Coastal Measurements in the Netherlands

ministerie van verkeer en waterstaat rijkswaterstaat

dienst weg- en waterbouwkunde afdeling advisering waterbouw

coastal measurements in the Netherlands

Jarkus

Introduction

Because a sandy coast is a very dynamic coastline, and because stability of the coastline is vital for the safety of the Netherlands against inundation by storm surges (dunes protect the polder-area, which is situated below sea le-vel), already in former centuries i t was necessary to get some knowledge about

the movements of the coastline. In the middle of the 19th century systematic coastal measurements started. On fixed intervals (approx. 250 m) the position of the dune-foot, the high water line and the low water line was measured every year. In order to have a fixed reference, monuments were placed on the beach. Around 1960 it became clear that more detailed information was necessary. Therefore the system of monuments and base lines was somewhat updated, and a detailed program for coastal measurements was set up.

Along the whole North Sea coastline from Cadzand in the south to Rottumeroog a fixed set of measuring lines was defined (see fig. 1). In total approx. 3000 lines are defined this way. The lines have a intermediate distance of 200 m and are perpendicular to the base line. The base line is parallel to the coast-line, and is positioned approx. near the high water line. On coastal sections with groins, the measuring lines are placed in the middle between the groins. Profiles are measured every year from 200 m landward of the base line to 800 m seaward of the base line (that means to an average depth of 8 m below mean sea level). Every five years every kilometer a line is measured until 2500 m from the base line. All measurements are made in the period between april and

september. ,

The measurements are performed in two parts (see fig. 2):

*

leveling above the low wat.er line

*

sounding below approx. mean sea level line leveling

The levelings are done by the survey department of Rijkswaterstaat, using aerial photography (see fig. 3). In the field only some calibration points are measured using conventional equipment. In flight direction 60 % overlap is used in order to get a good stereoscopic effect. In more than one strip is required, an overlap of 20 - 30 % between the strips is used. With the use of an analytic plotter the height is measured in the measuring lines. In this plotter the overlapping photographs are presented a a three dimensional image. The position of the bends in the profile are read as digital values. The program interpo-lated the data, and every 5 m .a height-point of the profile is stored in memory.

soundings

The soundings are done by survey vessels of various Rijkswaterstaat departments and by survey vessels of the waterboards. Profile data are automatically stored on board as x-y-z- values. Postprocessing equipment calculates the depths in the profile with intermediate distances of 10 m.

s t o r a g e

L e v e l i n g d a t a and s o u n d i n g d a t a a r e c o u p l e d and s t o r e d on a mainframe computer o f R i j k s w a t e r s t a a t . The d a t a a r e a v a i l a b l e t o c o a s t a l managers, r e s e a r c h e r s and everyone e l s e who i s i n t e r e s t e d . A l l d a t a a r e o n - l i n e a v a i l a b l e s i n c e 1963. T h i s data-base i s c a l l e d J a r k u s ( a b b r e v i a t i o n f r o m J A a R l i j k s e KUStmetingen, y e a r l y c o a s t a l measurements). Post p r o c e s s i n g s o f t w a r e i s a v a i l a b l e t o r e a d t h e d a t a , and t o p e r f o r m a d d i t i -o n a l c -o m p u t a t i -o n s . S t a n d a r d P -o s t p r -o c e s s i n g i s p l -o t t i n g p r -o f i l e s ( f i g . 4 ) and making t h r e e d i m e n s i o n a l p l o t s ( f i g . 5 ) . A l s o v o l u m e t r i c c a l c u l a t i o n s can be made. T h i s can be done i n h o r i z o n t a l and i n v e r t i c a l s l i c e s ( f i g . 6 ) . The v e r t i c a l s e c t i o n s a r e used f o r g e n t l e c o a s t l i n e s , w h i l e t h e h o r i z o n t a l s e c t i o n s

a r e used f o r c o a s t l i n e s f a c i n g r e l a t i v e l y s t e e p t i d a l c h a n n e l s .

