Gold Coast Beach replenishment program

GOLD COAST BEACH REPLENISHMENT PROGRAM REPORT No. 72

THE IMMEDIATE GROWTH OF SPINIFEX VEGETATION I N PUMPED-UP DUNES

(1) INTRODUCTION

I n 1974, t h e Gold Coast C i t y C o u n c i l embarked upon a major program o f beach r e p l e n i s h m e n t f o r t h e i r l o c a l beaches, t h a t had s u f f e r e d c o n s i d -e r a b l -e -e r o s i o n d u r i n g t h -e c y c l o n -e i n d u c -e d wav-e c o n d i t i o n s o f -e a r l y 1972 and e a r l y 1974. I n terms o f t h e Beach P r o t e c t i o n ' A c t 1968-1970 o f t h e S t a t e o f Queensland, these beach n o u r i s h m e n t p r o j e c t s were r e q u i r e d t o comply w i t h a s e r i e s o f c r i t e r i a , as l a i d down by t h e S t a t e ' s Beach P r o t e c t i o n A u t h o r i t y T w o p a r t i c u l a r c r i t e r i a were nominated as p a r t o f t h e beach r e p l a c e m e n t p r o c e s s . F i r s t l y , an a r t i f i c i a l dune o f s u b s t a n t i a l p r o p o r t i o n s was r e q u i r e d t o be cons-t r u c cons-t e d a cons-t cons-t h e r e a r o f cons-t h e beach, and s e c o n d l y , cons-t h a cons-t once c o n s cons-t r u c cons-t e d , t h i s new dune was t o be i m m e d i a t e l y p r o t e c t e d by t h e p l a n t i n g o f

S p i n i f e x g r a s s e s . T h i s r e p o r t t h e r e f o r e r e c o r d s t h e C i t y C o u n c i l ' s e f f o r t s t o meet these r e q u i r e m e n t s .

(2) THE NOURISHMENT WORKS

I n m i d 1974, two m a j o r n o u r i s h m e n t c o n t r a c t s were l e t by t h e C o u n c i l . At K i r r a , 1 x lO^m^ was dredged f r o m t h e Tweed R i v e r t o t h e K i r r a beaches, t o t h e N o r t h , 1.4 x lO^m^ was c o n c u r r e n t l y dredged f r o m t h e S o u t h p o r t B r o a d w a t e r t o Mainbeach and S u r f e r s P a r a d i s e . I n b o t h c o n t r a c t s , t h e i n i t i a l sand placement s t r i c t l y f o l l o w e d t h e c r i t e r i a nominated by t h e B.P.A. As d e s c r i b e d by Smith ( 1 9 7 7 ) , t h e r e q u i r e m e n t

t o r i g i d l y p l a c e a g i v e n r e n o u r i s h m e n t volume p e r u n i t l e n g t h , w h i l s t a t t h e same t i m e c o n s t r u c t i n g t h e " t h e o r e t i c a l " dune p r o f i l e , was f o u n d t o be o p e r a t i o n a l nonsense. I t was v e r y r a p i d l y o b s e r v e d t h a t N a t u r e was a much b e t t e r d e s i g n e r o f n o u r i s h m e n t w o r k s , t h a n was t h e B.P.A. N a t u r e , t h r o u g h h e r day t o day v a r i a b i l i t y o f t h e wave c l i m a t e

i n t e r a c t i n g w i t h t h e a p p l i e d s e d i m e n t , soon d e m o n s t r a t e d where t h e n o u r i s h m e n t volumes had t o be p l a c e d , t o g a i n a new beach o f e f f e c t i v e even p r o f i l e . C o u n c i l ' s e n g i n e e r s t h e r e f o r e r e - a d j u s t e d t h e beach placement ( w h i l s t s t i l l m a i n t a i n i n g a near c o n s t a n t v o l u m e / u n i t

l e n g t h d e p o s i t i o n ) t o a t t a i n a p a r a l l e l beach, where t h e a c t i o n was — i . e . i n t h e s u r f zone. Then i f any volume happened t o be l e f t - o v e r

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f r o m t h e s u r f zone, t h e n and o n l y t h e n , was an a r t i f i c i a l dune b u i l t - u p .

Thus, where i n i t i a l l y a f i n e l o o k i n g dune had been c o n s t r u c t e d a t t h e expense o f a ragged beach, t h e a l t e r n a t i v e end r e s u l t was a f i n e

l o o k i n g beach w i t h a ragged dune! C o u n c i l ' s e n g i n e e r s had no d o u b t as t o w h i c h approach was p r e f e r a b l e . To be f a i r t o C o u n c i l , i t must be r e c o r d e d t h a t when t h e s e a l t e r n a t i v e r e s u l t s were r e p o r t e d by phone t o B.P.A., t h e y e l i c i t e d an a b s o l u t e z e r o r e a c t i o n . There was not o n l y a l a c k o f any g u i d a n c e , b u t no r e a l response at a l l e i t h e r . C o u n c i l t h e r e f o r e p r e s s e d on and c o m p l e t e d t h e works on t h e even

p a r a l l e l beach a l t e r n a t i v e . A t t h e same t i m e , as any a r t i f i c i a l dune areas became a v a i l a b l e , t h e y were i m m e d i a t e l y p l a n t e d w i t h S p i n i f e x seed t o t h e B.P.A. r e q u i r e m e n t s , b u t t o t h e C o u n c i l ' s a s t o n i s h m e n t , p r a c t i c a l l y e v e r y c r o p f a i l e d . Many S p i n i f e x seeds g e r m i n a t e d , b u t as soon as t h e i r s h o o t s began t o appear, t h e y w i t h e r e d and d i e d .

A g a i n , t h e B.P.A. n e i t h e r o f f e r e d any e x p l a n a t i o n as t o why t h e c r o p s were f a i l i n g , n o r any a d v i c e as t o how t h e problem m i g h t be s o l v e d . The C o u n c i l t h e r e f o r e mounted i t s own i n v e s t i g a t i o n aimed a t d e t e r m -i n -i n g t h e reasons f o r t h e f a -i l u r e s .

(3) SPINIFEX GROWTH INVESTIGATION

I n O c t o b e r 1974, C o u n c i l s e t up a s e r i e s o f t r i a l p l a n t i n g s i n a pumpedup dune a t Mainbeach and a t t h e same t i m e , s t a k e d o u t e q u i v a l -ent areas o f a n a t u r a l dune on t h e S p i t . The g r o w t h o f r u n n e r s and seeds p l a n t e d on t h e pumped dune was t h e n compared w i t h t h e n a t u r a l . Sand t e m p e r a t u r e s , o r g a n i c s , m o i s t u r e c o n t e n t , pH v a l u e and s a l t

c o n t e n t , were a l s o measured on t h e two t r i a l a r e a s . I n i t i a l l y e x t r a t r i a l s were made i n t h e pumped dune t o a c c e l l e r a t e t h e S p i n i f e x by e x t r a m u l c h i n g and f e r t i l i s e r a p p l i c a t i o n s . I n a d d i t i o n some p l o t s were a l s o i r r i g a t e d w i t h f r e s h w a t e r f o r t h e same r e a s o n , b u t t h e s e were soon t e r m i n a t e d when t h e r e a l reasons f o r c r o p f a i l u r e became more o b v i o u s .

I n t h e pumped t e s t p l o t s , t h e number o f S p i n i f e x s h o o t s p e r u n i t a r e a were c o u n t e d p e r week and r o o t g r o w t h was a l s o assessed by e x c a v a t i n g u n i t a r e a s o f t h e p l o t s a t t h e same t i m e . Even a t t h e v e r y commence-ment o f t h e c o m p a r a t i v e t r i a l s , one o f t h e most ominous d i s c o v e r i e s was made d u r i n g t h e s i m p l e p l a n t i n g o p e r a t i o n . The pumped-up dune

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sand was v e r y much more compact and dense, t h a n t h e n a t u r a l dune. T h i s n a t u r a l l y f o l l o w e d f r o m t h e method o f p l a c e m e n t . A pumped dune sand, p l a c e d c o m p l e t e l y s a t u r a t e d , was a t maximum d e n s i t y , whereas t h e blown dune sand was a t minimum d e n s i t y — anybody who has c l i m b e d any n a t u r a l dune, can n o t i c e t h i s d i f f e r e n c e i m m e d i a t e l y . The r e s i s -t a n c e -t o r o o -t g r o w -t h w i -t h i n a sand body, i s p r o p o r -t i o n a l -t o i -t s d e n s i t y — b u t i t s w a t e r p e r c u l a t i o n and s a l t l e a c h i n g p r o p e r t i e s a r e i n v e r s l y p r o p o r t i o n a l t o i t s d e n s i t y . D e n s i t y however, makes l i t t l e d i f f e r e n c e t o t h e n e a r - s u r f a c e s o i l t e m p e r a t u r e , a l l sand d e p o s i t s a r e h e a t - s i n k s , b u t a h i g h e r a i r c o n t e n t reduces n i g h t - t i m e h e a t r e - r a d i a t i o n . (A) RESULTS

I n h i n d - s i g h t , a g r e a t d e a l o f work was expended on t h i s s t u d y t h a t was i n - e f f e c t u a l and w a s t e f u l . However, a t t h e t i m e t h e c r u c i a l p a r a m e t e r s t h a t i n h i b i t e d pumped dune S p i n i f e x g r o w t h , were q u i t e unknown. The f i r s t c o n s i d e r a t i o n was t o examine t h e d i f f e r e n c e s

between t h e pumped sand and t h e n a t u r a l dune. A s t a n d a r d B.S. s i e v e t r a i n a n a l y s i s d e m o n s t r a t e d t h a t t h e pumped sand D^^ was o.SOmm

compared w i t h t h e n a t u r a l dune D^^ o f 0.21mm. A l l t h a t t h i s i n d i c a t e d was t h a t t h e n a t u r a l dune sand was v e r y much " f i n e r " , b u t t h i s was o n l y t o be e x p e c t e d . Wind blown dune sand i s n e a r l y always d o m i n a t e d by t h e s m a l l e r g r a i n s winnowed f r o m t h e p a r e n t beach-sand p o p u l a t i o n . N o t h i n g s i g n i f i c a n t c o u l d be d e t e c t e d f r o m t h i s d a t a .

