University of Southern California Dissertations and Theses

Sand Transport By Shoaling Waves

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Sand Transport By Shoaling Waves

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Content 70-19,110 COOK, David Olney, 1942- SAND TRANSPORT BY SHOALING WAVES. University of Southern California, Ph.D., 1969 Geology University Microfilms, A XERQ\Company, Ann Arbor, Michigan THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED SAND TRANSPORT BY SHOALING WAVES by David Olney Cook A D i s s e r t a t i o n P r e s e n te d t o th e FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA i n P a r t i a l F u l f i l l m e n t o f th e R e q u ire m e n ts f o r t h e Degree DOCTOR OF PHILOSOPHY ( G e o lo g ic a l S c ie n c e s ) A ugust 1969 UNIVERSITY O F SOUTHERN CALIFORNIA T H E GRADUATE SCHOO L UNIVERSITY PARK L O S A N G ELES. CA LIFO RN IA 8 0 0 0 7 This dissertation, written by David Olney Cook under the direction of Dissertation Com mittee, and approved by all its members, has been presented to and accepted by The Gradu ate School, in partial fulfillment of require ments of the degree of D O C T O R OF P H IL O S O P H Y Dean n.,, A u g“at 1969 I p. Chairman A ...... DISSERTATION COMMITTEE CONTENTS C h a p te r Page ILLUSTRATIONS...................................................................... * . . . i v T A B L E S ................................................................................................ - . v l i A B STR A C T................................................................................................. v i l l INTRODUCTION....................................................................................... 1 G e n e ra l s ta te m e n t ................................................................... 1 P r e v io u s w o r k ............................................................................. 2 P h ilo s o p h y o f a p p ro a c h . . . . . . . . . . . 8 L i m i t a t i o n s .................................................................................. 10 Acknowledgments . . . . . .............................................. 10 METHODS...................................................................................................... 12 Wave m e a s u r e m e n t ................................................................... 12 C u r r e n t d e te r m i n a ti o n 12 Sedim ent t r a p s ........................................................................ 18 T o ta l l o a d ........................................................................ 18 Suspended se d im e n t ................................................... 24 O nshore and o f f s h o r e m ig r a tio n ..................... 29 R ip p le mark s a m p l i n g ........................................................ 32 G ra in s i z e a n a l y s i s .............................................................. 32 Dyed sand e x p e r i m e n t s ........................................................ 36 U n d e rw a te r p h o t o g r a p h y ................................................... 36 i i C h ap ter Page TEST SITES . . ............................................................................ 40 P a lo s V e r d e s ........................................................................... 40 S a n ta C a ta li n a I s l a n d ........................................................ 43 Redondo Beach ........................................................................... 43 RESULTS OR HYDRODYNAMIC STUDIES........................................ 45 D i s t r i b u t i o n o f c u r r e n t v e l o c i t i e s ..................... 45 Comparison o f o bserved and t h e o r e t i c a l v e l o c i t i e s ............................................................................ 46 Surge a s y m m e tr y ....................................................................... 59 RESULTS OR SEDIMENT TRANSPORT STUDIES .......................... 75 Sand m otion and r i p p l e m a r k s .................................... 75 Dim ensions o f r i p p l e marks ............................... 75 C i r c u l a t i o n and g r a in p a th s o v e r a r i p p l e d bed . . . * .............................................. 85 Sedim ent d i s t r i b u t i o n over r i p p l e marks . 90 R ip p le mark m i g r a t i o n .............................................. 95 T h resh o ld v e l o c i t i e s ........................................................ 97 T o ta l moving l o a d .................................................................. 101 Suspended s e d i m e n t ............................................................ 110 Sand m i g r a t i o n ....................................................................... 123 MODEL ROR NEARSHORE SAND TRANSPORT............................... I 37 CONCLUSIONS........................................................... 141 REFERENCES........................................................................................... 144 i l l ILLUSTRATIONS F ig u re Page 1 . C o n v e n tio n a l model f o r n e a r s h o r e w a te r c i r c u l a t i o n ............................................................................ 3 2. C u rre n t m e t e r ...................................................................... 13 3. R ecords from c u r r e n t m e t e r ................................... 1 6 4. The t h r e e ty p e s o f sedim ent t r a p s u t i l i z e d . 19 5. Trap in p o s i t i o n to I n t e r c e p t th e t o t a l moving l o a d ............................................................................ 21 6 . Trap i n p o s i t i o n to sample suspended sand . 27 7 . Trap In p o s i t i o n to I n t e r c e p t sedim ent b e in g moved o n - and o f f s h o r e ............................... 30 8 . P r e c i s io n o f s e t t l i n g tu b e a n a ly s e s . . • . 34 9. G r a in - s iz e d i s t r i b u t i o n o f f l u o r e s c e n t t r a c e r sands ....................................................................... 37 10. L o ca tio n m a p ...................................................................... 41 11. T y p ic a l d i s t r i b u t i o n s o f su rg e v e l o c i t y . . 46 12. T h e o r e ti c a l v e rs u s observed surge v e l o c i t i e s f o r s te e p c o n t i n e n t a l s h e lv e s . 50 13. Accuracy o f su rg e v e l o c i t i e s p r e d i c te d by t r o c h o i d a l wave th e o r y o v e r a g e n tl e c o n t i n e n t a l s h e l f . . . .............................................. 52 14 . Accuracy o f su rg e v e l o c i t i e s p r e d i c te d by s o l i t a r y wave th e o r y o v e r a g e n tle c o n t i n e n t a l s h e l f ............................................................. 54 15. Use o f s o l i t a r y and t r o c h o i d a l wave th e o r y to p r e d i c t su rg e v e l o c i t y o v e r g e n tle c o n t i n e n t a l s h e lv e s ........................................................ 57 iv F ig u r e Page 16. A t t e n u a ti o n o f su rg e w i t h i n c r e a s i n g d e p th 60 17. Asymmetry o f su rg e v e l o c i t i e s a t d i f f e r e n t d e p t h s ........................................................................ 63 18. Asymmetry o f s u rg e d u r a t i o n a t d i f f e r e n t d e p t h s ....................................................................................... 65 19. R e l a t i v e w a te r t r a n s p o r t a t d i f f e r e n t d e p t h s ....................................................................................... 67 20. P r e d i c t i o n o f bottom d r i f t by an e m p i r i c a l m e t h o d ....................................................................................... 72 21. R ip p le marks on t h e s h a llo w s e a f l o o r . . . 76 22 . In d e x o f r i p p l e s i n t h e o f f s h o r e zone . . . 78 23. R ip p le w a v e le n g th p l o t t e d a g a i n s t mean d ia m e te r o f botto m se d im e n t ..................................... 81 24. E f f e c t o f su rg e v e l o c i t y on r i p p l e wave l e n g t h ....................................................................................... 83 2 5 . G rain p a th s o v e r r i p p l e m arks d u rin g a low v e l o c i t y su rg e ................................................................... 86 26. Eddy d e v elo p ed i n th e l e e o f a r i p p l e c r e s t ............................................................................................. 88 27. G ra in p a th s o v e r r i p p l e m arks d u r in g a h i g h v e l o c i t y su rg e .................................................... 91 28 . S iz e o f g r a i n s c o n c e n tr a te d a t r i p p l e c r e s t s and t r o u g h s ......................................................... 93 29. O bserved t h r e s h o l d v e l o c i t i e s f o r sand t r a n s p o r t .................................................................................. 99 30. R e l a t i o n s h i p betw een s i z e o f sand t r a n s p o r te d and se d im e n t a v a i l a b l e ............................... 102 31. Average su rg e v e l o c i t y v e r s u s r a t e o f sand t r a n s p o r t ................................................................... 106 v F ig u re Page 3 2 , A verage su rg e v e l o c i t y v e r s u s a l t e r n a t e r a t e o f sand t r a n s p o r t ............................................... 108 3 3 . V e r t i c a l d i s t r i b u t i o n o f suspended sa n d . ,, 111 34* F a c to r s i n f l u e n c i n g t h e d i s t r i b u t i o n o f su spend ed sand ......................... 114 35* D i s t r i b u t i o n o f sa n d suspended by w aves, s tr e a m s , and wind . . . . . . . . . . . . . 116 36. R ate o f suspended sand t r a n s p o r t v e r s u s a v e ra g e s u rg e v e l o c i t y ............................................... 119 37. A l t e r n a t e r a t e o f suspended sand t r a n s p o r t v e r s u s a v e ra g e s u r g e v e l o c i t y . . . . . . . 121 3 8 . G rain s i z e d i s t r i b u t i o n o f suspended san d . 124 39. R e l a t i o n s h i p betw een s u rg e asymmetry and m ig r a ti o n o f c o a r s e t r a c e r sand ............................ 126 40 . R e l a t i o n s h i p betw een su rg e asymmetry and m ig r a ti o n o f f i n e t r a c e r s a n d ................................. 128 4 1 . R e l a t i o n s h i p betw een su rg e asymmetry and m ig r a ti o n o f i n s i t u bottom s a n d ....................... 131 42 . R e l a t i o n s h i p betw een asymmetry o f h i g h e r su rg e v e l o c i t i e s and i n s i t u sand m ig r a ti o n ............................................... . . . . . . . 133 v i TABLES T able Page I . A ccuracy o f m easurem ents o f sand t r a n s p o r t r a t e s ........................................................................ 25 I I . R e s u l t s o f K r u s k a l- W a llis t e s t s c o n c e rn in g su rg e a s y m m e t r y .................................................... 70 I I I . R e s u l t s o f K r u s k a l- W a llls t e s t s r e l a t i n g sand m ig r a ti o n t o su rg e asym m etry . . . . 136 vii ABSTRACT W a v e -g e n e ra te d c u r r e n t s and a t t e n d a n t sa n d t r a n s p o r t have been exam ined in th e o f f s h o r e zone a t v a r i o u s l o c a t i o n s in s o u th e r n C a l i f o r n i a . The stu d y r e f u t e s s e v e r a l p r e v i o u s l y re c o g n iz e d t h e o r i e s c o n c e rn in g s e d i m e n ta ry p r o c e s s e s in t h i s r e g i o n . A s e t o f p a s s i n g waves g e n e r a t e s o s c i l l a t i n g c u r r e n t s a t th e s e a f l o o r w hich have a sp e ctru m o f v e l o c i t i e s . The a v e ra g e c u r r e n t s t r e n g t h o ver b ro a d s h e lv e s i s p r e d i c t e d a c c u r a t e l y by c l a s s i c a l wave t h e o r y . Long p e r i o d s w e lls and o f f s h o r e b r e e z e s cause a n e t t r a n s p o r t o f bottom w a te r to w a rd s th e b e a c h , w h ile s h o r t p e rio d waves and o n sh o re w inds a re a s s o c i a t e d w ith n e u t r a l o r seaw ard flo w . The s h a llo w ocean f l o o r i s co v ered w i t h o s c i l l a t i o n r i p p l e marks whose d im en sio n s a r e a f u n c t i o n o f sand s i z e and c u r r e n t v e l o c i t y . T h e ir p r e s e n c e a llo w s a l l a v a i l a b l e g r a i n s i z e s t o be moved w ith e q u a l e a s e . R ip p le mark r e l i e f d e te rm in e s th e r e l a t i v e h e i g h t s t o w hich p a r t i c l e s a r e su sp en d e d . A r i p p l e rem a in s in th e same p o s i t i o n even thou gh b o th se d im e n t and w a t e r may be d i f f e r e n t i a l l y t r a n s p o r t e d . Most b o tto m san d m otion i s c o n fin e d t o t h e lo w e st few c e n t im e te r s o f th e w a te r colum n. The r a t e a t which p a r t i c l e s a r e t r a n s p o r t e d v a r i e s w i t h th e s q u a re o f c u r r e n t s t r e n g t h . The c o n c e p t o f t h r e s h o l d v e l o c i t i e s i s n o t r e a d i l y a p p l i c a b l e t o th e n e a r s h o r e en v iro n m en t b e c a u se o f v a r i a b i l i t y in su rg e c h a r a c t e r i s t i c s , boundary r o u g h n e s s , and se d im e n t c o h e s i v e n e s s . N et w a te r d r i f t a r^ r e l a t i v e c u r r e n t v e l o c i t y d e te rm in e th e d i r e c t i o n in w hich sa n d i s s h i f t e d . Moving g r a i n s a r e n o t s o r t e d by s u rg e s b u t r a t h e r th e whole s i z e p o p u la tio n i s t r a n s p o r t e d as a u n i t . In w i n t e r , r i p c u r r e n t s t r a n s p o r t b e a c h sand to th e o f f s h o r e . The p red o m in an t seaw ard o s c i l l a t i o n s caused by s t e e p waves and s t r o n g winds p r e v e n t th e r e t u r n o f p a r t i c l e s to th e b r e a k e r s . R ip s a r e n o t a c t i v e i n th e calm summer m o n th s, and lo n g p e r i o d s w e lls p ro d u ce s h o r e ward su rg e asym metry w hich r e p l e n i s h e s th e b e a c h w ith s e d im e n t. v i i i INTRODUCTION G e n e ra l S ta te m e n t T r a n s p o r t a t i o n o f se d im e n t by s h o a l i n g waves i s an I m p o r ta n t g e o lo g ic p r o c e s s . M a t e r i a l e ro d e d from th e c o n t i n e n t s must p a ss t h r o u g h th e zone o f wave a c t io n b e f o r e i t I s d e p o s ite d i n th e m arin e e n v iro n m e n t. S i l t and c la y move more o r l e s s d i r e c t l y to deep w a t e r , b u t sand I s f u n n e l l e d a lo n g t h e s h o r e l i n e . U l t i m a t e l y , th e sa n d i s d e p o s i te d on th e I n n e r c o n t i n e n t a l s h e l f o r i s i n t e r c e p te d by a subm arine canyon and t r a n s p o r t e d to the ocean b a s i n . Movement o f s e d im e n t in th e o f f s h o r e zone i s a l s o r e l e v a n t from a p r a c t i c a l s t a n d p o i n t . Sand eroded from b e a c h e s d u rin g storm s i s u s u a l l y d e p o s ite d o u t s i d e t h e b r e a k e r s . An u n d e r s t a n d i n g o f th e exchange o f sand b e tween th e beach and o f f s h o r e i s i n v a l u a b l e f o r th e p r e d i c t i o n and c o n t r o l o f s h o r e l i n e e r o s i o n . The i n t e r a c t i o n betw een waves and se d im e n t h a s been exam ined e x t e n s i v e l y i n th e l a b o r a t o r y w ith th e use o f wave t a n k s . B ecause o f s c a l i n g p ro b le m s, how ever, an e x t r a p o l a t i o n o f e x p e r i m e n t a l c o n c lu s io n s t o th e n a t u r a l e n v iro n m en t i s n o t a lw ay s v a l i d . Many f i e l d i n v e s t i g a t i o n s have been p e rfo rm ed on b e a c h e s , b u t d i r e c t o b s e r v a t i o n o f sand t r a n s p o r t In t h e o f f s h o r e was n o t p o s s i b l e u n t i l th e d ev elo p m en t o f s e l f - c o n t a i n e d u n d e rw a te r b r e a t h i n g a p p a r a t u s . ThlB stu d y c o m p rise s an in s i t u exam ina t i o n o f th e d i s t u r b a n c e o f th e s e a f l o o r by s h o a l in g w a v e s. P r e v io u s Work A d i s c u s s i o n o f s h o r e l i n e p r o c e s s e s m ust b e g in w i t h c o n s i d e r a t i o n o f w a te r m otion to p ro v id e t h e dynamic fram ew ork f o r se d im e n t t r a n s p o r t . The c o n v e n tio n a l m odel f o r w a te r c i r c u l a t i o n in th e l i t t o r a l zone e s t a b l i s h e d by S h e p a rd and Inman (1950) i s d i s p l a y e d on F ig u r e 1. The p r i n c i p a l c u r r e n t s a r e : 1. C o a s t a l c u r r e n t s , w hich flo w p a r a l l e l to s h o re o u t s i d e th e b r e a k e r s a t low v e l o c i t i e s . 