Shell dredging as a factor in estuarine sedimentation

CHAPTER 25

SHELL DREDGING AS A FACTOR IN ESTUARINE SEDIMENTATION

Frank D. Mas ch

Associate Professor of Civil Engineering The University of Texas, Austin, Texas

ABSTRACT

During the past few decades, industrial demands for buried shell have been rapidly increasing. Highway construction, the petro-chemical industries, and cement manufacturers are among the major users of shell. Semi-fossil oyster shell is found in tidal embayments in deposits of varying thickness and is dredged from depths varying from two or three feet up to forty feet by suction type hydraulic dredges which often discharge wash waters immediately into the surrounding embayment.

Because shell deposits are often covered with several feet of overburden, large quantities of silt and clay are pumped on board the dredge with the shell. On the dredge the shell is washed, screened, and loaded onto barges. Unwanted shell fragments and sediments are then pumped overboard with the wash water.

Considerable quantities of estuarine sediments are placed into motion by continuous dredging operations and uncontrolled dredging can lead to serious depositional problems. Expos ed shellfish produc-ing reefs can be damaged, small craft channels can be filled, and vital shore protection can be removed in instances where shell dredg-ing is uncontrolled.

INTRODUCTION

During the past several decades industrial demands for buried oyster shell have been increasing at a very rapid rate. Commercially shell has a multitude of uses with perhaps one of the most important being its use in the construction of road bases in coastal areas. Shell is also used as a surfacing material, as an aggregate in concrete, a s a pad for drilling operations, and it is used in the manufacture of chemi-cals, lime, cement, glass, plastics, and even in poultry feeds. Along with oil, natural gas, and commercial fisheries, semi-fossil oyster shell necessarily must be considered as one of the important and vi tal natural resources found in tidal embayments. Unlike commercial fish-eries, buried shell is a non-replenishable resource. Shell deposits have semi-fossil characteristics and several hundred years are required to

628 COASTAL ENGINEERING b u i l d d e p o s i t s of s u f f i c i e n t t h i c k n e s s to be e c o n o m i c a l l y d r e d g e d . L I T E R A T U R E R E V I E W D r e d g i n g f o r b u r i e d s h e l l has b e e n c a r r i e d on f o r m o r e t h a n f i f t y y e a r s i n m a n y of the t i d a l w a t e r s of the G u l f C o a s t s t a t e s . M o s t m a j o r s h e l l d r e d g i n g o p e r a t i o n s i n v o l v e m o r e t h a n one d r e d g e and a r e c a p a b l e of p r o d u c i n g m i l l i o n s of c u b i c y a r d s of s h e l l a n n u a l l y . P e r -m i t s g r a n t e d to s h e l l d r e d g e r s g e n e r a l l y c o n t a i n c l a u s e s a u t h o r i z i n g s t a t e c o n s e r v a t i o n agencies to c o n t r o l d r e d g i n g o p e r a t i o n s and to i n -s u r e t h a t no damage i -s done to o t h e r n a t u r a l r e -s o u r c e -s . W h i l e -s h e l l d r e d g i n g has h a d a l o n g and s u c c e s s f u l h i s t o r y , r e l a t i v e l y f e w s y s -t e m a -t i c i n v e s -t i g a -t i o n s have been c a r r i e d ou-t -to d e -t e r m i n e -the e f f e c -t s of s h e l l d r e d g i n g on o t h e r n a t u r a l r e s o u r c e s .

M a c k i n (1961) has t r a c e d the h i s t o r y of d r e d g i n g and i t s r e l a t i o n to o y s t e r r e e f s b a c k to the w o r k of G a r y i n L o u i s i a n a i n 1906. F o r the r e s u l t s and c o n c l u s i o n s of these e a r l y s t u d i e s the r e a d e r i s r e f e r r e d to the w o r k of M a c k i n and the e x t e n s i v e b i b l i o g r a p h y i n c l u d e d w i t h h i s p a p e r .

M o r e r e c e n t l y , i n a M a s t e r ' s t h e s i s , W i l s o n (1950) r e p o r t e d on a s t u d y of the e f f e c t s of s h e l l d r e d g i n g on o y s t e r s i n Copano B a y , T e x a s . B a s e d on h i s data, W i l s o n r e p o r t e d t h a t t u r b i d i t i e s of 4,000 p a r t s p e r m i l l i o n c o u l d be f o i m d c l o s e to the o u t l e t of a d r e d g e , and t h a t these t u r b i d i t i e s w e r e h a r m f u l to o y s t e r s . W i l s o n f u r t h e r s u m m a r i z e d f o u r f a c t o r s w h i c h d e t e r m i n e d the m o v e m e n t and d e p o s i t i o n of s u s p e n d e d m a t e r i a l i n Copano B a y to be: (1) The c o m p o s i t i o n of the m a t e r i a l ; (2) The f l o c c u l a t i o n of s i l t and c l a y i n sea w a t e r ; (3) W i n d a c t i o n ; and, (4) B o t t o m t o p o g r a p h y . W i l s o n f u r t h e r s u m m a r i z e d t h a t the m o v e m e n t of s u s p e n d e d m a t e r i a l and r e s u l t i n g s e d i m e n t a t i o n was s i g n i f i c a n t up to d i s t a n c e s a p p r o x i m a t e l y 300 y a r d s f r o m the d r e d g e and t h a t b o t t o m b a s k e t t r a p s w i t h i n 300 y a r d s of the d r e d g e w e r e a l w a y s c o v e r e d w i t h s e d i m e n t i f t h e y w e r e s e t at a l o c a t i o n t h a t was at the same d e p t h as the s u r r o u n d i n g b o t t o m . W h e n the t r a p s w e r e s e t above the b o t t o m t h e y w e r e n o t s i l t e d u n l e s s s p o i l was d e p o s i t e d d i r e c t l y on t h e m . W i l s o n f u r t h e r s u m m a r i z e d t h a t c o n t r o l l e d d r e d g i n g m a y be b e n e f i c i a l to o y s -t e r p r o d u c -t i o n and -t h a -t -the e x c a v a -t i o n s m a d e by -the d r e d g e f u r n i s h e d a b e t t e r s u p p l y of w a t e r f o r the o y s t e r s .