Records o f t h e J a r k u s - d a t a b a s e have a f i x e d f o r m a t ( f i g . 7 ) . T h i s f o r m a t has become t h e s t a n d a r d o f a l l c o a s t a l i n f o r m a t i o n systems and c o a s t a l computer models i n t h e N e t h e r l a n d s .

One measurement c o n s i s t s o f a header and d e p t h d a t a . The h e a d e r f o r m a t i s 716 and c o n t a i n s t h e f o l l o w i n g i n f o r m a t i o n : 1. C o a s t a l s e c t i o n 1 . Rottum 9. D e l f l a n d 2. Schiermonnikoog 10. M a a s v l a k t e 3. Ameland 1 1 . Voorne 4. T e r s c h e l l i n g 12. Goeree 5. V l i e l a n d 13. Schouwen 6. T e x e l 14. Noord B e v e l a n d 7. Noord H o l l a n d 15. W a l c h e r e n 8. R i j n l a n d 16. Zeeuwsch-Vlaanderen 2. y e a r , i n 4 d i g i t (1982) 3. p r o f i l e number ( w h i c h i s i n f a c t a d i s t a n c e f r o m a z e r o - p o i n t a l o n g t h e c o a s t ) . The p r o f i l e number i s g i v e n i n 10 m ( t h u s : p r o f i l e 07 - 234 i s 2.34 km s o u t h o f Den-Helder a l o n g t h e c o a s t o f Noord H o l l a n d ) . 4. P r o f i l e t y p e 0 - n o r m a l ( o f f i c i a l ) p r o f i l e 1 - a d d i t i o n a l p r o f i l e i n t h e a x i s o f a g r o i n 2 - a d d i t i o n a l p r o f i l e p a r a l l e l t o a g r o i n 5. d a t e o f measurement ( l e v e l i n g ) ddmm 6. d a t e o f s o u n d i n g ddmm 7. number o f p o i n t s The d e p t h l i n e s c o n t a i n t h e f o l l o w i n g i n f o r m a t i o n 5(2X6,11,3X)

8. h o r i z o n t a l d i s t a n c e i n meters f r o m base l i n e (seaward i s p o s i t i v e )

9. v e r t i c a l d i s t a n c e i n c e n t i m e t e r s r e l a t i v e t o N a t i o n a l Datum (NAP), w h i c h i s approx. a t Mean Sea L e v e l ( d e p t h a r e n e g a t i v e , h e i g h t s a r e p o s i t i v e ) . 10. c o n t r o l code ( f r o m t h e code i t c a n be seen i f d a t a a r e l e v e l i n g o r

soun-d i n g soun-d a t a ) .

Dune e r o s i o n

The J a r k u s database i s a t t h i s moment i n use f o r r o u t i n e checks o f t h e s a f e t y o f o u r dune c o a s t . Every y e a r a l l p r o f i l e s have t o be j u d g e d i f t h e p r o v i d e enough s a f e t y . The t e c h n i q u e t o do t h i s i s d e s c r i b e d i n t h e " g u i d e t o t h e assessment o f t h e s a f e t y o f dunes as a sea d e f e n s e " [ e n g l i s h e d i t i o n , CUR, 1 9 8 9 ] .