2. Wave d r i f t , th e mass t r a n s p o r t o f w a te r to w a rd s s h o r e su perim p o sed on th e o s c i l l a t o r y m otion caused by p a s s i n g w aves. 3. L o n gsho re c u r r e n t s , w hich move p a r a l l e l to th e c o a s t between t h e b r e a k e r s and s h o r e . 4. R ip c u r r e n t s , by w h ich th e w a te r t r a n s p o r t e d to w a rd s th e b e a c h by wave d r i f t i s r e t u r n e d t o th e o f f s h o r e . W av e -g e n e ra ted o s c i l l a t o r y c u r r e n t s c a u se th e movement o f s e a f l o o r se d im e n t and t h e r e f o r e d e se rv e c l o s e a t t e n t i o n . When a wave a p p ro a c h e s s h o r e , t h e m otion w h ich i t c a u se s e x te n d s to th e b o tto m when w a te r d e p th e q u a ls F ig u re 1. C o n v e n tio n al model f o r n e a r s h o r e w a te r c i r c u l a t i o n ( a f t e r S hep ard and Inman, 1 9 5 0 ). 3 4 COASTAL CURRENT WAVE DRIFT RIP ^ CURRENT LONGSHORE CURRENT <---------- < ■ 1 -----------7------------- 7------------- 7--------------7------------ 7 7 7 7 7 7 7 7 7 ? 7 BEACH o n e - h a l f th e wave l e n g t h . A ccording to th e o r y , energy i s c o n t i n u a l l y l o s t from a p a s s in g wave because o f bottom f r i c t i o n and p e r m e a b i l i t y , c a u s in g th e onshore Burge to exceed i n b o th v e l o c i t y and a m p litu d e th e o f f s h o r e com p o n e n t. In sh a llo w w a te r , o s c i l l a t i o n i s more i n t e n s e and in c r e a s e d f r i c t i o n a l f o r c e s c r e a t e a g r e a t e r d is c re p a n c y between th e s t r e n g t h s o f th e two d i r e c t i o n s o f s u r g e . As a r e s u l t , a n e t onshore flow o f w a te r , wave d r i f t , o c cu rs n e a r th e s e a f l o o r and i n c r e a s e s e x p o n e n t ia l ly tow ards s h o re . The c o n ce p t o f wave d r i f t waB an aly ze d from a t h e o r e t i c a l s ta n d p o in t by L o n gu et-H igg ins (1953) and ob se rv e d in wave ta n k s by Bagnold (194-7) and R u s s e l l and O sario (1 9 5 5 ). Longinov ( in Z enkovich, 1967) s t u d i e d wave m otion in th e B lack Sea u s in g p r e s s u r e t r a n s d u c e r s in sh a llo w w a te r . F o r th e s h o r t p e rio d waves w ith which he worked, shorew ard mass t r a n s p o r t o f w a te r o c c u rre d a t d e p th and I n c r e a s e d in v e l o c i t y t o a h ig h J u s t beyond th e b r e a k e r s . Inman and Nasu (1956) perform ed a s i m i l a r i n v e s t i g a t i o n in s o u th e rn C a l i f o r n i a where waves a r e m oderate to lo n g in p e r io d . They r e p o r t e d t h a t d r i f t u s u a l l y was to w ards sh o re , b u t o c c a s i o n a l l y flow ed Beaward. A tte n u a tio n o f su rg e w ith d ep th may be p r e d i c te d from c l a s s i c a l wave t h e o r y . A ttem pts to r e l a t e to th e o ry ob served c u r r e n t s (Inman and Nasu, 1956) and p r e s s u r e s (lo n g in o v i n Z enkovich, 1967, and D ra p er, 1957) a t th e ocean botto m , how ever, have produced u n c e r t a i n and o fte n 6 ! c o n f l i c t i n g r e s u l t s . Sand i s t h e m ost common sed im en t on t h e sh a llo w s e a f l o o r o f f exposed c o a s t s . The sand may be i n tr o d u c e d by r i v e r s o r wave e r o s i o n and i s t r a n s p o r t e d a lo n g th e s h o r e l i n e by lo n g s h o r e c u r r e n t s t o c r e a t e and m a i n ta in b e a c h e s . Sedim ent r e a c h e s t h e o f f s h o r e zone i n r i p c u r r e n t s and i s d e p o s i t e d i n th e form o f a wedge w hich p r o g r a d e s se aw ard . I n t e r a c t i o n o f s h o a l in g waves w ith b o tto m sand p r o v id e s s e v e r a l p o t e n t i a l a r e a s f o r r e s e a r c h . The com p e te n c y and c a p a c i t y o f o s c i l l a t i n g c u r r e n t s may be exam ined and compared t o s i m i l a r in f o r m a tio n f o r u n i d i r e c t i o n a l flo w . A few t h e o r e t i c a l l y d e r iv e d e q u a tio n s f o r se d im e n t t r a n s p o r t r a t e s and l a b o r a t o r y e x p e rim e n ts on t h r e s h o l d v e l o c i t i e s r e p r e s e n t th e o n ly i n v e s t i g a t i o n s o f t h e m ech an ics o f o f f s h o r e sand t r a n s p o r t a t i o n . R ip p le marks a re u b i q u i t o u s f e a t u r e s b o th on th e s h a llo w s e a f l o o r and i n a n c i e n t se d im e n ta ry d e p o s i t s . The c h a r a c t e r i s t i c s o f o s c i l l a t i o n r i p p l e m arks have been exam ined by B agnold (1 9 4 6 ), S c o t t (1 9 5 4 ), and Manohar (1955) i n th e l a b o r a t o r y » R isk ( 1 9 6 5 ) In Lake Huron, and Inman (1957) in t h e o c e a n . Y et th e m ig r a ti o n o f th e s e f e a t u r e s and t h e r e l a t i o n th e y b e a r to sand m otion have n o t been s c r u t i n i z e d i n t h e n a t u r a l e n v iro n m e n t. Most e f f o r t i n s t u d i e s o f n e a r s h o r e sed im en t m otio n h a s been d e v o te d t o t h e n e t m ig r a tio n o f sand norm al t o th e c o a s t . The Im pact o f t h i s s u b j e c t on b e a c h e r o s i o n i s o b v io u s . T hree m odels f o r n e t sand movement have been p ro p o se d : 1 . G ran t (1943) assumed t h a t b e ca u se o f th e t h e o r e t i c a l i n e q u a l i t y o f on sh o re and o f f s h o r e su rg e v e l o c i t i e s , a l l b ottom sand sh o u ld be t r a n s p o r t e d tow ard t h e b e a c h . A b a la n c e i s m a in ta in e d by s e a w a rd -flo w in g r i p c u r r e n t s la d e n w ith s e d im e n t. The d im e n sio n s o f i n coming waves d e te rm in e t h e s i z e and q u a n t i t y o f g r a i n s exchanged betw een th e b e a c h and t h e o f f s h o r e . S a to , I J i n a , and Tanaka (1963) l e n t s u p p o r t t o t h i s model by o b s e r v in g n e a r th e c o a s t o f Japan t h a t l a r g e q u a n t i t i e s o f i r r a d i a t e d sand w ere moved in t h e d i r e c t i o n o f wave a d v an c e. 2. A n o th e r t h e o r y presum es t h a t two o p p o sin g f o r c e s a c t on sedim en t g r a i n s on th e s h a llo w s e a f l o o r . In a d d i t i o n t o o n sh o re wave d r i f t , t h e s lo p e o f th e b o tto m p ro d u c e s a c o n s t a n t o f f s h o r e g r a v i t a t i o n a l com p o n e n t. F o r eac h g r a i n o f a c e r t a i n s i z e , s h a p e , and d e n s i t y t h e r e e x i s t s a p o s i t i o n o f e q u i l i b r i u m betw een seaw ard and shorew ard m o tio n . The n u l l p o i n t s f o r th e l a r g e s t p a r t i c l e s a re c l o s e s t t o s h o r e , y i e l d i n g a bottom se d im e n t s i z e d i s t r i b u t i o n w hich d e c r e a s e s In t h e o f f sh o re d i r e c t i o n . U sing a wave t a n k w ith a p la n e b o tto m , Ippen and E a g le so n (1955) co n firm ed th e e x i s t e n c e o f n u l l p o i n t s f o r i n d i v i d u a l g r a i n s . M i l l e r and Z e i g l e r (1958) in c o r p o r a t e d t h e n u l l p o i n t c o n c e p t in a model w hich t h e y propo sed f o r se d im e n t t r a n s p o r t i n th e s h a llo w n e r i t i c e n v iro n m e n t. Vernon (1966) made f i e l d m ea su re m e n ts,o n th e t r a n s p o r t a t i o n o f dyed sand on th e s e a f l o o r o f f s o u th e r n C a l i f o r n i a w hich s u p p o r te d t h i s t h e o r y . U nder s i m i l a r wave c o n d i t i o n s , shorew ard m ig r a tio n o c c u r r e d i n s h a llo w w a te r w h ereas a t d e p th e i t h e r n e u t r a l o r seaw ard d i s p l a c e ment was o b s e r v e d . Two s i z e d i s t r i b u t i o n s o f t e s t sa n d were used in th e e x p e r im e n ts , and t h e r e s u l t s i n d i c a t e d a s h a llo w e r n u l l p o i n t f o r t h e c o a r s e r m a t e r i a l , 3 . S e v e r a l r e c e n t i n v e s t i g a t o r s c o n clu d ed t h a t n e i t h e r wave d r i f t n o r th e seaw ard component o f g r a v i t y c o n t r o l th e d i s t r i b u t i o n o f sand i n t h e o f f s h o r e . Most t r a n s p o r t a t i o n i s th o u g h t to ta k e p la c e by d i f f u s i o n o f suspended s e d im e n t. U a g ata (1961) p e rfo rm ed an e x p e rim e n t on t h e c o a s t o f Japan and o b se rv e d t h a t su spend ed sand seemed to be moved i n a d i r e c t i o n o p p o s i t e to t h a t o f wave d r i f t . M urray (1967) d e c id e d t h a t t h e m o tio n o f f i n e dyed sand o f f a low e n erg y b each conformed t o th e p r e d i c t i o n s o f d i f f u s i o n t h e o r y . P a s s e g a , R i z z i n i , and B o r g h e t t i (1967) i n f e r r e d from s u b t l e t i e s in th e t e x t u r e o f se d im e n t o f f th e A d r i a t i c c o a s t t h a t i t had been d e p o s i te d from a s u s p e n s io n . P h ilo so p h y o f Approach The l i t e r a t u r e c o n c e rn in g th e t r a n s p o r t a t i o n o f sand by s h o a l i n g waves i s r e p l e t e w ith e x h a u s t i v e l a b o r a - t o r y i n v e s t i g a t i o n s , t u t complem entary f i e l d o b s e r v a tio n has te e n l i m i t e d . For t h i s r e a s o n , i t was d e cid ed t o use SCUBA equipm ent to conduct a com prehensive stu d y o f th e hydrodynam ics o f th e n e a r s h o r e environm ent and a t t e n d a n t sedim ent m otion i n th e s e a o f f s o u th e rn C a l i f o r n i a . A c u r r e n t m eter was d e sig n ed t o r e c o r d wave g e n e r a te d o s c i l l a t o r y s u rg e s n e a r th e ocean f l o o r . T his p e r m itte d an a sse ss m e n t o f wave d r i f t , a co n cep t which i s c e n t r a l to t h e o r i e s p r e d i c t i n g sand m ig r a ti o n . C o r r e l a t i o n s o f wave c h a r a c t e r i s t i c s w ith bottom d r i f t and c u r r e n t v e l o c i t y were a ls o p o s s i b l e . D evices t o i n t e r c e p t moving sedim ent were used to examine th e q u a n t i t y and n a t u r e o f sand t r a n s p o r t e d by w aves. The competency and c a p a c ity o f o s c i l l a t i n g c u r r e n t s as w e ll as th e su s p e n s io n o f sed im en t in th e w a te r column were exam ined. T raps which i n t e r c e p t e d sand b e in g moved shorew ard and seaward en ab led an e v a l u a t i o n o f models proposed to e x p la in t h i s m ig r a tio n p r o c e s s . R ip p le m arts were p h o tographed and sam pled t o d e te rm in e t h e i r r o l e in sed im en t t r a n s p o r t . I n d i v i d u a l o b s e r v a tio n s were acco m p lish ed o ver p e r io d s ra n g in g between o n e - h a l f and s i x h o u rs a t v a r i e t y o f bottom s i t e s . The stu d y e n t a i l e d o v e r 80 days in th e f i e l d and 350 d iv e s . 10 L im i ta t io n s The energy o f waves r e a c h in g t h e c o a s t o f so u th e rn C a l i f o r n i a may he c l a s s i f i e d as m o d era te . S w e lls c a u sin g th e w a te r and sedim ent m otion un der c o n s i d e r a t i o n had p e rio d s o f 6 t o 12 seconds and h e i g h t s between 0 .2 and 1 .5 m. O b s e rv a tio n s were perform ed a t d e p th s o f 3 to 30 m h a v in g r e l a t i v e wave h e i g h t (wave h e i g h t / w a t e r depth) v a lu e s betw een .015 and .0 8 . Average surge v e l o c i t i e s ranged from 5 to 35 c m /sec. Bottom sedim ent a t th e te B t s i t e s c o n s i s t e d e n t i r e l y o f sands w ith mean d ia m e te rs o f 0 ,0 1 t o 1 .0 mm. Acknowledgments The a u th o r i s i n d e b te d to D r. Donn G o r s lln e who s u p e r v is e d th e r e s e a r c h and p ro v id ed many h e l p f u l sug g e s t io n s and t o D octors R ich a rd Stone and R u s s e l Zimmer f o r c r i t i c a l l y r e a d in g t h e m a n u s c rip t. Dr, H aro ld Palmer perform ed an a s s o c i a t e d p r o j e c t on sc o u r i n t h e o f f s h o r e zone and c o l l a b o r a t e d on much o f th e f i e l d work and l a b o r a t o r y a n a l y s i s . Mr. H e r b e r t Summers c o n s t r u c t e d th e c u r r e n t m e te r used e x t e n s i v e l y i n t h e stu d y and prov id ed an u n d e rw a te r tim e -la p B e movie cam era. An a u t o m a t i c a l l y - r e c o r d i n g s e t t l i n g tube w hich g r e a t l y f a c i l i t a t e d t e x t u r a l a n a l y s i s o f sedim ent sam ples was b u i l t by Mr. David F e l i x . Many s t u d e n t s , i n c l u d i n g R ic h a rd L i t t l e , S ig u rd Murphy, and M ic h ae l R isk s e rv e d as d iv in g p a r t n e r s . The r e s e a r c h was p e rfo rm ed u s in g t h e U n i v e r s i t y o f S o u th e rn C a l i f o r n i a r e s e a r c h v e s s e l Ahoyoha I I I and t h e D epartm ent o f G e o lo g i c a l S c ie n c e s BoBton W h a le r. The p r o j e c t was g e n e r o u s ly s u p p o rte d by th e O f f i c e o f N aval R e s e a rc h u n d e r c o n t r a c t number N 00014-67-A -0269-0002. S t i p e n d s were p ro v id e d from O f f ic e o f N a v al R e s e a r c h c o n t r a c t s num ber 228 (1 7 ) and number N00014-67-A-0269-0009* and from th e N a t io n a l A e r o n a u tic s and Space A d m i n i s t r a t i o n . 12 METHODS Wave Measurement The f i r s t e v e n t d u rin g a day o f u n d e rw a te r o b s e rv a t i o n was an e v a lu a tio n o f waves r e a c h in g th e c o a s t and c a u s in g bottom sedim ent t o move. T his was accom plished w ith th e use o f a wave s t a f f . M easurem ents were perform ed in w a te r d e p th s e x c e e d in g 25 m so t h a t s w e lls would n o t be a f f e c t e d by f r i c t i o n a t th e s e a f l o o r . T h e r e fo r e , a l l v a lu e s o f wave h e ig h t a re v a l i d f o r deep w a te r o n ly . Because s w e lls r e a c h in g th e so u th e rn C a l i f o r n i a c o a s t a re g e n e ra te d h u n d red s o f m ile s away and have u n d e r gone d i s p e r s i o n , t h e i r c h a r a c t e r i s t i c s rem ain e s s e n t i a l l y c o n s t a n t f o r h o u rs and o f t e n d a y s. Thus, s e v e r a l m inu tes o f wave m easurem ents each f i e l d day y i e l d e d in fo rm a tio n w hich s u f f i c e d f o r th e su c c e e d in g p e rio d o f subm arine o b s e r v a t i o n . C u rre n t D eterm in a tio n O s c i l l a t i n g c u r r e n t s a t th e se a f l o o r g e n e ra te d by p a s s in g waves were r e c o rd e d on a m ec h an ica l c u r r e n t m eter ( P ig . 