CHAPTER 24
BY-PASSING AND BACKPASSING {nTH SPECIAL REFERENCE TO CONDITIONS IN FLORIDA
P. M. Bruun
Department of Coastal Engineering University of Florida Gainesville, Florida
ABSTRACT
By-passing by natural action is mentioned ~~ith special reference to Florida Inlets and to some other inlets in the United States and abroad. Natural by-passing at harbors on open shores is dealt with briefly. Present status of by-passing plant operations in Florida is reviewed. Inasmuch as it is evident that by-passing plants - partly because of the tidal flow which discharges material in the ocean and in the bay and partly as a result of the rise of sea level - will not be able to solve more than a certain part of a beach erosion problem--replenishment by sand from other sources is indispensable. The most logical source is offshore deposits. Material may be brought to shore by "backpassing" using an offshore scraper (useful for maintenance) or by a special hydraulic dredge (for major improvements). If the borrO\~ area is located close to shore, the question arises of whether the borrO\~ pit
I~ill fill up again by material from further offshor~ material from the
sides or from material dragged out by ~~aves from the beach. The report describes briefly tests on Jupiter Island using an offshore scraper. The success of this operation is checked by fluorescent tracers placed on the beach and on all sides of the borrow pit.
NATURAL BY-PASSING AT COASTAL INLETS GENERAL
Natural by-passing at several inlets in and outside the United States is dealt I~ith by P. Bruun and F. Gerritsen (~) and (4). Most inlets by-pass material partly by tidal flow action and partly by transfer of sand from one side of the inlet to the other on a shoal or offshore bar. Figure 1 shows a normal bottom profile without an inlet channel. The profile carries net M3/ year longshore. Figure 2 demonstrates the changes which occur in a bottom profile when a breakthrough has taken place and
the drift mainly takes place on a bar across the inlet entrance. It will be of interest to consider the sand drift budget at the inlet. If the total amount of material carried to the inlet from all sides is Mt = Mtotal and p per cent is transferred by inlet flO\~,
(1 - p) Mt must be by-passed on a bar or shoal (Figure 3). The inlet currents carry bottom material for and back in the inlet. If an equilibrium condition develops inlet currents are able to push the "surplus material" ~~hich entered in the inlet from the sides out of the inlet channel for depositing offshore or in the bay or for further migration on the ocean bottom. The dimensionless parameter Mnet/Qmax seems to be of significance for the magnitude of by-passing. The value of this ratio indicates whether by-passing is a predominately "bar" or a predominately "tidal flow transfer." By the latter material is
NORMAL P R O F I L E l o ï t o m F i g u r e 1. L i t t o r a l D r i f t (Mm /year) i n a N o r m a l Bottom P r o f i l e (Bruun and G e r r i t s e n , 1961) P R O F I L E AT ENTRANCE BAR F i g u r e 2. Change of L i t t o r a l D r i f t P a t t e r n caused by B r e a k -through of an Inlet (Bruun and G e r r i t s e n , 1961)
BYPASSING 563
B A Y
F i g u r e 3. Coastal M e t w i t h a Predominant B a r By-Passing ( B r u u n and G e r r i t s e n , 1961)
B a y y / / I I - I / I X I I O CO
.1
JK" I N L E T C H A N N E L ^1
I
I ¬
i
1
1
Ocean \ ^ 1 F i g u r e 4. " R o l l i n g Carpet" of Bottom M a t e r i a l i n I n l e t C h a n n e lBYPASSING 565
f l u s h e d out o f the i n l e t by ebb c u r r e n t s c a r r y i n g t h e m a t e r i a l away £rom_ the i n l e t entrance t o the o f f s h o r e area and p o s s i b l y i n d o w n d r i f t d i r e c t i o " ^
Reference i s made t o Table 1 (Reference 4 ) . ^^hen Qj^g^^ i s expressed i n cub.yds/sec. and Mnet i n cub.yds/yr., a v a l u e of Mnet = r between 5 and
Qmax 900 has been found f o r t h e i n l e t s c o n s i d e r e d .
From p r a c t i c a l experience about by-passing, the f o l l o w i n g r u l e may be used as a g u i d e :
r < 10 - 20 i n d i c a t e s predominant t i d a l f l o w by-passing ( l i t t l e or no bar f o r m a t i o n )
r > 200 - 300 i n d i c a t e s predominate bar by-passing w i t h t y p i c a l bar or shoal f o r m a t i o n
That Mnet i s small compared t o Qmax does n o t n e c e s s a r i l y mean t h a t c o n d i t i o n s are i d e a l f o r t i d a l f l o w by-passing. A l a r g e Q^ax) and t h e * ^ f o r e a s m a l l e r Mnet, may s t i l l mean u n s a t i s f a c t o r y by-passing o f m a t e r i ^ - ' '
Qmax
i f t h e t i d a l f l o w i s n o t u t i l i z e d p r o p e r l y f o r f l u s h i n g o f m a t e r i a l t o d o w n d r i f t beaches. I n s t e a d o f being c a r r i e d i n a d o w n d r i f t d i r e c t i o n ^ t i ^ m a t e r i a l may be j e t t e d out i n deep water and s e t t l e t h e r e i n a s h o a l
( F i g u r e s 7 and 9 ) .
I n t h e i n l e t channel bottom m a t e r i a l i s moving i n b o t h d i r e c t i o n s by t h e f l o o d and ebb c u r r e n t s which i n case o f no f r e s h water flov? a n d ^ l a r g e t i d a l p r i s m may be r a t h e r s j m m e t r i c a l . I n the i n i t i a l stage o f development when the i n l e t channel may be s h o r t and t h e i n l e t cross s e c t i o n i s expanding the s i t u a t i o n i s as d e p i c t e d i n F i g u r e 4, s h o w i n g a l o n g i t u d i n a l s e c t o r i n the i n l e t .
The s h e e t - l a y e r (bed l o a d ) m o t i o n may be compared t o t h e m o t i o n of " r o l l i n g c a r p e t s , " l e n g t h s b (bay) and o ( o c e a n ) . By the movement of these c a r p e t s , p a r t o f these i s l o s t on sea shoals (MQ = Mocean) and another p a r t on bay shoals (Mb = Mbay)• I f no m a t e r i a l a t a l l i s t r a n s f e r r e d t o t h e i n l e t channel by l i t t o r a l d r i f t from b o t h s i d e s , C It - ^ channel w i l l g r a d u a l l y deepen and widen u n t i l i t becomes n o n s c o u r i n g -I f Mb p l u s Mo = V = the f l u s h i n g a b i l i t y o f the i n l e t equals t h e amoui^-*-of d r i f t t o t h e i n l e t from the s i d e s an e q u i l i b r i u m c o n d i t i o n e x i s t s a l t h o u g h i t i s n o t e v e r l a s t i n g because o f the c o n t i n u a t i o n o f d e p o s i t ^ a t b o t h ends o f the channel. I f t h e i n l e t channel grows v e r y l o n g , a s i t u a t i o n may develop by which the i n l e t c u r r e n t g r a d u a l l y weakens and the cross s e c t i o n area o f the i n l e t gorge decreases s i m u l t a n e o u s l y ' ^ because o f decreases i n t i d a l p r i s m . This may f i n a l l y r e s u l t i n l a c l c of a b i l i t y o f the i n l e t channel t o f l u s h i t s e l f adequately f o r a l l t h ^ m a t e r i a l brought t o the i n l e t from the longshore d r i f t . C o n s i d e r i n g
the a l l - o v e r s t a b i l i t y i t seems t h a t one i s faced w i t h t h e f o l l o w i n g t h r e e cases:
T A B L E 1
F l o w and L i t t o r a l D r i f t C h a r a c t e r i s t i c s (Reference 4)
i n cub yds per y r
(Order o f Magnitude)
( 3 )
Amelandse Gat, H o l l a n d 36,000 IQÓ
A v e i r o , P o r t u g a l 9,000l > i o 6 B i g Pass, F l a . 720 B r i e l s e Maas, H o l l a n d ( b e f o r e c l o s i n g ) 2,7 AO 10^ Brouwershavense Gat, 2,7 AO 10^ H o l l a n d 30,000 106 C a l c a s i e u Pass, La, 2,600 105 East Pass, F l a , 1,720 l o s Eyerlandse Gat, H o l l a n d 19,000 106 F i g u e i r a Da Foz, P o r t u g a l 1,100 >106 F o r t P i e r c e I n l e t , F l a . 3,700 1/4 106 G a s p a r i l l a Pass, F l a . 910 1/4 105
Grays Harbor, Oreg. 48,000 10^
H a r l i n g v l i e t , H o l l a n d 25,000 106 I n l e t o f T e x e l , H o l l a n d 97,000 10& I n l e t o f V l i e , H o l l a n d 94,000 106 Longboat Pass, F l a , 1,430 105 M i s s i o n Bay, C a l i f . 1,430 105 ( b e f o r e d r e d g i n g ) 1,130 105 O o s t e r s c h e l d e , H o l l a n d 100,000 106 Oregon I n l e t , N, C, 5,100l 3/4 106 Ponce de Leon I n l e t , F l a . 1,450 1/2 106 P o r t Aransas, Tex. 1,870 1/2 105 St. A u g u s t i n e I n l e t , F l a , 2,700 1/2 106 San F r a n c i s c o , C a l i f , 210,000 1/2 106 Scheveningen, H o l l a n d s l u i c e s 3/4 10^ Thorsminde, Denmark s l u i c e s 1/2 10^ Thyboron, Denmark 7,450 1/2 106 I f l i i t e Sands, Denmark s l u i c e s 1/2 10^ Westerschelde, H o l l a n d 115,000 1/2 10^ i n cuE^yds per s e c I n l e t ( S p r i n g t i d e C o n d i t i o n s ) ( 1 ) ( 2 )
BYPASSING 567
1. Short channel: V > p M t o t a l . This w i l l cause an u n s t a b l e c o n d i t i o n . The i n l e t i s V 7 i d e n i n g and p r o b a b l y l e n g t h e n i n g .