R. I n g l e (1952) s t u d i e d d r e d g i n g i n s o m e F l o r i d a b a y s . A m o n g the p r i n c i p a l c o n c l u s i o n s of R. I n g l e ' s s t u d y w e r e t h a t the f l o w of w a t e r d e t e r m i n e s the d i r e c t i o n of t r a n s p o r t of s u s p e n d e d m a t e r i a l and, i n g e n e r a l , the d i s t a n c e s u s p e n d e d m a t e r i a l i s c a r r i e d depends o n the s i z e of the p a r t i c l e s w i t h i n the suspens o l d and the speed of the c u r r e n t w i t h w h i c h the p a r t i c l e s a r e t r a n s p o r t e d . I n g l e a l s o r e p o r t e d t h a t w a t e r

S H E L L DREDGING 629 m o v i n g at a g i v e n l e v e l m a y be d e f l e c t e d u p w a r d as i t passes o v e r a r i d g e c a u s i n g the s u s p e n s i o n of f i n e r p a r t i c l e s and e n h a n c i n g t r a n s -p o r t a b i l i t y . I n g l e r e -p o r t e d i t was l o g i c a l to a s s u m e t h a t f o r any d r e d g i n g s i t u a t i o n t h e r e e x i s t s a p o i n t b e y o n d w h i c h h e a v i e r p a r t i c l e s w i l l n o t be d e p o s i t e d i n any a p p r e c i a b l e a m o u n t . B e y o n d t h i s p o i n t I n g l e r e p o r t s t h a t o n l y f i n e m a t e r i a l s w o u l d be i n s u s p e n s i o n a n d c o u l d be e x p e c t e d to c o n t i n u e t o be t r a n s p o r t e d i n d e f i n i t e l y . F u r t h e r m o r e , s i n c e these f i n e s e d i m e n t s w o u l d p r o b a b l y not r e m a i n on o y s t e r r e e f s f o r any l e n g t h of t i m e t h e y w o u l d not have any e f f e c t on o y s t e r m o r -t a l i -t y . I n g l e f u r -t h e r r e c o m m e n d s -t h a -t " c o n -t r o l l e d " d r e d g i n g be u s e d b a s e d on l o c a l v a r i a t i o n s i n s e d i m e n t , t i d e s , d e p t h s , etc,

M a c k i n ' s s t u d y ( I 9 6 I ) was of c h a n n e l d r e d g i n g and s i l t i n g i n L o u i s i a n a and o f the e f f e c t s of d r e d g i n g on o y s t e r s i n s o m e s h a l l o w bays of L o u i s i a n a . The s t u d y c o n s i s t e d of t h r e e case h i s t o r i e s of d r e d g i n g ; f i r s t , t h a t of a h y d r a u l i c d r e d g e , s e c o n d l y , t h a t of a c l a m d r e d g e o p e r a t i n g i n a c h a n n e l , and t h i r d l y , t h a t of a h y d r a u l i c d r e d g e r e l e a s i n g s e d i m e n t i n a h a l f c i r c l e . M a c k i n c o n c l u d e d t h a t s i l t was c a r r i e d a m a x i m u m d i s t a n c e of 1,300 f e e t f r o m a w o r k i n g d r e d g e w i t h r a p i d d i s p e r s a l by d i l u t i o n and s e d i m e n t a t i o n . A m a x i m u m of one p e r -cent of the s e d i m e n t d r i f t e d a w a y f r o m the s i t e of d r e d g i n g o p e r a t i o n , and t u r b i d i t i e s b e y o n d a f e w h u n d r e d f e e t d i d n o t e x c e e d the n a t u r a l l e v e l of 20 t o 200 p a r t s p e r m i l l i o n . E x p e r i m e n t s s h o w e d no h a r m f u l e f f e c t s on o y s t e r s f o r t u r b i d i t i e s up to 700 p a r t s p e r m i l l i o n .

A l t h o u g h s o m e r a t h e r d e t a i l e d s t u d i e s have b e e n c a r r i e d o u t on the e f f e c t s of s h e l l d r e d g i n g , t h e r e i s l i t t l e u n a n i m i t y i n the c o n c l u s i o n s t h a t have been r e a c h e d . I t i s i n t e r e s t i n g to note i n the l i t e r a t u r e t h a t m a n y of the s t u d i e s have b e e n c a r r i e d on w i t h l i t t l e c o n c e r n f o r the b a s i c concepts of e s t u a r i n e s e d i m e n t a t i o n . M a n y a u t h o r s h a v e u s e d s o m e m e a s u r e of t u r b i d i t y as a n i n d i c a t i o n of s e d i m e n t m o v e m e n t a n d d e p o s i -t i o n . T u r b i d i -t y , h o w e v e r , i s n o -t a l w a y s dependable b e c a u s e s u s p e n s i o n s h a v i n g the s a m e c o n c e n t r a t i o n s of s e d i m e n t m a y g i v e e n t i r e l y d i f f e r e n t r e a d i n g s of t u r b i d i t y , as d e t e r m i n e d f o r e x a m p l e b y the J a c k s o n C a n d l e , s i m p l y b e c a u s e of c o l o r , m i n e r a l c o m p o s i t i o n , o r the p r e s e n c e of s u s -p e n d e d o r g a n i c m a t e r i a l .

O n l y l i t t l e m e n t i o n has been m a d e o f the p o s s i b i l i t i e s of m u d f l o w s or d e n s i t y c u r r e n t s of v e r y h i g h c o n c e n t r a t i o n s of s e d i m e n t . M a n y of the s t u d i e s on s h e l l d r e d g i n g have u s e d s u r f a c e w a t e r s a m p l e s as an i n -d i c a t o r of s e -d i m e n t -d e p o s i t i o n , an-d l i t t l e has b e e n s a i -d of the f l o c c u l a t i o n p r o c e s s so i m p o r t a n t to a l l e s t u a r i n e s e d i m e n t a t i o n p r o c e s s e s . F o r e x a m p l e , R. I n g l e ' s c o n c l u s i o n t h a t the d i r e c t i o n of f l o w of w a t e r c u r -r e n t d e t e -r m i n e s the d i -r e c t i o n of t -r a n s p o -r t of s u s p e n d e d m a t e -r i a l c a n be q u i t e m i s l e a d i n g . T h i s a u t h o r ' s e x p e r i e n c e has b e e n t h a t m a n y t i m e s w a t e r c u r r e n t s n e a r the b o t t o m of the b a y do n o t f o l l o w the s a m e d i r e c -t i o n as s u r f a c e c u r r e n -t s , and o n l y r a r e l y do ebb c u r r e n -t s f o l l o w -the s a m e

630 _{C O A S T A L ENGINEERING}

p a t h s as f l o o d c u r r e n t s i n w i d e s h a l l o w e m b a y m e n t s . I n the event t h a t s e d i m e n t s d i s c h a r g e d by d r e d g e s f l o c c u l a t e i n t o a d e n s i t y c u r r e n t , g r a v i t y c a n b e c o m e e q u a l l y i m p o r t a n t o r p e r h a p s m o r e i m p o r t a n t t h a n c u r -r e n t s , and the d i -r e c t i o n t h a t the s e d i m e n t s m o v e m a y be t o t a l l y d i f f e -r e n t t h a n i n d i c a t e d b y the s u r f a c e c u r r e n t s .