I n f a c t an s t o r m i n d u c e d e r o s i o n a n a l y s i s has t o be made f o r a s e r i e s o f p r o

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O GanI Location (10^ tPy _period 1 toaland 2,2 1980 2 Amelarri 0,31 1979 3 Texel, Eierlarei 3,05 1979 4 Texel, Eierland ' 2,85 1985 5 Texel, da Koog 3,02 1984 6 Callantsoog 0,35 1976-1977 7 Callantsocg 0,47 1979-1980 8 Scheveningen 0,045 1969 9 Scheveningen 0,7 1975 10 Scheveningen 0,33 1985

11a Heek V. Holland 18,94 1971-1972 11b Hoek V. Holland 1,50 1976-1977 12 Hoek V. Holland 0,87 1977-1978 13 B r i e l s a Gat Ceua 0,15 1979-1980 14 Voorra 0,11 1974 15 Voorna 0,15 1974 16 Voorna 1,10 1977 17 Voorna 0,44 1983 18 Voorna 3,40 1984-1985 19 Goeroa 0,40 1969-1970 20 Goeree 0,61 1971 21 Goeree 3,64 1973-1974 22 Goeree 1,27 1977 23 Goeree 0,33 1984 24 Goeree 0,53 1985 25 Schouwen 0,112 1975 26 Noord Beveland 0,21 1973 27 VÈuLcheren 0,10 1984 28 Vfalcheren 0,775 1952-1959 29 v l i s s i n g e n 0,05 1952 30 Vlissingen 0,032 1966 31 V l i s s i n g e n 0,045 ,1975 32 Breskens 0,206 1971 33 Callantsoog 1,3 1986 34 Texheijde 3,0 1986 35 Voome 1,0 1986 36 Conburg 0,23 1986 37 Kop V . Schouwen 1,83 1987 38 Westkapelle 0,025 1988 39 Westkapelle 0,41 1988 40 Zoutelande 0,15 1988 41 Cadzand 0,85 1988 42 Zwanenwater 1,85 1987 43 Hoek V. Hollard 0,2 1988 Totaal 59,14

f i l e measurements ( a p p r o x . 10 y e a r s ) . For each y e a r t h e c a l c u l a t i o n r e s u l t e d i n a maximum e r o s i o n p o i n t d u r i n g a d e s i g n s t o r m . Through t h e c a l c u l a t e d e r o s i o n p o i n t s ( p o i n t P ) , a r e g r e s s i o n l i n e has t o be drawn ( f i g . 8 ) . E x t r a p o l a t i o n o f t h i s l i n e u n t i l n e x t y e a r i n d i c a t e s i f t h e r e i s s t i l l s u f f i c i e n t s a f e t y a v a i l a -b l e . Computer programs on a PC a r e a v a i l a b l e t o p e r f o r m t h e c o m p u t a t i o n s , u s i n g t h e d a t a f r o m t h e J a r k u s database. A l l c o a s t a l managers have t o do t h e s e computa-t i o n s on a r o u computa-t i n e b a s i s ( f i g . 9 ) .

E v a l u a t i o n o f beach n o u r i s h m e n t p r o j e c t s

I n October 1987 a r e p o r t has been p r e p a r e d t o e v a l u a t e beach n o u r i s h m e n t p r o -j e c t s i n t h e N e t h e r l a n d s . For t h e p r e p a r a t i o n o f t h e C o a s t a l Memorandiim o f 1989 a number o f t e c h n i c a l r e p o r t s has been p r e p a r e d . The t e c h n i c a l r e p o r t n r . 12 (beach and dune n o u r i s h m e n t ) i s a f o l l o w up o f t h e r e p o r t t o t h e p a r l i a m e n t o f October 1987.

I n t h i s paper a summary o f t h e s e r e p o r t s i s g i v e n , w i t h some more d e t a i l e d i n f o r m a t i o n on a few s p e c i f i c p r o j e c t s .