The s i t u a t i o n may develop towards a "non-scouring" channel as discussed i n a f o l l o w i n g paragraph on i n l e t s t a b i l i t y . 2. Medium channel l e n g t h : V = p M t o t a l . This w i l l r e s u l t i n a
s t a b l e channel as long as V = pM i s v a l i d ,
3. Long channel: V < p M t o t a l , This w i l l r e s u l t i n an u n s t a b l e c o n d i t i o n . The i n l e t i s s h o a l i n g because m a t e r i a l i s p o u r i n g i n t h e i n l e t channel from b o t h s i d e s and i n l e t c u r r e n t s a r e n o t a b l e t o f l u s h t h e m a t e r i a l o u t . I t may a l s o happen t h a t V increases t o a maximum c a p a c i t y , b u t i f V s t i l l i s l e s s
than p M t o t a l t h e channel w i l l a g a i n s t a r t d e c r e a s i n g i t s f l u s h i n g a b i l i t y a t t h e same time as a bar o r s h o a l may develop a t t h e ocean entrance o f t h e i n l e t . The a b i l i t y t o t r a n s f e r m a t e r i a l over t h e b a r may then i n c r e a s e u n t i l a s t a t i o n a r y c o n d i t i o n develops by which ( 1 - p ) M t o t a l i s t r a n s f e r r e d over t h e b a r , w h i l e p M t o t a l i s f l u s h e d o u t on b o t h s i d e s o f t h e i n l e t channel f o r d e p o s i t i n g on s h o a l s , or i t i s perhaps m a i n l y f l u s h e d o u t on one s i d e , t h a t i s , on the ocean s i d e i f t h e ebb c u r r e n t i s t h e s t r o n g e s t (which u s u a l l y i s t h e c a s e ) . I f p i s r e l a t i v e l y s m a l l and the t i d a l p r i s m i s l a r g e enough t o meet temporary i n c r e a s e s of p d u r i n g extreme storms an e q u i l i b r i u m c o n d i t i o n may r e s u l t which may p r e s e r v e t h e i n l e t as a t i d a l channel f o r c e n t u r i e s . See t h e s e c t i o n s below on " F l o r i d a I n l e t s " and on " T i d a l I n l e t s S t a b i l i t y C o n s i d e r a t i o n s . "
The q u a n t i t y o f l i t t o r a l m a t e r i a l p o u r i n g i n t o t h e i n l e t f r o m t h e a d j a c e n t shores depends upon many p a r t l y i n t e r r e l a t e d f a c t o r s i n c l u d i n g the longshore component o f t h e wave energy, t h e g e o m e t r i c a l shape o f t h e beach and bottom p r o f i l e , t h e shore l i n e geometry, and m a t e r i a l c h a r a c t e r -i s t -i c s . There -i s , however, another -i m p o r t a n t f a c t o r wh-ich -i s t h e
a v a i l a b i l i t y o f m a t e r i a l . I t i s known t h a t c o a s t a l p r o t e c t i o n s t r u c t u r e s , whether g r o i n s or c e r t a i n types o f sea w a l l s , slow do\m t h e q u a n t i t y o f l i t t o r a l d r i f t . I n l e t s may sometimes cause severe decreases o f t h e l i t t o r a l d r i f t f o r some d i s t a n c e o r f o r s e v e r a l m i l e s d o w n d r i f t . I f t h e l i t t o r a l d r i f t i s s t r o n g , and t h e t i d a l p r i s m i s l e s s , more b r e a k t h r o u g h s may occur and they may s t a y open f o r a l o n g e r p e r i o d o f t i m e .
The U n i t e d States East Coast i n c l u d e s an almost continuous b a r r i e r coast w i t h numerous i n l e t s some o f w h i c h have stayed open as l o n g as they have been known. Others have opened and c l o s e d c o n t i n u o u s l y . The tendency toward breakthroughs i s u s u a l l y i n c r e a s i n g i n d o ^ m d r i f t
( u s u a l l y s o u t h ) d i r e c t i o n , s i m p l y because the l i t t o r a l d r i f t decreases w i t h the number o f i n l e t s accompanied by sea and bay shoals upon w h i c h m a t e r i a l d e p o s i t s t e m p o r a r i l y or permanently. The N o r t h C a r o l i n a Shore, n o r t h o f Cape H a t t e r a s has a t p r e s e n t o n l y one i n l e t (The Oregon I n l e t ) , b u t o t h e r s have e x i s t e d . The n e t s o u t h l i t t o r a l d r i f t i s p r o b a b l y above 500,000 cub.yds/year. O c c a s i o n a l l y new i n l e t s have broken t h r o u g h and c l o s e d a g a i n v e r y s h o r t l y .
Oregon I n l e t i s d e p i c t e d on E n g l i s h maps from t h e 1 6 t h Cent^^^'^^^i b u t o t h e r h i s t o r i c a l sources i n d i c a t e t h a t t h e present i n l e t was °^(.];opi" by a seiche generated i n the Pamlico Sound d u r i n g t h e passage o f ^ ^^^ted c a l storm i n September 1846. I n the p e r i o d 1846-1952, t h i s i n l e t
1.5 t o 2 m i l e s southward,
100,00° Maintenance by dredging t h e l a s t 3 t o 4 years has been about _ cub.yds/year. The ocean bar channel has a u t h o r i z e d p r o j e c t dimens'-of 400 f t , w i d t h and 14 f t , depth, b u t shoals 8 t o 9 f t , depth occ^ E i g h t t o n i n e m i l e s s o u t h o f t h e present l o c a t i o n o f Oregon I n l e t , ^\ose another i n l e t was l o c a t e d , p o s s i b l y f o r c e n t u r i e s . I t d i d however ^ i n January 1922 and was re-opened i n 1924 as "New I n l e t , " b u t c l " ^ ^ ^ a g a i n i n t h e 1930's, I n 1962, t h e March 9 t o 1 1 t h storm opened iiP j ^ ^ t new i n l e t j u s t n o r t h o f Buxton ( V i l l a g e o f Cape H a t t e r a s ) . T h i s ''"'^ylce was c l o s e d by a h y d r a u l i c dredge t h e f o l l o w i n g year, A
withdrawn-would have p r o h i b i t e d t h a t k i n d o f c o s t l y a c c i d e n t .
West o f Cape H a t t e r a s , t h e l i t t o r a l d r i f t i s undoubtedly o£ ^ ^ o p e " l e s s magnitude. The f i r s t i n l e t i s H a t t e r a s I n l e t which was f o u n ' * ^ i n 1585 and has remained open s i n c e t h e n . The i n l e t m i g r a t e d a b ^ ^ 3,600 f t , southwest between 1852 and 1905 and has l a t e r been r a t l ^ ^ ^ ^ ^ ^ e a s t a b l e i n l o c a t i o n . W i t h i t s 50,000 sq. f t , gorge cross s e c t i o n ^ - ' - ^ ^ i : , the H a t t e r a s I n l e t has swallowed huge q u a n t i t i e s o f sand. No " ^ " ^ ^ t h e r e f o r e , t h a t t h e n e x t i s l a n d , t h e Ocracokee I s l a n d , has suffei^® v e r y severe e r o s i o n by which a l l dunes have been washed o u t i n tt"-®^ n o r t h e r n p a r t . The s i t u a t i o n a t t h e s o u t h e r n p a r t o f t h e O c r a c o l ^ ^ ^ ^ , I s l a n d i s s i m i l a r . Since 1830, t h e s p i t has extended about 8,0OO i n t h e s o u t h w e s t e r l y d i r e c t i o n i n t h e next i n l e t , t h e Ocracokee and huge q u a n t i t i e s o f m a t e r i a l have accumulated i n shoals t h e r e t " ^ d e p r i v i n g d o i m d r i f t beaches.
The shore from here on d.o\m t o Cape Lookout c o n s i s t s o f w a s g l i o r e b a r r i e r s and i n l e t s causing continuous d r a i n o f m a t e r i a l from tii-^
f o r d e p o s i t i n g i n s h o a l s . Many i n l e t s however have n o t been a b 3 - ^ e n t r a n c e s t a y open because o f overs'/helming l i t t o r a l t r a n s p o r t t o t h e i n l ^ ^ ' ^ - j ^ y
compared t o t h e a v a i l a b l e t i d a l p r i s m . The shore between Cape ^ * ^ ^ t and Cape Lookout (about 200 m i l e s ) today has o n l y 3 open i n l e t s ^ 10 t o 12 " f o s s i l " i n l e t s which have been open a t v a r i o u s t i m e s .