T h e r e has been a g r e a t d e a l w r i t t e n and s a i d about the e f f e c t s of s h e l l d r e d g i n g on s h e l l and f i n f i s h r e s o u r c e s and t h e r e a r e n u m e r o u s u n p u b l i s h e d r e p o r t s , p a p e r s , and n e w s p a p e r a r t i c l e s a v a i l a b l e to t h e i n t e r e s t e d r e a d e r . H o w e v e r , m a n y of the c o n c l u s i o n s set f o r t h a r e i n -c o n s i s t e n t w i t h one a n o t h e r and a r e b a s e d o n l y on v e r y m e a g e r f i e l d d a t a . S H E L L D E P O S I T S S h e l l d e p o s i t s v a r y b o t h i n s i z e and i n depth. S u r f a c e a r e a s r a n g e f r o m a f e w a c r e s up to s e v e r a l s q u a r e m i l e s and depths v a r y f r o m a f e w f e e t up to at l e a s t f o r t y f e e t . The depths to w h i c h s o m e d e -p o s i t s e x t e n d i s n o t a c c u r a t e l y k n o w n , h o w e v e r , f o r t y f e e t i s a b o u t t h e m a x i m u m d e p t h f r o m w h i c h d r e d g e s c a n e f f e c t i v e l y take s h e l l . S h e l l d e p o s i t s m a y be c o m p l e t e l y b u r i e d b e n e a t h the f l o o r of an embayno-ent o r t h e y m a y be e x p o s e d and i n s o m e cases a c t u a l l y r i s e above the b o t t o m . W h e n the d e p o s i t s a r e e x p o s e d t h e y p r o v i d e p o t e n t i a l o y s t e r g r o w i n g a r e a s and i n n u m e r o u s i n s t a n c e s m a y be c o v e r e d b y l i v e o y s t e r s . S h e l l d e p o s i t s a l s o o c c u r i n l a y e r s s e p a r a t e d b y a f e w f e e t of f i n e s a n d s , s i l t s , a n d c l a y s . I t i s n o t u n c o m m o n to f i n d l a y e r e d d e p o s i t s i n w h i c h t h e r e m a y be a t h i n l a y e r of e x p o s e d s h e l l f o l l o w e d by two o r t h r e e f e e t o f s e d i m e n t s and t h e n t w e n t y t o t h i r t y f e e t of b u r i e d s h e l l d e p o s i t . B e c a u s e of the o y s t e r g r o w t h on these t h i n l a y e r s of s h e l l , v a l u a b l e d e p o s i t s a r e o f t e n l o c k e d up and n o t a v a i l a b l e f o r c o m m e r c i a l use. The g e n e r a l p a t t e r n o f s t r a t i g r a p h y i n the a r e a of e x p o s e d r e e f s i s o f t e n t h a t of a f a i r l y h o m o g e n e o u s sequence of f i n e - g r a i n e d d e t r i t a l m a t e r i a l s i n c l u d i n g sands, s i l t s , and c l a y s . T h i s sequence, e s p e c i a l l y a l o n g the f l a n k s of o y s t e r r e e f s , i s o c c a s i o n a l l y i n t e r - m i x e d w i t h f i n e t o c o a r s e s h e l l f r a g m e n t s . L a c k of d i s t i n c t b e d d i n g can be a t t r i b u t e d t o the a c t i v i t i e s of n u m e r o u s m a r i n e b u r r o w i n g o r g a n i s m s . W i t h c o n s t a n t b u r -r o w i n g , t u n n e l i n g , and m i x i n g of the s e d i m e n t s b y these o -r g a n i s m s , l i t t l e of a n y o r i g i n a l s t r a t i f i c a t i o n of the s e d i m e n t s is p r e s e r v e d . I n a d d i t i o n to the h o m o g e n e o u s sequence of m a t e r i a l s n o r m a l l y f o u n d , t h e r e a r e g e n e r a l l y n u m e r o u s o c c u r r e n c e s of f i n e to c o a r s e s h e l l f r a g m e n t s f o u n d w i t h i n c o r e s t a k e n n e a r o r on the f l a n k s of p r e s e i ^ t d a y r e e f s . T h e s e f r a g m e n t s a r e s h e l l d e b r i s f r o m the r e e f s and a r e d e r i v e d f r o m the r e e f b y w a v e a c t i o n o r b y o y s t e r h a r v e s t i n g b r e a k i n g u p s h e l l and s c a t t e r i n g the d e b r i s a l o n g the f l a n k s of the r e e f s .

S H E L L DREDGING 631

F i g u r e 1 i s a t r i a n g l e d i a g r a m s h o w i n g sand, s i l t , and c l a y content of the a c t i v e s e d i m e n t s i n c o r e s a m p l e s t a k e n f r o m the v i c i n i t y of r e e f s i n C e n t r a l and E a s t C e n t r a l Texas G u l f C o a s t b a y s . T y p i c a l l y these se ^ m e n t s a r e f o u n d i n the s i l t y - c l a y c l a s s w i t h l e s s e r a m o u n t s i n the s a n d ' ^ s i l t - c l a y , the c l a y - s a n d , and sand c l a s s e s . A c l o s e r e l a t i o n s h i p i s n o t e b e t w e e n the r e s u l t s i n F i g u r e 1 and those r e p o r t e d b y S h e p a r d and M o d ' ( I 9 6 0 ) .

The s e d i m e n t s f o u n d i n o v e r b u r d e n v a r y f r o m sands and f i n e s a n ' ^ ® d o w n to c l a y s . The change i n s e d i m e n t c o m p o s i t i o n c a n g e n e r a l l y be e5i p l a i n e d b y the e n e r g y e n v i r o n m e n t of the s a m p l e and the p a r t i a l s o u r c e of t h e s e d i m e n t s . T y p i c a l l y , i n s h a l l o w w a t e r n e a r b a y s h o r e s , w a v e a c t i o n e f f e c t i v e l y s o r t s out the f i n e r c l a y s and s i l t s i z e f r a c t i o n leaving b e h i n d sand p a r t i c l e s . A s w a t e r depths i n c r e a s e away f r o m the s h o r e > ^ the w a v e s b e c o m e l e s s e f f e c t i v e i n s o r t i n g out the f i n e r p a r t i c l e s a n d t q u i e t e r deep w a t e r i s c o n d u c i v e to the a c c u m u l a t i o n of s i l t and c l a y p a r t i d e s .