2 ) . Two p a d d le s , which move in re s p o n se to th e s u r g e , s c r i b e on a drum which com pletes one r o t a t i o n in 14 m in u te s . A s t a t i o n a r y pendulum on th e o p p o s ite s id e o f t h e drum d e f i n e s th e n u l l p o i n t. The m eter i s o r i e n t e d F ig u r e 2 . C u r r e n t m e t e r . P a d d le s move in r e s p o n s e t o su rg e and s c r i b e on th e r o t a t in g drum. 13 15 norm al t o th e o s c i l l a t i o n s , and t r i a l s were made in l o c a t i o n s w hich l a c k c u r r e n t s o t h e r th a n th o s e c au se d by p a s s in g w aves. The v e l o c i t y ran g e e x te n d s from 0 to 60 cm /se c. T y p ic a l r e c o r d s from th e I n s tr u m e n t a r e d is p l a y e d on F ig u r e 3 . B oth an e v a l u a t i o n o f a v e ra g e c u r r e n t s t r e n g t h and a com parison o f th e r e l a t i v e m agnitude and d u r a t i o n o f o nsh o re and o f f s h o r e p u l s e s i s p o s s i b l e f o r eac h t r i a l . V e l o c i ty d e te r m i n a t i o n s a re s i m p l i f i e d by th e use o f a t r a n s p a r e n t o v e r l a y on w h ich c u r r e n t s t r e n g t h i s marked in in c re m e n ts o f 10 c m /se c . To compute a v e r a g e s , th e number o f s u r g e s i n each i n t e r v a l i s m u l t i p l i e d by th e m id - p o in t o f th e i n t e r v a l . The p r o d u c ts a r e th e n summed and d iv id e d by th e t o t a l number o f i n d i v i d u a l p u l s e s . The d e g re e o f su rg e asymmetry i s i n d i c a t e d by o n s h o r e - o f f s h o r e r a t i o s o f v e l o c i t y and d u r a t i o n o f flo w . In com puting th e s e r a t i o s th e l a r g e r o f th e two q u a n t i t i e s i s p la c e d in t h e n u m e ra to r so t h a t th e v a lu e i s g r e a t e r th a n 1 . 0 . A p lu s s ig n d e n o te s asymmetry in t h e shorew ard d i r e c t i o n , a n e g a t i v e s ig n a predom inance o f seaw ard p u l s e s . By com bining r a t i o s o f c u r r e n t s t r e n g t h and d u r a t i o n , t h e d i r e c t i o n o f wave d r i f t may be a p p r a i s e d . The c u r r e n t m e te r was c a l i b r a t e d w i t h t h e u se o f a s i m i l a r I n s tr u m e n t k i n d l y lo a n e d f o r t h i s p u rp o se by s c i e n t i s t s a t th e U n ite d S t a t e s Navy U n d erw ater W a rfa re C e n te r . The N a v y - c o n s tr u c te d m e te r had been c a r e f u l l y t e s t e d i n a tow t a n k . B oth i n s t r u m e n t s w ere ru n c o n c u r - F ig u re 3. R eco rd s from c u r r e n t m e te r . S u rg e s on t h e r e c o r d to t h e r i g h t a r e weak and have a lo n g p e r i o d . Those in th e example a t th e l e f t a re s t r o n g e r and s h o r t e r in p e r i o d . Each r e p r e s e n t s 14 m in u te s o f ob s e r v a t i o n . 16 18 r e n t l y in th e o f f s h o r e and th e two r e c o r d s were compared. By c o n t r a s t i n g d isp la c e m e n ts o f th e p a d d le s w ith known v e l o c i t i e s from th e Navy m e te r, a g rap h r e l a t i n g c u r r e n t s t r e n g t h to d isp la c e m e n t was e a s i l y p re p a re d . C onfidence l i m i t s w hich were, computed f o r th e c a l i b r a t i o n curve i n d i c a t e t h a t a v erag e v e l o c i t i e s f o r i n d i v i d u a l t r i a l s a re a c c u r a t e to w ith in ± 2 cm /sec. S ince v a lu e s f o r average c u r r e n t s t r e n g t h rang ed from 5 to 32 c m /s e c ., in s tru m e n t e r r o r i s n o t a p p r e c i a b l e . Sedim ent T raps Three v a r i e t i e s o f t r a p s (P ig . 4-) to i n t e r c e p t moving sedim en t were used in th e s tu d y . They were d e s ig n ed to sample th e t o t a l l o a d , th e d i s t r i b u t i o n o f suspended sed im ent n e a r th e bottom , and th e volume o f sand moving onshore compared to t h a t moving o f f s h o r e . T o ta l Load The d e v ic e f o r sam pling a l l m a t e r i a l b ein g t r a n s p o r te d i s e x tre m e ly sim ple i n d e s ig n , c o n s i s t i n g o f a number 107A t i n can w ith a s l i t 1 .5 cm w id e . In p r a c t i c e , th e s l i t i s co vered w ith m asking t a p e , and th e can I s c a r e f u l l y b u r ie d in a r i p p l e tr o u g h so t h a t only t h e ta p e i s exposed. A f t e r an i n t e r v a l d u rin g w hich e q u ilib r iu m i s r e s t o r e d , th e ta p e i s removed and moving g r a in s s e t t l e i n t o th e can (P ig . 5)* Sand i s allo w ed t o accum ulate f o r F ig u re 4 . The t h r e e ty p e s o f se d im e n t t r a p s . From t o p t o b o tto m , th e y a r e f o r i n t e r c e p t i n g sand b e in g d i f f e r e n t i a l l y t r a n s p o r t e d , s u s pended p a r t i c l e s , and th e t o t a l moving l o a d . 19 F ig u r e 5. T rap i n p o s i t i o n t o i n t e r c e p t th e t o t a l moving l o a d . 21 22 , ’ ' . ‘ s.*' & * • • ■ + . '' » 23 1 t o 4 h o u rs b e fo r e t h e t r a p i s r e c o v e r e d . The a d v a n ta g e o f t h i s d e s ig n i s t h a t n o t h in g p r o t r u d e s above th e s e a f l o o r t o o b s t r u c t c u r r e n t flo w . S e v e r a l f u n n e l - l i k e t r a p s were t e s t e d , b u t t h e s e cau sed so much s c o u r and anom alous sand m otion t h a t t h e y were u n a c c e p t a b l e . However, a problem i n h e r e n t i n t h e su b s u r f a c e sa m p le r i s t h a t r o l l i n g p a r t i c l e s a r e I n t e r c e p t e d w h ile suspended g r a i n s b y -p a s s t h e s l i t . T h is d i f f i c u l t y i s m inim ized by th e manner i n w hich sand i s moved, low v e l o c i t y s u r g e s c a u se t r a n s p o r t a t i o n by t r a c t i o n and a l l sand r e a c h i n g t h e t r a p i s c a u g h t. S t r o n g e r c u r r e n t s sweep se d im e n t p a s t th e s l i t u n t i l th e wave p a s s e s , and th e n a l l p a r t i c l e s s e t t l e i n t o t h e can. T hus, sand moved by i n d i v i d u a l p u l s e s i s sam pled more o r l e s s r e p r e s e n t a t i v e l y . A lth o u g h a c e r t a i n amount o f s e l e c t i v i t y c h a r a c t e r i z e s th e b u r l e d - c a n d e s i g n , i t seems to be t h e beBt s o l u t i o n to th e p a r a d o x i c a l prob lem o f I n t e r c e p t i n g t h e t o t a l se d im e n t lo a d i n an o s c i l l a t i n g c u r r e n t . In th e l a b o r a t o r y th e c o n t e n t s o f th e can were d r i e d and w e ig h ed . D iv id in g t h i s w e ig h t by t h e tim e o f e x p o su re y i e l d s a se d im e n t t r a n s p o r t r a t e w hich i s e x p r e s s e d in term s o f grams p e r h o u r . The sam ples were th e n s p l i t and a n a ly z e d f o r g r a i n s i z e d i s t r i b u t i o n u s i n g an a u t o m a t i c a l l y - r e c o r d i n g s e t t l i n g t u b e . B ecause s e v e r a l c o n c lu s io n s a r e b ased on r e s u l t s from t h i s se d im e n t t r a p , th e m a g n itu d e o f sa m p lin g e r r o r 24 i s i m p o r t a n t . E r r o r s may r e s u l t from two s o u r c e s . S e d i m ent t r a n s p o r t i s c h a r a c t e r i z e d "by a d e g re e o f l o c a l v a r i a b i l i t y b e c a u se o f th e in h o m o g e n eity o f r i p p l e mark p a t t e r n s . A ls o , im p ro p e r p lac em en t o f th e t r a p i n th e r i p p l e tro u g h s l e a d s to i r r e g u l a r i t y i n th e r e s u l t s . The r e l a t i v e sa m p lin g e r r o r was examined on f o u r o c c a s i o n s . In e a c h c a se 6 t r a p s w ere p la c e d i n th e s e a f l o o r and a llo w e d t o c o l l e c t sa n d c o n c u r r e n t l y . The r e s u l t i n g g r a l n - s i z e d i s t r i b u t i o n s and s e d i m e n ta t io n r a t e s w ere th e n com pared. The mean d ia m e te rs d i s p l a y e d a v a r i a t i o n on th e same o r d e r o f m ag nitude a s t h e l a b o r a t o r y e r r o r o f a s e t t l i n g tu b e a n a l y s i s and may t h e r e f o r e b;e c o n s id e re d e q u i v a l e n t . Means and s t a n d a r d d e v i a t i o n s w ere c a l c u l a t e d f o r th e a c c u m u la tio n r a t e s and a r e shown on T ab le I . A lso t a b u l a t e d i s th e s t a n d a r d e r r o r , w hich i s d e f in e d as th e e r r o r exceeded i n o n e - h a l f t h e o b s e r v a t i o n s . Two sa m p le rs alw ays w ere used t o I n v e s t i g a t e t h e t o t a l moving l o a d , d e c r e a s i n g th e s t a n d a r d e r r o r . As su c h , t h e e r r o r i s r e a s o n a b l e c o n s i d e r i n g t h a t s e d im e n ta tio n r a t e s ran g e d from .5 t o 2 2 5 .0 g / h r . Suspended Sedim ent A t r a p c o n s i s t i n g o f v i a l s a r r a n g e d i n t i e r s was d e sig n e d t o sam ple sand suspended n e a r t h e b o tto m . The d e v ic e i s b u r ie d i n a r i p p l e tr o u g h so t h a t th e l i p o f th e l o w e s t v i a l i s even w i t h th e s e d im e n t- w a te r I n t e r f a c e . 25 TABLE I ACCURACY OP MEASUREMENTS OP SAND TRANSPORT RATES Average A ccum ulation R ate ( g / h r ) S ta n d ard D e v ia tio n * ( g / h r ) S ta n d a rd E r ro r * ( g / h r ) 5 .0 ± 1 .6 1 1 .1 6 .9 ± 2 .9 1 2 .0 18 1 .4 + 29.1 ± 1 9 .6 19 6.2 12 6 .2 ± 1 7 .7 * On th e b a s i s o f 6 c o n c u rre n t o b s e r v a t i o n s . 26 A f t e r "burial t h e c o n t a i n e r s a r e uncapped and m a t e r i a l s e t t l e s i n t o them a s t h e s u r g e s c a u s in g s u s p e n s io n wane ( F i g . 6 ) . Suspended sa n d i s sam pled a t th e f o llo w in g i n t e r v a l s : 0 - 1 . 2 , 1 . 3 - 7 . 6 , 7 . 7 - 1 5 - 2 , 1 5 . 3 - 2 2 . 8 , and 2 2 .9 - 3 0 .5 cm above th e s e a f l o o r . One t o 4 h o u rs a r e a llo w e d t o e la p s e b e f o r e th e t r a p i s rem oved. A f u n n e l - t y p e o f sa m p le r was used by N a g ata (1961) i n h i s work on t h e s u s p e n s io n o f se d im e n t by w aves. When a s i m i l a r model was t e s t e d i n t h i s s t u d y , i t was o b se rv e d t h a t th e r e t u r n su rg e washed o u t some sand p r e v i o u s l y c a r r i e d i n t o t h e f u n n e l . A lthough th e v i a l d e s ig n does n o t sam ple a l l sed im en t p a s s i n g t h e t r a p , i t does c o l l e c t r e p r e s e n t a t i v e a l i q u o t s o v e r a c o n tin u o u s r a n g e . Sand c o n ta in e d i n th e v i a l s was w eighed and a n a ly z e d f o r g r a i n s i z e d i s t r i b u t i o n . She amount o f s e d i ment su spend ed i n a h e i g h t i n t e r v a l i s e x p re s s e d in te rm s o f i t s r a t e o f a c c u m u la tio n . On o c c a s io n s when two s a m p le rs w ere used to c o l l e c t susp en d ed m a t t e r , i t was n o te d t h a t t h e s e d im e n ta tio n r a t e i n t h e i n t e r v a l a d j a c e n t t o th e s e a f l o o r showed c o n s i d e r a b l e v a r i a t i o n . T h is r e f l e c t s t h e d i f f i c u l t y o f b u r y in g t h e t r a p so t h a t t h e l i p o f th e lo w e r v i a l i s p e r f e c t l y f l u s h w i t h th e sand s u r f a c e . However, a c c u m u la tio n r a t e s a t h i g h e r l e v e l s d i f f e r e d by an a v e ra g e o f 14 p e r c e n t , w hich i s sm a ll compared t o t h e 100 p e r c e n t ra n g e encom pass in g o b se rv e d r a t e s . F ig u re 6. Trap in p o s i t i o n t o sample s u s pended sand. 27 29 Onshore and O ffs h o re M ig r a tio n The b u r ie d - c a n p r i n c i p l e was employed a g a in i n t h e d e s ig n o f a t r a p to i n t e r c e p t sand u n d e rg o in g shorew ard and seaw ard t r a n s p o r t . In t h i s c a s e , ho w ev er, a l o n g i t u d i n a l b a f f l e s e p a r a t e s th e c o n t a i n e r i n t o two h a l v e s . Hinged to t h e b a f f l e i s a g a te w hich i s moved back and f o r t h by th e f o r c e o f th e su rg e on a s a i l l o c a t e d s e v e r a l c e n t im e te r s above t h e b o tto m . The s a m p le r ( P ig . 7) i s b u r ie d in th e s e a f l o o r w ith i t s a x is n o rm al t o o s c i l l a t o r y s u r g e s . The g a te k eep s t h e a c c u m u la tin g sand s e g r e g a te d i n o n s h o re - and o f f s h o re -m o v in g co m partm ents. T h is d e sig n i s s u b j e c t to th e same a d v a n ta g e s and l i m i t a t i o n s a s t h e sa m p le r f o r th e t o t a l m oving lo a d and was s e l e c t e d o n ly a f t e r e x te n s iv e e x p e r i m e n t a t io n . In an e f f o r t to m inim ize t h e p o s s i b i l i t y o f e r r o r in c o r r e l a t i o n s o f sand m ig r a ti o n w ith w a t e r m o tio n , s e d i ment was c o l l e c t e d o n ly w h ile th e c u r r e n t m e te r was r e c o r d i n g . The s l i t i n th e t r a p was opened p r i o r to t h e b e g i n n in g o f a t e s t t o e l i m i n a t e su b s e q u e n t c o n ta m in a tio n from h a n d li n g . When th e c u r r e n t m e te r was s t a r t e d , f o r c e p s were u sed to p la c e a u x i l i a r y r e c e p t a c l e s i n th e t r a p com p a r tm e n ts . Im m ediately f o llo w in g th e c o m p le tio n o f c u r r e n t m easurem ent, th e t r a p s were rem oved, and sand was c o l l e c t e d from th e r e c e p t a c l e s . The sedim en t was l a t e r w eighed and s i z e d . P r e f e r e n - F ig u re 7. Trap In p o s i t i o n to i n t e r c e p t s e d i ment b e in g moved on- and o f f - s h o r e . 30 31 32 : t l a l movement I s e x p re s s e d in te rm s o f o n s h o r e - o f f s h o r e t r a n s p o r t r a t i o s . Two sa m p le rs were alw ays u se d in t e s t s , and th e r e s u l t s w ere r a r e l y i n p e r f e c t a g re e m e n t. D i f f e r e n c e s r e f l e c t i r r e g u l a r i t i e s i n s e a f l o o r m i c r o r e l i e f and im p e r f e c t t r a p p o s i t i o n i n g . In a t h i r d o f t h e t r i a l s , th e v a r i a t i o n exceeded 40 p e r c e n t , and th e s e c a s e s were d i s c a r d e d . A v e rag in g t h e two v a lu e s f u r t h e r r e d u c e s e r r o r . D e sp ite t h e i n e q u i t i e s w hich p lag u e d t h i s p o r t i o n o f th e s tu d y , t h e r e s u l t s J u s t i f y c e r t a i n c o n c l u s i o n s . R i p p l e Mark S a m p lin g O s c i l l a t i o n r i p p l e marks c o v e r th e s h a llo w s e a f l o o r . I n o rd e r to a s c e r t a i n t h e i r h e a r i n g on se d im e n t t r a n s p o r t , th e s e f e a t u r e s were s c r u t i n i z e d d u r i n g each day o f o b s e r v a t i o n . A r u l e r was u sed t o d e te rm in e th e a v e ra g e wave l e n g t h and h e i g h t . P re v io u s r e s e a r c h i n d i c a t e d t h a t r i p p l e m arks s e g r e g a t e se d im e n t by s i z e w ith ex tre m e s l o c a t e d a t th e c r e s t and tr o u g h . T h e r e fo r e c h a n n e l sam ples w ere ta k e n from s e v e r a l c r e s t s and tr o u g h s w i t h sm all v i a l s and l a t e r s u b j e c t e d to a s i z e a n a l y s i s . G r a in S i z e A n a l y s i s R e s u l t s o f g r a i n s i z e a n a l y s e s on s e d im e n t s a m p le s form t h e b a s i s f o r much o f t h e d i s c u s s i o n . F o r t h i s r e a s o n a b r i e f d e s c r i p t i o n o f t h e a u t o m a t i c a l l y - r e c o r d i n g 33 s e t t l i n g tu b e i s w a r r a n t e d . The tu b e I s 1 1 .2 cm i n d ia m e te r . A pan, suspended from a s t r a i n gauge, l i e s a t a d e p th o f 169 cm below th e w a te r s u r f a c e . The w e ig h t o f g r a i n s a c c u m u la tin g on th e pan c a u s e s th e s t r a i n gauge t o g e n e r a te an e l e c t r i c a l s i g n a l . T h is im p u lse I s a m p l i f i e d and fe d I n t o a c h a r t r e c o r d e r w hich draws a c u m u la tiv e cu rv e f o r th e sam ple. The c u rv e i s I n t e r p r e t e d w i t h th e use o f t r a n s p a r e n t o v e r l a y s . A d e t a i l e d re v ie w o f th e d e s ig n o f th e tu b e i s g iv e n by F e l i x (1 9 6 9 ). The r e s u l t s o f a s e t t l i n g tu b e a n a l y s i s a r e e x p r e s s ed in te rm s o f h y d r a u l i c e q u iv a le n c e . A c co rd in g to t h i s p a r l a n c e , t h e f a l l v e l o c i t y o f s e d im e n ts i s compared w ith t h a t o f q u a r t z s p h e r e s . Thus, t h e h y d r a u l i c a l l y e q u iv a l e n t mean d ia m e te r o f a sand i s i n f lu e n c e d by t h e s i z e , s p e c i f i c g r a v i t y , sh a p e , and s e t t l i n g i n t e r a c t i o n o f th e component g r a i n s . Mean d i a m e t e r s were computed u s in g th e p h i sy stem and t h e fo rm u la o f Inman (1 9 5 2 ). V alues were s u b s e q u e n tly c o n v e r te d t o m i l l i m e t e r s . C a l i b r a t i o n o f t h e tu b e was d is c u s s e d by Cook (1 9 6 9 ). The t o t a l e r r o r o f an a n a l y s i s i s s m a l l e r th a n t h a t o f s i e v i n g . C o n fid en ce l i m i t s f o r t h e p o s i t i o n o f mean d ia m e te r s d e te rm in e d w ith t h e s e t t l i n g tu b e a r e d i s p la y e d on F ig u r e 8 . F ig u re 8. P r e c i s i o n o f s e t t l i n g tu b e a n a l y s e s . 