Beach n o u r i s h m e n t s can be p l a c e d a t s e v e r a l s i t e s i n t h e c r o s s s e c t i o n a l p r o f i l e , see f i g u r e 10. Sand i s i n t h e N e t h e r l a n d s a l m o s t always p l a c e d by h y d r a u -l i c equipment. Dry h a u -l i n g i s h a r d -l y used. U s i n g h y d r a u -l i c dredges a q u a n t i t y o f 200 000 - 500 000 m-^ p e r week i s f e a s i b l e . The c o a s t o f a n o u r i s h m e n t work i n c r e a s e s i g n i f i c a n t l y i f t h e sand has t o be p l a c e d h i g h e r on t h e beach. I n f i g u r e 11 i s i n d i c a t e d t h e q u a n t i t y o f sand n o u r i s h e d on t h e Dutch c o a s t i n t h e f o r m e r y e a r s . T a b l e 1 g i v e s an o v e r v i e w o f t h e e x e c u t e d n o u r i s h m e n t w o r k s . For t h e e v a l u a t i o n a s e l e c t i o n was made f r o m a l l t h e p r o j e c t s . The c r i t e r i a f o r t h i s s e l e c t i o n were

- t h e s i z e o f t h e n o u r i s h m e n t , i n r e l a t i o n t o t h e n a t u r a l p r o c e s s o f e r o s i o n - t h e t y p e o f n o u r i s h m e n t ; dune improvements were n o t e v a l u a t e d .

The e v a l u a t i o n i s done by u s i n g t h e d a t a f r o m t h e J a r k u s - d a t a b a s e . I t i s assumed t h a t t h e boundary o f t h e a c t i v e zone l i e s w i t h i n 800 m f r o m t h e base l i n e s and t h a t c o n s e q u e n t l y t h e whole a c t i v e zone i s c o v e r e d b y t h e J a r k u s d a t a . Because o f t i d a l c h a n n e l s f o r t h e n o u r i s h m e n t s i n Z e e l a n d d a t a a r e o n l y used u n t i l t h e m i d d l e o f t h e t i d a l c h a n n e l .

On t h e b a s i s o f t h e d a t a i n t h e J a r k u s database t h e volume o f t h e c o a s t a l s e c t i o n i n f r o n t o f t h e n o u r i s h m e n t has been c a l c u l a t e d . T h i s has been done f o r a number o f y e a r s , b e f o r e and a f t e r t h e n o u r i s h m e n t . I n t h i s way i t i s p o s s i b l e t o d e t e r m i n e how much sand i s d i s a p p e a r i n g due t o t h e n a t u r a l e r o s i o n o f t h e c o a s t and o f t h e e f f e c t o f t h e n o u r i s h m e n t .

I n t h e f i r s t y e a r g e n e r a l l y sand moves q u i t e f a s t f r o m t h e d r y beach t o t h e r e g i o n a r o u n d t h e low w a t e r l i n e . The v i s u a l e f f e c t i s a q u i c k d e c r e a s e o f t h e beach. V o l u m e t r i c c o m p u t a t i o n show, however, t h a t n o t r e a l l y v e r y much sand d i s a p p e a r s f r o m t h e p r o f i l e . The a d a p t a t i o n t o t h e n a t u r a l p r o f i l e i s c o m p l e t e -l y f i -l t e r e d o u t , by making c o r r e c t v o -l u m e t r i c c o m p u t a t i o n s .

Nine p r o j e c t s were e v a l u a t e d more i n d e t a i l t o see i f t h e n o u r i s h m e n t s d i d f u l f i l l t h e e x p e c t a t i o n s . T a b l e 2 summarizes t h e s e p r o j e c t s :

p r o j e c t y e a r d e s i g n l i f e e x p e c t e d l i f e t i m e 1. Ameland 1980 10 y e a r s 1990 w i l l be r e a c h e d 2. T e x e l - n o r t h 1979 5 y e a r s f u l f i l l e d e x p e c t a t i o n s 3. T e x e l - n o r t h 1985 5 y e a r s had a l i f e t i m e o f 4 y e a r s 5. T e x e l - m i d d l e 1984 10 y e a r s p r o b a b l y 1994 w i l l be r e a c h e d 9. Scheveningen 1975 n o u r i s h m e n t o n l y i f n e c e s s a r y