The s i z e d i s t r i b u t i o n of the p a r t i c l e s i n the o v e r b u r d e n s e d i m e n t s is i m p o r t a n t i n e v a l u a t i n g the p o t e n t i a l d e p o s i t i o n f r o m d r e d g i n g a c t i - v ^ " t i e s . I n t a k i n g s h e l l , d r e d g e s r e t u r n a l l s e d i m e n t s to the e m b a y m e n t w a t e r s . I f the o v e r b u r d e n m a t e r i a l i s c o m p o s e d p r i m a r i l y of s a n d s , r a p i d d e p o s i t i o n w i l l t a k e p l a c e and the s e d i m e n t s w i l l n o t m o v e a p p r ® ' c i a b l e d i s t a n c e s a w a y f r o m the d r e d g e s . On the o t h e r h a n d w h e n s e d - i " m e n t s a r e c o m p o s e d m o s t l y of c l a y s and s i l t s , f l o c c u l a t i o n of the s e d ^ " m e n t s o c c u r s i n s a l t w a t e r . D e n s i t y c u r r e n t s of " f l u i d m u d " m a y f o r ^ ^ and m o v e m e n t f o r a p p r e c i a b l e d i s t a n c e s m a y o c c u r . S H E L L D R E D G E S S h e l l d r e d g e s a l l o p e r a t e i n e s s e n t i a l l y the s a m e m a n n e r . B a S a l -J IS c* l y t h e s h e l l is d r e d g e d f r o m b u r i e d d e p o s i t s b y a c u t t e r h e a d a t t a c h e d a l a d d e r at the bow of the d r e d g e . The b u r i e d s h e l l , a l o n g w i t h w a t e * " ^ s e d i m e n t s , i s t a k e n up b y a s u c t i o n p i p e l o c a t e d i m m e d i a t e l y b e h i n d t-T^^

c u t t e r b l a d e . S h e l l and s e d i m e n t s a r e t h e n p u m p e d on b o a r d the d r e d ^ w h e r e t h e y a r e w a s h e d and p a s s e d t h r o u g h a s e r i e s of s c r e e n s . I n s

i n s t a n c e s the s i z e of the s m a l l e s t s c r e e n i s p r e s c r i b e d b y the s t a t e c ^ s e r v a t i o n a g e n c y c h a r g e d w i t h m a n a g i n g s h e l l d r e d g i n g o p e r a t i o n s .

c o n t r o l l i n g the s i z e of the s c r e e n s , s h e l l of v a r i o u s s i z e s i s s e p a r a t e : ^-^-ly and t h e n e i t h e r t e m p o r a r i l y s t o r e d on b o a r d the d r e d g e o r l o a d e d d i r "

onto b a r g e s by c o n v e y e r s l o c a t e d on e i t h e r s i d e of the d r e d g e .

W h i l e i n the p r o c e s s of d i g g i n g f o r s h e l l , a d r e d g e m o v e s a h e a < ^ s w i n g i n g about one of t w o spuds l o c a t e d at the s t e r n of the d r e d g e a n p u l l i n g i t s e l f w i t h a n c h o r s m o v e d and set w i t h b o o m s l o c a t e d o n e i t h . ^ ^ side of the bow of the d r e d g e .

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The m a i n d i f f e r e n c e s i n s h e l l d r e d g e s l i e i n t h e i r s i z e and i n the m a n n e r i n w h i c h t h e y r e l e a s e w a s h w a t e r s . Some of the l a r g e s t s h e l l d r e d g e s a r e o v e r 300 f e e t l o n g and d i s p l a c e o v e r 3,000 t o n s . T h e s e l a r g e d r e d g e s a r e c a p a b l e of d i g g i n g f r o m depths up to 45 f e e t w i t h a l e n g t h of c u t o v e r 300 f e e t . D e p e n d i n g on the c a p a c i t y and n u m b e r of p u m p s , l a r g e s h e l l d r e d g e s d i s c h a r g e w a s h w a t e r at the r a t e o f o v e r 40,000 g a l l o n s p e r m i n u t e , and u n d e r c o n t i n u o u s o p e r a t i o n p r o d u c e u p -w a r d s of 300,000 c u b i c y a r d s of s h e l l p e r m o n t h . A s a g e n e r a l r u l e , s h e l l d r e d g e s d i s c h a r g e a l o n g w i t h the w a s h w a t e r s a l l s e d i m e n t s and s h e l l p a r t i c l e s f i n e r t h a n the s m a l l e s t s c r e e n

i m m e d i a t e l y b a c k to the w a t e r s of the e m b a y m e n t . Some d r e d g e s r e l e a s e a l l o r a p o r t i o n of the w a s h w a t e r d i r e c t l y t h r o u g h a n open d i s -c h a r g e p i p e . O t h e r d r e d g e s r e t u r n w a s h w a t e r s to the e m b a y m e n t t h r o u g h v e l o c i t y r e d u c t i o n b o x e s . V e l o c i t y r e d u c t i o n boxes a r e p r o b a b l y q u i t e e f f e c t i v e i n p r o m o t i n g the r a p i d d e p o s i t i o n of sands and s h e l l p a r -t i c l e s . H o w e v e r , -the r e d u c -t i o n i n v e l o c i -t y p r o b a b l y has l i -t -t l e e f f e c -t on the s e d i m e n t p a r t i c l e s i n the s i l t and c l a y r a n g e . R e g a r d l e s s of the type of d i s c h a r g e u s e d , t h e r e i s g e n e r a l l y a g r e a t d e a l of t u r b u l e n c e i n the w a t e r s i m m e d i a t e l y s u r r o u n d i n g the d r e d g e , so that the f i n e r s e d i m e n t s a r e k e p t s u s p e n d e d w h e r e t h e y a r e f r e e to be m o v e d away f r o m the d r e d g e by e x i s t i n g c u r r e n t s .

The a m o u n t of s e d i m e n t any g i v e n d r e d g e d i s c h a r g e s w h i l e d i g g i n g s h e l l depends p r i m a r i l y on the o v e r b u r d e n and the a m o u n t of s e d i m e n t m i x e d i n w i t h the s h e l l d e p o s i t . T a b l e 1 shows the r a n g e of s u s p e n d e d f i x e d s e d i m e n t c o n c e n t r a t i o n that c o u l d be e x p e c t e d f r o m t w o d i f f e r e n t d r e d g e s w o r k i n g u n d e r d i f f e r e n t c o n d i t i o n s . T a b l e 1. Suspended S e d i m e n t C o n c e n t r a t i o n i n D r e d g e W a s h W a t e r . Suspended D r e d g e Date F i x e d S o l i d s R e m a r k s m g / 1 A 1 2 - 2 3 - 6 4 7,000 Deep c l e a n s h e l l A 1 2 - 2 9 - 6 4 29,000 H e a v y o v e r b u r d e n B 1 2 - 2 3 - 6 4 14,000 L a y e r e d s h e l l B 6 - 2 5 - 6 5 3,000 Deep c l e a n s h e l l The r e p o r t e d s e d i m e n t c o n c e n t r a t i o n s i n T a b l e 1 a r e f o r f i x e d s e d i m e n t s and do n o t i n c l u d e the v o l a t i l e s o l i d s w h i c h m a y add a n o t h e r t h r e e t o s i x

634 COASTAL ENGINEERING

p e r c e n t to the t o t a l s u s p e n d e d s o l i d s s i n c e the s e d i m e n t s a r e t a k e n f r o m the b o t t o m .