34 35 .05 D C O L l < 0 t- UJ o 2 UJ 9 Li. 2 O o tn C D .125 ,250 500 MEAN GRAIN DIAMETER (MM) 36 Eyed Sand E x p e rim e n ts An a tt e m p t was made t o compare t h e m ig r a ti o n o f f l u o r e s c e n t dyed sand w ith w a te r m o tio n . The sand fo rm e r l y h ad been u sed by Vernon i n h i s s t u d i e s and was a v a i l a b le i n two s i z e s h a v in g mean d ia m e te r s o f .254 and .430 mm ( F i g . 9)* O b s e r v a tio n s w ere p re c e d e d by sm oothing an a r e a o f th e s e a f l o o r . When th e c u r r e n t m e te r was s t a r t e d , a s m a ll p o r t i o n o f dyed sand was po ured on th e c e n t e r o f th e f l a t a r e a and i t s p o s i t i o n marked w i t h a s t a k e . At t h e c o n c lu s io n o f c u r r e n t m easurem ents p l a s t i c c a r d s c o a te d w ith v a s e l i n e were f i r m l y p r e s s e d to th e b o tto m a t d i s t a n c e s o f 20 and 40 cm sh orew ard and seaw ard o f th e s t a k e . L a t e r a l m otion o f th e sand was n o t c o n s id e r e d im p o r t a n t . S u b s e q u e n tly , th e c a r d s w ere exam ined u n d e r f l u o r e s c e n t l i g h t so t h a t t h e dyed g r a i n s on e a c h c a r d c o u ld be c o u n te d . C o n to u rs o f g r a i n c o n c e n t r a t i o n w ere drawn, and d i s t a n c e s o f t h e s e from th e r e l e a s e p o i n t e n a b le d th e c o m p u tatio n o f o n s h o r e - o f f s h o r e t r a n s p o r t r a t e s (s e e Vernon, 1 9 6 6 ). U n d erw ater P h o to g ra p h y P h o to g ra p h y was u sed to docum ent th e r e l a t i o n s h i p betw een r i p p l e m arks and sand m o tio n . C lo s e -u p m otion p i c t u r e s p e r m i tt e d a d e l i n e a t i o n o f t h e p a th s ta k e n by F ig u r e 9 . G r a i n - s i z e d i s t r i b u t i o n o f f l u o r e s c e n t - d y e d t r a c e r sa n d s . 37 GRAIN SIZE (MM) CUMULATIVE WEIGHT PERCENT o * o D 0 1 o o V ro cn o U l 39 su spend ed sand g r a i n s , and th e m ig ra tio n o f r i p p l e s them s e l v e s was i n v e s t i g a t e d u s in g tim e - l a p s e te c h n iq u e s . The tim e - l a p s e mechanism on th e camera developed by Mr. Summers i s c o n t r o l l e d from a b o a t , and a n o th e r camera d e s ig n e d and used by Dr. Palm er i s c o m p le te ly s e l f - c o n ta in e d . 40 TEST SITES Three l o c a t i o n s on th e s o u th e rn C a l i f o r n i a c o a s t l i n e ( P i g . 10) s e rv e d a s s i t e s f o r u n d e rw a te r r e s e a r c h . Wave g e n e r a te d su rg e and sed im en t m otion w ere s t u d i e d o f f P a lo s V erdes and S a n ta C a t a l i n a I s l a n d , r e g i o n s w hich a r e c h a r a c t e r i z e d by n a rro w c o n t i n e n t a l s h e l v e s . Hence, an a r e a w ith g e n t l e "bathymetry was d e s i r a b l e f o r a d d i t i o n a l c u r r e n t m ea su re m e n ts. Redondo Beach was s e l e c t e d f o r t h i s p u rp o s e . P a lo s V erdes The P a lo s V erdes H i l l s l i e a t th e s o u th e r n end o f S a n ta Monica Bay. Sea c l i f f s , c o b b le b e a c h e s , and r e e n t r a n t s a r e p red o m in an t a lo n g t h i s p e n i n s u l a . O b serv a t i o n s were made In s e v e r a l bays where p a tc h e s o f f i n e sand a r e I n t e r s p e r s e d w ith r o c k o u tc r o p s In s h a llo w w a t e r . The b o tto m s lo p e s s t e e p l y seaw ard a t an a n g le o f 3 t o ,5 ° * Most waves r e a c h i n g th e c o a s t o f s o u th e r n C a l i f o r n i a a r e g e n e r a te d In t h e G ulf o f A la sk a and th e South P a c i f i c . S w e lls from sto rm s th u s a r r i v e r e s p e c t i v e l y from t h e n o r th w e s t and s o u th w e s t. E xposure to th e s e d i r e c t i o n s v a r i e s m ark ed ly a lo n g P a lo s V erd es and th e i r r e g u l a r s h o r e l i n e c a u s e s waves t o r e f r a c t . F ig u re 10. L o c a tio n o f t e s t s i t e s In s o u th e rn C a l i f o r n i a . 41 42 MONICA BAY REDONDO BEACH . PALOS VERDES •X HILLS CALIFORNIA SANTA CATALINA ISLAND 43 S a n ta C a ta li n a I s l a n d C a t a l i n a is 40 km so uth w est o f Los A ngeles. The rugged i s l a n d is bounded by h ig h c l i f f s and subm arine t a l u s s l o p e s s u p p o rtin g a lu s h grow th o f k e lp (M a c ro c y stis p y r i f e r a ) extend to d e p th s o f 3 to 30 m. There the b o u ld e rs a r e bordered by broad a r e a s o f f i n e t o c o a rse sand w hich have an o f f s h o r e dip o f 3 to 5°» S t u d i e s were p erfo rm ed b o th on th e n o r t h and n o r t h w est s i d e s o f the i s l a n d . The n o r th w e s t r e c e i v e s t h e f u l l fo rc e o f open ocean s w e l l , w hereas o nly r e f r a c t e d waves r e a c h t h e low energy n o r t h s h o re . C a t a l i n a i s renowned f o r th e c l a r i t y o f i t s c o a s t a l w a te r and was f r e q u e n t l y th e s i t e f o r u n d e rw ater p h oto grap hy. Redondo Beach Redondo Beach r e p r e s e n t s th e so u th ern m o st e x te n s io n of t h e b ro a d sandy b e a c h e s form ing th e S a n ta Monica Bay sh o re . The c o n t i n e n t a l s h e l f i s 15 km wide and bottom s lo p e s a r e l e s s th an 3 ° . Bine sand n e a r shore i s r e p la c e d by r e l i c t co arse sand and co b b les a t d e p th . The s o u th e rn end o f th e beach i s s h e l t e r e d from so u th w e st s w e ll, w h ile waves r e f r a c t over Redondo Submarine Canyon to th e n o r t h . To a v o id anom alies i n tr o d u c e d by th e s e f a c t o r s , e x p erim e n ts were co nd ucted in th e c e n t r a l p o r t i o n o f th e s t r a n d . The r e s u l t s o f t h i s d i s s e r t a t i o n a r e p a r t i c u l a r l y r e l e v a n t t o s h o r e l i n e s s i m i l a r t o Redondo. U n f o r tu n a te ly , th e t u r b i d w a te r o f f t h i s and o t h e r beaches p re c lu d e d a l l b u t c u r r e n t m easurem ent, th e most e le m e n ta ry o f th e u n d e rw a te r o p e r a t i o n s . RESULTS OP HYDRODYNAMIC STUDIES D i s t r i b u t i o n o f C u rr e n t V e l o c i t i e s Wave m otion i s o f t e n d e s c r i b e d in an i d e a l i z e d f a s h i o n . One e n v i s i o n s an i n f i n i t e s e r i e s o f I d e n t i c a l s w e l l s a p p ro a c h in g a b e ac h and c a u s in g th e w a t e r t o o s c i l l a t e u n ifo rm ly a t th e s e a f l o o r . In a c t u a l i t y , incom ing t waves a r r i v e In g ro u p s and v a ry b o th i n h e i g h t and p e r i o d . W ater m otion a t d e p th i s i r r e g u l a r , and c u r r e n t s have a sp e c tru m o f v e l o c i t i e s . T y p ic a l d i s t r i b u t i o n s o f su rg e v e l o c i t y a r e p r e s e n te d on P Ig u re 1 1 . The g ra p h s show th e p e r c e n t o f on s h o re and o f f s h o r e c u r r e n t s In in c re m e n ts o f 10 c m /sec. These f o u r common p a t t e r n s may be d i s t i n g u i s h e d from e ac h o t h e r as f o llo w s : 1* The c u r r e n t s m easured in th e f i r s t example a r e weak and d e m o n s tra te no d i r e c t i o n a l asym m etry. The se m i n o rm al d i s t r i b u t i o n o f su rg e s t r e n g t h s , w hich I s marked h e r e , c h a r a c t e r i z e s m ost r e c o r d i n g s . 2 . The seco nd example I n d i c a t e s t h a t w i t h s t r o n g e r c u r r e n t s th e ra n g e o f v e l o c i t i e s i s b r o a d e r . In t h i s I n s t a n c e t h e r e e x iB ts a d i s p a r i t y betw een o n sh o re and o f f s h o re flo w . The d i r e c t i o n a l . ' I n e q u a l i t y o f s u r g e s i s su b s e q u e n t l y d is c u s s e d a t l e n g t h . 3 . In example o a weak shorew ard flo w I s s u p e r - F ig u r e 1 1 . T y p ic a l d i s t r i b u t i o n s o f su rg e v e l o c i t y . Onshore and o f f s h o r e p u ls e s a r e p l o t t e d s e p a r a t e l y and v e l o c i t y i s shown i n in c re m e n ts o f 10 cm /se c. 46 PERCENT OF TOTAL SURGES » OFF ON tn o i r < m c « OFF ■ ON w o o o » ■ > t 48 1 imposed on th e w a v e -g e n e ra te d o s c i l l a t i o n s . T his c u r r e n t i s p ro b a b ly a s s o c i a t e d w ith th e p assage o f an i n t e r n a l wave. LaFond (1961) h as shown t h a t i n t e r n a l waves cause low v e l o c i t y onshore and o f f s h o r e m otion a lo n g th e sh a llo w s e a f l o o r . 4 . The complex p a t t e r n d is p la y e d by th e l a s t e x ample was caused by d i f f e r e n t waves a r r i v i n g from so u th e rn C a l i f o r n i a 's two g e n e r a t i n g a r e a s . U s u a lly one s e t o f s w e lls i s dom inant, b u t o c c a s i o n a l l y such i n t e r a c t i o n o c c u rs i n exposed l o c a t i o n s . Comparison o f Observed and T h e o r e t i c a l V e l o c i t i e s E q u a tio n s f o r p r e d i c t i n g th e v e l o c i t y o f o s c i l l a t in g c u r r e n t s a t th e ocean f l o o r have been d e riv e d from c l a s s i c a l wave t h e o r y . Deep w a te r s w e lls may be a p p r o x i m ated by a t r o c h o i d a l p r o f i l e and th e fo llo w in g e x p re s s io n a p p l i e s (Inman i n S h ep ard , 1963): u = H m T s ln h f 2 h) L where - maximum h o r i z o n t a l v e l o c i t y H = wave h e i g h t T = wave p e rio d h = w a te r d e p th L = wave l e n g t h 4 9 I As waves s h o a l , f r i c t i o n a t th e s e d im e n t- w a te r i n t e r f a c e c a u s e s t h e i r p r o p e r t i e s to c h an g e. A lth o u g h th e p e r io d re m a in s c o n s t a n t , h e i g h t i n c r e a s e s w h ile wave l e n g t h and v e l o c i t y d im i n i s h . When th e r e l a t i v e wave h e i g h t e x c e e d s .0 4 t o .06 s w e l l s a re no l o n g e r t r o c h o l d a l "but c o n s i s t o f i s o l a t e d c r e s t s s e p a r a t e d by f l a t t r o u g h s . W ater m otion i s d e s c r i b e d by s o l i t a r y wave t h e o r y , and a new e q u a tio n o b t a i n s f o r b o tto m v e l o c i t i e s (In m an ): = H ( </gh) m 2 h where g = a c c e l e r a t i o n o f g r a v i t y M arine g e o l o g i s t s f r e q u e n t l y use p r e d i c t e d su rg e v e l o c i t i e s t o a i d i n i n t e r p r e t i n g th e d i s t r i b u t i o n o f n e a r s h o r e s e d im e n ts . A com parison o f t h e o r y w i t h o b se rv ed v a lu e s i s t h e r e f o r e o f s i g n i f i c a n c e . V e l o c i t i e s c a l c u l a t e d from wave s t a f f m easu rem ents u s in g b o th t r o c h o l d a l and s o l i t a r y th e o r y a r e p l o t t e d a g a i n s t a v e ra g e su rg e s t r e n g t h s r e c o r d e d on t h e c u r r e n t m e te r on F ig u re s 12, 13, and 14. D ata from C a t a l i n a and P a lo s V erdes a r e c o n s id e r e d s e p a r a t e l y from i n f o r m a tio n f o r Redondo B each b e c a u se o f d i f f e r e n c e s In wave s h o a l i n g t r a n s f o r m a t i o n o v e r s t e e p and g e n t l e c o n t i n e n t a l s h e l v e s . A l a c k o f c o rre sp o n d e n c e e x i s t s betw een o b se rv e d and t h e o r e t i c a l v e l o c i t i e s from s t e e p s h e l v e s . The s c a t t e r i s p ro b a b ly a r e s u l t o f wave r e f r a c t i o n o v e r th e Figure 12. Theoretical versus observed surge velocities recorded over steep continental shelves (Palos Verdes and Catalina Island). 50 THEORETICAL VELOCITY - SOLITARY WAVES O c o c /> m < o < m r w o o o H U J ALL VELOCITIES I N C M /S E C OBSERVED VELOCITY THEORETICAL VELOCITY - TROCHOIDAL WAVES U 1 H F ig u re 13* Comparison o f su rg e v e l o c i t i e s c a l c u l a t e d u s i n g t r o c h o i d a l wave th e o r y w ith v a l u e s r e c o r d e d o v e r a g e n t l e con t i n e n t a l s h e l f (Redondo B e a c h ). 52 THEORETICAL VELOCITY - TROCHOIDAL WAVES (CJi/l/SEC) 53 70 SO 50 40 30 20 10 0 10 30 40 O 20 50 OBSERVED VELOCITY (CM/SEC) F ig u re 14. Comparison o f s u rg e v e l o c i t i e s c a l c u l a t e d u s i n g s o l i t a r y wave th e o r y w ith v a l u e s r e c o r d e d o v e r a g e n t l e c o n t i n e n t a l s h e l f (Redondo B e a c h ). 54 OBSERVED VELOCITY (CM/SEC) THEORETICAL VELOCITY - SOLITARY WAVES (CM/SEC) O t o O q O k o o < \ <b O Oo O ■ o \J1 Ui 56 i r r e g u l a r ‘ b a th y m e tr y o f P a l o s V e rd es and C a t a l i n a . Wave s t a f f r e a d i n g s w ere p e r fo r m ed o f f s h o r e from u n d e r w a te r t e s t s i t e s , and s w e l l d im e n s io n s a t t h e a d j a c e n t a r e a s c o u ld d i f f e r a p p r e c i a b l y . R e s u l t s from Redondo Beach a r e c o n s id e ra b ly more r e v e a l i n g . T h e o r e ti c a l v e l o c i t i e s f o r t r o c h o i d a l waves a r e c o n s i s t e n t l y a few c e n tim e te r s p e r second l a r g e r th a n re c o rd e d c u r r e n t s in th e 10 t o 25 cm /sec ra n g e . T h is a s s o c i a t i o n i s n o t t r u e a t h i g h e r su rg e s t r e n g t h s . The re a so n f o r th iB a b ru p t d e p a r t u r e o f v a lu e s i s th e t r a n s fo rm a tio n o f s w e lls to th e s o l i t a r y form in sh a llo w w a te r . P r e d i c t i o n s o f s o l i t a r y wave t h e o r y a g r e e w e l l w i t h o b s e r v a t i o n s o f B tro n g c u r r e n t s . T hey show l e s s c o r r e l a t i o n w i t h lo w e r r e a d i n g s ta k e n i n d e e p e r w a te r w h ere w a v es a r e s t i l l t r o c h o i d a l . These r e s u l t s i n d i c a t e t h a t t h e change o f s w e ll form t a k e s p la c e a t a r e l a t i v e wave h e i g h t o f 0 .0 4 . T his v a lu e i s u sed to i n t e g r a t e th e two t h e o r i e s on F ig u re 15. O b s e rv a tio n s ta k e n where r e l a t i v e wave h e i g h t s were lo w er th a n 0 .0 4 a re p l o t t e d a g a i n s t t r o c h o i d a l t h e o r y . These p r e d i c te d c u r r e n t s have been red uced by a f a c t o r o f 0 ,8 0 . S o l i t a r y t h e o r y i s compared w ith v e l o c i t i e s r e c o rd e d in s h a llo w e r w a t e r . In t h i s form th e a c c u ra c y o f c a l c u l a t e d v e l o c i t i e s i s rem ark ab le in com parison t o th e s t u d i e s o f Longinov ( i n Z enkevich, 1967)» D raper (1957)» and Inman and Nasu (1 9 5 6 ). F ig u r e 15 . Use o f two t h e o r i e s to p r e d i c t su rg e v e l o c i t i e s over g e n tl e c o n t i n e n t a l s h e l v e s . S o l i t a r y th e o ry i s used w ith r e l a t i v e wave h e ig h t s g r e a t e r than .040 and t r o c h o i d a l th e o ry a t g r e a t e r d e p th . 