16. Voorne 1977 5 y e a r s had a l i f e t i m e o f more t h a n 5 y e a r s 21. Goeree 1973 - had a l i f e t i m e o f more t h a n 5 y e a r s 22. Goeree 1977 5 y e a r s had a l i f e t i m e o f more t h a n 5 y e a r s 27. W e s t k a p e l l e 1984 5 y e a r s f u l f i l l e d e x p e c t a t i o n s T a b l e 2: r e s u l t s o f beach n o u r i s h m e n t e v a l u a t i o n From t h e e v a l u a t i o n f o l l o w e d t h a t : * From t h e 9 e v a l u a t e d n o u r i s h m e n t s , 5 d i d f u l f i l l t h e e x p e c t a t i o n s , t h r e e w i l l p r o b a b l y f u l f i l l t h e e x p e c t a t i o n s and one f a i l e d ( l i f e t i m e 4 y e a r s i n s t e a d o f 5 y e a r s ) . * I t has been p r o v e d t h a t i t i s p o s s i b l e w i t h i n t h e v a r y i n g s i t u a t i o n s a l o n g a n a t u r a l c o a s t l i n e , t o d e s i g n a beach n o u r i s h m e n t p r o j e c t w i t h a p r e - d e f i n e d l i f e t i m e .

* The a p p l i c a t i o n o f beach n o u r i s h m e n t p r o j e c t s as a method o f c o a s t a l de-f e n s e i s e de-f de-f i c i e n t . The o c c u r r i n g e r o s i o n i s a d e q u a t e l y compensated by t h e n o u r i s h e d sand, a t a r e l a t i v e l y low c o s t .

I n g e n e r a l a l l beach n o u r i s h m e n t s i n t h e N e t h e r l a n d s have been d e s i g n e d on b a s i s o f t h e known c o a s t a l r e c e s s i o n . C a l c u l a t e d i s t h e volume o f s e d i m e n t w h i c h d i s s a p p e a r e d d u r i n g t h e l a s t y e a r s f r o m t h e c o a s t ( i . e . f r o m t h e t o p o f

t h e dune t o a d i s t a n c e o f approx. 800 m o u t o f t h e base l i n e ) . Then a q u a n t i t y o f sand was p l a c e d on t h e beach, compensating t h e e r o s i o n d u r i n g a g i v e n number o f y e a r s . F r o n e c o n o m i c a l o p t i m a z t i o n f o l l o w s t h a t t h e o p t i m a l d e s i g n l i f e o f a beach n o u r i s h m e n t i n t h e N e t h e r l a n d s i s g e n e r a l l y i n t h e o r d e r o f 5 y e a r s . Every y e a r t h e volume o f sand i n t h e c o a s t a l s e c t i o n i s d e t e r m i n e d and p l o t t e d . I f t h i s q u a n t i t y becomes l o w e r t h a n a minimum q u a n t i t y , r e n o u r i s h m e n t i s neces-s a r y . See a l neces-s o f i g . 12.

Because t h e new c o a s t l i n e forms some k i n d o f a p r o m o n t o r y i n t o t h e sea, i n t h e f i r s t y e a r s some e x t r a e r o s i o n m i g h t be e x p e c t e d , e s p e c i a l l y i f t h e l e n g t h o f t h e n o u r i s h m e n t i s r e l a t i v e l y s m a l l . No d e t a i l e d i n f o r m a t i o n i s a v a i l a b l e on t h e r e q u i r e d e x t r a q u a n t i t y . M o s t l y an o v e r f i l l r a t i o o f 20 % i s u s e d f o r t h i s p u r p o s e . I t i s assumed t h a t t h i s o v e r f i l l r a t i o a l s o compensates m i n o r d i f f e -r e n c e s i n g -r a i n s i z e . The c o s t s o f b e a c h n o u r i s h m e n t

The c o s t s d i d v a r y v e r y much w i t h t h e b o r r o w i n g s i t e o f t h e sand f o r t h e n o u r i s h m e n t . Besides t h e b o r r o w i n g a r e a , a l s o t h e way o f d e t e r m i n i n g t h e p r i c e i s o f i m p o r t a n c e . Three s c e n a r i o s a r e used t o d e t e r m i n e t h e c o s t o f sand w i n -n i -n g .