D r e d g e s i z e has l i t t l e to do w i t h the c o n c e n t r a t i o n of the suspe^*^^ s o l i d s d i s c h a r g e . The m a i n d i f f e r e n c e i n d r e d g e s i z e r e s u l t s f r o m ^^'^ t i m e of o p e r a t i o n and r a t e of w a s h w a t e r d i s c h a r g e . G e n e r a l l y t h e ^ ^ ^ ^ d r e d g e s o p e r a t e w i t h f e w e r i n t e r r u p t i o n s and d i s c h a r g e w a s h w a t e r s ^ h i g h e r r a t e s . H e n c e , the d r e d g e w h i c h o p e r a t e s the m a x i m u m annoU^^''*' t i m e w i t h the h i g h e s t d i s c h a r g e w i l l i n t r o d u c e o r r e - s u s p e n d i n t h e ^ b a y m e n t w a t e r s the g r e a t e s t a m o u n t of s e d i m e n t s w h i c h e v e n t u a l l y l ^ ^ ^ be d e p o s i t e d .

I N S T R U M E N T A T I O N A N D O B S E R V A T I O N S O N S E D I M E N T M O V E M E N T F R O M S H E L L D R E D G E S

A n u m b e r of d i f f e r e n t types of d e v i c e s have b e e n u s e d to ohs^^^'^

s e d i m e n t m o v e m e n t and d e p o s i t i o n i n the a r e a of s h e l l d r e d g i n g a c t i " v i t i e s . A m o n g the m o s t c o m m o n s a m p l i n g d e v i c e s a r e f u n n e l t r a p s ' m u d b o a r d s , c o r e t u b e s , s i l t b a s k e t s , and w a t e r s a m p l e r s . F u n n e l '^^ have been d e s c r i b e d by W i l s o n (1950) a n d G a l t s o f f (1964). A n e f f e c ^ ^ ^ ^ ^ f i m n e l t r a p u s e d b y t h i s a u t h o r c o n s i s t e d of a 225 m l p o l y e t h y l e n e ^ f i t t e d w i t h a c o r k and a 2 - 1 / 4 i n c h p l a s t i c f u n n e l as s h o w n i n F i g u r The f u n n e l t r a p was a t t a c h e d to a h e a v y m e t a l s c r e e n s u s p e n d e d f r ' ^ - ' ^ ^ •jn-e r s . a 2 i n c h x 2 i n c h s t a k e d r i v e n i n t o the s h a l l o w b a y b o t t o m . O t h e r t ^ ' c o u l d a l s o be l o c a t e d at v a r i o u s depths b y s i m p l y a t t a c h i n g o t h e r C^' t a i n e r s to the s u p p o r t i n g r o p e and i n s e r t i n g the t r a p s i n t o the c o n t ^ ' ' ^ I n g e n e r a l the p r o c e d u r e f o r l o c a t i n g f u n n e l t r a p s t a t i o n s i n v o l v e s ^ ^ a v i s u a l o b s e r v a t i o n of the m o v e m e n t of s e d i m e n t f r o m the d r e d g e o v e -A f t e r s o m e data i s c o l l e c t e d and a n a l y z e d , the p a t t e r n of s e d i m e n t ;

m e n t c a n be d e t e r m i n e d and a d d i t i o n a l s t a t i o n s can be e s t a b l i s h e d b e t t e r d e f i n e the s e d i m e n t m o v e m e n t . I t s h o u l d be e m p h a s i z e d t h ^ * r e a d i n g s f r o m f u n n e l t r a p s a r e n o t m e a s u r e s of the a m o u n t of s e d i ^ " ^ * ^ ^ t h a t w o u l d d e p o s i t i n a g i v e n a r e a . A c t u a l l y , the t r a p s c a t c h a n d J n " ^

s e d i m e n t w h i c h w o u l d n o r m a l l y pass t h r o u g h the a r e a and p e r h a p s ^ w a s h e d a w a y b y t i d a l c u r r e n t s . The t r a p s a r e u s e f u l m a i n l y t o d e : f

the e x t e n t of s e d i m e n t m o v e m e n t and t o g e t an i n d i c a t i o n of the r e l ^ e d g e i n t e n s i t y of the m o v e m e n t t h r o u g h o u t the a r e a a f f e c t e d by a g i v e n * " ;i^ours F o r t h i s r e a s o n , i t i s d e s i r a b l e to l e a v e t r a p s out a p p r o x i m a t e l y 3 ^ m i n e c so t h a t the s e d i m e n t m o v e m e n t t h r o u g h o u t a t i d a l c y c l e c a n be d e t ^

A t y p i c a l m u d b o a r d f o r m e a s u r i n g the a c c u m u l a t i o n o f s e d i i s s h o w n i n F i g u r e 3. Such a b o a r d is u s e f u l f o r m e a s u r i n g the a a c c u m u l a t i o n of s e d i m e n t i n a g i v e n a r e a . H o w e v e r , the b o a r d d o to be set n e a r l y f l u s h w i t h the b o t t o m o f the e m b a y m e n t ; and s i n c e c a n n o t be l i f t e d w i t h o u t d i s t u r b i n g the a c c u m u l a t e d s e d i m e n t s , o b s t i o n s c a n be o b t a i n e d o n l y b y a d i v e r .