The n e x t t o p i c examined, was t h e r e s u l t s o f n a t u r a l r a i n - f a l l . Here i t was f o u n d t h a t on t h e pumped dune,9nim o f r a i n i n A h o u r s o n l y

p e n e t r a t e d 12mm below t h e sand s u r f a c e and 42.Amm o f r a i n i n t h e day o n l y p e n e t r a t e d 90mm. The same day's r a i n on t h e n a t u r a l dune penet r a penet e d some 200mm and s i n c e i penet was measured penet h a penet S p i n i f e x r o o penet s p r o s -pered o n l y between 100 and 150mm below t h e s u r f a c e i n t h e n a t u r a l dune, t h i s seemed t o r e p r e s e n t an i m p o r t a n t g r o w t h p a r a m e t e r . The n a t u r a l dunes, w i t h t h e i r sand i n a l o o s e p a c k i n g , h e l d a s u i t a b l e p e r m e a b i l i t y t o r a i n f a l l , t o a l l o w adequate r a i n m o i s t u r e t o r e a c h t h e S p i n i f e x r o o t l e v e l . The pumped sand on t h e o t h e r hand, d i d n o t .

A s e r i e s o f i r r i g a t i o n t e s t s on t h e pumped dune p l o t s , was t h e n mounted. An a r r a y o f d i f f e r e n t a g r i c u l t u r a l a e r i a l r o t a t i n g s p r a y d e v i c e s were t h e n employed. The e f f i c i e n c y o f t h e s e s p r a y d i s t r i b

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u t o r s was f o u n d t o be r e m a r k a b l y l o w , t h e most e f f i c i e n t o n l y d e l i v -e r -e d 45% o f i t s m-et-er-ed i n p u t w a t -e r o n t o t h -e a c t u a l sand s u r f a c -e . The r e s t o f t h e f r e s h w a t e r e v a p o r a t e d b e f o r e t h e r e s i d u e l a n d e d on the s u r f a c e . I t was f o u n d t h a t a n e t t s u r f a c e a p p l i c a t i o n r a t e ( i n one o p e r a t i o n ) o f 150gal/m^ was r e q u i r e d t o p e n e t r a t e t h e pumped sand

t o 150mm d e p t h t o S p i n i f e x r o o t l e v e l and l e a c h o u t t h e NaCl t o a c c e p t a b l e S p i n i f e x g r o w t h t o l e r a n c e l e v e l s . A r t i f i c i a l i r r i g a t i o n o f t h e pumped dune sands was c l e a r l y un-economic. T h i s i r r i g a t i o n dosage i t may be n o t e d , r e p r e s e n t e d t h e e q u i v a l e n t o f an e n t i r e y e a r s n a t u r a l r a i n f a l l f o r S o u t h p o r t , i . e . 800 t o 1000mm.

I n t h i s , t h e reason f o r t h e f a i l u r e o f a l l t h e m u l c h i n g and " b o o s t e r " f e r t i l i s i n g t r e a t m e n t s on t h e pumped sand t r i a l p l o t s became o b v i o u s . W i t h o u t s u f f i c i e n t sand p e r m e a b i l i t y i n t h e pumped m a t e r i a l , t h e r e was i n a d e q u a t e n a t u r a l m o i s t u r e a t r o o t l e v e l t o l e a c h - o u t excess s a l t , i n d u c e r o o t g r o w t h , and a c t i v a t e t h e p e l l e t f e r t i l i s e r .

The f i n a l r e s e a r c h c o n c l u s i o n was t h a t t h e c r u c i a l S p i n i f e x g r o w t h i n h i b i t o r was t h e s a l t c o n t e n t o f t h e sand. The b a s i c c o m p a r i s o n s between t h e pumped sand t r i a l p l o t s and t h e e q u i v a l e n t a r e a s o f t h e n a t u r a l dune where t h e S p i n i f e x g r o w t h was s p a r s e and where i t was good, i s s e t o u t i n T a b l e I . The s a l t c o n t e n t o f t h e sand i s

expressed i n p a r t s / t h o u s a n d o f s o l i d NaCl o v e r t h e d r i e d w e i g h t o f sand. I n t h i s , t h e pumped sand r e s i d u e h e l d o v e r a thousand t i m e s the s a l t c o n t e n t o f t h e b e s t n a t u r a l dune p l o t . I n d e e d , t h e pumped sand g r a i n s were a l m o s t c o m p l e t e l y c o v e r e d w i t h s a l t c r y s t a l s . From T a b l e I , i t can be deduced t h a t m o i s t u r e c o n t e n t was i r r e l e v e n t , o r g a n i c c o n t e n t and pH were s i g n i f i c a n t , b u t t h e dominant S p i n i f e x g r o w t h c o n t r o l was c l e a r l y e x e r t e d by t h e NaCl c o n t e n t .

T h i s c o n c l u s i o n was s u p p o r t e d by t h e beach e v e n t s o f 1976 ( i . e . a y e a r a f t e r t h e t r i a l s ) . Then t h e pumped beach sand was d r e n c h e d by c y c l o n i c r a i n s and blown by h i g h w i n d s o n t o t h e back-beach sand d r i f t f e n c e s . A l m o s t i m m e d i a t e l y a b u r s t o f S p i n i f e x g r o w t h d e v e l o p e d i n the n e w l y - b l o w n sand d e p o s i t s , t r i g g e r e d perhaps by t h e h i g h d e n s i t y o f seeds p l a n t e d t h e y e a r b e f o r e . A l l t h e S p i n i f e x seed had been p l a c e d w i t h a f e r t i l i s e r . I t seems t h e r e f o r e , t h a t t h e w i n d i n d u c e d r o l l i n g o f t h e sand g r a i n s removed n o t o n l y t h e excess s a l t c r y s t a l s , b u t a l s o e n s u r e d t h a t t h e y came t o r e s t a t a m i n i m a l p a c k i n g . I n

t h i s s t a t e , t h e pumped sand had r e p r o d u c e d t h e n a t u r a l dune c o n d i t -i o n s and by a l l o w -i n g easy m o -i s t u r e p e n e t r a t -i o n and l o w r o o t g r o w t h r e s i s t a n c e , i t had r e v e r t e d t o an e f f e c t i v e medium f o r S p i n i f e x g r o w t h . The a d d i t i o n a l b e n e f i t o f a p p l i e d f e r t i l i s e r was a l s o p r o b a b l y advantageous. I t was a l s o n o t e d t h a t a sudden S p i n i f e x " d e l a y e d " success a l s o o c c u r r e d a t t h e two o t h e r beach r e p l e n i s h m e n t p l a c e m e n t s ( e f f e c t e d i n 197A-75) a t K i r r a and Palm Beach, a f t e r t h e same 1976 c y c l o n i c w e a t h e r .

( 5 ) CONCLUSIONS

A l l o f C o u n c i l ' s r e s e a r c h i n d i c a t e s t h a t i t i s i m p r a c t i c a b l e t o grow " i n s t a n t " S p i n i f e x i n v e r y r e c e n t l y p l a c e d a r t i f i c i a l dunes, f o r m e d f r o m sands pumped d i r e c t l y i n t o p o s i t i o n w i t h s a l t w a t e r . The p r i n c i p a l i n h i b i t i n g agency i s t h e h i g h NaCl c o n t e n t , h e l d by t h e pumped m a t e r i a l . U n t i l t h i s excess s a l t i s removed by a b r a s i o n and n a t u r a l l e a c h i n g , i t i s p o i n t l e s s t o e x p e c t any s u c c e s s f u l c r o p .

The B.P.A. were c o r r e c t i n r e q u i r i n g t h e beach n o u r i s h m e n t dunes t o be s t a b i l i s e d w i t h S p i n i f e x , b u t v e r y much i n e r r o r i n t h e i r s t r i c t i n s t r u c t i o n s as t o how — and more c r i t i c a l l y — when t h e S p i n i f e x was t o be c u l t u r e d . I f t h e B.P.A. d i d n o t leam t h i s l e s s o n , a t

l e a s t t h e C i t y C o u n c i l d i d . From a b o u t h a l f way t h r o u g h t h e S u r f e r s P a r a d i s e n o u r i s h m e n t c o n t r a c t , no f u r t h e r dunes were formed by

d i r e c t pumping. F o r t h e l a s t h a l f o f t h e c o n t r a c t , any s p a r e sand l e f t f r o m m e e t i n g an even s u r f - z o n e beach, was l e f t a t mid-beach and t h e n dozed i n t o t h e back-beach, t o f o r m a dune p r e f e r a b l y a f t e r r a i n b u t n e a r l y always a f t e r some n a t u r a l d r y i n g . I n t h i s way, t h e

maximum o p e r a t i o n a l s a l t r e m o v a l by l e a c h i n g and a b r a s i o n was a t t a i n ed. A f t e r t h i s p r o c e d u r e was i n s t i t u t e d , S p i n i f e x g r o w t h was t h e r e -a f t e r much more r -a p i d -and e f f e c t i v e .

( 6 ) ACKNOWLEDGEMENTS

The m o n i t o r i n g and s i t e e x p e r i m e n t s e x e c u t e d on t h e t r i a l dune a r e a s were c a r r i e d o u t by T e c h n i c i a n R.W. W a l d o f , t h e n o f Gold Coast C i t y C o u n c i l ' s s o i l s l a b o r a t o r y . The dune p l a n t i n g o p e r a t i o n s and a l l S p i n i f e x m o n i t o r i n g e f f o r t s , a f t e r mid-1975, were c o n d u c t e d by Foreman R.W. Matthews, who a l s o a s s i s t e d i n p r e p a r i n g t h i s r e p o r t .