The C a l i f o r n i a shore has o n l y a few r a t h e r s h o r t b a r r i e r s ^ ^ b l e s s e d w i t h one o f t h e l a r g e s t t i d a l i n l e t s i n e x i s t e n c e . The
F r a n c i s c o Harbor has a t i d a l p r i s m o f 2,880 sq, m i l e s times f t . , ^ ^ - - j a x ) f l o w i n g through t h e Golden Gate (875,000 sq, f t , ) . The r ( M n e t /
v a l u e i s 5 ( T a b l e 1 ) , The l i t t o r a l d r i f t i s n o t v e r y p r e d o m i n a * ^ ^ érial b u t s t r o n g t i d a l c u r r e n t s and heavy wave a c t i o n t o g e t h e r w i t h va.^' -|_£-from t h e bay and the n o r t h shore a r e r e s p o n s i b l e f o r t h e huge h - ^ " 5 ) . moon shaped o f f s h o r e bar w i t h depths o f from 12 t o 18 f t , ( F i g u ^ * ^
Some l i t t o r a l d r i f t m a t e r i a l passes across t h e (now 50 f t . ) d e e ^ C ^ t h e n a v i g a t i o n channel n o u r i s h i n g t h e beaches s o u t h o f C l i f f House
San F r a n c i s c o s i d e . Studies o f t h e d i s t r i b u t i o n o f g r a i n s i z e s ^ Schatz of heavy m i n e r a l s i n t h e bar and i n l e t area as d e s c r i b e d by B y r
BYPASSING 569
F i g u r e 5. The Golden Gate and i t s Ocean Shoals (U. S. A r m y C of Engineers Annual Report)
and o t h e r s (15) demonstrated c l e a r l y t h a t g r a i n s i z e diameters r a t h e r than heavy m i n e r a l s may be h e l p f u l i n d e t e r m i n i n g the d i r e c t i o n o f sand t r a n s
-Going f o r a v i s i t abroad, a s m a l l but i n t e r e s t i n g case has been i n
?r^f'-?N
" Thorsminde (Thors I n l e t ) on the Danish N o r t h Sea Coast ( T a b l e 1 ) . F i g . 6 ( a ) and F i g . 6 ( b ) are c h a r t s o f the i n l e t , which has n a v i g a t i o n l o c k s as w e l l as l o c k s f o r g e n e r a l f l o w . T i d a l range i s a p p r o x i m a t e l y 1 f t , and wave a c t i o n o f t e n heavy w i t h up t o 12 t o 15 f t waves L i t t o r a l d r i f t i s e s t i m a t e d t o be o f the o r d e r 1/2 m i l l i o n t o "loT/ ^""^-y^^^y^- The i n l e t i s p r o t e c t e d by two j e t t i e s . U n t i l
1944 b o t h were 500 f t . l o n g (Reference 4 ) . The f o l l o w i n g years t h e n o r t h j e t t y was extended 200 f t , and d u r i n g the p e r i o d 1942 - 1947 two g r o i n s were b u i l t on the u p d r i f t s i d e o f the i n l e t . The g r o i n s s h o u l d c a t c h excessive amounts o f m a t e r i a l m i g r a t i n g toward the i n l e t and the j e t t y e x t e n s i o n should cause m a t e r i a l t o be by-passed f a r t h e r seaward. F i g . 6 ( a ) shows the s i t u a t i o n on June 23, 1941, when the i n l e t c o n d i t i o n s were p a r t i c u l a r l y bad, w i t h depths l e s s than 3 f t , between the j e t t i e s ( n o r m a l l y 6 f t , t o 7 f t . ) . I t can be seen t h a t t h e r e i s no bar i n f o ^ f " i ' ^ ' ' ^^S- ^ '^^^ '1^^ c o n d i t i o n s on November 7 t o 11,
•ZZ'., "'"^ ^^"^ ^^"^ time the i n l e t c o n d i t i o n s are good w i t h depth o f about 8 f t . between t h e j e t t i e s . The problem of s h o a l i n g which always takes p l a c e when the bar d i s a p p e a r s , may be e x p l a i n e d by
the d i f f e r e n t d i s t r i b u t i o n o f l i t t o r a l d r i f t i n a p r o f i l e x ^ i t h a bar and a p r o f i l e w i t h o u t a bar. At the f i r s t mentioned p r o f i l e , much sand by-passes on the bar where waves break. A t the o t h e r p r o f i l e most m a t e r i a l m i g r a t e s c l o s e t o the shore l i n e causing r a p i d s h o a l i n g as soon as i t meets an o b s t r u c t i o n , as f o r example, the i n l e t e n t r a n c e . D u r i n g and a f t e r World War I I the n o r t h j e t t y was extended about 130 f t , C40m). The r e l i e f was o f temporary n a t u r e o n l y and f u r t h e r improvements became necessary. I n 1958/1959 a 550 f t . (160m) j e t t y was e r e c t e d about 500 f t . (150m) on t h e u p d r i f t s i d e o f the i n l e t . Before t h i s improvement s t a r t e d , t h e depth on the bar v a r i e d from about 6 t o a p p r o x i m a t e l y 8 f t . The j e t t y , however, i s a p p a r e n t l y l o c a t e d too f a r from t h e entrance and so f a r the improvements a r e not s a t i s f a c t o r y because the bar c o n t i n u e s t o "creep around" the j e t t y ,
H / ^ ^ ^ ' " ^ l ^ ^ / i t " a t i o n e x i s t s a t another i n l e t p r o v i d e d w i t h s l u i c e s and 1,000 f t . l o n g entrance j e t t i e s and l o c a t e d about 50 m i l e s s o u t h of Thorsminde a t Hvide Sande (IThite Sands). Model experiments were conducted on t h i s i n s t a l l a t i o n i n t h e 1920's and was used f o r d e s i g n of t h e c o n f i g u r a t i o n o f t h e entrance area and the j e t t i e s . The depth on t h e entrance bar was u s u a l l y 10 t o 12 f t , , but l e s s a f t e r storms.
The e r e c t i o n of a sand t r a p j e t t y was s t a r t e d i n 1961, This 600m l o n g j e t t y was l o c a t e d 100m n o r t h o f t h e entrance. Before c o n s t r u c t i o n s t a r t e d the entrance channel had t o pass over an o f f s h o r e bar w i t h depths o f about 2.5m. The j e t t y pushed t h e bar somewhat seaward and upon c o m p l e t i o n o f the j e t t y i n 1963 depth was g r e a t e r than 5m over the bar w h i l e depth o f about 4m o c c u r r e d i n the entrance channel i n s i d e the bar. I n 1964 shoals had reformed w i t h depth down t o 3.1 - 3.3m
i n t h e entrance area o u t s i d e t h e sand t r a p j e t t y and t h e c o n d i t i o n was s i m i l a r i n 1965. The improvement t h e r e f o r e has n o t been t o o g r e a t vrhen compared t o 1961 b u t n a v i g a t i o n i s s t i l l g r e a t l y improved f o r W storms.
I t i s apparent from these two cases t h a t by-passing may occur s a t i s f a c t o r i l y and w i t h o u t g i v i n g r i s e t o l e e - s i d e e r o s i o n , on a bar o f a c e r t a i n depth and v;idth. However, i r r e g u l a r i t i e s i n the amount of l i t t o r a l d r i f t may momentarily r e s u l t i n s h o a l i n g of the r e s p e c t i v e i n l e t s t h a t must be c l e a r e d by f l u s h i n g by o p e r a t i o n of t h e i r s l u i c e gates and/or by d r e d g i n g .
FLORIDA INLETS
C o n s i d e r i n g F l o r i d a I n l e t s (Reference 2) i n f o r m a t i o n i s a v a i l a b l e from h y d r o g r a p h i c surveys and d r e d g i n g o p e r a t i o n s . Q u a n t i t i e s o f m a t e r i a l by-passed by n a t u r a l a c t i o n and q u a n t i t i e s o f m a t e r i a l which s e t t l e d d o ™ i n t h e i n l e t and i t s a d j o i n i n g entrance areas a r e l i s t e d i n Table 2. Most data a r e d e r i v e d from Corps of Engineers, J a c k s o n v i l l e D i s t r i c t , r e p o r t s . These data s h o u l d , needless t o say, s t i l l be c o n s i d e r e d as a p p r o x i m a t i v e .
T A B L E 2
Predominant D r i f t A and By-Passed D r i f t B at F l o r i d a A t l a n t i c I n l e t s , (cub. yds/yr. U. S. A r m y Corps of Engineers, Jacksonville D i s t r i c t )
I n l e t or Entrance Predominant D r i f t A By-Passed B (cub.yds/year) ( c u b . y d s / y e a r ) St. Mary's R i v e r 500,000 unknown S t , John's R i v e r 500,000 unknoxm St. A u g u s t i n e I n l e t 500,000 unkno™ Matanzas I n l e t 500,000 almost a l l Ponce De Leon I n l e t 500,000 350,000
Canaveral Harbor (no t i d a l f l o w ) 350,000 v e r y l i t t l e
S e b a s t i a n I n l e t 300,000 200,000
F t , P i e r c e I n l e t 250,000 150,000
St. L u c i e I n l e t 200 ,000-250,000 30,000
J u p i t e r I n l e t 200 ,000-250,000 150,000
Palm Beach I n l e t 200 ,000-225,000 v e r y l i t t l e
South Lake Worth I n l e t 150 ,000-200,000 40,000
perhaps 30,000 H i l l s b o r o I n l e t 100,000 40,000 perhaps 30,000 Everglades I n l e t 50,000 v e r y l i t t l e Bakers Haulover I n l e t 50,000 v e r y l i t t l e
Government Cut, Miami Beach 20,000 v e r y l i t t l e
South o f Cape Kennedy, which i s " t h e b i g r o b b e r " of m a t e r i a l f o r the lower East Coast, the number o f i n l e t s i n c r e a s e . The q u a n t i t y o f d r i f t decreases from a p p r o x i m a t e l y 250,000 cub,yds/yr, predominant s o u t h a t t h e F o r t P i e r c e I n l e t t o perhaps 10,000 t o 20,000 cub.yds/yr. a t Government Cut (Miami Beach),
BYPASSING 573
I t may be s a i d t h a t the number o f i n l e t s (and r i v e r s ) on t h e upper East Coast o f F l o r i d a w i t h i t s h e a v i e r d r i f t i s one per 40 m i l e s , w h i l e the number o f i n l e t s on t h e lower East Coast w i t h l e s s d r i f t i s one p e r 20 m i l e s ( i n c l u d i n g some i n l e t s which were c u t by man r e p l a c i n g e a r l i e r breakthroughs or i n l e t s c u t by n a t u r e ) .
On t h e lower Gulf Coast t h e predominant l i t t o r a l d r i f t i s v e r y l i m i t e d (about 50,000 cub.yds/yr.) thanks t o low wave energy i n p u t . I n f o u r lower G u l f Coast c o u n t i e s (about 150 m i l e s o f shore) t h e r e i s a t p r e s e n t one i n l e t per 10 t o 15 m i l e s .