57 58 3 5 n 25 - 20 - o tiJ H o o U J a : 0 - 10 - BEST-FIT LINE 30 35 20 25 15 1 0 OBSERVED VELOCITY (CM/SEC.) RELATIVE WAVE HT THEORY SYMBOL GREATER THAN . 0 4 SOLITARY 0 LESS THAN . 0 4 TROCHOIDAL X .8 O 59 F ig u re 15 le n d s c o n fid e n c e to p r e d i c t i o n s o f "bottom c u r r e n t s g e n e r a te d by waves w ith m oderate h e i g h t s and p e r i o d s . Computed v e l o c i t i e s a r e w ith in p lu s o r minus 5 cm /sec o f th e c o r r e c t v a lu e . However, c a u tio n must be e x e r c i s e d when a p p ly in g th e e q u a tio n s in a r e a s o f a b ru p t and i r r e g u l a r b a th y m e try . F ig u re 16 was drawn t o i l l u s t r a t e th e d e c re a s e o f su rg e v e l o c i t y w ith d e p th . Average c u r r e n t s o bserved a t Redondo Beach a re p l o t t e d a s a f u n c t i o n o f th e r e l a t i v e wave h e i g h t . Some s c a t t e r i s I n e v i t a b l e b e cau se w hile th e s w e lls a t Redondo Beach had a p p ro x im a te ly th e same h e ig h t on f i e l d d a y s, t h e i r p e rio d ran ged between 7 and 11 se co n d s. N e v e r t h e l e s s , a tr e n d i s e v i d e n t , and th e a t t e n u a ti o n c o rre sp o n d s more c l o s e l y t o th e p r e d i c t i o n f o r s o l i t a r y waveB th an f o r t r o c h o i d a l s w e l l s . T h is i s re a s o n a b le because most c u r r e n t s were m easured a t s i t e s where th e r e l a t i v e wave h e ig h t was g r e a t e r th a n .040. S u rg e Asym m etry N e t w a t e r t r a n s p o r t s u p e r im p o s e d on w a v e -in d u c e d o s c i l l a t i o n s i s e x p r e s s e d on a c u r r e n t m e te r r e c o r d a s asym m etry o f o n s h o r e and o f f s h o r e p u l s e s . E i t h e r an i n e q u a l i t y o f a v e r a g e v e l o c i t y o r d u r a t io n o f f l o w may c a u s e d r i f t a l o n g t h e s e a f l o o r . In t h e o r y , o n s h o r e s u r g e s s h o u ld be s t r o n g e r , w h i l e o f f s h o r e f l o w o c c u p ie s a l o n g e r tim e I n t e r v a l . As t h e b o tto m s h o a l s , t h e s e i n - Figure 16. Attenuation of surge with in creasing depth. 60 61 o U l c o 28 - h § 24 £ K 2 0 o: 3 O o 1 1 1 > cr uj co CO o 16 - O ,05 ,075 RELATIVE WAVE HEIGHT 62 1 e q u i t i e s sh o u ld i n c r e a s e . R e s u l t s from th e stu d y did n o t c o m p le te ly s u p p o r t t h i s c o n c e p t b u t d id r e v e a l some i n t e r e s t i n g r e l a t i o n s h i p s . The d i s t r i b u t i o n o f o n s h o r e - o f f s h o r e r a t i o s f o r a v e ra g e c u r r e n t v e l o c i t y and p e r i o d o f flo w a r e demon s t r a t e d on F i g u r e s 17 and 18 w here t h e s e v a lu e s a re g rap h e d a g a i n s t r e l a t i v e wave h e i g h t . I n s p e c t i o n o f t h e f i g u r e s le a d s to two im m ediate c o n c l u s i o n s : 1. The r a t i o s show b o th shorew ard and seaward asym metry and a re c h a r a c t e r i z e d by a p p r e c i a b l e v a r i a b i l i t y . The o n ly v i s i b l e t r e n d i s a te n d e n c y o f o n sh o re v e l o c i t i e s to p re d o m in a te a t Redondo B each. 2. The s c a t t e r o f v a lu e s overshadow s any c o r r e l a t i o n betw een d i r e c t i o n a l su rg e d i s p a r i t i e s and d e p th . The o n s h o r e - o f f s h o r e r a t i o s were combined to c r e a t e a f a c t o r r e p r e s e n t i n g n e t w a te r d r i f t on F ig u r e 19. As m ig h t be a n t i c i p a t e d , bo ttom d r i f t i s a l s o v a r i a b l e b o t h i n d i r e c t i o n and m agnitud e a t a l l d e p th s . The su rg e asym m etries o b se rv e d i n t h i s I n v e s t i g a t i o n a r e th u s by no means a sim p le f u n c t i o n o f wave s h o a l in g . The o n ly o t h e r e x te n s iv e f i e l d e x a m in a tio n o f o s c i l l a t i n g c u r r e n t s , p erfo rm ed by L ong inov , y i e l d e d r e s u l t s w hich c o in c id e d w i t h t h e o r e t i c a l e x p e c t a t i o n s . However, L o n g in o v 's work was u n d e rta k e n In an a r e a o f low wave e n e rg y , which was n o t s u b j e c t e d t o th e m e t e o r o l o g i c a l r i g o r s o f an expo sed c o a s t . The l o c a t i o n was i d e a l f o r Figure 17. Asymmetry of surge velocities at different depths. 63 RELATIVE W AVE HEIGHT RATIO OF SURGE VELOCITY OFFSHORE / ONSHORE ONSHORE / OFFSHORE o ro o < n co « o ro o o o < T > GOO *o co « V9 Figure 18. Asymmetry of surge duration at different depths. 65 RELATIVE WAVE HEIGHT RATIO OF SURGE DURATION OFFSHORE/ONSHORE ONSHORE /OFFSHORE 66 co o ro o 0> o a> < ? > O o o fo w --------------1 1 i 1 1 o O II n C O c / > m H x m — ! m r X m to in x X m m r- 0 r~ < < m O m < /> V) u 00 J o o <p o 0 ° o o o ° o o o o o Co C O o o o F ig u re 19. R e la ti v e t r a n s p o r t o f w a te r a t d i f f e r e n t d e p th s . 67 RATIO O F WATER MOTION L U c c o X to L_ U l o s U J c c o T - t i 5 2: o 2,0 - U J o c o X to 1.5 - 1 .0 00 o o 1.5 o o o o — O ' U J c c o X C O u- l l 0 2.0 O = STEEP SHELVES O = GENTLE SHELVES 0 .02 .04 RELATIVE WAVE HEIGHT O o o e r > t) u o — o o o o o o I .06 — V' .08 69 i s o l a t i n g th e e f f e c t s o f waves on s u rg e i n e q u a l i t i e s , y e t m ost o c e a n ic s h o r e l i n e s e x p e r ie n c e t i d a l f l u x and s t r o n g b r e e z e s . N o n -p a ra m e tric s t a t i s t i c s were used to i n s p e c t f o r p o s s i b l e r e l a t i o n s h i p s betw een c u r r e n t asymmetry and wave p e r i o d , th e t i d e , and w in d . A K r u s k a l- W a llls one-way t e s t f o r v a r ia n c e by ra n k s ( S i e g e l , 1956) was employed f o r th e p u r p o s e . T his ty p e o f t e s t i s u s e f u l when q u a n t i t a t i v e d a t a , such as o n s h o r e - o f f s h o r e su rg e r a t i o s , a r e c o r r e l a t e d w ith q u a l i t a t i v e in f o r m a ti o n such as wind d i r e c t i o n . The d a ta a r e r a n k e d , th en t h e ra n k s i n d i f f e r e n t c l a s s e s a r e summed and used in an e q u a t i o n . R e s u l t s a r e e x p re s s e d I n term s o f c o n fid e n c e l e v e l s w ith a h i g h v a lu e i n d i c a t i n g a p ro b a b le a s s o c i a t i o n . The c o r r e l a t i o n s h a v in g a c o n fid e n c e l e v e l o f 80 p e r c e n t o r more a r e l i s t e d on T able I I . The a b s e n c e o f r e l a t i o n s h i p s i n t h e d a ta from C a t a l i n a and P a l o s V erdes may be e x p la in e d by t h e f a c t t h a t anom alous c u r r e n t s o f t e n a re e n c o u n te re d n e a r i r r e g u l a r s h o r e l i n e s . The f o l lo w in g I n f e r e n c e s a r e drawn from c u r r e n t m easurem ents a t Redondo Beach: 1. T i d a l f l u x d o e s n o t have any b e a r in g on su rg e asym m etry. The ran g e betw een h ig h and low w a te r n e a r Los A n g e les i s m o d e ra te (2 m ), and d i f f e r e n t r e s u l t s m ig h t be e x p e c te d in r e g i o n s w ith g r e a t e r t i d a l r a n g e s . 2. The wind a f f e c t s r a t i o s f o r d u r a t i o n o f c u r r e n t 70 TABLE II RESULTS OP KRUSICAL-W ALLIS TESTS CONCERNING SURGE ASYMMETRY Surge Asymmetry F a c to r A verage o n sh o re and o f f s h o r e v e l o c i t i e s Average o n sh o re and o f f s h o r e d u r a t i o n o f flow Bottom d r i f t C onfidence L e v e ls D ir e c tio n * ___________ P e rio d * * §2 .0% Q8.0J& 99-5% 95.0% 99.5% 9 7 .5 # * Wind d i r e c t i o n r e l a t i n g i n v e r s e l y to surge i n e q u a l i t i e s . ** Long p e r io d s w e l l s (10-11 se co n d s) c a u s in g more sh orew ard asym metry th a n s h o r t p e r io d waves (7-8 s e c o n d s ) . C o r r e l a t i o n s o f t i d a l f l u x w ith su rg e i n e q u i t i e s were lo w e r th an t h e 80^ c o n fid e n c e l e v e l . 71 flow and n e t bottom d r i f t . An onshore b re e z e causes shorew ard t r a n s p o r t o f s u r f a c e w a te r and com pensatory o f f sh o re flow a t d e p th . As a r e s u l t seaward su rg e s occupy th e g r e a t e s t amount o f tim e . An o f f s h o r e w ind, on th e o t h e r hand, g e n e r a te s s u b - s u r f a c e d r i f t tow ards th e beach. 3. S w ell c h a r a c t e r i s t i c s a ls o p la y a r o l e in d e te r m in in g w a te r d r i f t . W ith lo n g p e rio d waves, shorew ard p u ls e s p r e v a i l te m p o r a r ily c a u sin g n e t m otion in th e same d i r e c t i o n . T h is c o n c lu sio n h a s i n t e r e s t i n g im p l i c a ti o n s c o n c e rn in g th e an n u al c y c le o f e ro s io n and d e p o s itio n on so u th e rn C a l i f o r n i a beaches and w i l l be d is c u s s e d l a t e r . The asymmetry o f surge v e l o c i t i e s showed s i m i l a r p o s s ib l e c o r r e l a t i o n s w ith wind d i r e c t i o n and wave p e rio d b u t a t a lo w e r l e v e l o f c o n fid e n c e . T h e re fo re , th e s e a s s o c i a t i o n s m ust be re g a rd e d as t e n t a t i v e . The te rm "wave d r i f t " im p lie s a g e n e t i c r e l a t i o n s h i p between n e t t r a n s p o r t superim posed on o s c i l l a t i n g su rg e s and s h o a lin g s w e l l s . Because o t h e r f a c t o r s I n f lu e n c e th e s e c u r r e n t s , the name "bottom d r i f t " may be more a p p r o p r i a t e . I t i s im p o s s ib le to c o n s t r u c t an i n t r i c a t e model f o r su rg e asymmetry s in c e wind v e l o c i t i e s were n o t m easured, n e v e r t h e l e s s , bottom d r i f t may be e s tim a te d e m p i r ic a ll y by a d j u s t i n g an o n s h o r e - o f f s h o r e r a t i o f o r wave p e rio d and wind d i r e c t i o n . The method i s d is p la y e d on F ig u re 20. As an exam ple, i f th e wind were blow ing tow ard sho re and F ig u r e 20. P r e d i c t i o n o f bottom d r i f t by an e m p ir ic a l m ethod. 72 73 + 1.4 T PREDICTED WATER MOTION RATIO O N SH O R E /O F FS H O R E O F FS H O R E /O N S H O R E PREDICTION METHOD - AMEND RATIO OF 1,0 AS FOLLOWS! WAVE PERIOD (SECONDS) 7 - 8 * ,05 8 - 9 *.10 9 - 1 0 *,15 10-11 * 2 0 WIND ONSHORE - OFFSHORE -.15 *.15 f “ 1.4 s w e l l s w ith a p e r io d o f 7 se co n d s w ere s t r i k i n g th e b e a c h , t h e r a t i o o f n e t w a te r m otio n would be - 1 , 0 - 0 .1 5 + .0 5 = - 1 . 1 0 . Thus, seaw ard p u l s e s would p re d o m in a te in th e s e c o n d i t i o n s , w hich a re common d u r in g w i n t e r a lo n g s o u th e r n C a l i f o r n i a . In summer, th e wind i s o f t e n n i l and waves have p e r i o d s o f 10 to 11 se c o n d s . F o r t h i s c a se th e b o tto m d r i f t r a t i o would be 1 .0 + 0 + .20 = + 1 .2 0 , and sh orew ard w a te r m otion would o c c u r . W hile t h i s method i s n o t q u a n t i t a t i v e l y s a t i s f y i n g , i t was used to p r e d i c t s u c c e s s f u l l y th e d i r e c t i o n o f b o tto m d r i f t in 86 p e r c e n t o f t h e o b se rv e d caseB . RESULTS OF SEDIMENT TRANSPORT STUDIES Sand Motion and R ip p le Marks O s c i l l a t i o n r i p p l e marks o c c u r w h e rev e r c u r r e n t s g e n e r a te d "by p a s s in g waves impinge on a sandy b o tto m . R ip p le s a re a r e s u l t o f sed im en t m o tio n , y e t once form ed, th e y e x e r t a p ro fo u n d i n f l u e n c e on th e t r a n s p o r t o f d e t r i t a l g r a i n s . An a n a l y s i s o f s u b - l i t t o r a l se d im e n ta ry p r o c e s s e s i s n o t p o s s i b l e w ith o u t c o g n iz a n c e o f th e s e u b i q u it o u s bed form s. D im ensions o f R ip p le Marks O s c i l l a t i o n r i p p l e marks have a s y m m e tric a l p r o f i l e in c o n t r a s t to forms c r e a t e d by u n i d i r e c t i o n a l flo w . T h e ir s h a rp b i f u r c a t i n g c r e s t s a r e s e p a r a t e d by g e n t l y - c u r v i n g tr o u g h s ( F ig . 2 1 ). A ltho ugh th e p h y s i c a l s i z e o f r i p p l e s in t h e o f f s h o r e zone may v a ry , t h e i r sh a p es a r e n e a r l y i d e n t i c a l . The r e l a t i v e c o n sta n c y o f t h e r a t i o betw een h e i g h t and w a v e le n g th i s i n d i c a t e d on F ig u re 22. This r a t i o , known a s th e r i p p l e in d e x , s e r v e s to d i f f e r e n t i a t e en v iro n m en ts o f d e p o s i t i o n . Tanner ( 1 9 6 7 ) made a compre h e n s iv e c l a s s i f i c a t i o n o f r i p p l e m arks a c c o rd in g t o t h e i r shape and o r i g i n . H is system c o r r e c t l y a t t r i b u t e s th e r i p p l e s o b se rv e d in th e o f f s h o r e zone o f s o u th e r n C a l i f o r n i a to fo rm a tio n by waves on th e b a s i s o f t h e i r symmetry F ig u re 21. R ip p le marks on th e sh allo w sea f l o o r . M e g a rip p le s in th e fo re g ro u n d l i e a d j a c e n t to s m a ll e r forms in f i n e sand. Note th e sc o u r p i t developed around a sm a ll o u tc r o p . 76 ■ ? ■ f j - ' V V ■ '' aV ' ,*j F ig u r e 22. In d e x o f r i p p l e s in th e o f f s h o r e zon e. 78 79 x H o 2 U J -J U J > < cc < llJ _J C L o . 2 10 0 1 C O ® 75 - 50 o o o 0 10 20 RIPPLE MARK HEIGHT (CM) 80 ; and in d e x o f 5 . I n v e s t i g a t o r s o f o s c i l l a t i o n r i p p l e m arks in th e l a b o r a t o r y (E v an s, 194-2 and B a g n o ld , 1 9 4 6 ), and in th e f i e l d (R is k , 1965 and Inman, 195?) found t h a t t h e i r d im e n sio n s depend on sedim ent s i z e and c u r r e n t v e l o c i t y . T h is s tu d y s u b s t a n t i a t e s th e s e c o n c l u s i o n s . A d i r e c t r e l a t i o n s h i p betw een r i p p l e w a v e le n g th and g r a i n d ia m e te r i s d e m o n s tra te d on F ig u r e 2 3 . The l a r g e s t fo rm s, mega- r i p p l e s , o c c u r i n c o a r s e sand and have w a v e le n g th s o f a m e te r o r m ore. In f i n e sa n d , r i p p l e c r e s t s a r e spaced s e v e r a l c e n t i m e t e r s a p a r t . The s u b o r d i n a te e f f e c t s o f c u r r e n t v e l o c i t y a r e i l l u s t r a t e d by d a ta f o r a l i m i t e d g r a i n s i z e p l o t t e d on F ig u re 2 4 . R ip p le d im en sio n s and s u rg e I n t e n s i t y a r e shown to be r e l a t e d i n v e r s e l y , and t h i s c a u s e s an i n c r e a s e in s e a f l o o r m ic ro to p o g ra p h y i n th e o f f s h o r e d i r e c t i o n . L o c a l v a r i a t i o n s in r i p p l e s i z e were examined on s e v e r a l o c c a s i o n s by making r e p e a t e d m easurem ents o f w ave l e n g t h . In a l l c a s e s th e s t a n d a r d d e v i a t i o n o f l e n g t h s was 20 t o 30 p e r c e n t o f th e a v e r a g e . Harms ( 1 9 6 9 ) r e p o r t e d t h a t o s c i l l a t i o n r i p p l e marks form ed in th e l a b o r a t o r y w ere more r e g u l a r l y sp a ce d th a n c u r r e n t r i p p l e s . V a r i a b i l i t y in w a v e le n g th s o b s e rv e d in th e o f f s h o r e i s s i m i l a r to t h a t r e c o r d e d by Harms f o r w a v e -g e n e ra te d r i p p l e s , l e n d in g c o n fid e n c e t o th e u se o f t h i s p a r a i e t e r f o r u n r a v e l l i n g d e p o s i t i o n a l h i s t o r i e s . F ig u re 23. E ip p le Tfavelengtla p l o t t e d a g a i n s t mean d ia m e te r o f b ottom se d im e n t. 81 RIPPLE M ARK WAVELENGTH (CM) 82' 100 7 5 - o o 5 0 - 2 5 - 1 . 