6 3 6 _{COASTAL ENGINEERING}

M Q A . ^ ""T^ """"^ e f f e c t i v e c o r i n g tube has been d e v e l o p e d b y M a c k i n (1961) to d e t e r m i n e r e c e n t d e p o s i t i o n i n s h a l l o w e m b a y m e n t s w i t h s o f t b o t t o m s . T h i s c o r i n g tube s h o w n i n F i g u r e 4 c o n s i s t s of a p l a s t i c tube about I i n c h e s m d i a m e t e r and a p p r o x i m a t e l y 4 f e e t i n l e n g t h . The tube I S a t t a c h e d to a h a n d l e w h i c h c a n be m a d e e i t h e r of w o o d or 1/2 i n c h g a l v a n i z e d p i p e . A l s o m o u n t e d on the h a n d l e is a g u i d e f o r d i r e c t i n g a s t o p p e r i n t o the u p p e r end of the tube. The b o t t o m end of the tube m a y be f i t t e d w i t h a s h a r p e n e d s t e e l edge to f a c i l i t a t e f o r c i n g the tube i n t o the bottona m u d s . A s the tube i s i n s e r t e d i n t o the b o t t o m , w a t e r passes u p t h r o u g h the v a l v e a t the top o f the tube. W h e n the c o r e tube is w i t h -d r a w n f r o m the b o t t o m the s t o p p e r seats i n the t o p of the tube a l l o w i n g s o m e o f the b o t t o m m u d s and the c o l u m n o f w a t e r above t h e s e m u d s to be w i t h d r a w n . The p r i n c i p a l d i s a d v a n t a g e i n the use of t h i s c o r i n g tube

s t h a s o m e of the b o t t o m s e d i m e n t s o f t e n f a l l out f r o m the b o t t o m of the tube, and d i s t u r b the w a t e r c o l u m n c o l l e c t e d above the s e d i m e n t s a m

-A f o u r t h v e r y s i m p l e m e t h o d that can be u s e d to d e t e r m i n e the r e l a t i v e i n t e n s i t y of d e p o s i t i o n a r e s i l t b a s k e t s . T h e b a s k e t s a r e m a d e up o± a 1 i n c h m e s h c h i c k e n w i r e f o l d e d to f o r m a b a s k e t about 2 f e e t s q u a r e . T h e b a s k e t i s f i l l e d w i t h c l e a n o y s t e r s h e l l and i s a t t a c h e d b y a n y l o n r o p e to a 2 i n c h x 2 i n c h stake d r i v e n i n the bay b o t t o m . I n o r d e r to o b t a i n a n i n d i c a t i o n of the r e l a t i v e i n t e n s i t y of s e d i m e n t a c t i v i t y o b -s e r v e d on the b a -s k e t -s , i t i-s r e c o m m e n d e d t h a t n u m b e r -s be a -s -s i g n e d f o r d i f f e r e n t s i l t b a s k e t r e a d i n g s , s u c h as d e f i n e d b y M u r r a y ( 1 9 6 5 ) , T a b l e 2 T a b l e 2. Scale f o r S e d i m e n t B a s k e t R e a d i n g s Scale _{D e s c r i p t i o n} 0 _{C l e a n} 1 _{T r a c e} Z _{C o a t i n g} 3 _{L a y e r} 4 _{H e a v y L a y e r} 5 ' B u r i e d F i g u r e 5 i s a p h o t o g r a p h of a s i l t basket w i t h a s c a l e r e a d i n g o f 2 The s c a l e I S o b v i o u s l y s u b j e c t to p e r s o n a l i n t e r p r e t a t i o n . H o w e v e r , i t does p r o v i d e s o m e s t a n d a r d i z a t i o n and gives a s e d i m e n t a c t i v i t y . v e r y q u i c k i n d i c a t i o n of r e l a t i v e

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w i t h the s a m p l e r s h o w n i n F i g u r e 6. T h i s s a m p l e r c o n s i s t e d of an 8 i n c h l o n g x 2 i n c h d i a m e t e r L u c i t e tube w i t h a s m a l l r u b b e r v a l v e a t -t a c h e d -to -the b o -t -t o m of -the -tube. The u p p e r end of -the c y l i n d e r was c l o s e d w i t h a r u b b e r s t o p p e r w h i c h was f i t t e d w i t h S a r a n t u b i n g and an a q u a r i u m v a l v e . W a t e r s a m p l e s w e r e c o l l e c t e d b y f i r s t c l o s i n g the e m p t y p l a s t i c tube w i t h the s t o p p e r , t h e r e b y b u i l d i n g up p r e s s u r e w i t h -i n the tube. The c l o s e d tube was a d j u s t e d on a m e t a l p -i p e so t h a t the b o t t o m of the s a m p l e r was a t the depth f r o m w h i c h a w a t e r s a m p l e w a s d e s i r e d . The a q u a r i u m v a l v e was t h e n opened, the i n s i d e p r e s s u r e was r e l i e v e d , and w a t e r f l o w e d i n t o t h e tube t h r o u g h the b a l l v a l v e . T h e s e s a m p l e r s w e r e t e s t e d i n a q u a r i u m s c o n t a i n i n g s o f t c l a y b o t t o m s to d e t e r m i n e w h e t h e r s e d i m e n t s w e r e s u c k e d f r o m the b o t t o m . T h e s e t e s t s s h o w e d t h a t i n g e n e r a l no s e d i m e n t s w e r e s u c k e d f r o m the b o t t o m b y the s a m p l e r even t h o u g h the s a m p l e r was w i t h i n l e s s t h a n an i n c h f r o m the b o t t o m . T h e s e s a m p l e r s h a d the added advantage t h a t s e v e r a l of t h e m c o u l d be a t t a c h e d to a s i n g l e 1/2 i n c h pipe so t h a t i t was p o s s i b l e to take p o i n t w a t e r s a m p l e s o v e r the e n t i r e d e p t h s i m u l t a n e o u s l y . DISCUSSION A N D C O N C L U S I O N S T w o f a c t o r s s e e m to be p a r t i c u l a r l y p e r t i n e n t to the m o v e m e n t of s e d i m e n t s f r o m s h e l l d r e d g i n g o p e r a t i o n s . F i r s t , f o r a p p r e c i a b l e s e d i -m e n t a t i o n to o c c u r on e x p o s e d r e e f a r e a s , the r e e f -m u s t be c o v e r e d b y a l a y e r of s e d i m e n t s e v e r a l i n c h e s i n t h i c k n e s s . T h i s l a y e r m u s t be c o m p o s e d of f i n e s e d i m e n t c o n c e n t r a t i o n s g r e a t e r t h a n about 10 g r a m s p e r l i t e r so t h a t the s e t t l i n g w i t h i n the l a y e r i s h i n d e r e d . F i g u r e 7 shows a s u s p e n d e d s e d i m e n t c o n c e n t r a t i o n p r o f i l e f o r a d r e d g e w o r k i n g i n G a l v e s t o n B a y , T e x a s . T h i s p r o f i l e w a s t a k e n i n 10 f e e t of w a t e r and at a d i s t a n c e of 600 f e e t f r o m the d r e d g e . The p r o f i l e was o b t a i n e d u s i n g the w a t e r s a m p l e r of F i g u r e 6. The t h i c k n e s s of the m u d l a y e r was a b o u t 10 i n c h e s and c o n c e n t r a t i o n s w i t h i n the l a y e r w e r e i n e x c e s s o f 100,000 m g / 1 at 6 i n c h e s above the b o t t o m , and m o r e t h a n 160,000 m g / 1 at 3 i n c h e s above the b o t t o m . The c o n c e n t r a t i o n s of s e d i m e n t f o u n d w i t h i n the d e n s i t y c u r r e n t a r e o v e r the 10 g r a m s p e r l i t e r r e q u i r e d f o r h i n d e r e d s e t t l i n g . I t is p r i m a r i l y due to t h i s h i n d e r e d s e t t l i n g and the i n t e r -f e r e n c e o-f p a r t i c l e s o r -f l o e s w i t h one a n o t h e r t h a t p r e v e n t s the c o m p l e t e s e t t l e m e n t and a l l o w s the s e d i m e n t s to f l o w as d e n s i t y c u r r e n t s f o r r e l a -t i v e l y l o n g p e r i o d s of -t i m e . Such m u d l a y e r s g e n e r a l l y f o r m o n l y w h e n d r e d g e s o p e r a t e i n s h e l l c o v e r e d b y r e l a t i v e l y l a r g e o v e r b u r d e n o r i n l a y e r e d s h e l l d e p o s i t s s e p a r a t e d b y t w o o r t h r e e f e e t of m u d . F u r t h e r m o r e , i t a p p e a r s t h a t f o r the l a y e r to f o r m , the o v e r b u r d e n , o r m u d l a y e r s , m u s t c o n s i s t of m o r e t h a n about 80 p e r c e n t b y w e i g h t o f p a r t i c l e s s m a l l e r t h a n . 0625 m m , of w h i c h about 50 p e r c e n t b y w e i g h t o r m o r e s h o u l d be i n the c l a y r a n g e o r l e s s t h a n . 0039 m m . W h e n d r e d g e s o p e r a t e i n a r e a s i n w h i c h s e d i m e n t s c o n t a i n l a r g e q u a n t i t i e s of sand, d e p o s i t i o n of the sands o c c u r s v e r y