The upper West Coast o f F l o r i d a has a few i n l e t s o n l y . The p r e -dominant d r i f t i s perhaps o f the order 150,000 cub.yds/yr., and t h e t i d a l prisms v a r y s t r o n g l y , thanks t o t h e d i u r n a l t i d e . I t may even be non-e x i s t i n g f o r a wnon-enon-ek or two. Such s i t u a t i o n lnon-eavnon-es t h non-e o p p o r t u n i t y f o r a few l a r g e r i n l e t s t o s t a y open w h i l e a l l s m a l l e r breakthroughs must c l o s e .
I n F l o r i d a , as elsewhere, numerous i n l e t s opened up as a r e s u l t of breakthroughs caused by h u r r i c a n e s or major storms. A r e c e n t example on t h e East Coast i s t h e i n l e t which the March 9 - 1 1 storm i n 1952 cut t h r o u g h the n o r t h e r n p a r t o f J u p i t e r I s l a n d (20 m i l e s n o r t h o f Palm Beach, F l o r i d a ) . I t was n o t t h e f i r s t time t h a t an i n l e t broke
through i n t h i s area b u t they a l l c l o s e d . The 1962 i n l e t c o n t i n u e d expanding thanks t o a r a t h e r l a r g e t i d a l p r i s m . Because o f t h e f a c t t h a t t h e i n l e t a t t h e same time robbed the a d j o i n i n g seashore f o r an i n c r e a s i n g amount o f beach sand, i t was decided t o c l o s e t h e i n l e t by a h y d r a u l i c dredge. The lower G u l f Coast b a r r i e r s i n F l o r i d a have as mentioned e a r l i e r many breakthroughs and t h e r e i s h a r d l y a p l a c e on t h e b a r r i e r s w h i c h has n o t experienced a breakthrough. Some i n l e t s r e c e i v e d t i d a l p r i s m enough t o s t a y open. Longboat Pass ( F i g u r e 7) l o c a t e d n o r t h o f Sarasota i s such example. According t o Table 1 i t s r - f a c t o r i s 70. I t was r e c e n t l y (1958) improved by a j e t t y on t h e n o r t h s i d e and w i l l undoubtedly s u r v i v e f o r a c o n s i d e r a b l e p e r i o d o f t i m e i n t h e f u t u r e . I t has some bay and sea shoals b u t t i d a l f l o w i s r a t h e r s t r o n g . Mean t i d a l range i s about 2 f t . The i n l e t by-passes l i t t l e m a t e r i a l and beaches on b o t h sides have s u f f e r e d . This i s p a r t i c u l a r l y t r u e f o r the s o u t h shore on Longboat Key.
The s i t u a t i o n i s d i f f e r e n t a t B i g Pass l o c a t e d f u r t h e r n o r t h on the b a r r i e r s a t C l e a r i r a t e r . The r - v a l u e a c c o r d i n g t o Table 1 i s 139 which means t h a t t h e Pass i s u n s t a b l e and must by-pass most m a t e r i a l
on a b a r or s h o a l . S h o a l i n g has a c c e l e r a t e d i n r e c e n t y e a r s , p a r t l y because o f bay developments causing l o s s o f t i d a l p r i s m and B i g Pass i s v i r t u a l l y c l o s i n g .
A few i n l e t s i n F l o r i d a should be o f f e r e d separate a t t e n t i o n f o r t e c h n i c a l and h i s t o r i c reasons.
An example o f a v e r y tough i n l e t which, r e g a r d l e s s o f t h e f a c t t h a t i t i s a t y p i c a l b a r by-passer, has been a b l e t o s t a y open f o r c e n t u r i e s i s t h e Matanzas I n l e t (Reference 4) a p p r o x i m a t e l y 15 m i l e s s o u t h o f S t . A u g u s t i n e on the F l o r i d a A t l a n t i c Shore ( F i g u r e 8) . The
i n l e t has a v e r y s u b s t a n t i a l ocean s h o a l a t i t s entrance w i t h 2 t o 6 f t . depth on the sea shoals. There i s , however, always a channel p e n e t r a t i n g t h r o u g h the s h o a l s . The depth i n t h e channel may v a r y from 4 t o 8 f t . , b u t i t i s deep enough t o a l l o w a v e s s e l o f the type used by the Spanish Navy o f the 16th and 17th c e n t u r y t o pass through t o the lagoon a t h i g h t i d e . This channel i s u s u a l l y a s i n g l e channel which because o f t h e southward d r i f t moves slov?ly from n o r t h t o south and t h e n , v;hen an extreme s o u t h e r n p o s i t i o n has p r e v a i l e d f o r some time and a severe storm u s u a l l y from the N.E, occurs, w i l l experience a new breakthrough i n the n o r t h e r n p a r t o f the ocean s h o a l . The new channel w i l l then take over t h e f l o w and t h e s o u t h e r n channel w i l l c l o s e . By t h i s process a l a r g e q u a n t i t y of m a t e r i a l w i l l be t r a n s f e r r e d a t one time s u b s t i t u t i n g f o r s e v e r a l years o f accumulation on the u p d r i f t s i d e o f the channel which g r a d u a l l y f o r c e d the channel d o w n d r i f t .
Ponce de Leon I n l e t , F i g u r e 9, i s l o c a t e d i n V o l u s i a County on t h e east coast of F l o r i d a , about 65 m i l e s south of S t . Augustine and 57 m i l e s n o r t h of Canaveral Harbor. Mean ocean t i d a l range i s about 4.1 f t . a t t h e Coast Guard S t a t i o n ; i n s i d e the i n l e t i s 2.3 f t ; mean range i s 2.7 f t . The i n l e t i s a n a t u r a l waterway c o n n e c t i n g the A t l a n t i c Ocean w i t h the H a l i f a x R i v e r and t h e I n d i a n R i v e r N o r t h . According t o h i s
-t o r i c a l accoun-ts -the i n l e -t has been used f o r n a v i g a -t i o n f o r more -t h a n 200 y e a r s . I t i s another example o f a grand s c a l e n a t u r a l bar by-passer. A fan-shaped sand bar l i e s across t h e ocean entrance. The main channel across the bar changes f r e q u e n t l y i n depth, w i d t h , p o s i t i o n , and a l i g n m e n t . I n September 1962 the i n l e t channel extended i n an e a s t e r l y d i r e c t i o n w i t h depths r a n g i n g from l e s s than 6 f t . across the bar t o 35 f t . i n the gorge between the l a n d p o i n t s . I n A p r i l 1950 the channel extended due east w i t h a c o n t r o l l i n g depth o f 4.5 f t . I n May 1949, the main channel extended n o r t h e a s t e r l y w i t h a c o n t r o l l i n g depth of about 4 f t .
The l i t t o r a l d r i f t i n t h e v i c i n i t y o f Ponce de Leon I n l e t i s p r e d o m i n a n t l y s o u t h e r l y ; n e t southward movement i s e s t i m a t e d t o be about 500,000 cub.yds a n n u a l l y . Gross annual d r i f t r a t e s are e s t i m a t e d to be about 600,000 cubic years s o u t h e r l y , 100,000 cub.yds. n o r t h e r l y . A v a i l a b l e r e c o r d s from 1936 t o 1962 show t h e r e has been e r o s i o n n o r t h
of t h e i n l e t and both e r o s i o n and a c c r e t i o n south o f the i n l e t . Much of t h e e r o s i o n i s c o n c e n t r a t e d near t h e i n l e t where the s h o r e l i n e on b o t h sides has receded. Estimated l i t t o r a l d r i f t d i s t r i b u t i o n f o r ex-i s t ex-i n g c o n d ex-i t ex-i o n s ex-i s summarex-ized ex-i n Table 3. The d ex-i s t r ex-i b u t ex-i o n ex-i s based on t h e e s t i m a t e d net s o u t h e r l y d r i f t r a t e . The maximum discharge Qmax i s about 1,450 cub.yds/sec. which as i n d i c a t e d i n Table 1 gives an r v a l u e o f 345. This i s i n d i c a t i v e of the f a c t t h a t the l a r g e h a l f
-moonshaped bar i n the ocean i n f r o n t o f t h e Ponce de Leon I n l e t t r a n s f e r the g r e a t e r p a r t of t h e longshore d r i f t .
T A B L E 3
L i t t o r a l D r i f t D i s t r i b u t i o n at the Ponce De Leon I n l e t Average Annual Volume - 1 , 0 0 0 cub. yds. (U. S. A r m y Corps of Engineers, Jacksonville D i s t r i c t )
D i s t r i b u t i o n s Condi'^^°" Net southv7ard l i t t o r a l d r i f t t o i n l e t
T r a n s f e r t o south beach By n a t u r a l processes By pumping
From i n i t i a l channel dredging From i n i t i a l b a s i n d r e d g i n g From i n l e t channel maintenance From i n t e r i o r channel maintenance From b a s i n maintenance
S u b t o t a l t o s o u t h beach Not t r a n s f e r r e d t o south beach
Retained i n i n l e t area L o s t o f f s h o r e S u b t o t a l , l o s t t o south beach T o t a l 500 350
The U. S. Army Corps o f Engineers i n 1963 recommended t h a t "^°p and Leon I n l e t , F l o r i d a , be Improved t o p r o v i d e : A channel 1 5 f t . "^^^^ thence
2 0 0 f t . wide from deep water i n t h e A t l a n t i c Ocean i n t o the i n l ^ s ' ' - '
1 2 by 2 0 0 f t . and 1 2 by 1 0 0 f t . t o I n d i a n River N o r t h ; 1 2 by l O O * '
southward t o t h e I n t r a c o a s t a l Wateriray; ocean j e t t i e s 4 , 2 0 0 f t . " ' ' e c t i v e l y . and 2 , 7 0 0 f t . l o n g on the n o r t h and south sides o f t h e i n l e t ^^^^^e^ctton The n o r t h j e t t y f o r t h e s o - c a l l e d '^7eir p r o j e c t " i n c l u d e s a w e H ^ i f t i n g
2 0 0 f t . long w i t h c r e s t a t e l e v a t i o n zero a t M.L.W. Southward-<d-^
beach m a t e r i a l would pass n a t u r a l l y over the w e i r and s e t t l e i n ^ g r o w a r d impoundment b a s i n i n s i d e t h e i n l e t ( l i k e the H i l l s b o r o u g h I n l e t ^
County, southeast c o a s t , F l o r i d a , which i s mentioned l a t e r ) . A ^ i y t o v e n t i o n a l p i p e l i n e dredge would excavate 5 0 0 , 0 0 0 cub.yds. i n i t l - ^ QQQ
c r e a t e the Impoundment b a s i n . The dredge would remove about 3 1 C> ' - [ M a t e r i a l cub.yds. a n n u a l l y i n r e - d r e d g i n g the b a s i n t o a depth o f 1 8 f t .