0 .5 .25 ,125 MEAN SEOIM ENT SIZE [MM J F ig u r e 24. E f f e c t o f s u rg e v e l o c i t y on r i p p l e w a v e le n g th . The s i z e o f sand i s a p p ro x im a te ly c o n s t a n t w ith mean d ia m e te rs l y i n g 'between .177 and .250 mm. 83 RIPPLE MARK WAVELENGTH (CM) 84 3 0 - i IQ - 2 5 0 1 0 15 20 3 0 AVERAGE CURRENT VELOCITY (CM/SEC) 85 C i r c u l a t i o n and G rain P a th s Over a R ip p le d Bed The p re s e n c e o f r i p p l e m arks c a u s e s t h e boundary betw een se d im e n t and moving w a t e r to be h y d ro d y n a m ic a lly ro u g h . As a r e s u l t , flo w c o n d i t i o n s n e a r t h e s e a f l o o r a r e t u r b u l e n t even w i t h low v e l o c i t y c u r r e n t s . C lo se -u p c in e m a to g ra p h y was used to examine c i r c u l a t i o n o v e r r i p p l e marks and t h e r e s u l t i n g m otion o f sand g r a i n s . D i s t i n c t se d im e n t t r a n s p o r t p a t t e r n s were d e v elo p ed f o r c o n d i t i o n s o f weak and s t r o n g s u r g e , and t h e s e o b t a i n w i t h r i p p l e s o f a l l s i z e s . G rain p a th s o b se rv e d u n d e r low v e l o c i t y c u r r e n t s a re d i s p l a y e d on F ig u re 25 and a r e c a t e g o r i z e d a s f o l l o w s : 1 . As th e oncoming s u rg e flo w s o v e r a r i p p l e c r e s t , p a r t i c l e s on th e s t o s s s i d e a r e sw ept o v e r th e to p and c a sc a d e down t h e l e e f a c e . 2. A l a r g e eddy ( F ig . 26) i s e s t a b l i s h e d i n r i p p l e t r o u g h s . Where th e eddy im pinges a g a i n s t th e se d im e n t, s u r f a c e g r a i n s a r e c a r r i e d up th e r i p p l e l e e i n a d i r e c t i o n o p p o s i t e t o g e n e r a l c u r r e n t f lo w . The s u b s e q u e n t d e p a r t u r e o f th e eddy i s marked by a s l i g h t i n f l e c t i o n o f th e r i p p l e s u r f a c e below th e c r e s t . O c c a s i o n a l l y , e d d ie s c o l l i d e w ith th e b o tto m and c au se an a p p a r e n t e x p lo s io n o f tro u g h g r a i n s in a l l d i r e c t i o n s . 3 . S m all h o r i z o n t a l e d d ie s r e s e m b lin g d e s e r t d u s t d e v i l s su spend w isp s o f se d im e n t n e a r t h e r i p p l e c r e s t . F ig u re 25. G ra in p a th s o v e r r i p p l e m arks d u r in g a low v e l o c i t y s u r g e . 86 LOW VELOCITY SURGE F ig u re 26. Eddy developed in th e l e e o f a r i p p l e c r e s t . Surge i s flo w in g from r i g h t to l e f t and th e r i p p l e w a v ele n g th i s 90 cm. (P h o to g ra p h by H. P a lm e r.) 88 90 T r a n s p o r t p a t t e r n s a t t e n d i n g h ig h v e l o c i t y s u r g e s a r e shown on F ig u re 27. In t h i s c ase g r a i n s moving o v e r t h e c r e s t a r e " la u n c h e d ” i n t o s u s p e n s io n . The v e r t i c a l e d d ie s a r e w e ll-d e v e lo p e d and dom inate c i r c u l a t i o n In r i p p l e t r o u g h s . Suspended p a r t i c l e s b o th from c r e s t s and tr o u g h s a r e w h ir le d a b o u t and e v e n t u a l l y p r o j e c t e d h ig h enough to be c a r r i e d o v e r t h e n e x t c r e s t . As th e su rg e w anes, e d d ie s i n c r e a s e in d ia m e te r and b r e a k down, r e l e a s in g a cloud o f g r a i n s w hich a re c a r r i e d o v e r th e s e a f l o o r . These a r e d e p o s ite d d u r in g s u c c e e d in g p u l s e s . A lle n (1968) a n a ly z e d c i r c u l a t i o n and se d im e n t t r a n s p o r t f o r r i p p l e s In a flu m e. H is p a t t e r n s a re s i m i l a r to th o s e o b se rv ed a t th e s e a f l o o r where s t r o n g o s c i l l a t i n g c u r r e n t s e x i s t . Because t h e Burge changes d i r e c t i o n e v e ry few s e c o n d s , how ever, t u r b u l e n c e i s n o t a t t a i n e d a t h i g h e r l e v e l s i n th e w a te r column. Under calm c o n d i t i o n s even th e e d d ie s In r i p p l e tr o u g h s a r e n o t f i r m l y e s t a b l i s h e d . S e d im e n t D i s t r i b u t i o n o v e r R i p p l e M arks D i f f e r e n c e s In t h e i n t e n s i t y o f c u r r e n t flow o v e r th e r i p p l e form cause a s e g r e g a t io n o f g r a i n s by h y d r a u l i c s i z e . E xtrem es o f mean d ia m e te r e x i s t a t th e c re B t and t r o u g h . T his ran g e in g r a i n s i z e i s i l l u s t r a t e d on F ig u re 28 where I t i s graphed a g a i n s t a v e ra g e d i a m e t e r . W ith f i n e sa n d , and hence s m a ll r i p p l e s , c r e s t se d im e n t i s F ig u r e 27. G rain p a th s o v e r r i p p l e marks d u r in g a h ig h v e l o c i t y s u r g e . 91 92 HIGH VELOCITY SURGE F ig u re 28. C o n t r a s t in t h e s l 2e o f g r a i n s c o n c e n tr a te d a t r i p p l e c r e s t s and tr o u g h s . 93 M EAN CREST SIZE MINUS M EAN TROUGH SIZE (MM) 9 4 CREST COARSER 00 .10 - 2 0 - T T T 1.0 .5 .25 .125 AVERAGE MEAN GRAIN SIZE OF RIPPLED SEDIMENT (MM) 95 s l i g h t l y c o a r s e r th a n t h a t In th e t r o u g h . T h is may he e x p la in e d by winnowing o f sm a ll p a r t i c l e s from t h e exposed r i p p l e to p and t h e i r d e p o s i t i o n i n t h e more s h e l t e r e d tr o u g h . On m e g a rip p le s c o n s i s t i n g o f c o a rs e s a n d , how e v e r , th e tr o u g h g r a i n s a r e t h e l a r g e s t . Inman (1957) a l s o o b se rv e d t h i s te n d e n c y and a t t r i b u t e d i t t o i n s u f f i c i e n c y o f a v a i l a b l e m ed lu m -size san d to c o m p le te ly c o v e r p r e - e x i s t i n g g r a v e l p a v em en ts. I t i s t r u e t h a t r e g i o n s o f c o a rs e se d im e n t on t h e s e a f l o o r a r e u s u a l l y n o t th e r e c i p i e n t s o f ab u n d an t d e t r i t u s . An a l t e r n a t e e x p l a n a t i o n i s t h a t t h e m a t e r i a l in q u e s t io n i s in tr o d u c e d to th e o f f sh o re as a u n i t d u r in g p e r i o d s o f h ig h s u r f . U nder lo w e r en erg y c o n d i t i o n s p a r t o f th e lo a d i s to o c o a r s e t o be t r a n s p o r t e d by s u s p e n s io n and i s c o n fin e d by s l o p i n g r i p p l e s i d e s t o th e t r o u g h . R ip p le Mark M ig r a t io n C hanges i n t h e p o s i t i o n o f o s c i l l a t i o n r i p p l e marks may be c o n s i d e r e d from two t e m p o r a l s t a n d p o i n t s . D u rin g a p e r i o d o f s e c o n d s , a c r e s t can m i g r a t e a s much a s 2 cm in th e d i r e c t i o n o f s u r g e f l o w . T h is i s a r e s u l t o f t h e a f o r e m e n tio n e d t r a n s f e r o f g r a i n s from t h e s t o s s t o t h e l e e and im p a r t s a d e g r e e o f asym m etry t o t h e r i p p l e fo r m . When t h e c u r r e n t c h a n g e s d i r e c t i o n , t h e asym m etry i s r e v e r s e d a s th e c r e s t r e t u r n s to i t s fo r m e r p o s i t i o n . One m ig h t assu m e t h a t a d i r e c t i o n a l i n e q u a l i t y o f 96 s u r g e s would cause g r a d u a l and perm anent m ig r a ti o n o f r i p p l e m arks in t h e d i r e c t i o n o f n e t w a t e r m o tio n . On s h o re movement o f r i p p l e s h a s been o b se rv e d in a wave ta n k by S c o t t (1954) and Bagnold (1947)» and i n Lake Huron by R is k ( 1 9 6 5 ). Inman and Bagnold (1963) pro p o sed a fo rm u la f o r s u b - l i t t o r a l s e d im e n t t r a n s p o r t b ased on r i p p l e s s h i f t in g u n d e r th e I n f l u e n c e o f su p p o s e d ly sho rew ard wave d r i f t . The m ig r a ti o n o f r i p p l e m arks on th e s h a llo w s e a f l o o r was examined by Vernon ( 1 9 6 6 ) and by t h e a u t h o r in th e p r e s e n t stu d y u s i n g t i m e - l a p s e p h o to g ra p h y . The p o s i t i o n o f r i p p l e c r e s t s was m o n ito re d o v e r p e r io d s ra n g in g from 1 t o 8 h o u r s . In a l l c a s e s I t was found t h a t a s i d e from s h o r t term " c h a t t e r i n g " o f c r e s t s no s i g n i f i c a n t move ment o f th e r i p p l e s o c c u r r e d . T h is s t a b i l i t y was m a i n t a i n ed in s p i t e o f s u r g e i n e q u i t i e s and n e t t r a n s p o r t a t i o n o f b o tto m s e d im e n t. T hus, th e c o n c e p t o f sh o re w a rd -m a rch in g r i p p l e m arks and In m a n 's fo rm u la a p p e a r t o be i n v a l i d a t e d . These o b s e r v a t i o n s demand an e x p la n a t i o n w ith r e f e r ence t o t h e c o n f l i c t i n g in f o r m a ti o n from b o th t h e l a b o r a t o r y and th e l a c u s t r i n e e n v iro n m e n t. The problem may be r e s o l v e d by n o t i n g t h a t sho rew ard su rg e v e l o c i t i e s g e n e r a te d by s m a ll waves exceed th e o f f s h o r e component by a f a c t o r o f a t l e a s t 2 ( S c o t t , 1 9 5 4 ). W ith o c e a n ic s w e lls th e r a t i o o f s t r e n g t h s i s u s u a l l y l e s s th a n 1 : 1 . 3 . Ap p a r e n t l y a l a r g e r v e l o c i t y d i f f e r e n t i a l i s r e q u i r e d to c au se a s h i f t i n p o s i t i o n o f th e e n t i r e wedge o f sand com- 97 p o sin g a r i p p l e c r e s t . T h resh o ld V e l o c i t i e s The c o n c e p t o f t h r e s h o l d v e l o c i t y has g r e a t im p o r t ance f o r g e o l o g i s t s a tt e m p t in g t o i n t e r p r e t a n c i e n t f d e p o s i t s . A p l e t h o r a o f l a b o r a t o r y s t u d i e s on t h i s su b je c t* how ever, h a s produced r e s u l t s w hich d i f f e r by a lm o st an o r d e r o f m a g n itu d e . A p r e c i s e s ta te m e n t on t h r e s h o l d v e l o c i t i e s in t h e o f f s h o r e i s a l s o im p o s s ib le f o r t h e f o llo w in g r e a s o n s : 1 . A v a r i e t y o f c u r r e n t s t r e n g t h s i s produced by a s e t o f p a s s i n g w aves. Even u n d e r low en erg y c o n d i t i o n s one p u l s e out o f e v e ry 10 or 20 p ro b a b ly w i l l be c a p a b le o f moving s e d im e n t. 2. The c o n t i n u a l a c c e l e r a t i o n and d im in is h in g o f su rg e v e l o c i t y p la y s a r o l e in i n i t i a t i n g g r a i n t r a n s p o r t . T his f a c t o r i s e x tre m e ly d i f f i c u l t to m easure and q u a n t i f y . 3 . T u rb u le n c e n e a r t h e s e a f l o o r i s a f u n c t i o n o f r i p p l e d im e n sio n s w hich v a ry even w ith in a g iv e n g r a i n s i z e r a n g e . In a r e a s w here c o a rs e m e g a rip p le d san d i s a d j a c e n t t o f i n e s e d im e n t, t h e l a r g e r m a t e r i a l i s commonly s u b j e c t t o v i o l e n t m o tio n , w hereas calm c o n d it i o n s p r e v a i l i n th e a r e a o f sm a ll r i p p l e m arks. 4 . The p o r o s i t y o f b o tto m san d , w hich a f f e c t s th e e x p o su re o f I n d i v i d u a l p a r t i c l e s to c u r r e n t f le w , i s n o t c o n s t a n t . D e te r m in a tio n s o f p o r o s i t y w ere made on 13 98 o c c a s io n s by c a r e f u l l y c o l l e c t i n g and l a t e r w e ig h in g a known volume o f s e d im e n t. V alues f o r f i n e sand f e l l "be tween 44 and 54 p e r c e n t . H am ilton and o t h e r s (1956) r e co rd ed s i m i l a r p o r o s i t i e s f o r sa n d s on th e c o n t i n e n t a l s h e l f n e a r San D iego, C a l i f o r n i a . 5 . D uring p e r i o d s o f q u i e s c e n c e , o f f s h o r e se d im e n t "becomes "cemented** by o r g a n ic d e b r i s . In t h i s s t a t e t r a n s p o r t i s i n h i b i t e d u n t i l th e sand h a s been c o m p le te ly r e w orked. In s p i t e o f t h e s e c o m p lic a tin g f a c t o r s , a tt e m p t s w ere made to d e f i n e a p p ro x im ate t h r e s h o l d v e l o c i t i e s f o r d i f f e r e n t g r a i n s i z e s . T his was a c c o m p lish e d by c o u n tin g t h e number o f p u l s e s c a u s in g bed lo a d t r a n s p o r t and s u s p e n s io n d u rin g c u r r e n t m e te r m ea su re m e n ts. C r i t i c a l v e l o c i t i e s were l a t e r d e te rm in e d from th e s u rg e r e c o r d . T h is s u b j e c t i v e m ethod i s open t o c r i t i c i s m s in c e l o c a l v a r i a t i o n s in r i p p l e r e l i e f may c a u se some g r a i n s t o be susp en d ed w h ile o t h e r s a r e r o l l e d o r n o t moved a t a l l by t h e same p u l s e . N e v e r t h e l e s s , r e s u l t s were r e a s o n a b ly con s i s t e n t , and a v e ra g e s f o r 14 t r i a l s a r e p l o t t e d on F ig u re 29. I t I s I n t e r e s t i n g t h a t t h r e s h o l d v e l o c i t i e s f o r a l l g r a i n s i z e s a re p r a c t i c a l l y I d e n t i c a l . P a r t i c l e m otion commences w ith a c u r r e n t flo w in g 18 to 21 c m /se c , and s u s p e n s io n i s w id e sp re a d a t 27 to 50 c m /se c . A p p a r e n tly t h e developm ent o f l a r g e r r i p p l e m arks com pensates f o r i n c r e a s e s i n botto m se d im e n t s i z e . The v e l o c i t i e s f o r F ig u r e 29. Observed t h r e s h o ld v e l o c i t i e s f o r sand t r a n s p o r t . 99 AVERAGE THRESHOLD VELOCITY (CM /SEC) 100 35 3 0 - 20 SUSPENSION 0 o 25 H N o \ TRACTION ° - o ^ ----------------------------------- 15 ------------1 ------------------------ 1 ------------------------1 ------------------------ 1 ------------------------ 1 ------------------------ 1 70? 500 3 5 4 250 177 125 MEAN GRAIN SIZE OP BOTTOM SEDIMENT (MM) 101 g e n e r a l I n i t i a t i o n o f "bed l o a d t r a n s p o r t a r e c l o s e s t t o t h o s e p r e s e n t e d by B a g n o ld ( 1 9 4 6 ) , who o s c i l l a t e d a s a n d - c o v e r e d p l a t e in s t i l l w a t e r . T o t a l M oving Load The t r a n s p o r t a t i o n o f b ottom sand by w a v es w i l l be c o n s i d e r e d w i t h r e f e r e n c e t o t h e d i f f e r e n t v a r i e t i e s o f s e d im e n t t r a p s . The s l i t t e d can d e s i g n i n t e r c e p t s t h e t o t a l lo a d m o v in g a t a l l l e v e l s and i n any d i r e c t i o n . The g r a i n s i z e and q u a n t i t y o f sand r e c o v e r e d from t h i s t r a p fo rm s th e b a s i s f o r t h e f o l l o w i n g d i s c u s s i o n . I t was p r e v i o u s l y n o t e d t h a t o s c i l l a t i o n r i p p l e m arks s e g r e g a t e s e d im e n t by s i z e w i t h e x tr e m e s l o c a t e d a t t h e c r e s t s and t r o u g h s . T h u s, i f g r a i n s i z e d i s t r i b u t i o n s from t h e s e p o r t i o n s o f r i p p l e s a r e p l o t t e d a s c u m u la t iv e c u r v e s , an e n v e l o p e i s c r e a t e d w h ic h d e f i n e s s i z e s a v a i l a b l e f o r m ovem ent. A c u m u la t iv e c u r v e r e p r e s e n t i n g t h e s i z e d i s t r i b u t i o n o f t h e t o t a l l o a d moved by w aves c h a r a c t e r i s t i c a l l y l i e s w i t h i n t h i s e n v e l o p e . T h is r e l a t i o n s h i p o b t a i n s r e g a r d l e s s o f a v e r a g e c u r r e n t v e l o c i t y and e x t e n d s t o t h e t a i l s o f t h e c u r v e . F i g u r e 3 0 shows t y p i c a l e x a m p le s . One I n t u i t i v e l y e x p e c t s t h a t f i n e r s i z e s w ould be moved w i t h g r e a t e r f a c i l i t y th an c o a r s e m a t e r i a l . T h a t a l l a v a i l a b l e s e d im e n t i s t r a n s p o r t e d w i t h e q u a l e a s e may b e e x p l a i n e d as f o l l o w s : N e a r s h o r e sa n d s a r e w e l l - s o r t e d , and v a r i a t i o n s in t h r e s h o l d v e l o c i t y f o r a F ig u r e 30. Exam ples o f th e r e l a t i o n s h i p b e tween th e g r a i n s i z e o f san d t r a n s p o r t e d and se d im e n t a v a i l a b l e f o r m o tio n . The d a t a r e p r e s e n t 3 i n d i v i d u a l o b s e r v a t i o n s . 