1 P r e s e n t b o r r o w areas more near t o t h e c o a s t l i n e

2 A l t e r n a t i v e b o r r o w i n g s i t e s , based on newest m o r p h o l o g i c a l knowledge 3 P r e s e n t b o r r o w a r e a s , u s i n g a h i g h c a l c u l a t i o n v a l u e

I n t a b l e 3 can be seen t h a t t h e p r i c e ( i n c l u d i n g a l l t a x e s ) v a r i e s f r o m US $ 1.50 p e r m"^, i f sand can be b o r r o w e d f o r m areas near t h e c o a s t , up t o US $ 5.35, i f t h e b o r r o w a r e a i s f a r away ( a t a d e p t h o f 20 m, 20 km o f f s h o r e ) and a h i g h c a l c u l a t i o n v a l u e i s used.

s c e n a r i o f o r sand w i n n i n g wadden c o a s t o f p r i c e i n US $ p e r m a r e a H o l l a n d c o a s t o f Z e e l a n d p r e s e n t b o r r o w areas 1.50 4.80 2.40 a l t e r n a t i v e b o r r o w areas 2.25 2.40 2.40 p r e s e n t b o r r o w areas and 2.90 5.35 3.20 a h i g h c a l c u l a t i o n v a l u e T a b l e 3: p r i c e o f sand o Dune n o u r i s h m e n t i s US $ 2.00 p e r m more e x p e n s i v e . I n d e t e r m i n i n g t h e s e p r i c e s m a r k e t e f f e c t s a r e n o t t a k e n i n t o a c c o u n t . Because o f o v e r p r o d u c t i o n t h e d r e d g i n g p r i c e s on t h e w o r l d market a r e l o w e r a t t h i s moment, b u t f o r b a s i s o f a l o n g t e r m p o l i c y , t h i s s h o u l d n o t be t a k e n i n t o a c c o u n t . beach n o u r i s h m e n t on t h e i s l a n d o f T e x e l

The n o r t h e r n s e c t i i o n o f t h e i s l a n d o f T e x e l has been n o r i s h e d t w i c e , i n 1979 and i n 1985. F i g u r e 13 shows t h e e f f e c t s o f t h e n o u r i s m e n t on t h e volume o f sand. I t can be seen t h a t a f t e r t h e n o u r i s h m e n t o f 1979 t h e e r o s i o n r a t e was a l m o s t i d e n t i c a l t o t h e e r o s i o n r a t e b e f o r 1979.

beach n o u r i s h m e n t on t h e i s l a n d o f Goereee

F i g u r e 14 shows t h e n o u r i s m e n t s on t h e i s l a n d o f Geree. T h i s i s on a f i r s t v i e w a s t r a n g e n o u r i s h m e n t , because i t i s on an a c c r e t i n g c o a s t . The r e a s o n o f t h i s n o u r i s m e n t was t h a t t h e dunes d i d n o t g i v e enough s a f e t y a g a i n s t i n u n d a t i o n o f t h e p o l d e r s b e h i n d t h e dunes, and more sand was r e q u i r e d on t h e dunef r o n t and t h e ( d r y ) beach. T h i s sand was p l a c e d t h e r e i n 1974, 1978 and 1985. The beach n o u r i s m e n t s caused t h a t t o o much sand came i n t h e a c t i v e zone, and t h e s u r p l u s o f sand eroded away.