V 7 o u l d then be pumped t o the s o u t h beach about 2 , 0 0 0 f t . s o u t h o s o u t h j e t t y . This should p r e v e n t any l a r g e s c a l e e r o s i o n .
The F o r t P i e r c e I n l e t on t h e lower east coast o f F l o r i d a ( -y an 10) s h o u l d be mentioned here because i n t h i s case t i d a l c u r r e n t s p l - j_g i m p o r t a n t r o l e i n i t s n a t u r a l t r a n s f e r arrangement. Meanwhile, ^ r a t h e r u n l i k e l y t h a t sand would be t r a n s f e r r e d w i t h o u t t h e e x i s t e n c e o of t h e wide r o c k r e e f w i t h 1 0 f t . t o 1 2 f t . depths on the d o w n d r l f t s i
I n v e s t i g a t i o n conducted by t h e Coastal E n g i n e e r i n g L a b o r a t o r y of t h e U n i v e r s i t y o f F l o r i d a ^ ) a r e mentioned belov? because of i t s a t t e m p t t o e s t a b l i s h q u a n t i t a t i v e balance e q u a t i o n s . The s i t u a t i o n a t t h e I n l e t i s d e p i c t e d i n F i g . 11. N o t a t i o n s r e f e r t o q u a n t i t y per year as f o l l o w s :
N = Net amount o f l i t t o r a l d r i f t m a t e r i a l e n t e r i n g the area from the n o r t h .
S = Net amount o f l i t t o r a l d r i f t m a t e r i a l l e a v i n g t h e area f o r southward d r i f t .
s = Amount o f l i t t o r a l d r i f t m a t e r i a l d e p o s i t e d permanently i n area n o r t h o f n o r t h e r n j e t t y .
b = Amount o f l i t t o r a l d r i f t m a t e r i a l passing t h r o u g h and over t h e n o r t h e r n j e t t y i n t o t h e i n l e t .
c = Amount o f l i t t o r a l d r i f t m a t e r i a l sucked i n t o t h e i n l e t by f l o o d c u r r e n t s .
d = T o t a l amount o f l i t t o r a l d r i f t m a t e r i a l d e p o s i t e d i n t h e i n l e t channel and on t h e bay shoals ( d = d-j+d^+d-^). e = Amount o f l i t t o r a l d r i f t m a t e r i a l j e t t e d o u t i n t o t h e sea
by t h e i n l e t e b b - c u r r e n t s .
f = Amount o f l i t t o r a l d r i f t m a t e r i a l brought o u t by i n l e t e b b - c u r r e n t s and d e p o s i t e d i n deep water o u t s i d e t h e l i t t o r a l zone.
g = Net amount o f l i t t o r a l d r i f t m a t e r i a l i n t h e o f f s h o r e area south of t h e i n l e t which by-passed t h e i n l e t . h = Amount o f l i t t o r a l d r i f t m a t e r i a l passing t h r o u g h or
over the s o u t h e r n j e t t y i n t o t h e i n l e t .
k = Net amount o f m a t e r i a l eroded from the beach and near o f f s h o r e area s o u t h of t h e i n l e t from t h e s o u t h e r n j e t t y t o t h e p o i n t where normal l i t t o r a l d r i f t has been r e - e s t a b l i s h e d and l e e - s i d e e r o s i o n i s n o t e v i d e n t . The f o l l o v / i n g " m a t e r i a l - b a l a n c e e q u a t i o n s " can be w r i t t e n :
N = S (under the assumptions: s i m i l a r p r o f i l e s , m a t e r i a l and wave a c t i o n on both sides o f the i n l e t )
N = S = g + k N = a + d + f + g b + c + h = d + e This g i v e s :
BYPASSING 581 e = b + c + h - d f = k - a - d e - f = b + c + h + a - k ( t h e amount o f m a t e r i a l which i s d e l i v e r e d back t o t h e l i t t o r a l c o a s t a l zone by t h e e b b - c u r r e n t through t h e i n l e t ) .
The d i f f e r e n t q u a n t i t i e s p r e s e n t e d i n these equations a r e n o t w e l l known b u t based on e a r l i e r surveys i n c l u d i n g i n f o r m a t i o n by t h e U, S. Army Corps o f Engineers on dredging q u a n t i t i e s a t t h e F o r t P i e r c e I n l e t ,
the f o l l o w i n g data a r e c o n s i d e r e d f a i r l y r e l i a b l e :
N = S = 200,000 t o 250,000 cub.yds. ( = n e t southward d r i f t ) k = 100,000 cub.yds.
a = 20,000 cub.yds.
d = 40,000 cub.yds. (26,000 cub.yds. dredged per year and the r e s t e s t i m a t e d t o be d e p o s i t e d i n t h e bay s h o a l s )
g = 100,000 t o 150,000 cub.yds.
W i t h N = 200,000 cub.yds. and a = 20,000 cub.yds. one has: f = 100,000 - 20,000 - 40,000 = 40,000 cub.yds. e = ( b + c + h ) - 40,000
This means t h a t 40,000 cub.yds. a year are l o s t i n deep w a t e r ; and t h a t because o f t h e f a c t t h a t "e" must have a p o s i t i v e v a l u e g r e a t e r than " f " ( f i s o n l y a p a r t of e ) , t h e q u a n t i t y (b + c + h ) e n t e r i n g the i n l e t t h r o u g h the j e t t i e s and through t h e entrance i s a t l e a s t 80,000 cub.yds/yr., and p r o b a b l y more, because t h e amount o f m a t e r i a l p a s s i n g the extreme end o f t h e n o r t h j e t t y l o c a t e d a t about 18 f t . d e p t h must be l i m i t e d , and t h e b u l k o f t h e n e t 100,000 cub.yds. o f m a t e r i a l ( g ) which a p p a r e n t l y i s d e l i v e r e d back t o t h e l i t t o r a l zone s o u t h o f t h e
i n l e t must be t r a n s f e r r e d m a i n l y by i n l e t e b b - c u r r e n t s . W i t h a q u a n t i t y of 40,000 cub.yds. t o be d e p o s i t e d i n the i n l e t and on bay s h o a l s , t h i s means t h a t t h e amount o f sand passing through and over t h e n o r t h j e t t y must be v e r y h i g h , p r o b a b l y about 140,000 t o 180,000 cub.yds. p e r year.
The d i s t r i b u t i o n o f l i t t o r a l d r i f t along t h e beach and o f f s h o r e bottom p r o f i l e n o r t h o f t h e i n l e t i s n o t known b u t t h e o f f s h o r e p r o f i l e s are g e n t l y s l o p i n g . I t i s t h e r e f o r e assumed t h a t a t l e a s t 80 p e r cent of t h e l i t t o r a l d r i f t or 160,000 cub.yds. ( w i t h 200,000 cub.yds. t o t a l d r i f t ) m i g r a t e w i t h i n t h e 18 f t . depth contour which i s r o u g h l y l o c a t e d at t h e extreme end o f the n o r t h j e t t y . W i t h o n l y 20,000 cub.yds per year d e p o s i t e d n o r t h o f t h e i n l e t , 140,000 cub.yds. pass t h r o u g h or over the n o r t h j e t t y . The amount o f m a t e r i a l passing t h e s o u t h j e t t y must be
much s m a l l e r than t h e amount passing the n o r t h j e t t y because o£ l e s s wave a c t i o n and lower storm t i d e s from the southeast quadrant. The south^ j e t t y seems a l s o t o l e a k l e s s m a t e r i a l than t h e n o r t h e r n one. Assuming t h a t 20,000 cub.yds. pass t h r o u g h the s o u t h j e t t y , t h e t o t a l amount o f m a t e r i a l p a s s i n g t h r o u g h t h e j e t t i e s i n t o t h e i n l e t i s 160,000 cub.yds.
Out of these 160,000 cub.yds. p l u s t h e q u a n t i t y "c" which was sucked i n t h r o u g h the e n t r a n c e , 40,000 cub.yds. i s d e p o s i t e d i n t h e i n l e t and on t h e bay shoals w h i l e t h e balance o f the m a t e r i a l amounting t o 120,000 + c = e cub.yds. i s d e l i v e r e d back t o t h e ocean. I f " c " ( t h e t o t a l amount o f l i t t o r a l d r i f t o u t s i d e the 18 f t . depth c o n t o u r ) e q u a l s 40,000 cub.yds. a t o t a l of 160,000 cub.yds. i s d e l i v e r e d back t o the ocean where 40,000 cub.yds. are l o s t t o deep w a t e r , w h i l e 100,000 cub.yds. d r i f t southward and 20,000 cub.yds. ("h") i s c a r r i e d back i n t o t h e i n l e t t h r o u g h the south j e t t y .
Under t h e assumption o f a n e t southward l i t t o r a l d r i f t o f 250,000 cub.yds. a new s e t o f values i s o b t a i n e d as g i v e n i n Table 4.