1 102 X o U J U J > 1- < 3 o io o n 5 0 - CREST, T R O U G H T .5 .2 5 GRAIN SIZE (MM) .1 2 5 = SAND II 100 i P o uj 5 0 - > ,5 .2 5 J 2 5 GRAIN S IZ E (MM) 100 n CREST. u j 50 - > T R O U G H .25 .125 GR A IN SIZE (MM) 103 MOTION 104 given g r a i n p o p u la tio n a re s m a ll. A lso , r i p p l e marks c r e a te a spectrum o f energy n ic h e s so t h a t p a r t i c l e s ojf d i f f e r e n t h y d r a u l i c s i z e may be a t e q u il i b r i u m a t th e same bottom l o c a t i o n . The sh a llo w s e a f l o o r i s a dynamic s u r fa c e in t h i s r e s p e c t . R ip p le d im ensio ns a d j u s t to surge v e l o c i t y so t h a t th e e n t i r e range o f a v a i l a b l e sand may be p laced i n m o tio n . In fo rm a tio n on th e r e l a t i o n s h i p between t r a n s p o r t e d lo a d and t h e sedim en t a v a i l a b l e f o r m otion i s la c k in g in s t u d i e s o f d e p o s i ti o n i n o t h e r n a t u r a l e n v iro n m e n ts. Bag n o ld (1968) r e c e n t l y o bserved t h a t such d a ta m ight e l u c i d a t e th e p ro c e s s o f s e d im e n ta tio n . In th e l a b o r a t o r y Bagnold (1954) and E a g le so n , G lenne, and Dracup (1961) r e s p e c t i v e l y a llo w ed wind and sm a ll waves to a f f e c t a m ix tu re o f g r a in s i z e s . The t r a n s p o r t e d lo a d was b e t t e r s o r te d th a n th e p a r e n t m a t e r i a l as m ight be e x p e c te d . The s i t u a t i o n i n the o f f s h o r e zone i s n o t a n a la g o u s , however, because th e sand r e a c h in g t h i s a r e a by means o f r i p c u r r e n t s a l r e a d y i s c o n fin e d to a narrow s i z e rang e (Cook, 1968). The r a t e o f sand t r a n s p o r t may be e x p re ss e d as e i t h e r th e w eig h t o r th e number o f g r a i n s i n t e r c e p t e d by a sedim ent t r a p p e r u n i t o f tim e . Inasm uch as an a c t u a l count o f p a r t i c l e s i s i m p r a c t i c a l , th e number o f g r a in s was c a l c u l a t e d from th e t o t a l w eig h t by assum ing a l l were sp h e re s w ith a s p e c i f i c g r a v i t y o f 2 . 6 5 . T his assum ption 105! I s r e a s o n a b ly sound a s th e sands in q u e s tio n c o n s is te d l a r g e l y o f q u a r tz and f e l d s p a r . These a l t e r n a t e s e d i m e n ta tio n r a t e s a re p l o t t e d a g a i n s t averag e c u r r e n t v e l o c i t y on F ig u re s 51 and 32. Three o b s e r v a tio n s w a rra n t s p e c i a l c o n s i d e r a t i o n : 1 . T r a n s p o r t a ti o n a p p e a rs to o c cu r w ith c u r r e n t s much lo w e r th an th e e x p e rim e n ta l t h r e s h o ld v e l o c i t i e s . T his a p p a re n t anomaly may be r e s o lv e d by n o t in g t h a t v e l o c i t i e s on th e g rap h a re a v erag e v a lu e s . The spectrum o f c u r r e n t s g e n e ra te d by a s e t o f p a s s in g waves i s s u f f i c i e n t l y broad so t h a t t h e u pper p a r t o f t h e ran g e exceeds 20 cm /sec. A p p a ren tly sedim en t m otion i s n e g l i g i b l e when averag e v e l o c i t i e s a r e lo w er th a n 10 cm /sec. 2. A lthough th e cu rv es r e v e a l a g e n e r a l tr e n d , c o n s id e r a b le s c a t t e r i s in e v id e n c e . V a r ia tio n in th e r a t e o f sand t r a n s p o r t w ith a g iven c u r r e n t r e f l e c t s s e v e r a l u n d e fin e d f a c t o r s . These in c lu d e surge a c c e l e r a t i o n , i n e q u a l i t i e s o f r i p p l e s i z e , " c e m e n ta tio n 1 1 by o rg a n ic m at t e r , and sedim ent p o r o s i t y . 3 . E m p iric a lly -d ra w n c u rv e s i n d i c a t e t h a t th e r a t e o f sedim en t movement v a r i e s as a p p ro x im a te ly t h e sq uare o f ex ce ss c u r r e n t v e l o c i t y above th e a v erag e su rg e s t r e n g t h a t which sand b e g in s t o move. However, t h e o r e t i c a l con s i d e r a t i o n s , e x p erim e n ts i n flum es ( G i l b e r t , 1 9 1 4 ), d a ta from n a t u r a l stre am s (C olby, 1 9 6 4 ), and in f o rm a tio n f o r a e o lia n t r a n s p o r t (B agnold, 1954) i n d i c a t e t h a t c a p a c ity F ig u re 31. A verage s u rg e v e l o c i t y v e r s u s r a t e o f sa n d t r a n s p o r t . T r a n s p o r t i s e x p re s s e d by grams o f se d im e n t I n t e r c e p t e d i n an h o u r by a t r a p . D ata I s f o r sands w ith mean d ia m e te r s r a n g i n g betw een .1 2 5 and .250 mm. 106 AVERAGE SURGE VELOCITY {C M /S E C ) * V o o o o I o O q 8 o o 8 107 F ig u re 32. Average su rg e v e l o c i t y v e rs u s a l t e r n a t e r a t e o f sand t r a n s p o r t . T ra n sp o rt i s e x p re ss e d in term s o f th e number o f g r a in s i n t e r c e p t e d p e r h o u r by a t r a p . Data a re f o r san ds w ith mean d ia m e te rs ra n g in g between .1 2 5 and .250 mm. 108 RATE O F SAN D TRANSPORT (ORA1NS P E R H O U R /IO AVERAGE SURGE VELOCITY (C M /S E C ) = V o fu - O J 1 1 109 110 I n c r e a s e s w ith th e cube o f v e l o c i t y o f th e t r a n s p o r t i n g a g e n t. T h is d i f f e r e n c e may r e s u l t from in c o m p le te d e v e lo p ment o f tu r b u le n c e th ro u g h o u t t h e w a t e r column w i t h o s c i l l a t o r y f lo w , A ls o , t h e s iz e o f r i p p l e m arks, and hence th e m ag n itu d e o f b o u n d ary r o u g h n e s s , a re i n v e r s e l y r e l a t e d to c u r r e n t s t r e n g t h . Suspended S edim ent S u rg e s w ith v e l o c i t i e s e x c e e d in g 27 t o 30 cm /sec cause tem p o rary s u s p e n s io n o f sand g r a i n s a t t h e s e a f l o o r . Whenever t h e se d im e n t t r a n s p o r t r a t e i s m o derate to h ig h , a s i g n i f i c a n t q u a n t i t y o f m a t e r i a l i s moved i n t h i s m anner. The s e d im e n t t r a p c o n s i s t i n g o f v i a l s a rra n g e d i n t i e r s i n t e r c e p t s p a r t i c l e s c a r r i e d a t h e i g h t s up t o 30 cm above th e b o tto m . T y p ic a l exam ples o f th e v e r t i c a l d i s t r i b u t i o n o f suspended sedim en t a r e i l l u s t r a t e d on F ig u re 3 3 . I t i s obvious t h a t t h e c o n c e n t r a t i o n o f p a r t i c l e s d im in is h e s e x p o n e n t i a l l y w ith i n c r e a s i n g h e i g h t . The p e r c e n t o f th e t o t a l m oving lo a d t r a n s p o r t e d a t l e v e l s g r e a t e r th a n 30 cm i s n e g l i g i b l e . Hom-ma and H orikaw a (1963) sam pled s u s pended se d im e n t a t i n t e r v a l s o f 6 0 cm in h ig h e n e rg y con d i t i o n s a lo n g th e c o a s t o f Ja p a n . They r e p o r t e d a s i m i l a r r a t e o f d e c r e a s e i n c o n c e n t r a t i o n th ro u g h o u t t h e e n t i r e w a te r colum n. A l l e n (1968) c o n s t r u c t e d a model f o r s a l t a t i o n o f F ig u re 33- Exam ples o f th e v e r t i c a l d i s t r i b u t i o n o f su s p e n d e d sa n d . The c u rv e s r e p r e s e n t 3 i n d i v i d u a l o b s e r v a t i o n s . HEIGHT ABOVE BOTTOM (C M I RELATIVE CONCENTRATION ' OF SUSPENDED SANO o o G J 112 g r a i n s i n a u n i d i r e c t i o n a l c u r r e n t . The p r i n c i p a l v a r i a b l e s were flo w v e l o c i t y , se d im e n t s i z e , and r i p p l e d im e n s io n s . I t i s l o g i c a l t h a t t h e same f a c t o r s sh o u ld c o n t r o l th e d i s t r i b u t i o n o f p a r t i c l e s above t h e s h a llo w s e a f l o o r . To t e s t t h i s h y p o t h e s i s t h e s e v a r i a b l e s a r e p l o t t e d a g a i n s t th e p e r c e n t o f t h e suspended lo a d i n t e r c e p te d by t h e two u p p erm o st v i a l s o f th e se d im e n t t r a p on F ig u re 3 4 . T his p e r c e n ta g e i s p r o p o r t i o n a l to t h e s lo p e o f a curve r e l a t i n g t h e amount o f su spend ed sand t o d i s ta n c e from th e b o tto m . I n t e r e s t i n g l y enough, c u r r e n t s t r e n g t h does n o t c o r r e l a t e w ith changes in t h e c o n c e n t r a t i o n o f g r a i n s . P a r t i c l e d ia m e te r shows a vague c o r r e s pondence w ith t h i s p a ra m e te r , b u t c a r e f u l i n s p e c t i o n r e v e a l s t h a t i t i s i n t h e wrong d i r e c t i o n . C oarse se d im e n t a p p e a rs t o be l o f t e d t o g r e a t e r h e i g h t s th a n f i n e r m a t e r i a l . T h is anomaly i s e x p la in e d by th e f a c t t h a t r i p p l e r e l i e f , w hich i s r e l a t e d d i r e c t l y t o san d s i z e , seems t o be th e dom inant c a u se o f s u s p e n s io n . An i n c r e a s e i n r i p p l e d im e n sio n s i s accom panied by more i n t e n s e t u r b u le n c e w hich c a r r i e s p a r t i c l e s f a r t h e r from th e s e a f l o o r . F u r t h e r c o n f ir m a tio n i s a t t a i n e d by p l o t t i n g a v e ra g e d i s t r i b u t i o n s o f suspended san d f o r d i f f e r e n t r i p p l e s i z e s on F ig u re 35* A lso shown on F ig u re 35 i s a n a lo g o u s i n f o r m a tio n f o r a e o l l a n and s tr e a m t r a n s p o r t . B ecause o f t h e low d e n s i t y o f a i r , sand i s n o t c a r r i e d f a r above th e ground s u r f a c e by w ind. On t h e o t h e r hand, t u r b u l e n c e i s f u l l y d e v e lo p e d F ig u re 34. E f f e c t s o f surge v e l o c i t y , bottom sed im en t s i z e , and r i p p l e wavelength, on th e d i s t r i b u t i o n o f suspended sa n d . The r e l a t i v e su sp e n sio n o f sand i s I n d i c a te d by th e p e rc e n t o f th e suspended lo a d c a r r i e d h i g h e r th a n 1 5 .3 cm above th e se a f l o o r . 114 RIFPLE MARK WAVELENGTH (CM) ro O ’ UJ O BOTTOM SAND SIZE (MM) ro ui tr j — , * _ ro O G J o O O O rp o ° o °o Lsst %, SAND SUSPENDED ABOVE 15.3 CM SURGE VELOCITY (CM/SEC) _ i \ j <j‘ O o o O- ro O J J O H M L n F ig u re 35* D i s t r i b u t i o n o f su spend ed sand t r a n s p o r t e d by w aves, s tr e a m s , and w ind. Average c u rv e s have been d e te rm in e d f o r d i f f e r e n t r i p p l e s i z e s i n t h e o f f s h o r e z o n e . 116 HEIGHT ABOVE SEDIMENT SURFACE (CM) 3 0 20 ■ 10- 100 75 5 0 25 0 RELATIVE CONCENTRATION OF SUSPENDED SAND RIPPLE WAVELENGTH (CM) SMALL = 7 - 1 0 INTERMEDIATE = 15 ~ 25 MEGARIPPLES = 5 0 " 9 0 118 1 and c o n tin u o u s i n s tr e a m s , c a u s in g se d im e n t to be c a r r i e d a t h i g h e r l e v e l s th a n i n t h e o f f s h o r e . The f a i l u r e o f t h e se d im e n t t r a p t o i n t e r c e p t a c o n s i s t e n t q u a n t i t y o f p a r t i c l e s a t th e l e v e l a d j a c e n t to th e ocean f l o o r was p r e v i o u s ly d i s c u s s e d . T h is o b v i a t e s a p r e c i s e e v a l u a t i o n o f bed lo a d a s compared t o su sp en d ed t r a n s p o r t u n d e r d i f f e r e n t c o n d i t i o n s . However, t h e amount o f sand r e c o v e r e d in th e 0 to 1 .3 cm i n t e r v a l , w hich m ight be d e fin e d a s bed l o a d , alw ays exceeded 80 p e r c e n t o f th e t o t a l l o a d . At l e a s t 95 p e r c e n t o f th e t r a n s p o r t o c c u rre d w i t h i n 15 cm o f t h e bo tto m . The r a t e a t w hich su spend ed g r a i n s a c c u m u la te d in th e t r a p i s graphed a g a i n s t a v e r a g e c u r r e n t v e l o c i t y on F ig u r e s 36 and 37* D e s p ite t h e s c a t t e r , a l i n e a r c o r r e l a t i o n i s s u g g e s t e d . T his c o n t r a s t s w ith th e e x p o n e n tia l r e l a t i o n s h i p betw een c u r r e n t s t r e n g t h and th e t o t a l s e d i ment t r a n s p o r t r a t e ( F i g s . 31 and 3 2 ) . From t h e s e two c a p a c i t y d iag ra m s one may i n f e r t h a t an I n c r e a s e in su rg e I n t e n s i t y i s accom panied by a r e d u c t i o n i n t h e p r o p o r t io n o f sand t r a n s p o r t e d i n s u s p e n s io n . C redence i s l e n t to t h i s u n e x p e c te d i n f e r e n c e by two o b s e r v a t i o n s . At h i g h e r c u r r e n t v e l o c i t i e s , r i p p l e mark r e l i e f d e c r e a s e s and t r a n s l a t i o n a l flo w i s r e - e s t a b l i s h e d . Both o f t h e s e e v e n ts enhance bed lo a d t r a n s p o r t a t i o n . The g r a i n s i z e o f se d im e n t c a r r i e d by s u r g e s i s f i n e r w ith d i s t a n c e from th e ocean f l o o r . T y p ic a l cum ula- F ig u re 3 6 . R a te o f su sp en d ed sand t r a n s p o r t v e r s u s a v e ra g e su rg e v e l o c i t y . T r a n s p o r t i s I n d i c a t e d by t h e w e ig h t o f se d im e n t s e t t l i n g i n t o a t r a p in one h o u r. 119 1 2 0 30n o o tE o 10- RATE OF SUSPENDED SAND TRANSPORT (GRAMS PE R HOUR) F ig u re 37. A l t e r n a t e r a t e o f su sp en d ed sand t r a n s p o r t v e r s u s a v e ra g e su rg e v e l o c i t y . T r a n s p o r t I s e x p re s s e d In term s o f t h e number o f g r a i n s a c c u m u la tin g in a t r a p p e r h o u r . 121 AVERAGE SURGE VELOCITY (CM/SEC) 122 30 -i 10- 100 50 O RATE OF SUSPENDED SAND TRANSPORT (GRAINS PER HOUR/I03) 123 t l v e c u rv e s r e p r e s e n t i n g t e d l o a d and d i f f e r e n t l e v e l s o f s u s p e n s io n a r e d e m o n s tra te d on F ig u re 3 8 . T his r e l a t i o n s h ip o b t a i n s r e g a r d l e s s o f c u r r e n t s t r e n g t h . Sand M ig r a tio n The n e t t r a n s p o r t a t i o n o f se a f l o o r se d im e n t by- waves was i n v e s t i g a t e d b o th w i t h th e u s e o f f l u o r e s c e n t dyed sand and d i r e c t i o n a l t r a p s . In l i g h t o f V e rn o n 's (1966) e x t e n s i v e u se o f t r a c e r m a t e r i a l , m ost em phasis was p la c e d on exam in in g t h e m i g r a t i o n o f i n s i t u sa n d . U sin g c u r r e n t m e te r r e c o r d s , o n s h o r e - o f f s h o r e r a t i o s f o r s e d i ment t r a n s p o r t may be compared w ith i n e q u i t i e s in s u r g e v e l o c i t y , d u r a t i o n o f flow , a n d bottom d r i f t . The s lo p e o f th e ocean f l o o r v a r i e d o n ly between 3 and 5° and can be c o n s id e r e d u n ifo rm . I t may be r e c a l l e d t h a t th e dyed san d e x i s t e d i n two s t a n d a r d g r a i n - s i z e p o p u l a t i o n s . M ig r a tio n d a t a f o r th e s e a r e p l o t t e d a g a i n s t r a t i o s r e p r e s e n t i n g c o n c u r r e n t w a te r m o tio n on F ig u r e s 39 a n d 40. S h i f t i n g o f t h e c o a rs e t r a c e r , w hich rem a in s in c l o s e c o n ta c t w i t h th e s e a f l o o r , c o rre sp o n d ^ c l o s e l y to n e t b o tto m d r i f t . I t seems l o g i c a l t h a t r o l l i n g and boun cing p a r t i c l e s s h o u ld be moved in c o n c e r t w i t h th e o v e r ly in g w a t e r . The s l i g h t o f f s e t o f th e l i n e a r r e l a t i o n s h i p from th e o r i g i n r e f l e c t s t h e seaw ard component o f g r a v i t y . T h is e f f e c t was u n d o u b te d ly e n hanced by sm oo th in g th e r i p p l e d se d im e n t s u r f a c e p r i o r to Figure 38. Grain size distribution of sand suspended to different heights above the sea floor. 124 125 100 n 75- ai o cr U J C l t- 50- x o U J UJ > = > s 25 - Z > u .062 .250 .125 1 . 0 .500 GRAIN SIZE (MM) HEIGHT ABOVE BOTTOM (CM) ------- = 0 * 1.3 -------- = 13 - 7.6 -------- = 7.6 " 30.5 i Figure 39* Relationship between surge asym metry and migration of coarse tracer sand. 126 oft\> o VELOCITY RATIO H . 2 5 ■ ■ -1.25 127 -O-!