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r a p i d l y and s e t t l e out i n the i m m e d i a t e a r e a of the d r e d g e u n l e s s w a t e r c u r r e n t s and t u r b u l e n c e i n the a r e a a r e of s u f f i c i e n t m a g n i t u d e to k e e p the sand p a r t i c l e s suspended w h e r e t h e y c a n be m o v e d by c u r r e n t s .

A s e c o n d f a c t o r w h i c h a p p e a r s to be of s i g n i f i c a n c e to r e e f d e p o s i -t i o n is r e e f -t o p o g r a p h y . S h e l l r e e f s w h i c h c o n -t a i n a r i s e of a f o o -t o r m o r e above the bay b o t t o m s e e m to be l e s s s u s c e p t i b l e to d e p o s i t i o n t h a n t h o s e t h a t do n o t r i s e above the b o t t o m of the e m b a y m e n t . The r e a s o n f o r t h i s can be r e l a t e d to the f l o w of the s e d i m e n t as a d e n s i t y c u r r e n t . W h e n d e n -s i t y c u r r e n t -s e n c o u n t e r a b a r r i e r -s u c h a-s w o u l d be p r o v i d e d b y the r i -s e i n a r e e f , t h e y a r e d e f l e c t e d i n an u p w a r d d i r e c t i o n , the h e i g h t to w h i c h t h e y r i s e b e i n g d e t e r m i n e d by the d i f f e r e n c e i n d e n s i t y b e t w e e n the s e d i -m e n t l a y e r and the s u r r o u n d i n g w a t e r s . I n the p r e s e n c e of a r i s e i n the r e e f e i t h e r one of t w o t h i n g s a p p e a r s to h a p p e n . I f the r i s e of the d e n s i t y c u r r e n t is h i g h e r t h a n the top e l e v a t i o n of the r e e f the s e d i m e n t l a y e r is s w e p t o v e r the top of the r e e f b y the a c c e l e r a t e d f l o w . U s u a l l y s u c h an o c c u r r e n c e i s a c c o m p a n i e d by s e p a r a t i o n and t u r b u l e n c e so t h a t the f l o e s a r e b r o k e n u p and the s e d i m e n t is r c - s u s p e n d e d . T h i s c o m b i n a t i o n of t u r b u l e n c e and a c c e l e r a t e d f l o w keeps the s e d i m e n t f r o m d e p o s i t i n g on the r e e f . I f the d e n s i t y c u r r e n t does n o t r i s e above the r e e f i t p i l e s up and t h e n f l o w s b y g r a v i t y a l o n g the s i d e s of the r e e f and e v e n b a c k i n the d i r e c t i o n f r o m w h i c h i t c a m e . F i g u r e 8 shows the c o n c e n t r a t i o n d i s t r i -b u t i o n of s e d i m e n t f r o m f u n n e l t r a p s along the edge of a r e e f c o n t a i n i n g a one f o o t r i s e . W h e n r e e f s have no r i s e above the b o t t o m o f the bay, t h e y a r e v u l n e r a b l e to d e p o s i t i o n f r o m s e d i m e n t d e n s i t y c u r r e n t s . Since the r e e f s p r o v i d e no b a r r i e r the d e n s i t y c u r r e n t c a n f l o w o v e r the r e e f c o v e r i n g a r e a s of o v e r 50 a c r e s at a t i m e w i t h a m u d l a y e r 6 to 8 i n c h e s t h i c k . S e d i m e n t s w h i c h m o v e onto r e e f s i n t h i s m a n n e r can be w a s h e d o f f i n s o m e c a s e s . H o w e v e r , s u c h w a s h i n g of s e d i m e n t s on and o f f of r e e f s i s d a n g e r o u s , and i f a l l o w e d to go u n c h e c k e d w i l l g e n e r a l l y r e s u l t i n a c -t u a l d e p o s i -t i o n w i -t h i n a c o u p l e of days. E v e n -t h o u g h s e -t -t l i n g i s h i n d e r e d i n the d e n s i t y c u r r e n t , c o n s o l i d a t i o n of the f l o c c u l a t e d p a r t i c l e s t a k e s p l a c e and the l a y e r g a i n s s t r e n g t h . W i t h i n a c o u p l e of days the l a y e r u s u a l l y has s u f f i c i e n t s t r e n g t h to r e s i s t the s h e a r p r o d u c e d b y l o w m a g n i t u d e t i d a l c u r r e n t s and i s no l o n g e r capable of b e i n g m o v e d a r o u n d b y c u r r e n t s . C o n t i n u e d d r e d g i n g i n an a r e a w h e r e s u c h w a s h i n g o c c u r s c a n o n l y a g g r a -v a t e the p r o b l e m as m o r e s e d i m e n t s a r e f e d i n t o the d e n s i t y l a y e r . D r e d g i n g on e i t h e r an i n c o m i n g o r o u t g o i n g t i d e has b e e n p r o p o s e d as a m e t h o d to r e d u c e r e e f d e p o s i t i o n . H o w e v e r , f o r s u c h m e t h o d s to be e f f e c t i v e s e d i m e n t s m u s t be g o t t e n r i d of on the t i d e d u r i n g w h i c h the d r e d g i s i n o p e r a t i o n . I n the e v e n t a d e n s i t y c u r r e n t f o r m s and i s n o t c a r r i e d a w a y f r o m the a r e a to be p r o t e c t e d i t m a y w a s h b a c k onto f l a t r e e f a r e a s d u r i n g the r e v e r s e t i d e even t h o u g h the d r e d g e i s n o t i n o p e r a t i o n .