T A B L E 4 L i t t o r a l D r i f t Quantities at E o r t P i e r c e M e t N o t a t i o n Q u a n t i t y Q u a n t i t y (cub.yds.) ( c u b . y d s . ) N 200,000 250, 0 0 0 a 20,000 20, 0 0 0 (b + h) 160,000 200, OOO d e c £ ^ „ , „ „ „ g 100,000 150, 0 0 0 h 20,000 20, 0 0 0 k 100,000 100, OOO 40,000 40, 0 0 0 120,000 + c 160,000 + c 40,000 50, 0 0 0 40,000 40, 0 0 0
Table 4 c l e a r l y i n d i c a t e s the importance of t h e t r a n s p o r t °f m a t e r i a t h r o u g h the j e t t i e s and t h e l o s s of m a t e r i a l t o deep water by e 6 6 j e t s . The p e c u l i a r shape o f t h e o f f s h o r e bottom p r o f i l e w i t h an a l m o s t h o r i -z o n t a l p l a t f o r m a t 10 t o 12 f t . depth i s p r o b a b l y r e s p o n s i b l e £°'^ '-"^ f a c t t h a t t h e i n l e t , t o a c o n s i d e r a b l e e x t e n t , vrorks as a "nat; ""Ji^al san t r a n s f e r p l a n t . " On t h e d o w n d r i f t s i d e o f t h e i n l e t e n t r a n c e , m a t e r i a a p p a r e n t l y i s pushed ashore by wave a c t i o n on t h e r o c k r e e f pX a . t f orm,
w h i c h i s c l o s e r t o t h e s h o r e l i n e f a r t h e r south. I f t h e botton-i T^^^ had been steeper on t h e d o w n d r i f t s i d e more m a t e r i a l would h a v ^ ^ been os t o deep sea and l a r g e shoals may have developed southeast o f l^e m l e . Such shoals V7ere n o t r e v e a l e d by t h i s or o t h e r surveys. The 3_ i t t o r a d r i f t m a t e r i a l d e l i v e r e d back t o t h e o f f s h o r e l i t t o r a l zone o n - t h e p l a t f o r m s t a b i l i z e s same and decreases d e s t r u c t i v e wave a c t i o n - > ^^^here y o f f e r i n g some p r o t e c t i o n t o t h e beach area. I n r e g a r d t o t h i s t r a n s f e r r i n g - m a t e r i a l a c t i o n , " i t must be c o n s i d e r e d most f o r t u n a t e ^ t h a t t e
s o u t h j e t t y i s r a t h e r s h o r t w i t h the extreme end l o c a t e d i n o n l y 10 f t . o f w a t e r . W i t h somewhat g r e a t e r depth such a c t i o n would n o t be v e r y l i k e l y . A c u r v e d j e t t y entrance would have a s i m i l a r e f f e c t , b u t i s n o t always p r a c t i c a l .
The e r o s i o n s o u t h o f t h e i n l e t i s s t i l l severe. A f e d e r a l p r o j e c t has recommended a l a r g e s c a l e a r t i f i c i a l nourishment program w i t h m a t e r i a l from t h e bay bottom.
BY-PASSING AT HARBORS ON OPEN SHORES
G e n e r a l l y i t may be s a i d t h a t by-passing a t harbors l o c a t e d on open shores w i l l h a r d l y ever take o l a c e i n a way which i s agreeable w i t h t h e r e q u i r e m e n t s by n a v i g a t i o n .
( 3 ) and ( 4 )
An example may be found on the N o r t h Sea (Skagerrak) c o a s t o f Denmark a t H i r t s h a l s Harbor, F i g . 12 ( 1 9 4 7 ) . A heavy l i t t o r a l d r i f t , perhaps 500,000 cub.yds. t o 1,000,000 cub.yds/yr., comes from t h e west b y a s t r o n g wave a c t i o n . P a r t o f t h e d r i f t i n g sand i s d e p o s i t e d i n " t o n g u e s " a l o n g t h e u p d r i f t j e t t y , w h i l e a g r e a t p a r t passes the extended u p d r i f t j e t t y and d e p o s i t s , by a l a r g e c l o c k w i s e eddy c u r r e n t , i n a l a r g e s h o a l on t h e d o x r a d r i f t s i d e . This shoal i s g r a d u a l l y growing l a r g e r by d e p o s i t s r a n g i n g between 50,000 cub.yds. and 200,000 cub.yds/yr. Maintenance d r e d g i n g i s necessary i n the 7m t o 8 m (25 f t . ) deep e n t r a n c e channel t o the h a r b o r . The development i n r e c e n t years shows decreasing depths on the dovzndrift shoal (10 f t . t o 13 f t . ) . A t the same t i m e t h e s h o a l has extended f a r t h e r d o x r a d r i f t w i t h the r e s u l t t h a t the l e e s i d e shore i s now b e i n g n o u r i s h e d from the shoal due t o s w e l l a c t i o n , which a p p a r e n t l y b r i n g s the c o a r s e s t sand m a t e r i a l back t o t h e shore. Accumulations h o v 7
-ever c o n t i n u e on the u p d r i f t s i d e where depth contours move seaward and the p o r t w i l l p r o b a b l y experience an i n c r e a s i n g maintenance d r e d g i n g .
Another i n t e r e s t i n g example o f by-passing sand by n a t u r a l a c t i o n a t a harbor i s found a t the h a r b o r o f La Guaira i n Venezuela. This h a r b o r has the head o f i t s n a i l - s h a p e d u p d r i f t j e t t y l o c a t e d a t 18m (60 f t . ) depth. There i s c o n s i d e r a b l e l i t t o r a l d r i f t from t h e east t o the west caused by heavy wave a c t i o n (waves up t o 20 f t . from n o r t h e a s t ) . Some years ago a tanker r a n aground midway out on the u p d r i f t j e t t y a t 30 f t . t o 40 f t . depth and accumulated, i n a s h o r t t i m e , a g r e a t amount of sand b e h i n d i t , d e m o n s t r a t i n g the e x i s t e n c e o f a heavy d r i f t . Mean-w h i l e , t h e r e has been no a c c u m u l a t i o n a t the end o f the j e t t y and i t i s b e l i e v e d t h a t the g r e a t depth and o f f s h o r e bottom steepness may be r e s p o n s i b l e f o r t h i s . Reference i s , i n t h i s r e s p e c t , made t o C o r n a g l i a ' s t h e o r y ( I t a l y , about 1900). Based on the experience w i t h e r o s i o n o f steep shores and g e n t l y s l o p i n g shores, C o r n a g l i a c l a i m e d t h a t a n e u t r a l l i n e o r depth e x i s t s f o r any c o n d i t i o n o f wave a c t i o n . O u t s i d e t h e n e u t r a l l i n e , d r i f t moves seav/ard, i n s i d e i t moves shoreward. Some l a b o r a t o r y experiments ( e . g . a t the MIT, Eagleson, 1961) have i n d i c a t e d c e r t a i n agreements w i t h C o r n a g l i a ' s t h e o r i e s , which are i n f a c t a l s o i n agreement w i t h f i e l d experience from r e l a t i v e l y steep shores a t deep w a t e r c o a s t s .
The normal case however i s t h a t m a t e r i a l w h i c h t r i e s t o by-pass a J e t t i e d harbor entrance may come t o r e s t i n the (dredged) e n t r a n c e channel ( P o r t o f Palm Beach, F l o r i d a , P o r t Everglades, F l o r i d a and Government Cut, Miami, F l o r i d a ) , or j u s t i n s i d e a s i n g l e s t r a i g h t or curved j e t t y on a " s t a n d a r d s h o a l " (Santa Barbara, C a l i f o r n i a ) .
Numerous model experiments have, however, been c a r r i e d o u t t h r o u g h the years a t t e m p t i n g t o g i v e harbor e n t r a n c e s , whether f l u s h e d by t i d a l c u r r e n t s o r n o t , such g e o m e t r i c a l slope t h a t a t l e a s t p a r t o f t h e m a t e r i a l may by-pass the harbor by n a t u r e ' s o ™ f o r c e s . Bruun and G e r r i t s e n C ^ ) mention s e v e r a l examples o f t h a t n a t u r e . One o f the most r e c e n t i s the im-provement o f the Ymuiden entrance i n H o l l a n d (Amsterdam s h i p c a n a l ) .
The Zeebrugge Harbor i n Belgium ( F i g . 13) s h o u l d be mentioned i n t h i s c o n n e c t i o n . This h a r b o r , p r o t e c t e d by a 4,500 f t . long n a i l - s h a p e d j e t t y was, f o r a long t i m e , g r e a t l y bothered by s i l t d e p o s i t s amounting t o approx m a t e l y 5,000,000 cub.yd/yr. The t i d a l range i s a p p r o x i m a t e l y 12 f t . and
the t i d a l c u r r e n t s o u t s i d e the harbor up t o 5 fps t o 6 f p s . F o r some t i m e the harbor was equipped w i t h a 1,300 f t . opening ( c l a i r e - v o i e ) per-m i t t i n g t i d a l c u r r e n t s t o f l o w through the harbor b a s i n . This was u n s a t i s f a c t o r y . Heavy d e p o s i t s , m a i n l y s i l t , c o n t i n u e d and d r e d g i n g of t h e d e p o s i t s endangered the economy o f the h a r b o r .
I n order t o improve t h i s s i t u a t i o n model experiments x^ere conducted a f t e r World War I I i n Belgium (Waterbouwkundig L a b o r a t o r i u m ) and i n H o l l a n d (Waterloopkundig L a b o r a t o r i u m ) . F i g . 13 shows t h e g e n e r a l c u r r e n t p a t t e r n w i t h a s t r o n g f l o o d c u r r e n t . The r e s u l t o f t h e c o n s t r u c t i o n o f the b i g c i r c u l a r j e t t y on t h e s h o r e - s i d e was the e l i m i n a t i o n o f a l a r g e s i l t - d e p o s i t i n g eddy c u r r e n t i n the h a r b o r b a s i n . The amount o f s i l t d e p o s i t s was reduced t o l e s s than 50%. The remainder o f the m a t e r i a l ( m a i n l y s i l t ) by-passes the harbor w i t h the t i d a l c u r r e n t s .