------ SAND MOTION RATIO + ■ 2 FLOW DURATION RATIO + 1.25- o -1.25 SAND MOTION RATIO BOTTOM DRIFT RATIO O Oo SAND MOTION RATIO O F F S H O R E /O N S H O R E Figure 40. Relationship between surge asym metry and migration of fine tracer sand. 128 .VELOCITY RATIO 129 -2 H SAND MOTION FLOW DURATION RATIO -2 +2 — 1.25 -f SAND MOTION BOTTOM DRIFT RATIO + 1 .2 5 -2 H— o — f SAND MOTION + -2 RATIO RATIO RATIO -■-1,25 + = O N S H O R E /O F F S H O R E - = OFFSHORE / ONSHORE 130 : t h e t r i a l s . Net t r a n s p o r t o f th e f i n e t r a c e r sand does n o t c o r r e l a t e c l o s e l y w ith any o f th e hydrodynam ic p a ra m e te r s . The d i r e c t i o n o f m ig r a tio n a g re e s b e s t w ith th e r a t i o of onshore and o f f s h o r e c u r r e n t s t r e n g t h s , d o u b tle s s because t h e s m a lle r p a r t i c l e s have a s e t t l i n g v e l o c i t y w hich i s o n e - h a l f t o o n e - t h i r d t h a t o f th e c o a rse dyed g r a i n s . Once suspended, th e y do n o t r e t u r n to th e bo ttom i n s t a n t a n e o u sly , b u t o v e r a p e rio d o f s e v e r a l se co n d s. The le n g th o f tim e sm a ll g r a in s a re c a r r i e d by a su rg e depends on th e h e i g h t to w hich th e y a r e lau nched r a t h e r th an flo w d u ra t i o n . Thus, v e l o c i t y e x e r t s th e most im p o rta n t in f lu e n c e on th e q u a n t i t y o f f i n e sand moved by a p u ls e . Because su rg e d u r a tio n a s w e l l as a c c e l e r a t i o n p lay a l i m i t e d r o l e i n t h i s p r o c e s s , a d e g re e o f s c a t t e r i s in tr o d u c e d to the g raph r e l a t i n g c u r r e n t s t r e n g th s to sedim ent t r a n s p o r t . The in s i t u sands examined i n t h i s s e r i e s o f e x p e r im e n ts .were o f a g r a in s i z e s i m i l a r to th e f i n e t r a c e r . When t h e i r m ig ra tio n as d e te rm in e d w ith sedim ent t r a p s i s compared to w a te r m otion on F ig u re 41, a s i m i l a r p a t t e r n o f b e h a v io r em erges. The most c o n s i s t e n t r e l a t i o n s h ip i s w ith th e v e l o c i t y f a c t o r . In an e f f o r t to improve agreem en t, o n s h o r e - o f f s h o r e v e l o c i t y r a t i o s were c a l c u l a t e d u s in g o n ly th e pulB es s u f f i c i e n t l y s t r o n g to move sand. T h is p a ra m e te r (F ig . 42) shows a c o n v in c in g c o r r e l a t i o n w ith b o th th e m ag nitu de and d i r e c t i o n o f se d im e n t t r a n s p o r t . Figure 41. R elationship between surge asym metry and migration o f in s itu bottom sand. 131 .VELOCITY RATIO 132 + 1 . 2 5 - -2 ■ ■ -1.25 +2 SAND MOTION FLOW DURATION RATIO + 1.25 -- SAND MOTION BOTTOM DRIFT RATIO O -M.25- O O + 2 —F - SAND MOTION RATIO RATIO RATIO O O - ' - 1 . 2 5 +-= O N SH O R E/O FFSH O R E - = OFFSHORE ONSHORE F ig u r e 4 2 , R e l a t i o n s h i p betw een i n s i t u sand m ig r a ti o n and asymmetry o f th o s e su rg e v e l o c i t i e s c ap a b le o f t r a n s p o r t i n g g r a i n s . The t h r e s h o l d v e l o c i t y f o r p a r t i c l e m o tio n i s 20 c m /se c , and bottom se d im e n t c o n s i s t s o f medium to f i n e san d . 133 SAND M OTION * 3 RATIO + = ONSHORE / OFFSHORE - = OFFSHORE / ONSHORE 135' G ra v ity a p p e a rs to have l i t t l e In f lu e n c e on sm a ll p a r t i c l e s c a r r i e d o v e r a r i p p l e d s u r f a c e . A K ru sk al-Y Ja llis one way t e s t f o r v a r ia n c e ( S i e g e l , 1956) was used to s t a t i s t i c a l l y r e l a t e th e d i r e c t i o n o f in s i t u g r a i n m ig r a tio n t o th e hydrodynam ic r a t i o s . Re s u l t s a re l i s t e d on T ab le I I I . I t may be s a i d w ith r e a s o n a b le c o n fid e n c e t h a t th e r e l a t i v e v e l o c i t y o f shorew ard and seaward p u ls e s i s r e s p o n s i b l e f o r s h i f t i n g f i n e bottom san d . The g r a i n - s l z e d i s t r i b u t i o n s o f sedim en t p r e f e r e n t i a l l y t r a n s p o r t e d on shore and o f f s h o r e were i d e n t i c a l in a l l c a s e s . The ab sence o f s o r t i n g norm al t o th e beach complements th e f o llo w in g o b s e r v a t i o n s : 1 . Surges o f v a ry in g s t r e n g t h s a re com petent to move a l l a v a i l a b l e p a r t i c l e s i z e s w ith e q u a l f a c i l i t y . 2. D i r e c t i o n a l i n e q u a l i t i e s in c u r r e n t s t r e n g t h a re sm a ll in r e l a t i o n to th e t o t a l spectru m o f v e l o c i t i e s . 3. The o f f s h o r e component o f g r a v i t y I s p rob ab ly n e g l i g i b l e in th e p re s e n c e o f r i p p l e m arks. 4 . Sand r e a c h in g th e o f f s h o r e I s c o n fin e d to a narro w s i z e ra n g e . TABLE III RESULTS OR KRUSKAL-WALLIS TESTS RELATING SAND MIGRATION TO SURGE ASYMMETRY Surge Asymmetry F a c to r Average o n sh o re and o f f s h o r e v e l o c i t i e s Average o n sh o re and o f f s h o r e v e l o c i t i e s e x c e e d in g 20 cm /sec A verage d u r a t i o n o f on sh o re and o f f s h o r e flow Bottom d r i f t C onfidence L e v e ls f o r D i r e c t C o r r e l a t i o n s w ith D i r e c t i o n o f Sand M ig r a tio n 95% 99% 35% 11% MODEL EOR NEARSHORE SAND TRANSPORT The r e s u l t s o f t h i s i n v e s t i g a t i o n p e rm it an e v a l u a t i o n o f p re v io u s t h e o r i e s propo sed t o e x p la in th e exchange o f s a n d "between th e "beach and th e o f f s h o r e . I t a p p e a rs t h a t a l l t h r e e o f th e m odels a re founded upon i n v a l i d a s s u m p tio n s . G ra n t (194-3) presumed t h a t a l l n e a r s h o r e "bottom se d im e n t i s moved to w a rd s th e b e a c h u n d e r th e i n f l u e n c e o f sh o rew ard wave d r i f t . However, c u r r e n t m e te r m ea su re m ents have r e v e a l e d t h a t bottom d r i f t i s v a r i a b l e in d i r e c t i o n . The n u l l p o i n t c o n c e p t i s a l s o p r e d i c a t e d on a n e t o n sh o re t r a n s p o r t o f w a t e r o v e r th e s e a f l o o r . The o t h e r t e n e t o f t h i s th e o r y i s t h a t th e seaw ard component o f g r a v i t y p la y s an a p p r e c i a b l e r o l e in d e te rm in in g sand m i g r a t i o n . T his may be th e case f o r l a r g e p a r t i c l e s on a s t e e p l y - s l o p i n g b o tto m . However, f i n e g r a i n s moving a c r o s s r i p p l e marks do n o t seem t o be a f f e c t e d by g r a v i t y . I f a h e te ro g e n e o u s se d im e n t lo a d w ere dumped on a s t e e p s h e l f a t a tim e when b o tto m d r i f t happened t o be s h o r e - w a rd s, a n u l l p o i n t ty p e o f s o r t i n g m ig h t o c c u r . But such an e v e n t i s u n l i k e l y b ecau se sand r e a c h i n g t h e o f f s h o r e zone h a s been so w e l l - s o r t e d t h a t i t i s moved a s a u n i t . Vernon ( 1 9 6 6 ) was n o t aware o f th e s h o rtc o m in g s o f 138! c o n v e n t io n a l wave d r i f t t h e o r y when he I n t e r p r e t e d h i s f i e l d o b s e r v a t i o n s . He r e p o r t e d t h a t th e r a t e and d i r e c t i o n o f dyed sand m ig r a ti o n was a f u n c t i o n o f w a te r d e p th i n su c h a m anner a s to s u p p o r t th e n u l l p o i n t con c e p t . T h is c o n c lu s io n i s te n u o u s c o n s i d e r i n g th e s c a t t e r o f h i s d a t a and i t s c o l l e c t i o n from s h e lv e s w i t h d i v e r s e seaw ard s l o p e s . The c o n fin e m e n t o f most moving b o tto m sand t o th e lo w e s t few c e n t i m e t e r s o f th e w a t e r column p r e c lu d e s e x t e n s i v e d i f f u s i o n o f t h e s e p a r t i c l e s . However, r i p c u r r e n t s w hich i n tr o d u c e se d im e n t t o th e o f f s h o r e a re d en se s u s p e n s io n s In w hich c o n c e n t r a t i o n g r a d i e n t s c au se g r a i n s to d i s p e r s e . Thus d i f f u s i o n I n f l u e n c e s th e o r i g i n a l d i s t r i b u t i o n o f n e a r s h o r e s a n d s , b u t i s n o t a p a r t o f th e r e w o rk in g p r o c e s s . The r e s u l t s o f t h i s s tu d y le a d to th e c o n s t r u c t i o n o f a r e l a t i v e l y sim p le model f o r th e n e a r s h o r e sand b u d g e t. The f i r s t s t e p i n t h e p r o c e s s i s th e c o n s t r u c t i o n o f a b e a c h . L i t t o r a l s e d im e n t i s d e riv e d from r i v e r s o r wave e r o s i o n and s p re a d a lo n g t h e c o a s t by lo n g s h o r e d r i f t . Rip c u r r e n t s s o r t o u t f i n e and medium sand from th e f o r e s h o r e and c a r r y i t beyond th e b r e a k e r s . The l a r g e s t p a r t i c l e s a r e th e f i r s t t o s e t t l e o u t o f th e s e a w a r d - f lo w in g r i p , and se d im e n t r e a c h i n g t h e ocean f l o o r d im in is h e s i n g r a i n s i z e w ith d i s t a n c e from t h e b e a c h . Once d e p o s i t e d , th e p a r t i c l e s m ig r a te a s a u n i t in th e 139 d i r e c t i o n o f p re d o m in a n t su rg e v e l o c i t i e s . R e g io n s o f c o a r s e sand a r e common on many s h e lv e s and r e p r e s e n t r e l i c t P l e i s t o c e n e s t r a n d l i n e s . In t h e s e a r e a s se d im e n t i s t r a n s p o r t e d by b o tto m d r i f t . D urin g t h e w i n t e r s e a s o n , l o c a l sto rm s cause h ig h , s h o r t p e r io d waves and s t r o n g o n sh o re w in d s. L arge r i p c u r r e n t s c a r r y sand from th e b e a c h t o th e i n n e r s h e l f . A f t e r th e g r a i n s a r e d e p o s i t e d on t h e b o tto m , th e y e i t h e r t e n d t o rem a in i n p l a c e o r m ig r a te o f f s h o r e u n d e r th e i n f l u e n c e o f seaw ard s u r g e asym m etry. The r e s u l t i s an a c c u m u la tio n o f se d im e n t o u t s i d e t h e b r e a k e r s a t t h e e x p en se o f t h e b e a c h . In summer t h e r i g o r o u s c o n d i t i o n s a m e l i o r a t e , w inds blow w ith l e s s f o r c e , an d lo n g p e r i o d s w e l l s g e n e r a te d a t g r e a t d i s t a n c e s r e a c h t h e c o a s t . Bottom san d i s moved to w a rd s l a n d by dom inant on sho re s u r g e s and r e - e n t e r s t h e l i t t o r a l z o n e . R ip c u r r e n t s a r e weak, and th e b e ac h i s r e p l e n i s h e d w h ile t h e o f f s h o r e r e s e r v o i r o f sand i s d e p l e t e d . The s h o r e l i n e o f s o u th e rn C a l i f o r n i a u n d erg o es an a n n u a l c y c le o f w i n t e r e ro s io n and summer d e p o s i t i o n w hich i s e x p la in e d by t h i s m o d el. S h ep ard and LaPond (1940) m o n ito re d s e d im e n t l e v e l on th e b e a c h and i n n e r s h e l f a t La J o l l a , C a l i f o r n i a . They c o n firm e d t h a t t h e d i s t r i b u t i o n o f m a t e r i a l betw een th e s e a d j a c e n t e n v iro n m e n ts was i n dynamic e q u i l i b r i u m w ith wave c h a r a c t e r i s t i c s . T h is exchange o f sand i n a d i r e c t i o n norm al t o th e c o a s t i s superim posed on th e c o n tin u o u s lo n g sh o re t r a n s p o r t I n s i d e th e b r e a k e r s . E v e n tu a lly a l l sand e i t h e r i s p e rm an en tly d e p o s ite d on th e s h e l f o r i s f u n n e lle d th ro u g h a subm arine canyon t o th e ocean b a s i n s . CONCLUSIONS As a r e s u l t o f th e s tu d y o f w a v e -g e n e ra te d b o tto m su rg e and i t s e f f e c t on sa n d t r a n s p o r t i n th e o f f s h o r e z o n e, th e f o l lo w in g c o n c lu s io n s a r e made: 1 . O s c i l l a t i n g c u r r e n t s a t t h e s e a f l o o r g e n e r a te d by a s e t o f p a s s i n g waves a r e n o t u n ifo rm b u t have a sp e c tru m o f v e l o c i t i e s . 2 . T r o c h o id a l wave t h e o r y may be used w i t h con f i d e n c e t o p r e d i c t a v e ra g e s u r g e s t r e n g t h s o v e r g e n t l y - s l o p i n g c o n t i n e n t a l s h e lv e s w here t h e r e l a t i v e wave h e i g h t i s l e s s th a n .0 4 0 , w h ile s o l i t a r y t h e o r y i s more a c c u r a t e in s h a l lo w e r w a t e r . 3. The c o n v e n tio n a l c o n c e p t o f wave d r i f t i s i n a d e q u a te s i n c e a n e t t r a n s p o r t o f w a t e r f r e q u e n t l y t a k e s p la c e to w ard s t h e o f f s h o r e , 4 . Wind d r i f t a t th e ocean s u r f a c e i s b a la n c e d by a co m p en sato ry flo w i n th e o p p o s i t e d i r e c t i o n o v e r th e s e a f l o o r . 5. Long p e r i o d s w e l l s c au se sho rew ard s u r g e s t o p red o m in ate and w a te r t o move to w a rd s t h e c o a s t . 6. The d im e n sio n s o f o s c i l l a t i o n r i p p l e m arks in b o tto m se d im e n t a r e d i r e c t l y r e l a t e d t o sand s i z e and v a ry I n v e r s e l y w i t h c u r r e n t s t r e n g t h . 7 . R ip p le marks i n t h e o f f s h o r e s e g r e g a t e g r a i n s 142 i a c c o r d in g to h y d r a u l i c s i z e and c o n c e n t r a t e e x tre m e s a t th e c r e s t s and t r o u g h s . 8 , D e s p ite t h e o c c u rre n c e o f b o th n e t w a te r and se d im e n t t r a n s p o r t , o s c i l l a t i o n r i p p l e s do n o t m ig r a te a lo n g th e s e a f l o o r . 9 . T h re s h o ld v e l o c i t i e s f o r se d im e n t m otion a re d i f f i c u l t to a p p ly i n s t u d i e s o f t h e s u b l i t t o r a l zone b e c au se o f v a r i a t i o n s i n su rg e s t r e n g t h and a c c e l e r a t i o n , r i p p l e r e l i e f , sand p o r o s i t y , and o r g a n i c a c c u m u la tio n s . 10 . R ip p le mark c h a r a c t e r i s t i c s come to e q u i l i b r i u m w ith c u r r e n t v e l o c i t y and bottom se d im e n t e n a b l in g a l l a v a i l a b l e g r a i n s i z e s t o be t r a n s p o r t e d w i t h e q u a l f a c i l i t y , 11. The r a t e a t w hich n e a r s h o r e sand i s moved i n c r e a s e s w ith th e s q u a re o f c u r r e n t v e l o c i t y . 12. P r a c t i c a l l y a l l t r a n s p o r t o f s e a f l o o r sand by o s c i l l a t i n g c u r r e n t s o c c u rs w i t h in l e s s th a n 1 m from th e b o tto m . 13. The v e r t i c a l d i s t r i b u t i o n o f susp en d ed g r a i n s i s p r i n c i p a l l y a f u n c t i o n o f r i p p l e mark r e l i e f . 1 4 . C oarse b ottom sand i s s h i f t e d i n t h e d i r e c t i o n o f wave d r i f t w h ile f i n e sand m i g r a t e s in r e s p o n s e t o i n e q u a l i t i e s i n t h e v e l o c i t y o f o n sh o re and o f f s h o r e s u r g e s , 15 . 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S o u th e rn C a l i f o r n i a , 135 P* Z enk ovich, V. P . , 1967, P r o c e s s e s o f c o a s t a l d e v elo p m e n t: I n t e r s c i e n c e P r e s s , 738 p. — ■ B a r

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Distribution And Ecology Of Two Families Of Natant Decapod Crustacea -- Oplophoridae And Pasiphaeidae -- In Waters Off Southern California

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Paleoecology And Stratigraphy Of Pre-Kaibab Permian Rocks In The Ely Basin, Nevada And Utah

Asset Metadata

Creator Cook, David Olney (author)

Core Title Sand Transport By Shoaling Waves

Degree Doctor of Philosophy

Degree Program Geology

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Gorsline, Donn S. (committee chair), Stone, Richard O. (committee member), Zimmer, Russel L. (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c18-409668

Unique identifier UC11362267

Identifier 7019110.pdf (filename),usctheses-c18-409668 (legacy record id)

Legacy Identifier 7019110.pdf

Dmrecord 409668

Document Type Dissertation

Rights Cook, David Olney

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA