (1971). T h e r e i n on ly n e a r - b e d v e l o c i t i e s f o r low t o medium d i s c h a r g e s w e r e m e a s u r e d a l o ng t w o c r o s s - s e c t i o n s , r e s p e c t i v e l y in a poo l and o v e r a r i f f l e . Fo r low d i s c ha r ge s , Ke l l e r f o und t h e b o t t o m v e l o c i t i e s (wh i ch he a s s u m e d p r o p o r t i o n a l t o t h e t r a c t i v e f o r c e a c t i n g on t h e bed) t o be l a r g e r o v e r t h e r i f f l e t h a n in t h e p o o l . H o w e v e r , K e l l e r n o t e d a t e n d e n c y f o r t h e b o t t o m v e l o c i t i e s t o equa l i z e in t h e poo l and t h e r i f f l e a t medium d i s c h a r g e and h y p o t h e s i z e d t h a t a r e v e r s a l ( i .e . t h e b o t t o m v e l o c i t y in t h e poo l e x c eed i ng t h a t o v e r t h e r i f f l e ) may o c cu r f o r t h e h i g h e s t , f o r m a t i v e d i s c h a r g e s . A r e v e r s a l in t e r m s o f t r a c t i v e f o r c e was n e v e r s y s t e m a t i c a l l y m e a s u r e d whi le his s e t o f o r i g i n a l b o t t o m v e l o c i t y d a t a ( K e l l e r , 1969), t a k e n a t 1.5 cm f r o m t h e bed (which has a D 5 0 o f abou t 32 mm o v e r t h e r i f f l e ) , s h o w e d r a t h e r l a r g e l o c a l v a r i a b i l i t y and p o s s i b l y i n d i v i d u a l p a r t i c l e e f f e c t s . N e v e r t h e l e s s s ome a u t h o r s h a ve a r g u e d t h a t t h e a p p a r e n t s o r t i n g o f s u r f a c e b e d m a t e r i a l , r i f f l e m a t e r i a l b e i n g in g ene r a l c o a r s e r t h a n t h a t o f a d j a c e n t poo l , cou ld r e s u l t f r o m a r e v e r s a l in f l o w c o m p e t e n c e b e t w e e n poo l s and r i f f l e s ( L i s l e , 1979 ; H i r s c h and Ab r ahams , 1981). A n d r e w s (1979) r e p o r t e d a 148 r e v e r s a l o f t h e mean v e l o c i t i e s f o r t h e E a s t Fork - r i v e r , a p o o l - r i f f l e s t r e a m . L i s l e ( 1979 ) a c t u a l l y r e p o r t e d f o r t h e same r i v e r a r e v e r s a l o f a v e r a g e s h e a r s t r e s s ( equa t i on 2.16) a t h igh f l o w s . A r e c e n t s t u d y by P e t i t (1987) in a smal l p e b b l e - b e d s t r e a m s e e m e d n o t t o s u p p o r t t h e h y p o t h e s i s o f b o t t o m v e l o c i t y r e v e r s a l . P e t i t a l s o e s t i m a t e d t h e t r a c t i v e f o r c e on t h e bed and conc l uded t h a t a r e v e r s a l o f t h e l a t t e r q u a n t i t y o c c u r r e d . H i s a p p r o a c h h o w e v e r s u f f e r s f r o m w e a k n e s s e s r e g a r d i n g t h e way t h e s t r e s s was a c t ua l l y e s t i m a t e d . Only l o c a l v e l o c i t y p r o f i l e m e a s u r e m e n t s wh ich wou ld a l l ow bed s h e a r s t r e s s e s t o be p r o p e r l y e s t i m a t e d t h r o u g h t h e s e q u e n c e c o u l d c l a r i f y t h e v e l o c i t y o r s h e a r s t r e s s r e v e r s a l i s sue . T h e o c c u r r e n c e o r s e d i m e n t o l o g i c a l i m p o r t a n c e o f t h e v e l o c i t y o r shea r s t r e s s r e v e r s a l has a l so been r e f u t e d by some a u t h o r s ( T e l e k i , 1972 ; Bhowmik and D e m i s s i e , 1982 ) . M i lne (1982b) p o i n t e d o u t t h a t l a t e r a l s o r t i n g b e t w e e n t h e poo l and t h e a d j a c e n t b a r in a r e l a t i v e l y s i nuous c h a n n e l was gene ra l l y m o r e s i g n i f i c a n t t h a n l o n g i t u d i n a l s o r t i n g f r o m poo l t o r i f f l e . The m o s t a ppea l i n g a s p e c t o f t h e r e v e r s a l h y p o t h e s i s rema ins -t h a t i t c a n a c c o u n t f o r t h e m a i n t e n a n c e a l b e i t n o t t h e i n i t i a t i o n o f t h e p o o l - r i f f l e s e q u e n c e ( L i s l e , 1979 ; K e l l e r , 1971 , 1 9 8 3 ) . The i n i t i a t i o n o f t h e p o o l - r i f f l e s e q u e n c e is a d i f f i c u l t p rob l em t o s t ud y due t o n a t u r a l f e e d b a c k b e t w e e n bed morpho logy and f l o w s t r u c t u r e a t f o r m a t i v e d i s c ha r ge . S e v e r a l a t t e m p t s a t e x p l a n a t i o n h a ve n e v e r t h e l e s s b e en o f f e r e d b u t no s y s t e m a t i c p h y s i c a l o b s e r v a t i o n s h a ve b e e n c o l l e c t e d . Two a l t e r n a t e v i ews a r e p o s s i b l e ; p o o l - r i f f l e s equences a r e v i ewed as e i t h e r a f l o w o r s e d i m e n t t r a n s p o r t p h e n o m e n o n . On t h e one hand, Langbe in and Leopo ld (1968) p r o po s ed t h a t p o o l - r i f f l e s e q u e n c e i n i t i a t i o n was d e t e r m i n e d by some p r o p e r t y o f t h e moving bed l oad ma t e r i a l s which would t r a v e l as k i nemat i c w a v e s . T hompson (1986) a r g u e d t h a t p o o l - r i f f l e s p a c i n g may e f f e c t i v e l y be under c o n t r o l o f such a mechan ism. Lekach and S c h i c k (1983) o b s e r v e d t h a t b e d l o a d t r a n s p o r t o v e r a n o n - u n d u l a t i n g c o n f i n e d r i v e r b ed had a p u l s e - l i k e c h a r a c t e r . The f a c t t h a t a p o o l - r i f f l e s e q u e n c e d e v e l o p e d some d i s t a n c e d o w n s t r e a m f r o m t h e c a n y o n r e a c h was t a k e n as an i nd i ca t i on t h a t k i n e m a t i c w a v e t r a n s p o r t ( w h i c h c o u l d r e p r e s e n t an i n h e r e n t s e d i m e n t t r a n s f e r phenomenon ) c ou l d be r e s p o n s i b l e f o r p o o l - r i f f l e i n i t i a t i o n . W a v e l i k e t r a n s p o r t w h i c h i s c h a r a c t e r i s t i c o f g r a v e l - b e d r i v e r s has b e e n n o t e d in o t h e r p o o l - r i f f l e s e q u e n c e s ( e . g . C a m p b e l l a nd S i d l e , 1985 ; Mi l l igan e t a l , 1980) . In t h i s c a s e , t h e c r e a t i o n o f o r gan i z ed b e d f o r m s wou ld be p r e r e q u i s i t e t o t h e o r g a n i z a t i o n o f f l o w pa t te rns . On t h e o t h e r hand , s e v e r a l ideas abou t how p a t t e r n s o f v e l o c i t y o r s t r e s s c ou l d e s t a b l i s h in t h e f l o w have a l so been p r o p o s e d . S o m e a u t h o r s a l s o a p p e a l e d t o m i n i m i z a t i o n p r i n c i p l e s (Yang , 1971 ; C h e r k a u e r , 1973) b u t t h e i r a r g u m e n t s w e r e n o t v a l i d u n d e r c h a n n e l - f o r m i n g c o n d i t i o n s . In c o n t r a s t , o t h e r a u t h o r s a t t e m p t e d t o a c c o u n t f o r t h e or ig in o f d i s t i n c t b u t p e r i o d i c f l o w s t r u c t u r e in p o o l s and r i f f l e s , as d e s c r i b e d by K e l l e r and Me l ho rn (1973) , F e r g u s o n and W e r r i t y (1983) and T h o m p s o n ( 1986 ) . R i c h a r d s ( 1 976a ) a d a p t e d Y a l i n ' s (1971b) p r o b a b i l i s t i c a r g u m e n t s t o t h e a p p a r e n t p e r i o d i c o c c u r r e n c e o f z o n e s o f r e l a t i v e l y h i g h e r a n d l o w e r s t r e s s e s in t h e l o n g i t u d i n a l d i r e c t i o n . N e a r l y a d e c a d e e a r l i e r , E i n s t e i n a nd Shen (1964) p r o p o s e d t h a t s e c o n d a r y c i r c u l a t i o n o f a l t e r n a t i n g n a t u r e c o u l d o r i g i n a t e f r o m t h e g r o w t h and d e c a y o f wa l l t u r b u l e n c e p r o d u c e d by a v o r t i c i t y g e n e r a t i n g m e c h a n i s m . A m o r e s o p h i s t i c a t e d and d i s t i n c t t r e a t m e n t by P a r k e r ( 1 9 7 6 ) , u s i n g i n s t a b i l i t y a n a l y s i s , s u g g e s t e d t h a t s e d i m e n t t r a n s p o r t a n d f r i c t i o n b u t n o t h e l i c i t y w e r e n e c e s s a r y f o r t h e d e v e l o p m e n t o f a l t e r n a t i n g z o n e s o f s c o u r a n d d e p o s i t i o n . However , none o f . t h e f o r e g o i n g ideas have been w e l l - t e s t e d . F o r t h e c a s e o f s t r e a m s i d e o b s t r u c t i o n - c o n t r o l l e d p o o l - r i f f l e s e q u e n c e d e v e l o p m e n t , a q u a l i t a t i v e a l b e i t t h e o r e t i c a l t r e a t m e n t is p r o p o s e d in G a l l i n a t t i ( 1984 ) . N e a r an o b s t r u c t i o n t h e imp i ng i n g f l o w g e n e r a t e s h i gh p r e s s u r e w h e r e a s t h e p r e s s u r e is l o w e r n e a r t h e o p p o s i t e bank . Th i s p r e s s u r e g r a d i e n t , c omb i n ed w i t h an a d v e r s e p r e s s u r e g r a d i e n t t h r o u g h t h e w a t e r d e p t h a t t h e f a c e o f t h e o b s t r u c t i o n , r e s u l t s in t h e f o r m a t i o n o f an he l i ca l f l o w s t r u c t u r e d i r e c t e d downward a t t h e o b s t r u c t i o n and upward nea r t h e inner bank. The shape and e x t e n t o f t h i s f l o w s t r u c t u r e wil l depend on t h e p a r t i c u l a r c o n f i g u r a t i o n o f t h e r e a c h . S c o u r a p p e a r s a l o n g t h e o b s t r u c t i o n . The c i r c u l a t i o n p e r s i s t s f o r some d i s t a n c e a f t e r t h e o b s t r u c t i o n dur ing which t h e a d v e c t i o n o f t h e high v e l o c i t y c o r e and t u r b u l e n t d i f f u s i o n g r a d u a l l y p r o m o t e t h e c r e a t i o n o f a l o b a t e d e p o s i t i o n a r e a which r e s emb l e s an a l t e r n a t e ba r . 151 A.4 P o o l - r i f f l e s e q u e n c e s : p e r s p e c t i v e s and p r o s p e c t s T h e m e c h a n i s m s p r o p o s e d f o r a l l u v i a l p o o l - r i f f l e o r i n c i p i e n t m e a n d e r i n g d e v e l o p m e n t o r m a i n t e n a n c e do n o t p r o d u c e u n i v e r s a l a g r e e m e n t . The p h y s i c a l p r o c e s s e s i n v o l v e d in t h e d e v e l o p m e n t o f t h e p o o l - r i f f l e s e q u e n c e a r e i n d e e d e x t r a o r d i n a r i l y d i f f i c u l t t o m e a s u r e , s p a t i a l l y d i s t r i b u t e d q u a n t i t i e s be i ng i n v o l v e d in t h i s t w o - p h a s e p h e n o m e n o n . F r o m t h i s s i t u a t i o n f o l l o w i n s t r u m e n t a l a n d d a t a r e s o l u t i o n c o n s t r a i n t s . In t h i s p e r s p e c t i v e , i t is a l s o i n t e r e s t i n g t o n o t e t h a t many s i m p l e r a s p e c t s o f p o o l - r i f f l e s e q u e n c e b e h a v i o u r i n t r o d u c e d in t h i s c h a p t e r a r e n o t y e t we l l u n d e r s t o o d , l i ke t h e c o n t r o l s on t h e i r v a r y i n g m o r p h o l o g y , t h e c r i t e r i a f o r t h e i r o c c u r r e n c e and t h e i r h y d r o d y n a m i c s a t equ i l i b r i um . Some o f t h e a b o v e r e m a r k s , n o t a b l y t h o s e a b o u t t h e p r o b l e m s o f c l a s s i f i c a t i o n , i l l u s t r a t e t h a t i t is n e c e s s a r y t o b e g i n t o s t u d y t h e p o o l - r i f f l e p henomenon by add r e s s i ng some f u n d a m e n t a l que s t i o n s , hence pe rhaps t o i gnore m o s t o f t h e work done un t i l now. A r e f l e x i o n a bou t t h e a c t u a l l e v e l o f know ledge on t h e p o o l - r i f f l e p r o b l e m and t h e n e ed f o r e x t e n s i v e and d e t a i l e d i n f o r m a t i o n l e a d t o c o n s i d e r a t i o n o f t h e p o s s i b i l i t y o f w o r k i n g u n d e r c o n t r o l l e d l a b o r a t o r y c o n d i t i o n s , S u c h w o r k i n g c o n d i t i o n s w o u l d a l l o w w i d e l y v a r y i n g c o m b i n a t i o n s o f s o m e i n f l u e n t i a l h y d r o d y n a m i c a nd s e d i m e n t o l o g i c a l p a r a m e t e r s as we l l as d e t a i l e d h y d r o d y n a m i c m e a s u r e m e n t s in o r d e r t o e x p l o r e s e v e r a l c r u c i a l i s s u e s . The w r i t e r b e l i e v e s t h a t r a p i d s c i e n t i f i c p r o g r e s s c ou l d be made under such work ing c ond i t i o n s . If i t is d e m o n s t r a t e d a p r i o r i t h a t t h e p h y s i c a l p r o c e s s e s a c t i n g in f u l l - s c a l e s y s t e m s c an be a p p r o p r i a t e l y r e p r o d u c e d a t a s m a l l e r s c a l e , a c c o r d i n g t o t h e t h e o r y i n t r o d u c e d in c h a p t e r 2, l a b o r a t o r y r e s u l t s can be t r a n s f e r r e d t o t h e f i e l d w i t h o u t s c e p t i c i s m . The l a t t e r s t a t e m e n t d e s c r i b e s t h e m o t i v a t i o n and r a t i o n a l e b e h i n d t h e c u r r e n t r e s e a r c h d e s i g n . In t h a t s e n s e , t h e c u r r e n t p r o j e c t r e p r e s e n t s a f i r s t , n e c e s s a r y s t e p on t h e way t o t r u l y f u n d a m e n t a l r e s e a r c h r e g a r d i n g t h e b ed m o r p h o l o g y in r i v e r s . 

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