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TABLE I

COMPARATIVE SPINIFEX GROWTH ENVIRONMENTS

Sand L o c a t i o n S p i n i f e x S a l t C o n t e n t O r g a n i c M o i s t u r e Growth p a r t s / 1 0 0 0 C o n t e n t % C o n t e n t % pH Pumped Dune N a t u r a l Dune N a t u r a l Dune F a i l u r e Sparse Good 100-400 2.0 0.1 n e g l i g i b l e 0.01 0.5 2.5-6.0 0.5 3.0 6.5-8 8.5-8.7 8.5-8.7 REFERENCE

S m i t h , A.W. "The S u r f e r s P a r a d i s e Beach Replenishment P r o j e c t — A Case Study" 3 r d . A u s t . Conf. on C o a s t a l and Ocean Eng. I . E . A u s t . Nat. Conf. Pub. No. 77/2 ( 1 9 7 7 ) .

2 5 t h May, 1985. A.W. Sam S m i t h

QL'/HENSLANO IMSTITUTE OF= T E C H N O L O G Y

Gold Coast Beach Replenishment Program REPORT No. 82

THE MANUAL ON ARTIFICIAL BEACH NOURISHMENT

(1) T h i s r e p o r t r e p r e s e n t s a p r e l i m i n a r y r e v i e w o f t h e above manual, i n terms o f c u r r e n t r e s e a r c h and e x p e r i e n c e on t h e Gold Coast. The manual was p r e p a r e d by a w o r k i n g g r o u p , g e n e r a l l y drawn f r o m R i j k s w a t e r s t a a t and t h e D e l f t H y d r a u l i c s L a b o r a t o r y , b o t h o f t h e N e t h e r -l a n d s . The manua-l has drawn upon w o r -l d - w i d e d a t a , b u t u n f o r t u n a t e -l y t h e Gold Coast e x p e r i e n c e i s r e p o r t e d i n o n l y f r a g m e n t a r y and i n a d e -q u a t e terms. T h i s would n o t be s u r p r i s i n g , most o f t h e work c a r r i e d o u t on t h e Gold Coast, has never been p r o p e r l y documented and no d e f i n i t i v e t e x t y e t e x i s t s . F u r t h e r m o r e , w i t h one e x c e p t i o n , none o f t h e p u b l i s h e d a c c o u n t s have been w r i t t e n by t h e e n g i n e e r s who a c t u a l l y e f f e c t e d t h e works and were a b l e t o m o n i t o r t h e i r b e h a v i o u r f o r many y e a r s t h e r e a f t e r . I t now seems u n l i k e l y t h a t a f u l l r e v i e w o f t h e Gold Coast n o u r i s h m e n t works w i l l ever be p r e p a r e d , b u t some documents and d a t a g o i n g back t o 1972 d o . e x i s t . They a r e however i n o n l y a p a r t i a l l y processed s t a t e , and much o f t h e d a t a i s s t i l l q u i t e raw.

(2) The most complete r e v i e w o f t h e s i t e l e s s o n s , l e a r n e d f r o m t h e 1973-75 Gold Coast w o r k s , i s a s e t o f l e c t u r e n o t e s t h a t f o r m p a r t o f t h e " C o a s t a l E n g i n e e r i n g " f i n a l y e a r e n g i n e e r i n g e l e c t i v e a t

Queensland I n s t i t u t e o f T e c h n o l o g y , i n B r i s b a n e . The n o t e s a r e used f o r l e c t u r e No. 9 and a copy i s a t t a c h e d as Appendix A, t o t h i s

r e p o r t . I n terms o f 1986, we would see l i t t l e t h a t need be changed s i n c e t h e n o t e s were w r i t t e n back i n 1983, b u t t h e y a r e be no means complete n o r f u l l y up t o d a t e . S i n c e t h e l e c t u r e n o t e s a r e l a r g e l y s e l f e x p l a n a t o r y , we w i l l n o t d i s c u s s t h e m . i n d e t a i l and t h e r e a d e r can draw h i s own c o n c l u s i o n s about them. We t h e r e f o r e d i s c u s s more g e n e r a l m a t t e r s t h a t may be absent o r o n l y p a r t i a l l y e x p l o r e d , i n

t h e "Manual" and/or t h e a t t a c h e d n o t e s .

(3) L o o k i n g back, t h e most i m p o r t a n t l e s s o n we now a p p r e c i a t e t o be t h e m a t t e r o f t h e " l i f e - s p a n " o f a n o u r i s h m e n t s l u g , on a l i t t o r a l d r i f t beach. Our f i e l d o b s e r v a t i o n s , show us t h a t a n o u r i s h m e n t s l u g , p l a c e d on a h i g h l i t t o r a l d r i f t beach, m i g r a t e s b o d i l y a l o n g

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w i t h t h e d r i f t b u t d i f f u s i n g s l o w l y as i t does so. The p r o c e s s i s s l o w , and t h e e f f e c t i s p a r t i a l l y o b s c u r e d by t h e n o i s e g e n e r a t e d by day t o day beach response t o t h e v a r i a b l e ambient wave c l i m a t e . The S u r f e r s P a r a d i s e s l u g o f 1.5 x 10 m^ was t r a c k e d f r o m i t s c o l o u r . The n o u r i s h m e n t m a t e r i a l h e l d a h i g h s h e l l c o n t e n t and f r o m t h e a i r , i t made t h e sand i n t h e swash zone orange, i n s t e a d o f the more u s u a l y e l l o w . The K i r r a s l u g o f 1.0 x 10 m' h e l d no c o l o u r d i f f e r e n c e , b u t i t c o u l d be r e a d i l y f o l l o w e d on a e r i a l p h o t o g r a p h s , d i r e c t o b s e r -v a t i o n and s u r -v e y s . From t h e s e and o t h e r s t u d i e s , we c o n c l u d e t h a t t h e l o c a l average l o n g term l i t t o r a l t r a n s l a t i o n v e l o c i t y , i s c l o s e t o 1.4kra/year. The s l u g p l a c e d a t K i r r a , i s c u r r e n t l y (1986) s i t e d w i t h i n t h e N o r t h e r n zone o f Palm Beach, and t h e S u r f e r s s l u g i s now p a r t i a l l y i n t h e B r o a d w a t e r , w i t h t h e r e s t , f u r t h e r N o r t h on S t r a d -b r o k e I s l a n d . The m i g r a t i o n o f t h e S u r f e r s P a r a d i s e s l u g , has n o t been p a r t i c u l a r l y n o t i c e a b l e as f a r as t h e t o t a l beach i s c o n c e r n e d . The s l u g has m e r e l y been r e p l a c e d by a n o t h e r l o n g n a t u r a l s l u g f r o m t h e S o u t h , where t h e beaches were much w i d e r i n 1975 t h a n t h e S u r f e r s beach, p r i o r t o n o u r i s h m e n t . The e f f e c t s a t K i r r a on t h e o t h e r hand, have been d r a m a t i c . W i t h a l m o s t t h e t o t a l l i t t o r a l d r i f t i n t o t h e Gold Coast, b e i n g t r a p p e d by t h e Tweed T r a i n i n g W a l l s , t h e o r i g i n a l n o u r i s h m e n t s i t e i s now an extreme s t a r v a t i o n shadow zone, t h e s l u g i s l o n g gone.

(4) We n®w b e l i e v e t h a t t h e o v e r a l l economy o f a beach n o u r i s h m e n t , depends h e a v i l y upon t h e s l u g ' s d r i f t r a t e and t h e t i m e i t may

r e m a i n , w i t h i n t h e l o c a l beach " c e l l " . A s l u g p l a c e d a t K i r r a , may take 25 y e a r s t o r e a c h S u r f e r s P a r a d i s e , b u t i t w i l l b e n e f i t e v e r y p a r t o f t h e beach c e l l f o r a w h i l e , d u r i n g i t s m i g r a t i o n p a s t each p o i n t . I n terras o f t o t a l beach w i d t h g a i n by t i m e , a K i r r a s l u g w i l l be 5 o r 6 t i m e s more e f f i c i e n t than a s l u g p l a c e d a t S u r f e r s . On a l i t t o r a l d r i f t c o a s t t h e r e f o r e , t h e most economic n o u r i s h m e n t w i l l f o l l o w f r o m a placement s i t e a t t h e extreme u p - d r i f t p o i n t o f

t h e embayment. Then t h e s l u g w i l l a t t a i n a maximum o p e r a t i o n a l " l i f e " . (5) A n o t h e r a l l i e d f a c t o r t h a t c u r r e n t l y c o n c e r n s us, i s t h e p e r c e n -tage o f a l i t t o r a l s l u g t h a t i s s i t e d w i t h i n t h e l i t t o r a l d r i f t c r o s s s e c t i o n a l p r i s m . I f we l o o k a t F i g . 5 o f Appendix A, m a t e r i a l p l a c e d i n Zone A, w i l l be c o m p l e t e l y c l e a r o f t h e l i t t o r a l p r i s m , so i t w i l l s u f f e r no m i g r a t i o n a t a l l , u n t i l s t o r m waves wash i t i n t o

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the p r i s m . A placement i n Zone B, w i l l have perhaps h a l f o f i t s s l u g o u t s i d e t h e p r i s m , b u t t h e o t h e r h a l f w i l l be i n a m i g r a t i n g s t a t e c o n t i n u o u s l y . . I t s d r i f t w i l l be a maximum. M a t e r i a l p l a c e d i n Zone C, on t h e o t h e r hand, w i l l p r o b a b l y remain seawards o f t h e l i t t o r a l p r i s m , so i t w i l l remain unmoved u n t i l t h e r i g h t wave c o n -d i t i o n s b e g i n t o sweep i t shorewar-ds. We see h e r e t h e r e f o r e , a new h i g h l y v a r i a b l e f a c t o r t h a t s h o u l d a l s o be c o n s i d e r e d . The s e l e c t i o n o f t h e optimum n o u r i s h m e n t p o s i t i o n , i t s l i f e , i t s t r a n s p o r t a b i l i t y and i t s p r o t e c t i v e c a p a c i t y , c l e a r l y f o r m s a h i g h v a r i a b i l -i t y e f f e c t -i v e n e s s m a t r -i x . We have however, made a s t a r t -i n a s s e s s -i n g the l i t t o r a l d r i f t p r i s m , by t h e s i m p l e a p p l i c a t i o n o f Q = AV, b u t we as y e t know n o t h i n g about t h e d e t a i l e d v a r i a t i o n i n t h e t r a n s p o r t ' v e l o c i t y w i t h i n t h e b o u n d a r i e s o f t h e p r i s m . We have been h i g h l y concerned on t h e o t h e r hand, t o l e a r n t h a t measurements t a k e n on t h e v i s i b l e beach, t e l l us n o t h i n g a b o u t t h e b e h a v i o u r o f t h e l i t t o r a l p r i s m , o t h e r t h a n o v e r t h e v e r y l o n g t i m e span i . e . > 10 t o 20 y e a r s . We a l s o have c o n c l u d e d t h a t t h e r e e x i s t s a t h r e s h o l d t o l i t t o r a l t r a n s p o r t , t h a t i s p r i n c i p a l l y r e l a t e d t o t h e s e d i m e n t " s i z e " and the wave p e r i o d as w e l l as t h e wave h e i g h t . T h i s r e q u i r e s m o n i t o r -i n g s u r v e y s o f beach n o u r -i s h m e n t p r o f -i l e s , t o be t a k e n o f f s h o r e t o w a t e r d e p t h s o f 20 t o 25 m e t r e s — an e x p e n s i v e o p e r a t i o n .