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cut, the h e a d i n g of the d r e d g e , the t y p e of w a s t e d i s c h a r g e , a m o n g o t h e r t h i n g s , a r e i m p o r t a n t to the u n d e r s t a n d i n g of s e d i m e n t m o v e m e n t f r o m d r e d g e s . I t s e e m s t h a t m o s t a l l e x a m p l e s of a c t u a l r e e f d e p o s i t i o n can be e x p l a i n e d i n t e r m s of the d e n s i t y c u r r e n t and the r e e f and s u r r o u n d i n g t o p o g r a p h y . The m a i n e f f e c t of d r e d g e h e a d i n g and d i r e c t i o n and l e n g t h of cut w o u l d be o n l y i n the l o c a t i o n of the d i s c h a r g e p o i n t . B e c a u s e of the s i z e of m a n y s h e l l d r e d g e s , the p o i n t of w a s h w a t e r d i s c h a r g e can v a r y up to a c o u p l e h u n d r e d f e e t . V e l o c i t y r e d u c t i o n boxes w o u l d be u s e -f u l o n l y -f o r s p e e d i n g u p the d e p o s i t i o n o-f sand and s h e l l p a r t i c l e s , and u n l e s s d e p o s i t e d r i g h t on a r e e f so as to b u r y e x p o s e d s h e l l , sand and s h e l l p a r t i c l e s a r e r e l a t i v e l y u n i m p o r t a n t to the m o v e m e n t of d r e d g e s e d i -m e n t s . I n s t e a d i t i s the f i n e s i l t and c l a y p a r t i c l e s t h a t a r e a f f e c t e d -m o r e b y e l e c t r o c h e m i c a l f o r c e s t h a n by w e i g h t f o r c e s , t h a t d e t e r m i n e the e x -t e n -t of -the m o v e m e n -t of d r e d g e s e d i m e n -t s . F i n a l l y , i t s h o u l d be n o t e d t h a t the d e p o s i t i o n of s o m e l o o s e c l a y s e d i m e n t s is n o t n e c e s s a r i l y d a m a g i n g to an e x p o s e d r e e f f r o m a p h y s i c a l p o i n t of v i e w . H o w e v e r , f r o m a b i o l o g i c a l b a s i s o t h e r p r o b l e m s m a y a r i s e d u r i n g the d e p o s i t i o n p e r i o d w h i c h a f f e c t the e n t i r e e c o l o g y o f the f o o d c h a i n and o t h e r n a t u r a l b i o l o g i c a l p r o c e s s e s and p r o d u c e d a m a g e to l i v e o y s t e r s . P h y s i c a l l y , h o w e v e r , those s e d i m e n t s w h i c h m a y be s e v e r a l i n -ches i n t h i c k n e s s a t the t i m e of d e p o s i t i o n c o n s o l i d a t e and c o m p a c t u n d e r n a t u r a l h y d r o - s t a t i c p r e s s u r e to a f r a c t i o n of t h e i r o r i g i n a l t h i c k n e s s . H e n c e , i n a s h o r t t i m e even t h o u g h the s e d i m e n t s do n o t w a s h o f f , t h e y m a y c o m p a c t to n e g l i g i b l e t h i c k n e s s and p r o d u c e no p h y s i c a l d a m a g e p r o v i d e d the d r e d g i n g o p e r a t i o n w h i c h p r o d u c e d the s e d i m e n t was s t o p p e d and no m o r e s e d i m e n t r e - s u s p e n d e d i n the i m m e d i a t e a r e a . R E F E R E N C E S G a l t s o f f , P. S. (1964). E n v i r o n m e n t a l f a c t o r s a f f e c t i n g o y s t e r p o p u l a t i o n s ; The A m e r i c a n o y s t e r , c h a p t e r X V I I I : F i s h e r y B u l l e t i n , v o l . 64, U . S. D e p t . of I n t e r i o r , F i s h and W i l d l i f e S e r v i c e , B u r e a u of C o m -m e r c i a l F i s h e r i e s , pp. 3 9 7 - 4 5 6 . I n g l e , R. M . (19 52). Studies of the e f f e c t of d r e d g i n g o p e r a t i o n s u p o n f i s h and s h e l l f i s h : T e c h . S e r i e s N o . 5, F l o r i d a State B o a r d of C o n s e r v a -t i o n , 26 pp. M a c k i n , J . G. ( I 9 6 I ) . C a n a l d r e d g i n g and s i l t i n g i n L o u i s i a n a B a y s : P u b l i -c a t i o n s of the I n s t i t u t e of M a r i n e S -c i e n -c e , v o l . 7 f o r 1 9 6 1 , p u b l i s h e d by the I n s t i t u t e of M a r i n e S c i e n c e , The U n i v e r s i t y of T e x a s , P o r t A r a n s a s , T e x a s , J u l y 1962, pp. 2 6 2 - 3 1 4 . M u r r a y , F . A . (1965). P e r s o n a l c o m m u n i c a t i o n . S h e p a r d , F . P . and M o o r e , D. G. ( I 9 6 0 ) . B a y s of c e n t r a l T e x a s c o a s t ; Recent s e d i m e n t s , n o r t h w e s t G u l f of M e x i c o , 1 9 5 1 - 1 9 5 8 : A m e r i c a n A s s o c i a t i o n of P e t r o l e u m G e o l o g i s t s , T u l s a , O k l a h o m a , pp. 1 1 7 - 1 5 2 .

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W i l s o n , W. B . (19 50). The e f f e c t s of s e d i m e n t a t i o n due to d r e d g i n g o p e r a t i o n s on o y s t e r s i n Copano B a y , T e x a s : T h e s i s s u b m i t t e d to the A and M C o l l e g e of T e x a s f o r the d e g r e e of M a s t e r of S c i e n c e , 1Z8 pp.