SAND BY-PASSING PLANTS I N FLORIDA
F l o r i d a has o n l y two by-passing p l a n t s ; namely, the p l a n t s a t t h e South Lake Worth I n l e t and a t the Lake Worth (Palm Beach) I n l e t .
SOUTH LAKE WORTH INLET ( F i g . 14)
The f i x e d d r e d g i n g i n s t a l l a t i o n a t the South Lake Worth I n l e t i s l o c a t e d on the seaward end o f the N o r t h j e t t y , or about 250 f t . eastward of t h e M.S.L. shore l i n e n o r t h o f the i n l e t . I t was i n s t a l l e d i n 1929, as a means o f i n t e r c e p t i n g the southward l i t t o r a l d r i f t and b y - p a s s i n g the m a t e r i a l across t h e i n l e t d e p o s i t i n g i t on the shore l i n e s o u t h o f the South j e t t y . The o p e r a t i o n was p r i m a r i l y i n t e n d e d t o s u p p l y
s u f f i c i e n t m a t e r i a l t o n o u r i s h the h e a v i l y e r o d i n g shore south o f t h e i n l e t and s e c o n d a r i l y , t o reduce s h o a l i n g a t b o t h ends o f the n a v i g a t i o n channel where the f l o o d and ebb t i d e v e l o c i t y was reduced t o a p o i n t where d e p o s i t i o n took p l a c e . The i n l e t , i t s e l f , was c o n s t r u c t e d i n 1927
t o p r o v i d e exchange o f bay waters and sea water f o r t h e south end o f the Lake Worth and t o g i v e access t o the ocean f o r f i s h i n g boats and p l e a s u r e c r a f t s . The channel i s a p p r o x i m a t e l y 125 f t . wide, and 600 f t . l o n g . I t accommodates c r a f t s drawing up t o 6 o r 8 f t . The t o p
BYPASSING 587
O 500
F i g u r e 13. Zeebrugge Harbor, B e l g i u mBYPASSING §89
e l e v a t i o n o f t h e s t e e l sheet p i l i n g and c o n c r e t e j e t t i e s i s about 12 f t . above mean low w a t e r .
The sand t r a n s f e r f a c i l i t y was i n i t i a l l y c o n s t r u c t e d i n 1929. I n 1937, t h e p l a n t V7as r e c o n s t r u c t e d , and a t t h i s time i t was p r o v i d e d w i t h an 8 i n c h s u c t i o n , 6 i n c h discharge, and 65 horsepower d i e s e l - d r i v e n c e n t r i f u g a l pump, and a p p r o x i m a t e l y 1,200 f t . o f 6-inch d i s c h a r g e l i n e . The d i s c h a r g e l i n e i s c a r r i e d across t h e i n l e t on a c o n c r e t e b r i d g e (highway A I A ) . The pump was operated c o n t i n u o u s l y on an as-needed b a s i s from 1937 u n t i l 1942, a t v/hich time t h e pumping was ceased due t o a f u e l shortage d u r i n g World War I I . A t t h e end o f World War I I ( 1 9 4 5 ) , pumping V7as resumed, and i n 1948, t h e pump was r e c o n s t r u c t e d t o a l a r g e r s i z e and p r o v i d e d w i t h a 10 i n c h s u c t i o n , 8 i n c h d i s c h a r g e , and an a p p r o x i m a t e l y 300 horsepov7er d i e s e l engine, and 700 t o 750 f t . o f mechanical j o i n t c a s t - i r o n d i s c h a r g e l i n e . The power p l a n t has been r e p l a c e d once s i n c e 1938 (1955) and pump p a r t s have been r e p l a c e d on a p r e v e n t a t i v e maintenance b a s i s . The d i s c h a r g e p i p e l i n e on t h i s p l a n t i s r o t a t e d a p p r o x i m a t e l y on a two-year i n t e r v a l and i s g e n e r a l l y r e p l a c e d e n t i r e l y a f t e r t h r e e r o t a t i o n s , or s i x years. The p l a n t has proved t o be a dependable a s s e t i n t r a n s f e r o p e r a t i o n s , n o t w i t h s t a n d i n g i t s " u g l y d u c k l i n g " l o o k .
T A B L E 5
South Lalie W o r t h Inlet By-Passed Quantities, 1960-1965 P e r i o d Cub.Yds. D u r i n g P e r i o d A c c u m u l a t i v e Cub. Yds. Oct. 1960 t h r u Sept. 1961 31,737 31,737 Oct. 1961 t h r u Sept. 1962 45,339 77,076 Oct. 1962 t h r u Sept. 1963 88,366 165,442 Oct. 1963 t h r u Sept. 1964 70,300 235,742 Oct. 1964 t h r u March 1965 20,520 256 , 26 2
The o p e r a t i o n a l costs d u r i n g t h e 1960 t o 1965 p e r i o d has been about $25,000 per year. The p r o d u c t i o n r e c o r d d u r i n g t h i s p e r i o d i s s e t f o r t h
BYPASSING
on Table 5. An e s t i m a t e of r e c o n s t r u c t i o n of t h i s p l a n t t o today's s i z e and a t today's c o n s t r u c t i o n c o s t s , approximates a t l e a s t $100,000.00. The o p e r a t i o n a l u n i t c o s t f o r t h e c u r r e n t expendi tures f o r t h e s e l e c t e d p e r i o d (1960-1965) i s $.41 per cub.yd. I f the p l a n t i s a m o r t i z e d over a 40-year p e r i o d , u s i n g c u r r e n t c o n s t r u c t i o n c o s t s , f i v e cents must be added t o the c u r r e n t o p e r a t i o n a l f i g u r e s b r i n g i n g t h e approximate t o t a l u n i t c o s t t o $.46 per cub.yd. This amount compares v e r y f a v o r a b l y t o the c o n t r a c t c o s t o f doing dredging work i n approximate 100,000 cub.yds. q u a n t i t i e s i n t i d a l i n l e t w a t e r s . The o p e r a t i o n must be s a i d t o have been s u c c e s s f u l . Shore l i n e recessions f o r a 10,000 f t . s e c t i o n south and n o r t h of t h e i n l e t f o r t h e p e r i o d 1929 t o 1955 a r e i n d i c a t e d i n Tables 6 and 7. The average r e c e s s i o n was 58 f t . south of t h e i n l e t and 33 f t . n o r t h of t h e i n l e t . These values (2-3 f t / y r ) correspond c l o s e l y to the shore l i n e r e c e s s i o n c a l c u l a t e d by Bruun as a r e s u l t o f t h e sea l e v e l r i s e of about 1/4 i n c h per year d u r i n g the p e r i o d 1930-1950, ( 5 ) and ( 8 ) . P a r t l y because a recommended enlargement of the p l a n t r e c e n t l y gave r i s e t o c o n s i d e r a b l e c o n t r o v e r s y between the m u n c i p a l i t i e s on b o t h sides o f the i n l e t and p a r t l y because of i t s age and the p i o n e e r v7ork done on the development and p r o p e r f u n c t i o n o f i t , t h e m a t e r i a l balance a t the i n l e t i n c l u d i n g the by-passing, which takes p l a c e p a r t l y by the pumping p l a n t and p a r t l y by h y d r a u l i c dredge from the bay shoals i s mentioned i n d e t a i l belov7.
Movements o f shore l i n e and depth contours on b o t h s i d e s o f t h e i n l e t Table 6 shows shore l i n e movements i n v a r i o u s s e c t i o n s t o t a l i n g 10,000 f t . of t h e shore n o r t h o f the i n l e t f o r the 1929-1955 p e r i o d . A s i m i l a r comprehensive survey has not been made s i n c e 1955.
I t may be seen t h a t shore l i n e has moved seaward f o r a s h o r t d i s t a n c e (1,250 f t . ) n o r t h o f the i n l e t . Next f o l l o w s a n e u t r a l area and then r e c e s s i o n . The 6 f t . contour shows t h e same p a t t e r n e n l a r g e d . A c c u m u l a t i o n on the u p d r i f t s i d e o f a l i t t o r a l b a r r i e r w i l l almost always cause a l o c a l shore l i n e r e c e s s i o n on the u p d r i f t s i d e beyond a d i s t a n c e o f 4 t o 6 times the b a r r i e r l e n g t h .
Table 7 shows s i m i l a r f i g u r e s f o r a 10,000 f t . s e c t i o n s o u t h
of t h e i n l e t . J u s t s o u t h of t h e i n l e t the m a t e r i a l by-passed has caused seaward movement o f t h e shore l i n e , b u t shore l i n e r e c e s s i o n has t a k e n p l a c e s o u t h of here f o r the major p a r t of t h e 10,000 f t . s e c t i o n . The r e c e s s i o n has t h e same order o f magnitude as n o r t h of t h e i n l e t . The movement o f t h e 6 f t . contour shows t h e f o r m a t i o n o f a p l a t e a u ( s h o a l ) s o u t h o f t h e i n l e t (about 1,200 f t . ) . For the r e m a i n i n g p a r t , t h e 6 f t . contour has receded i n some exaggerated s c a l e compared t o t h e shore l i n e . This i s a q u i t e normal development.
The f i g u r e s o f Tables 6 and 7 r e v e a l t h a t the d i r e c t i n f l u e n c e of t h e i n l e t has m a i n l y been l o c a l . As a l r e a d y mentioned above, t h e average shore l i n e r e c e s s i o n n o r t h o f t h e i n l e t was o n l y 33 f t ; s o u t h of t h e i n l e t 58 f t . i n 1929-1955. The s i t u a t i o n may have changed i n the d i s f a v o r o f t h e n o r t h beaches i n r e c e n t y e a r s , but e r o s i o n has a c c e l e r a t e d on a l l shores on the East Coast s i n c e 1960, u n d o u b t e d l y as a r e s u l t o f t h e r i s e o f sea l e v e l b e f o r e 1960 ( 5 ) .
T A B L E 6
Shore Line Movements North of the Inlet, 1929-1955 (Corps of Engineers, 1956) D i s t a n c e from N o r t h J e t t y 0 - 1,250 1,250 - 2,500 2,500 - 5,000 5,000 - 10,000 f t . Changes +62 0 -41 -62 f t .
Note: - means moving shoreward + means moving seaward
T A B L E 7
Shore L i n e Movements South of the Inlet, 1929-1955 (Corps of Engineers, 1956) D i s t a n c e from South J e t t y 0 - 1,300 1,300 - 10,000 f t . Changes f t . +105 -83 An i n d i r e c t i n f l u e n c e o f the i n l e t i s t h e l e e - s i d e e r o s i o n w h i c h occurs when n o r t h w a r d d r i f t f o r a c e r t a i n p e r i o d o f time p r e v a i l s and m a t e r i a l i s t r a n s f e r r e d across t h e i n l e t i n q u a n t i t i e s which may be
i n excess o f what i t s h o u l d be d u r i n g the abnormal c o n d i t i o n s . M a t e r i a l balance a t the i n l e t - Outside t h e 1,000 f t . shore on b o t h s i d e s o f t h e i n l e t .
W i t h r e f e r e n c e t o F i g . 15, t h e e q u i l i b r i u m c o n d i t i o n f o r t h e sand budget a t t h e South Lake Worth I n l e t i s e s t a b l i s h e d u s i n g t h e f o l l o v ? i n g t e r m i n o l o g i e s :
A = southward d r i f t i n cub.yds/year B = n o r t h w a r d d r i f t i n cub.yds/year
A = a = the r a t i o betv7een southward and n o r t h w a r d d r i f t , B
BYPASSING 593
g - balance o f m a t e r i a l which i s by-passing the i n l e t by n a t u r a l a c t i o n south and n o r t h i n cub.yds/year
t x = m a t e r i a l by-passed by dredging from bay shoals (cub. yds/year)
t 2 = m a t e r i a l by-passed by pumping from the n o r t h j e t t y (cub. yds/year)
f = m a t e r i a l l o s t t o deep water because o f f l u s h i n g by ebb c u r r e n t s (cub.yds/year)
E q u i l i b r i u m e q u a t i o n f o r t h e North Shore i s
(A-B) -g - t ^ - t 2 > 0
E q u i l i b r i u m e q u a t i o n f o r the South Shore i s
-(A-B) +g + t i + t 2 - f > 0
Inasmuch as A and B v a r y and t h i s may have some i n f l u e n c e on g and f , i t i s n o t p o s s i b l e t o f u l f i l l these equations a l l the t i m e , b u t they s h o u l d n o t d e v i a t e too much from 0 any t i m e . I t w i l l always be a d e f i c i t thanks t o f ( l o s s of m a t e r i a l t o deep w a t e r ) .
I n r e p o r t on model study f o r the South Lake Worth I n l e t by the C o a s t a l E n g i n e e r i n g Department of the U n i v e r s i t y of F l o r i d a , t h e
f o l l o w i n g f i g u r e s a r e used based on e x p e r i e n c e , i n c l u d i n g data p u b l i s h e d i n t h e c o o p e r a t i v e beach e r o s i o n study r e p o r t by the U. S. Army Corps of Engineers and the Palm Beach County (1947 and 1956) and data on d r e d g i n g by the I n l e t D i s t r i c t : Predominant d r i f t , o r d e r o f magnitude - a p p r o x i m a t e l y 200,000 cub.yds/year (may drop t o 150,000 cub.yds/year) t r a n s f e r o f m a t e r i a l from bay shoals 40,000 cub.yds/year ( v a r y i n g 30,000 cub.yds/year t o 90,000 cub.yds/year) and t r a n s f e r by the
p r e s e n t by-passing p l a n t 70,000 cub.yds/year ( v a r y i n g + 10,000 cub,yds/ y e a r ) . See Table 5,
D i f f e r e n t p o s s i b i l i t i e s are now c o n s i d e r e d . Table 8 g i v e s the q u a n t i t i e s of m a t e r i a l moving south (A) and n o r t h (B) under the assumption t h a t A-B = 240,000 cub,yds/year, 210,000 cub.yds/year, 180,000 cub.yds/year, and 150,000 cub.yds/year w i t h a = A v a r y i n g
T A B L E 8
Assumed L i t t o r a l D r i f t Quantities M o v i n g N o r t h and South of P a l m Beach, F l o r i d a
(cub. yds p e r yeax') a = Ratio of D r i f t - | 2 1 l t h w a r ^ = A N o r t h w a r d B 240,000 I s I t A-B 210,000 A B r e a l i s t i c 180,000 7 150,000 240,000 480,000 240,000 No Ct ^ 2 210,000 420,000 210,000 No 1 180,000 360,000 180,000 No 150,000 300,000 150,000 1 240,000 360,000 120,000 No a 3 210,000 315,000 105,000 No a 1 180,000 270,000 90,000 Yes 150,000 225,000 75,000 Yes 240,000 320,000 80,000 No = 4 210,000 280,000 70,000 Yes a = 4 180,000 240,000 60,000 Yes 1 150,000 200,000 50,000 Yes
Only f i v e o f t h e combinations l i s t e d i n Table 8 seem t o be r e a l i s t i c i n c l u d i n g one A-B = 210,000 cub.yds/year (a = 4 ) , two A-B =
180,000 cub.yds/year ( a = 3 or 4) and two A - B = 150,000 cub.yds/year
(a = 3 or 4 ) .
These p o s s i b i l i t i e s a r e c o n s i d e r e d i n Table 9 r e f e r r i n g t o t h e p r e s e n t s i t u a t i o n assuming t h a t t-^ + t2 v a r i e s from 100,000 cub.yds/year t o 160,000 cub.yds/year, g = 50,000 cub.yds/year (which may be on t h e h i g h s i d e ) and f ( l o s t t o deep w a t e r ) v a r y i n g from 30,000 cub.yds/year to 10,000 cub.yds/year as t x + t 2 i n c r e a s e s . Comparing t h e e q u i l i b r i u m c o n d i t i o n f o r t h e n o r t h and t h e south shore t h i s t a b l e r e v e a l s t h a t the c o n d i t i o n w i t h t x + t 2 = 130,000 cub.yds/year, g = 50,000 cub.yds/ year and f = 20,000 cub.yds/year seems t o be t o l e r a b l e . I f f > 20,000 cub.yds/year, t h e q u a n t i t y by-passed south c o u l d be i n c r e a s e d t o improve the n o r t h / s o u t h balance.
Present c o n d i t i o n i s t h a t about 70,000 cub.yds/year a v e r a g l y i s by-passed by t h e pumping p l a n t and about 45,000 cub.yds./year i s by-passe from the bay shoals (perhaps a l i t t l e less or 40,000 based on data going back t o 1950) t o t a l 115,000 cub.yds/year. I t , t h e r e f o r e , does n o t seem t o be necessary t o c u t dovm q u a n t i t i e s by-passed by t h e p l a n t below 70,000 cub.yds/year.
T A B L E 9
M a t e r i a l Balance at the South Lake W o r t h Inlet Present Conditions
Quantities i n cub. yds/year
N o r t h South (A-B) -g - t l - t 2 -(A-B) +g + t i +2 - f NO OK 0K(?) NO OK 0K(?) A-B = q u a n t i t y cub.yds/yr= a=A B t l + t 2 =100,000 g = 50,000 t l + t 2 =130,000 g = 50,000 t l + t 2 =160,000 g = 50,000 t l + t 2 =100,000 g = 50,000 f ( l o s t ) 30,000 t l + t 2 =130,000 g = 50,000 f ( l o s t ) 20,000 t l + t 2 =160,000 g = 50,000 f ( l o s t ) 10,000 A-B = 210,000 +60,000 +30,000 -0,000 -90,000 -50,000 -10,000 a = 4 no no yes no no yes A-B = 180,000 +30,000 -0,000 -30,000 -60,000 -20,000 +20,000
a = 3 no yes yes no yes yes
A-B = 150,000 -0,000 -30,000 -60,000 -30,000 +10,000 +50,000
a = 3 yes yes no yes yes no
A-B = 180,000 +30,000 -0,000 -30,000 -60,000 -20,000 +20,000
a = 4 no yes yes no yes yes
A-B = 150,000 -0,000 -30,000 -60,000 -30,000 +10,000 +50,000
a = 4 yes yes no yes yes no
1
o
No means: cannot be accepted
Yes: a c c e p t a b l e w i t h c e r t a i n p r e c a u t i o n s OK: seems i n g e n e r a l t o be a c c e p t a b l e 0 K ( ? ) : q u e s t i o n a b l e , s h o u l d p r o b a b l y n o t be accepted cn CO cn
I m p r o v e d Conditions Quantities i n cub. yds/year
N o r t h South (A-B) -g - t l - t 2 -(A-B) +g
+H
+ t 2 No OK No OK t l + t 2 t l + t 2 t l + t 2 t l +H
=130,000 =160,000 =130,000 =160,000 CO A g = 20,000 g = 20,000 g = 20,000 g = 20,000 CO A f ( l o s t ) 10,000 f ( l o s t ) 10,000 CO A w A-B = 210,000 +60,000 +30,000 -70,000 -40,000? TA L :a =
4 pump 170,000 TA L : A-B = 180,000 +30,000 -0,000 -40,000 -10,000 Ma.
=
3 GI N A-B = 150,000 -0,000 -30,000 -10,000 +20,000 td t?da =
3 A-B = 180,000 +30,000 -0,000 -40,000 -10,000a =
4 A-B = 150,000 -0,000 -30,000 -10,000 +20,000 a = 4No means: cannot be accepted
Yes: acceptable w i t h c e r t a i n p r e c a u t i o n s OK: seems i n g e n e r a l t o be a c c e p t a b l e