(6) I n Appendix A, we proposed a beach f i l l c o m p a t a b i l i t y c r i t e r i a w i t h t h e n a t i v e , based upon t h e p a r a m e t e r o f S p e c i f i c S u r f a c e .

Sub-sequent work c a r r i e d o u t i n 1985, has p r o v i d e d some e v i d e n c e t h a t t h e r e does e x i s t a d i r e c t r e l a t i o n s h i p ( a c t u a l l y L o g - l i n e a r ) between the seabed s l o p e o f a n a t u r a l beach and t h e F^q s e d i m e n t v a l u e . The d a t a f o r K i r r a Beach i s t h u s shown p l o t t e d as F i g . 1 a t t a c h e d . ' T h i s i s o n l y a f i r s t s t e p o f c o u r s e , b u t i t seems t o be a t l e a s t e n c o u r a g -i n g . We t h -i n k t h a t t h e c o n c e p t s h o u l d m e r -i t a g r e a t d e a l more s t u d y i n t h e f u t u r e .

(7) T h i s must r e m a i n a b r i e f and v e r y p r o v i s i o n a l n o t e , much p r o t o t y p e d a t a remains t o be processed and a n a l y s e d , b u t we may a t l e a s t t a k e t h i s t o be a s t a r t . N o r m a l l y we w o u l d w a i t u n t i l t h i s d a t a p r o c e s s i n g had been c o m p l e t e d , b u t what we have w r i t t e n , may r e p r e s e n t an i n t e r i m r e p l y t o t h e r e q u e s t f r o m D r s . P i l a r c z y k , when he was r e c e n t l y on t h e Gold Coast. He asked f o r a t l e a s t some comment on h i s d r a f t manual.

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T H E G O L D C O A S T

FIGURE 2

To Brisbane OCEAN Gold Coast Tugun Bilinga North Kirra Kirro Coolangatta Greenmount Rainbow Bay 0 >j ^ l^ • l l J NEW SOUTH W A L E S

, APPENDIX A CEB 561 LECTURE 9 BEACH NOTJEISHMENT ( l ) FONCTION

The h a s l c o b j e c t i v e of beach noTxcishment i s t o r e p a i r the ravages of p a s t e r o s i o n and s e c u l a r r e c e s s i o n by c o n s t r u c t i n g a new beach seawards o f t h e e x i s t i n g , and u s i n g sediments from elsewhere than t h e p a r t i c r i l a r beach system. T h i s concept i s extremely d i r e c t and s i m p l e , any e x t r a sediment i n t r o d u c e d i n t o a beach system must be of b e n e f i t t o m a i n t a i n i n g the beach and i n c r e a s i n g i t s amenity.

On t h e o t h e r hand, t h e s u c c e s s r a t e of major beach nourishment e x e r c i s e s h a s not been v e r y h i g h . I n many c a s e s m i l l i o n s of d o l l a r s have been expended on

l a r g e new a r t i f i c i a l beaches, only f o r t h e added sediments t o almost c o m p l e t e l y v a n i s h w i t h i n a y e a r o r two. The r e a s o n s f o r t h e s e f a i l u r e s h a s o f t e n n o t been a t a l l c l e a r a t the time. Perhaps t h i s s h o u l d not be s u r p r i s i n g , i f many o f t h e p h y s i c a l p r o c e s s e s a f f e c t i n g n a t u r a l beaches a r e b a r e l y understood, t h e n i t

may become i m p o s s i b l e t o p r e d i c t t h e s t a b i l i t y of a new beach, c o n s t r u c t e d from d i f f e r e n t sediments, under t h e same p r o c e s s e s .

To d a t e , even a f t e r s i x y e a r s p o s t - c o n s t r u c t i o n r e s e a r c h , i n t o the b e h a v i o u r of l a r g e nourishment p r o j e c t s on t h e Gold Coast,- "the main c o n c l u s i o n r e a c h e d , i s t h a t t h e d e s i g n of a nourishment beach i s v e r y complex and b e s e t by h i g h a r e a s of u n c e r t a i n t y .

The concept of beach nourishment however, remains a n extremely a t t r a c t i v e o p t i o n t o a l l C o a s t a l E n g i n e e r s , i t i s c l e a r l y a " s o f t " option much more comp a t i b l e w i t h Nature, t h a n the "hard" ocomptions of groynes, revetments axid b r e a k -w a t e r s . The s i d e e f f e c t s of an a r t i f i c i a l beach, shotild a l s o be v e r y much

l e s s than those generated by h a r d s t r u c t u r e s , and p o t e n t i a l d e t r i m e n t a l con-s t r u c t i o n e f f e c t con-s upon beach amenity con-should be almocon-st z e r o .

The problem i s t h a t Nature's h y d r a u l i c p r o c e s s e s a r e v a r i a b l e and c o n t i n u o u s . They may a l s o be q u i t e i m p l a c a b l e , i f Nature i s p r e s e n t e d w i t h an a r t i f i c i a l

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beach she d o e s n ' t l i k e , t h e n she w i l l v e r y r a p i d l y change i t - u n t i l she does. Such i s t h e h i s t o r y o f a c t u a l hea.ch n o u r i s h m e n t c

( 2 ) TIPES OE HODEISBMEET

A l l beach n o u r i s h m e n t c o n s i s t s o f m e r e l y p r o v i d i n g t h e beach v / i t h more s e d -i m e n t , b u t w -i t h -i n t h -i s , t h e r e a r e two m a -i n c l a s s e s o f n o u r -i s l m e n t vrorlc. These depend upon t h e t y p e o f t h e na/fctiral beach b e i n g a d d r e s s e d , b e i n g ;

-( a ) C a p t i v e Beaches.

E o r t h i s c l a s s i f i c a t i o n , a c a p t i v e b e a c h may be d e f i n e d as one t h a t i s f u l l y c o n t a i n e d between h e a d l a n d s s u c h t h a t t h e r e i s no s i g n i f i c a n t l i t t o r a l d r i f t e i t h e r i n t o , o r o u t o f t h e p a r t i c u l a r s y s t e m . P o r s u c h a b e a c h , t h e volume o f new s e d i m e n t t o a t t a i n any g i v e n a v e r a g e e x t r a beach w i d t h s h o u l d be f i n i t e , and l o s s e s f r o m t h e system s h o u l d t h e o r e t

-i c a l l y be n -i l . A c a p t -i v e beach n o u r -i s h m e n t s h o u l d o n l y be exposed t o o n s h o r e - o f f s h o r e s e d i m e n t t r a n s p o r t , and m a i n t e n a n c e demands s h o u l d be v e r y l o w .

I n p r a c t i c e however, s i g n i f i c a n t s e d i m e n t l o s s e s may be g e n e r a t e d i n

two ways. I f t h e new s e d i m e n t c o n t a i n s a h i g h e r p r o p o r t i o n o f f i n e s t h a n the n a t i v e , t h e n wave e n e r g y may a b s t r a c t t h e s e f r o m t h e new beach and t r a n s p o r t them i n t o deep w a t e r o f f s h o r e , where t h e y may be l o s t . Then, i f t h e n o u r i s h m e n t m a t e r i a l volume i s v e r y l a r g e , t h e beach may p r o g r a d e s u f f i c i e n t l y o u t t o t h e h e a d l a n d s , t h a t l i t t o r a l t r a n s p o r t becomes r e -a c t i v -a t e d b y N -a t u r e , -and l o s s e s w i l l e v e n t u -a t e -a r o u n d t h e d o w n d r i f t h e a d l a n d . Any p r e - a s s e s s m e n t o f t h e s e f a c t o r s however, i s e x t r e m e l y d i f f i c u l t .

( b ) L i t t o r a l D r i f t Beaches.

t h e s e a r e beaches where t h e s e d i m e n t s move i n t h e o n s h o r e - o f f s h o r e mode, b u t t h a t i n a d d i t i o n , t h e r e i s a n e t l i t t o r a l d r i f t i n one d i r -e c t i o n t h a t i s s i g n i f i c a n t . A b o u t h a l f t h -e N.S.W. c o a s t l i n -e w o u l d f a l l

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i n t o t h i s c a t e g o r y . The a c t u a l l i t t o r a l d r i f t mast he d r i v e n by t h e o b l i q u i t y o f t h e ocea,n s w e l l t o t h e bea.ch, b u t i t i s a p r o c e s s n o t sjnenable t o r e l i a . b l e mathematj.cal c a l c u l a t i o n . A beach n o u r i s h m e n t s l u g p l a c e d on such a c o a s t miist r e s p o n d i n t h e same manner as t h e n a t i x r a l beach d i d b e f o r e h a n d . Under v a r i a b l e wave c l i m a t e s s e d i m e n t w i l l always move onshore and o f f s h o r e , b u t t h e r e w i l l a l s o be a. n e t t l o s s o u t o f t h e s l u g due t o l i t t o r a J . d r i f t .

The amorait o f s e d i m e n t r e q u i r e d - f o r a g i v e n a r e a o f b e a c h "gaJ.n" on a l i t t o r a l d r i f t c o a s t may c a l l f o r much l a r g e r volumes t h a n f o r a cap-t i v e b e a c h , cap-t o malce up f o r cap-t h e s e l i cap-t cap-t o r a l l o s s e s . O b s e r v a cap-t i o n s on cap-the G o l d Coast i n d i c a t e t h a t u n d e r s t r o n g l i t t o r a l d r i f t c o n d i t i o n s , n o u r i s h -ment s l u g s n o t o n l y d i f f u s e l o n g i t u d i n a l l y ( o r smear t h e m s e l v e s on t o t h e b e a c h ) b u t t h e e n t i r e s l u g a l s o m i g r a t e s v f i t h t h e n e t d r i f t . Semi-isola/bed s l u g s o f nouj?ishment m a t e r i a l , even i f t h e y a r e 3 t o 5 1cm. l o n g t h u s d e m o n s t r a t e a l i m i t e d " l i f e " , on a l o n g ocean beach. I f a n o u r i s h -ment s l u g p l a c e d i n f r o n t o f any p a r t i c u l a r beach zone, i s g o i n g t o

m i g r a t e away f r o m t h a t zone v j i t h i n a few y e a r s , t h e n i f t h e d e s i r e d beach w i d t h "improvement" i s t o be m a i n t a i n e d , t h e n p r o g r e s s i v e r e - n o u r i s h m e n t becomes a n e c e s s i t y .

I n economic t e r m s , t h e s e c o n d i t i o n s a r e v e r y d i f f e r e n t f r o m c a p t i v e beaches. Not o n l y i s n o u r i s h m e n t an e x p e n s i v e o p e r a t i o n i n t h e f i r s t p l a c e , b u t u s u a l l y where n o u r i s l i m e n t i s a v i a b l e economic o p t i o n , because

o f t h e v a l u e o f a s s e t s i n h a z a r d , t h e n i n t h e s e a r e a s t h e r e a r e f e w dep-o s i t s dep-o f s u i t a b l e n dep-o u r i s h m e n t m a t e r i a l a v a i l a b l e . Mdep-ore t h a n l i k e l y , by t h e t i m e t h e m a t e r i a l i s most üeeded, i t i s f o u n d t h a t a l l t h e s u i t a b l e d e p o s i t s have a l r e a d y been b u i l t u p o n I

C l e a r l y , i n t e r m s o f s e n s i b l e and r a t i o n a l c o a s t a l p l a n n i n g and management, a p r e r e q u i s i t e o f any c o a s t a l s u b d i v i s i o n s on s u s p e c t a r e a s c o a s t -l i n e , must be t h e i d e n t i f i c a t i o n o f s u i t a b -l e a r e a s where d e p o s i t s o f

4

e f f e c t i v e n o u r i s h m e n t s e d i m e n t s a c t u a l l y e x i t . These a r e a s and d e p o s i t s , s h o u l d t h e n become r e s e r v e d f o r f u t i i r e u s e b y d e l i n e a t i n g them 8,s p u b l i c reserve o r open space on t h e l o c a l l a n d p l a n n i n g schemes. A l l d e v e l o p e r s x - i i t h i n t h e l o c a l i t y s h o u l d t h e n be r e q u i r e d t o d o n a t e t h e l a n d o v e r t h e s u i t a b l e d e p o s i t s , o r i n l i e u c o n t r i b u t e i n cash so t h a t t h e L o c a l A u t h -o r i t y may resume t h e s e c r i t i c a l a r e a s . U n f -o r t u n a t e l y , i n many a r e a s i n N.S.W., i t i s now t o o l a t e , b u t t h i s i s no r e a s o n f o r n o t a p p l y i n g t h i s c o n c e p t i n t h e futvxe.

I t may be n o t e d t h a t once t h e b e a c h n o u r i s h m e n t r e s e r v e d e p o s i t s have a c t u a l l y been u t i l i s e d , and p l a c e d on t h e b e a c h , t h e most e c o n o m i c a l p r o c e d u r e i s t o c o m p l e t e l y remove t h e m a t e r i a l and a l l o w a l a k e t o r e p l a c e i t . V / h i l s t s u c h

e x t r a c t i v e e x c a v a t i o n s may p r o v i d e a v a l u a b l e avenue f o r L o c a l A u t h o r i t y r u b b i s h d i s p o s a l , t h e n o i m a l f a c t i s t h a t a l a k e p r o v i d e s j u s t as v a l u a b l e a p u b l i c

r e s e r v e as does a more s t a t i c " p a r k " . I n d e e d much e x p e r i e n c e s u g g e s t s t h a t l a J c e f r o n t p r o p e r t i e s i n u r b a n a r e a s r e a c h s i m i l a r ra;bing v a l u e s , as do p r o p -e r t i -e s a c t u a l l y f r o n t i n g t h -e b-each i t s -e l f . Such l a k -e s a l s o h o l d h i g j i v a l u -e f o r a c t i n g as " p o n d i n g b a s i n s " w i t h i n l o c a l a r e a s , t o d e l a y peak u r b a n r a i n -f a l l r u n - o -f -f , and m i t i g a t e -f l a s h -f l o o d i n g . I t i s a l l a s i m p l e m a t t e r o -f com-p r e h e n s i v e h y d r a u l i c com-p l a n n i n g and d e s i g n . The t o t a l economic b e n e f i t s t o t h e L o c a l A u t h o r i t y may however, a l l be p o s i t i v e , p a r t i c u l a r y when l o o k e d a t o v e r

t h e medium t o l o n g t e r m . I n c r e a s e d l o n g t e r m r a t e s may well o f f s e t some o f t h e c a p i t a l c o s t s o f t h e i n i t i a l e x t r a c t i o n , and l a k e m a i n t e n a n c e s h o u l d be a b s o l u t e l y m i n i m a l .

( 5 ) BEACH NOURISHMENT BEBAVIOUR

I n t h e o r y , i f a r t i f i c i a l l y p l a c e d b e a c h i s composed o f e x a c t l y t h e same s e d -i m e n t s as t h e n a t u r a l b e a c h , t h e n -i t s h o u l d behave -i n e x a c t l y t h e same manner, and d i s p l a y e x a c t l y t h e same g e o m e t r y — e x c e p t t h a t i t i s more seawards. U n f o r t u n a t e l y i n N a t u r e , t h i s i s a l m o s t an i m p o s s i b i l i t y .

On r e a l ocean beaches, p a r t i c u l a r l y t h o s e exposed t o a r e a s o n a b l y h i g h wave e n e r g y c l i m a t e , I t i s f o u n d t h a t t h e s e d i m e n t p r o p e r t i e s t e n d t o v a r y a c r o s s t h e beach and dovm i n t o t h e sea-bed t o w a t e r d e p t h s r e a d i l y r e a c h i n g seven

m e t r e s , P i g . 2 f o r example d e m o n s t r a t e s t h e v a r i a t i o n i n t h e a v e r a g e sand g r a d i n g ba.sed on D50 ( b y s i e v e a n a l y s i s ) o f t h e t o p 5 m e t r e s o f seabed a l o n g f i v e

w i d e l y spaced s u r v e y l i n e s on t h e G o l d Coast. The sand samples were a l l r e c o v -e r -e d w i t h i n a, f-ew days o f -each o t h -e r , and d u r i n g a p -e r i o d o f n o r m a l l y f i n -e w e a t h e r , y e t t h e y a r e a l l f r o m w i t h i n a s i n g l e z e t a f o r m embayment. The s p r e a d

o f p a r t i c l e s i z e i s c e r t a i n l y v e r y s i g n i f i c a n t , b u t a l l w i t h a g e n e r a l t r e n d f o r t h e p a r t i c l e s t o become f i n e r , t h e f u r t h e r o f f s h o r e .

I n some s t r a n g e manner, n o t a t a l l c u r r e n t l y u n d e r s t o o d , N a t u r e has some c a l l t o s o r t i t s beach s e d i m e n t s , and i n p a r t i c u l a r , t o p l a c e t h e l a r g e s t where, on t h e a v e r a g e , t h e h y d r a u l i c e n e r g y / s e d i m e n t i n t e r f a c e i s t h e s t r o n g e s t . C l e a r l y i f a n o u r i s h m e n t sand r e s o u r c e d e p o s i t tha.t i s o f even p a r t i c l e s i z e were t o be p l a c e d on any o f t h e s e n a t i v e beaches, i t c o u l d n o t be i m m e d i a t e l y i n e q u i l i b i ' ' i \ i m w i t h them. To a t t a i n e q u i l i b r i u m , t h e nev7 n o u r i s h m e n t sand v/ould a l s o r e q u i r e t o be s o r t e d , and i t s e l f migra/fce t o t h e n a t u r a l bea.ch zones o f t h e same g r a d i n g .

One o f t h e m a j o r p r o b l e m s o f u s i n g s u i t a b l e n o u r i s h m e n t r e s o u r c e d e p o s i t s i s t h a t t h e y a r e u s u a l l y l o c a t e d i n a d j o i n i n g e s t u a r i e s , as dunes, o r i n t h e i m m e d i a t e o f f s h o r e d e e p w a t e r seabed. I n p l a c i n g t h e s e d e p o s i t s where e v e r t h e y have f i n a l l y c o l l e c t e d , N a t u r e has a l s o e x e r t e d s o r t i n g and v a r i a t i o n upon them. H y d r a u l i c s o r t i n g o c c u r s i n e s t u a r i n e s h o a l s , dunes u s u a l l y c o n s i s t o f t h e more r o u n d e d s e d i m e n t g r a i n s t h a t have been s e p a r a t e d b y w i n d , and

deepv/ater seabed d e p o s i t s a r e e i t h e r t h e f i n e s moved o f f s h o r e f r o m t h e e x i s t i n g b e a c h , o r c o a r s e r s e d i m e n t s l e f t b e h i n d f r o m p r e v i o u s b e a c h f o r m s m i g r a t i n g up t h e seabed f r o m a l o n g t e r m p r o g r e s s i v e r i s e i n s e a ^ - l e v e l . The chance o f f i n d i n g an economic n o u r i s h m e n t s o u r c e , tha.t i s p e r f e c t l y b a l a n c e d v / i t h the a m b i e n t a c t i v e beach zone, must t h e r e f o r e be somewhat r e m o t e .

6

The r e s u l t s may be r e a d i l y p r e d i c t a b l e . I f the nourishment work i s c a r r i e d out d u r i n g o r d i n a r y m i l d weather, then a r e s o u r c e m a t e r i a l c o a r s e r than the average n a t i v e , w i l l tend to remain perched a t the top of the beach. On the other hand, a much f i n e r r e s o u r c e m a t e r i a l must be expected to migrate o f f -shore immediately, s e e k i n g the deeper water c o n d i t i o n s t h a t s u i t i t . On the o t h e r hand, as soon a s the next g r e a t storm a r r i v e s , a l l the seabed w i l l become mobile, and the e x t r e m e l y h i g h h y d r a u l i c energy w i l l ens-ure t h a t a l l the beach and seabed sediments w i l l become completely i n t e r - m i x e d and the r e s u l t a n t w i l l be a sediment d i s p l a y i n g the average of both the n a t i v e , and the nourishment

sediment p o p u l a t i o n s . The f i n a l post-storm beach p r o p e r t i e s w i l l thus tend to demonstrate the f i n a l r e s u l t a n t of the i n t e r a c t i o n between n a t i v e and nou-rishment m a t e r i a l s . U n t i l the next major storm a r r i v e s however, the l o c a l r e s o u r c e manager may p e r c i e v e the nourishment o p e r a t i o n a s what i n t r u t h i s e i t h e r an exaggerated temporary s u c c e s s , or as an apparant t o t a l l o s s . Y e t the end mean r e s \ i l t can only be b e n e f i c i a l a f t e r the storm. There i s i n f a c t , n o t h i n g t h a t the l o c a l r e s o u r c e manager can do about i t anyway, but a t l e a s t t h e r e i s now a l a r g e r volume of sediment a v a i l a b l e w i t h i n the swept p r i s m . The b a s i c f a c t then, i s t h a t more sediment a v a i l a b l e , then the work a b s o r b t i o n c a p a c i t y of the t o t a l beach system can only be i n c r e a s e d .

Although i t cannot be c u r r e n t l y regarded as c o n c l u s i v e i n any way, enough ev-idence has accummulated over r e c e n t y e a r s , to suggest t h a t a n e x c e s s of f i n e s a t the bottom of the swept p r i s m , may perhaps be capable of f i l t e r i n g out a n e q u i v a l e n t volume of i n c i d e n t wave energy, as a n e x c e s s of c o a r s e m a t e r i a l n e a r the top of the beach. U n f o r t u n a t e l y r e s e a r c h i n t o t h i s phenomenon i s a g a i n i n i t s i n f a n c y , and much b a s i c work remains to be i n v e s t i g a t e d . Y e t , t h i s m i ^ t w e l l be one of N a t i i r e ' s s i m p l e mechanisms f o r a b s o r b i n g wave energy, w i t h

what-s o e v e r what-seabed what-sedimentwhat-s, t h a t might be a c t u a l l y a v a i l a b l e . I t i what-s c e r t a i n l y a l r e a d y e s t a b l i s h e d t h a t the slope of a beach, f o r c o n s t a n t wave energy, i s p r o p o r t i o n a l to the s i z e of the beach p a r t i c l e s . G r a v e l beaches a r e s t e e p , c o a r s e sand i s in-between, and f i n e sand beaches tend to be much f l a t t e r .

Y e t al3- t h e s e c o n d i t i o n s mean n o t h i n g t o t h e noxTnal b e a c h - f r o n t r e s i d e n t , h i s L o c a l A u t h o r i t y and o f t e n even t o h i s S t a t e Government D e p a r t m e n t , To a l l o f t h e s e , o n l y so o f t e n , t h e o n l y c r i t e r i a o f success o f a beach n o u r i s h -ment p r o j e c t , i s an i m m e d i a t e and d r a j n a t i c i n c r e a s e i n t h e w i d t h o f t h e

A d s i b l e beach. T h i s may, o r may n o t happen o f c o u r s e , d e p e n d i n g upon t h e

nourislrjment r e s c o u r c e a c t u a l l y u t i l i s e d , b u t t h e publ.ic u s u a l l y demand o n l y t h e g r e a t e s t v i s u a l " i m p a c t " , and n o t the g r e a t e s t t o t a l optimum c o a s t a l " p r o t e c t i o n " . F o r as l o n g as t h i s s o c i a l c o n f l i c t c o n t i n u e s , t h e n a l l b e a c h nonz'ishment e x e r c i s e s may w e l l be plagu.ed b y t h i s c l a s s o f p u b l i c a c c e p t a n c e , p a r t i c u l a r l y a f t e r w a r d s !

I t may a l s o be a truism,5 t h a t many beach n o u r i s h m e n t e x e r c i s e s i n t h e p a s t , have been mounted f o r s i m p l e i m m e d i a t e p o l i t i c a l r e a s o n s , and w i t h o u t any

s e r i o u s r e s e a r c h i n t o t h e i r p o t e n t i a l consequences. They have o f t e n been r u s h e d i n t o f r u i t i o n p u r e l y on' t h e s i m p l e h y p o t h e s i s t h a t " s e d i m e n t i s s e d -i m e n t " , and even -i f beach w -i d e n -i n g v a n -i s h e s a f t e r a few y e a r s , t h e n t h e

r a t e - p a y e r s w i l l have f o r g o t t e n a b o u t i t a l l anyv/ay. T h i s r e p r e s e n t s o f c o u r s e a l m o s t i r r e s p o n s i b l e e n g i n e e r i n g . As d i s c u s s e d b e l o w , t h e a c t u a l d e s i g n p r o -cesses n e c e s s a r y t o e v a l u a t e a r a t i o n a l b e a c h n o u r i s h m e n t p r o j e c t , a r e com-p l e x , f r u s t r a t i n g , and i n t h e end, based a l m o s t e n t i r e l y ucom-pon c o n c e com-p t s o f p r o b a b i l i t y , t h a t w i t h i n c u r r e n t k n o w l e d g e , c a n n o t be r e l i a b l y d e f i n e d . As w i t h many o t h e r f r o n t i e r d i s c i p l i n e s , beach n o u r i s h m e n t r e m a i n s much more an " a r t " t h a n a " s c i e n c e " .

I n s u m m a r i s i n g , t h e r e s u l t s o f any beach n o u r i s h m e n t e x e r c i s e , i t must be r e p e a t e d t h a t r e g a r d l e s s o f t h e D e s i g n e r ' s i n t e n t . N a t u r e w i l l s e l e c t f o r h e r

s e l f wh^t she w i l l do w i t h t h e e x t r a s e d i m e n t o f f e r e d t o h e r . She w i l l c e r t -a i n l y c o n s t r u . c t -a new v -a r i -a b l e " r e g i m e " b e -a c h f r o m t h e new s e d i m e n t i n p u t . B u t t h e n t h e b e a c h may f l a t t e n o r s t e e p e n i n a q u i t e u n p r e d i c t a b l e manner,

i t depends upon t h e n e x t a m b i e n t wave c l i m a t e , and t h e degree o f i n t e r - m i x i n g between n a t i v e and nourisloment m a t e r i a l . These a r e a l l , i n t h e c u r r e n t s t a t e

o f lmowledg8f a l m o s t c o m p l e t e i n t a n g i b l e s ; r e l i a b l e d a t a on t h e a c t u a l b e h a v -i o u r o f n o u r -i s h m e n t -i n j e c t -i o n s t o n a t u r a j beaches, r e m a -i n s r e m a r k a b l y spa.rse. Much has been o b s e r v e d , yet l i t t l e i s r e a l l y -understood.

Y e t t h e b a s i c p r i n c i p l e r e m a i n s , Nat-ure w i l l a l w a y s m a i n t a i n a s u i t a b l e dynami beach f r o m any beach n o u r i s h m e n t m a t e r i a l , i t may w e l l v a r y t h e f o r m b f t h e new beach, b u t t h e end r e s u l t w i l l a l w a y s r e p r e s e n t an e f f e c t i v e maximum e n e r g y a b s o r b t i o n system. As s u c h , N a t u r e w i l l a l w a y s a d j u s t t h e b e h a v i o u r o f M l " a r t i f i c i a l " beach t o s u i t h e r needs, b u t t h e f i n a l beach behaviour-must a l w a y s be " n a t u r a l " . A beach behaviour-must a l w a y s r e m a i n a beach, and i t w i l l a l w a y s r e a c t t o - n a t u r a l p r o c e s s e s . I t w i l l a l w a y s r e a c t t o n a t u r a l h y d r a u l i c f o r c e s i n an optimum manner, b u t f o r an economy o f words, t h e s e p o t e n t i a l r e a c t i o n s a r e d i s c u s s e d b e l o w , u n d e r t h e c o n s i d e r a t i o n s o f t h e D e s i g n o f N o u r i s h m e n t Works.

( 4 ) P A I L W E MODE

As d i s c u s s e d above, beach n o u r i s h m e n t can n e v e r r e s u l t i n a " f a i l u r e " . E v e r y c u b i c m e t r e o f e x t r a s e d i m e n t can o n l y I m p r o v e t h e r e s i s t a n c e o f a beach t o b o t h s h o r t , and l o n g t e r m e r o s i o n . The more s e d i m e n t a v a i l a b l e , t h e n t h e more must be t h e t o t a l e n e r g y a b s o r b t i o n c a E a c i t j a beach. On t h e o t h e r h a n d , t h e r e a r e l i m i t s t o t h i s c o n c e p t . I f a v e r y f i n e n o u r i s h m e n t m a t e r i a l i s a p p l i e d t o a c o a r s e n a t i v e beach, most w i l l m i g r a t e i n t o deep w a t e r . Then n o t h i n g o f t h e new viork may r e m a i n on t h e v i s i b l e beach a t a l l . I n t h i s case i t i s more i n t e r m s o f human a m e n i t y , t h a t t h e e x e r c i s e may be r e g a r d e d as a f a i l u r e . The b e a c h may have been s t r e n g t h e n e d , b u t n o t e f f e c t i v e l y , because t h e added m a t e r i a l was u n s u i t a b l e f o r t h e i m m e d i a t e t a s k .

I n o t h e r c a s e s , beach n o u r i s h m e n t p r o j e c t s have been p o p u l a r l y r e g a r d e d as f a i l u r e s , s i m p l y as a r e s u l t o f t h e manner t h a t t h e work was c a r r i e d o u t . I f a l a r g e volume o f new s e d i m e n t i s p l a c e d on t h e to£ s e c t i o n o f an e x i s t i n g beach d u r i n g f i n e w e a t h e r , -then t h e s m a l l e s t waves cannot g a i n access t o t h e

9

materia,!, aaid r e - d i s t r i b u t e i t i n t o t h e wave e n e r g y zones t h a t need i t most. The new m a t e r i a l w i l l r e m a i n as a p e r c h e d s l i x g o f sediment o v e r t h e above w a t e r beach. The p u b l i c t h e n v i e w s t h i s as a marked v i s i b l e s u c c e s s . Most

o f t h e s e d i m e n t i s t h e r e , i t can be seen t o have b u i l t up t h e beach. Then, when h i g h e r e n e r g y waves o c c u r , t h e p e r c h e d s l u g i s r a p i d l y d i s t r i b u t e d i n t o t h e swept p r i s m . Tiie beach has s u d d e n l y become n a r r o w e d above sea l e v e l , so t h e p u b l i c i m m e d i a t e l y p e r c i e v e s t h i s as a.n " e r o s i o n " , and as a " l o s s " . I n t h e s e t e r m s , t h e c o n s t r u c t o r s and d e s i g n e r s o f t h e scheme, have g e n e r a t e d t h e i r o m p u b l i c r e l a t i o n s d i s a s t e r . Perhaps u n w i t t i n g l y , t h e y have l e d t h e p u b l i c t o e x p e c t t h a t once t h e new s e d i m e n t has appeared on t h e beach, t h e n

i . t w i l l s t a y t h e r e f o r e v e r . T h i s i s n o t t h e b e h a v i o u r mode o f dynamic beaches, b u t u n d e r t h e c i r c u m s t a n c e s , who c a n r e a l l y blame t h e p u b l i c f o r regaa^ding

t h e work as an a p p a r a n t f a i l u r e ?

( 5 ) DESIGN OP NOUEISHiyiEWT WOEKS

W i t h i n c u r r e n t s t a t e s o f c o a s t a l p r o c e s s Icnovjledge, no beach n o u r i s l i m e n t p r o j e c t c a n be p r o p e r l y and f u l l y d e s i g n e d , n o r c a n t h e p o s t p l a c e m e n t b e -h a v i o u r o f t -h e new s e d i m e n t be a c c u r a t e l y p r e d i c t e d . However, even w i t -h i n o u r v e r y i m p e r f e c t u n d e r s t a n d i n g o f d e t a i l e d bea.ch p r o c e s s e s , many o f t h e l i k e l y s e d i m e n t r e s p o n s e modes c a n be c o n s i d e r e d i n advance, b u t l a r g e l y o n l y i n t e r m s o f p o s s i b i l i t i e s and p r o b a b i l i t i e s .

Over r e c e n t y e a r s many n o u r i s h m e n t p r o j e c t s have been c a r e f u l l y o b s e r v e d and m o n i t o r e d d u r i n g t h e c o n s t r u c t i o n phase, and sometimes f o r a f e w y e a r s a f t e r -wards. Subsequent a n a l a y s e s o f t h e s e o p e r a t i o n s however, has o n l y d e m o n s t r a t e d t h a t w h i l s t many nou.rishment p r o c e s s e s have i d e n t i f i e d , a s u f f i c i e n t v o l u n e o f d a t a t o q u a n t i f y them, h a s n e v e r become a v a i l a b l e . T h i s s h o u l d n o t b e

s u r p r i s i n g . The e q u i v a l e n t d a t a f o r t h e d e t a i l e d p r o c e s s e s on n a t u r a l beaches does n o t e x i s t e i t h e r , and i t w i l l be many y e a r s b e f o r e i t can be a c c u m u l a t e d i n any case. Then, a t t h e p o l i t i c a l l e v e l , t h e n o u r i s h m e n t works c o n s t r u c t i o n , and f u n d i n g a u t h o r i t i e s , u s u a l l y p e r c i e v e t h e work t o be so s i m p l e and o b y i o u s ,

10

t h a t t h e y a r e e x t r e m e l y r e l u c t a n t t o expend e x t r a monies on e x t e n s i v e da;ba c o l l e c t i o n . N e v e r t h e l e s s , w i t h i n a l l t h e s e c o n s t r a i n t s , t h e r e a r e a t l e a s t seven f a c t o r s t h a t s h o u l d be c o n s i d e r e d b e f o r e moixnting a beach n o u r i s h m e n t e x e r c i s e . As s u c h , t h e c o n s i d e r a t i o n o f t h e s e f a c t o r s , p a r t i c u l a r l y i n p r o b a b i l i s t i c t e n n s , t o g e t h e r w i t h t h e i r p o t e n t i a l i n t e r - r e l a t i o n , 3?epresents V7hat " d e s i g n " s t e p s as a r e a c t u a l l y a v a i l a b l e f o r t h i s c l a s s o f work. These a r e : -( a ) M a t e r i a l Type. T h i s r e m a i n s t h e d o m i n a n t d e s i g n p a r a m e t e r , and i n f a c t o f t e n a l m o s t t h e o n l y one c o n s i d e r e d . The e n e r g y a b s o r b t i o n , o r s t a b i l i t y f a c t o r , o f any s e d -i m e n t , -i s g e n e r a l l y d -i r e c t l y p r o p o r t -i o n a l t o -i t s s -i z e . I n N a t u r e , as d -i s c u s s e d above, t h e l a r g e r t h e p a r t i c l e t h e n t h e s t e e p e r t h e beach, pjid c o n v e r s e l y s m a l l e r t h e p a r t i c l e , t h e f l a t t e r t h e b e a c h , a l l f o r t h e same wave e n e r g y c l i m a t e .

F o r n a . t u r a l beach p a r t i c l e s t h a t may v a r y i n s i z e , b u t h o l d a s i m i l a r shape, o r g e o m e t r y , t h e r e i s much e v i d e n c e t o s u g g e s t t h a t t h i s i s an a l m o s t u n i v e r s a l n a t u r a l "law" f o r b e a c h b e h a v i o u r . F o r p a r t i c l e s o f an unusxial shape h o w e v e r ,

t h i s " l a w " a p p e a r s t o l a p s e t o much e x t e n t . I t i s c o n v e n t i o n a l t o " s i z e " beach p a r t i c l e s b y s i e v i n g , o r p a s s i n g them t h r o u g h d e c r e a s i n g l y s i z e d a p e r t u r e s . T h i s p r o c e s s a c t u a l l y "measures" t h e second l a r g e s t " w i d t h " o f a p a r t i c l e , b u t as l o n g as t h e p a r t i c l e s a l l m a i n t a i n a s i m i l a r shape, t h e n i t r e p r e s e n t s a ' r e a s o n a b l y r e l i a b l e i n d i c a t o r o f i t s " s i z e " . F o r l o n g f l a l c e y , h i g h l y a n g u l a r , o r n a t u r a l s h e l l - l i k e p a r t i c l e s however, s i e v i n g i s n o t e f f e c t i v e i n m e a s u r i n g t h e p a r t i c l e " s i z e " . Under t h e s e c i r c u m s t a n c e s i t becomes n e c e s s a r y t o i n v o k e a more b a s i c p a r a m e t e r o f p a r t i c l e shape — such as i t s s p e c i f i c s u r f a c e . Hovjever, i f t h e p a r a m e t e r o f s p e c i f i c s u r f a c e . i s u s e d t o c l a s s i f y s e d i m e n t p a r t i c l e s , t h e n a g a i n , t h e r e appears t o be a c o r r e l a t i o n , t h e h i g l i e r t h e s p e c i f i c s u r f a c e , t h e n t h e f l a t t e r t h e b e a c h , and v i c e v e r s a . I t may be n o t e d t h a t s m a l l p a r t i c l e s h o l d a h i g h s p e c i f i c s u r f a c e , and l a r g e p a r t i c l e s a l o w one .

11

I n p r a c t i c a l t e r m s what may t h u s be eacpected, i s t h a t i f t h e n o u r i s h m e n t m a t e r i a l i s c o a r s e r t h a n t h e n a t i v e , t h e n n o u r i s h m e n t w i l l r e s u l t i n a s t e e p -e n i n g o f t h -e n a t u r a l b -e a c h , and f i n -e r n o u r i s l i m -e n t m a t -e r i a l w i l l r -e s u l t i n a f l a t t e r beach. A t t h i s s t a g e , i t may be r e l e v e n t t o c o n s i d e r t h e e x t r e m e s . I f c o a r s e g r a v e l i s dumped a t t h e t o p o f a f i n e b e a c h , t h e n i t w i l l t e n d t o s t a y t h e r e i n t h e f o r m o f a s t e e p s c a r p , because t h e i n n e r m o s t wave e n e r g y i s i n c a p a b l e o f m o v i n g i t . On t h e o t h e r hand, i f a l o a d o f v e r y f i n e sand o r s i l t , i s dumped on t o t h e b e a c h , i t w i l l v e r y r a p i d l y be c o m p l e t e l y washed away i n t o d e e p - w a t e r . I n t h i s case, t h e s e d i m e n t i s so f i n e , and r e q u i r e s such a f l a t beach s l o p e t o l e t i t e x i s t i n any s t a t i o n a r y p l a c e a t a l l , t h a t t h e a m b i e n t wave e n e r g y w i l l m a i n t a i n i t i n s u s p e n s i o n u n t i l i t i s l o s t f r o m t h e system.

F o r t h e l a s t t h i r t y y e a r s t h e r e f o r e , t h e o p e r a t i o n o f any beach n o u r i s h m e n t scheme has been b a s e d upon t h e ' i n t e n t t o u t i l i s e n o u r i s h m e n t m a t e r i a l t h a t was e i t h e r t h e same, o r c o a r s e r i n " s i z e " t h a n t h e n a t i v e . I n s o c i a l t e r m s ,

t h i s i s sound e n g i n e e r i n g . I f a c o a r s e r n o u r i s h m e n t m a t e r i a l r e s u l t s i n a s t e e p e r a v e r a g e b e a c h , t h e n i t must a l s o r e s u l t i n a w i d e r " v i s i b l e " b e a c h . T h i s i s what t h e p u b l i c and t h e r a t e p a y e r s e x p e c t , and on t h i s b a s i s t h e y w i l l r e g a r d t h e n o u r i s h m e n t e x e r c i s e as a s u c c e s s . Then, t h e y may even be p r e p a r e d t o pay f o r t h e n e x t e x e r c i s e .

I n many l o c a l i t i e s however, t h e p o t e n t i a l s o u r c e s o f n o u r i s h m e n t m a t e r i a l , a r e " f i n e r " t h a n t h e n a t i v e . Then t h e p r o j e c t d e s i g n e r i s f a c e d w i t h t h e u s u a l q u a n d r y . S h o u l d he recommend n o u r i s h m e n t , s i m p l y because i t must h e l p

i n any c a s e , o r s h o u l d he c o n s i d e r t h e v i s u a l i m p a c t o f t h e p o t e n t i a l l y new f l a t t e n e d b e a c h , and recommend t h a t n o u r i s h m e n t i s n o t a v i a b l e o p t i o n ? I n d e e d , s h o u l d he recommend " h a r d " o p t i o n s , such as g r o y n e s o r revetments,- when he knows t h a t t h e y may v / e l l be l e s s s u c c e s s f u l o v e r a l l ? U n f o r t u n a t e l y , t h i s i s

j u s t -bhe s t u f f t h a t C o a s t a l E n g i n e e r i n g i s made f r o m .

may s t i l l r e s t o r e t h e beach t o a g i v e n v i s i b l e w i d t h . I t i s j u s t t h a t much more n o t i r i s h m e n t m a t e r i a l w i l l be r e q u i r e d — t h e n i t may become a s i m p l e m a t t e r o f economics. I f f i n e r n o u r i s l i m e n t m a t e r i a l c a n be o b t a i n e d a t much l e s s e r c o s t t h a n c o a r s e r m a t e r i a l , t h e n i t may become r a t i o n a l t o u t i l i s e much l a r g e r volumes o f f i n e m a t e r i a l t o a t t a i n t h e same end r e s u l t . Then, i f no c o a r s e r m a t e r i a l i s a v a i l a b l e , t h e u s e o f f i n e m a t e r i a l a l o n e may r e m a i n t h e c m l ^ o p t i o n .

Over t h e y e a r s , many t h e o r e t i c a l s t u d i e s have been made, i n an e f f o r t t o p r e d i c t t h e v a r i o u s amounts o f n o u r i s h m e n t , o r as t h e Americans c a l l i t

" b o r r o w " m a t e r i a l t o a c h i e v e a g i v e n volume o r v j i d t h f o r t h e new r e p l e n i s h e d beach. As f a r as i s Icnowi, none o f t h e s e t h e o r e t i c a l r e l a t i o n s h i p s have e v e r been c a l i b r a t e d a g a i n s t any r e a l p r o t o t y p e , so a g r e a t d e a l o f u n c e r t a i n t y i s i n v o l v e d i n t h e a p p l i c a t i o n o f t h e s e r e l a t i o n s h i p s . Much o f t h e e a r l y " u n d e r f i l l " and " o v e r f i l l " r a t i o r e s e a r c h was b a s e d upon a complex s e r i e s

o f p r o p e r t y moments d e r i v e d f r o m s i m p l e s i e v e t e s t i n g . Recent r e s e a r c h however, has s u g g e s t e d t h a t p a r t i c l e p r o p e r t i e s b a s e d upon s i m p l e s i e v i n g , may be

somewhat u n r e l i a b l e , vfhen a p p l i e d t o r e a l beach p a r t i c l e s .

The most r e c e n t t h e o r e t i c a l a p p r o a c h t o t h i s m a t t e r w i t h i n A u s t r a l i a , b a s e d on t h e w o r k o f S m i t h & Gordon t h e r e f o r e , has been t o c o n s i d e r t h e p a r t i c l e p r o p e r t y o f s p e c i f i c s u r f a c e , i n s t e a d o f some n o m i n a l s i e v e p a r a m e t e r . W i t h i n t h i s r e a l m , t h e o n l y b a s i c a s s u m p t i o n made i n t h e t h e o r y , i s t h a t t h e b e a c h s l o p e w i l l be p r o p o r t i o n a l t o a c o m b i n a t i o n o f t h e wave e n e r g y and t h e s e d i m e n t s p e c i f i c s u r f a c e . T h i s a s s u m p t i o n i s c e r t a i n l y no w o r s e , t h a n t h o s e i n v o l v e d i n o t h e r t h e o r i e s , i t i s a t l e a s t s i m p l e and d i r e c t , and i t r e m a i n s q u i t e open f o r f u t u r e p r o t o t y p e p r o o f . I n f a c t n o t h i n g b e t t e r i s y e t a v a i l a b l e , so i t a t l e a s t r e p r e s e n t s some s t a r t i n g p o i n t . f o r a s s e s s i n g beach n o u r i s h m e n t p r o b a b i l i t i e s . T h i s p a r t i c u l a r t h e o r y , o f c o u r s e , l i k e a l l t h e o t h e r s e x t a n t , has y e t t o be r i g o r o u s l y s u b s t a n t i a t e d i n t h e f i e l d , so i t must s t i l l r e m a i n c u r r e n t l y , as j u s t a n o t h e r t h e o r y .

15

On t h e o t h e r hand, t h i s t h e o r y a l l o w s f o r a d i r e c t e s t i m a t e t o be made f o r any

given

b o r r o w m a t e r i a l ' s beach slope-; compared w i t h t h e n a t i v e . I n f a c t t h e natd.ve m a t e r i a l a J r e a d y e x i s t s , and i t s r e g i m e beach s l o p e s c a n be d i r e c t l y measured be s u r v e y . The t h e o r y t h u s m e r e l y a l l o w s f o r some r e a s o n a b l y r a t i o n a l a p p r o a c h , f o r c a l c u l a t i n g a beach s l o p e cjiange, and does n o t r e l y on p o s t u -l a t i n g a b s o -l u t e beach s -l o p e s f r o m f i r s t p r i n c i p -l e s , and a -l -l i n t h e f i r s t p l a c e . C l e a r l y , once a beach s l o p e change, has been e s t i m a t e d , t h e n an a p -p r o x i m a t e v o l - u m e t r i c r e q u i r e m e n t c a l c u l a t i o n i s easy, f o r any g r a d e o f b o r r o w m a t e r i a l .

As an example, F i g . 5 d e m o n s t r a t e s how t h e t h e o r y may be a p p l i e d , f o r t h e c o m p a r i s i o n o f two p o t e n t i a l b o r r o w m a t e r i a l s , b o t h o f v e r y d i f f e r e n t s e d -i m e n t shape p r o p e r t -i e s . T h -i s d -i a g r a m -i s c l e a x l y s -i m p l -i s t -i c , b u t -i t a t l e a s t r e p r e s e n t s a s t a r t i n g p o i n t , t h a t may w e l l be o f p r a c t i c a l c o n s i d e r a t i o n . I n p a r t i c u l a r , t h e d i a g r a m assumes a n o u r i s h m e n t volume l a r g e r t h a n c r i t i c a l , and t h a t no i n t e r m i x i n g has h a d t i m e t o o c c u r (see b e l o w ) . These p a r t i c u l a r f a c t o r s however, may be o f c o n s i d e r a b l e s i g n i f i c a n c e w i t h i n any a c t u a l b e a c h n o u r i s h m e n t e x e r c i s e t h a t may be mounted, so a g a i n t h i s c l a s s o f s l o p e e s t i m a t e must be c o n s i d e r e d as r e p r e s e n t i n g p o t e n t i a l l i m i t s t o t h e b e a c h s y s t e m r e s p o n s e . Or a g a i n , a m a t t e r o f p l a i n p r o b a b i l i t y . I n many e n g i n e e r i n g e n t e r p r i s e s ,

i t i s o f t e n j u s t as v a l u a b l e , t o know w i t h i n w h i c h " b a l l - p a r k " one i s o p e r a t i n g , t h a n t o Imow o f any i n d i v i d u a l p a r t i c u l a r " s t a t e " t h a t may happen t r a n s i a n t l y f r o m t i m e t o t i m e . U n t i l s u c h t i m e t h a t r i g o r o u s r e s e a r c h p r o v i d e s a more r e l i a b l e c o n c e p t , t h e n t h e beach n o u r i s h m e n t d e s i g n e r d e s i g n e r may w e l l n o t e x p e c t a n y t h i n g b e t t e r . ( b ) I n t e r m i x i n g . , • I t i s i m p o s s i b l e t o p l a c e l a r g e volumes o f n o u r i s h m e n t m a t e r i a l i n s t a n t a n e o u s l y , and t h e r e a l r a t e s o f d e p o s i t i o n a r e c o m p a r a t i v e l y s l o w . S i n c e t h e seabed o f any beach i s m o b i l i s e d b y a l l wave a c t i o n , t h e n as soon as new b o r r o w m a t e r i a l