beach n o u r i s h m e n t n e a r W e s t k a p e l l e

The f o r m e r i s l a n d o f Walcheren (nowadays i t i s c o n n e c t e d t o t h e m a i n l a n d b y dams and r e c l a m a t i o n s ) i s s i t u a t e d i n t h e s o u t h - w e s t e r n p a r t o f t h e N e t h e r l a n d s (see f i g . 15) . Very near t o t h e s o u t h - w e s t c o a s t o f t h e i s l a n d , a deep and r e l a t i v e l y n a r r o w t i d a l c h a n n e l , can be f o u n d , t h e O o s t g a t . Because o f t h e g e n e r a l m o r p h o l o g i c a l dynamics i n t h e a r e a ( i t i s t h e mouth o f t h e r i v e r S c h e l d t ) t h i s c h a n n e l t e n d s t o move i n a s o u t h - e a s t e r n d i r e c t i o n , and conse-q u e n t l y e r o d i n g t h e c o a s t . T h i s i s a r a t h e r slow p r o c e s s , because b o u l d e r - c l a y l a y e r s and o t h e r d e n s e l y packed f o r m u l a t i o n s p r e v e n t t h e e r o s i o n . A l s o , a l o n g t h i s c o a s t l i n e a number o f g r o i n s has been b u i l d , each w i t h a g r o i n - h e a d w i t h s t o n e - p r o t e c t i o n e x t e n d i n g t o a g r e a t d e p t h . These s t o n y "headlands" p r e v e n t movement o f t h e c h a n n e l . Because o f t h e s e p r o c e s s e s and man's r e a c t i o n on i t ,

t h e c o a s t i s v e r y s t e e p and t h e beach r e l a t i v e l y narrow. The o r i e n t a t i o n o f t h e c o a s t causes t h a t a l l waves come f r o m t h e same d i r e c t i o n , c o n s e q u e n t l y t h e r e i s a c l e a r waveinduced l o n g s h o r e t r a n s p o r t f r o m t h e v i l l a g e o f W e s t k a p e l l e t o -wards t h e town o f V l i s s i n g e n i n t h e s o u t h o f Walcheren. The e x i s t i n g g r o i n s

( l o w s t o n e - g r o i n s w i t h p i l e s ( s e e f i g . 16) do n o t s t o p t h e l o n g s h o r e t r a n s p o r t . A l m o s t t h e e n t i r e c o a s t i s a sandy c o a s t w i t h a s i n g l e row o f dunes. However n e a r W e s t k a p e l l e no dunes a r e p r e s e n t and a heavy sea d i k e has been c o n s t r u c -t e d . Because o f -t h i s c o n f i g u r a -t i o n , -t h e r e i s no n a -t u r a l n o u r i s h m e n -t f o r -t h e l o n g s h o r e t r a n s p o r t near W e s t k a p e l l e and c o n s e q u e n t l y t h e dunes j u s t s o u t h o f W e s t k a p e l l e s u f f e r f r o m e r o s i o n . I n . t h e p a s t t h i s was t h e r e a s o n t o e x t e n d t h e

s e a d i k e i n a more s o u t h e r n d i r e c t i o n . Because t h e p o l d e r - a r e a b e h i n d t h e dunes i s l o w e r t h a n Mean H i g h Water, t h e dunes a l s o have a f u n c t i o n as seadefense. A b r e a k t h r o u g h w i l l cause i n u n d a t i o n o f t h e i s l a n d ( I n f a c t i n 1944 a l l i e d bom-b e r s d e s t r o y e d t h e d i k e a t W e s t k a p e l l e i n o r d e r t h e i n u n d a t e t h e i s l a n d f o r

s t r a t e g i c r e a s o n s . A f t e r s e v e r a l r a i d s t h e succeeded and d u r i n g t h e w i n t e r 1944/1945 t h e i s l a n d was drowned). F i g u r e 16 shows a p r o f i l e o f t h e c o a s t j u s t s o u t h o f W e s t k a p e l l e and i t can be seen e a s i l y t h a t t h e r e m a i n i n g dune does n o t g i v e much s a f e t y .

I n such cases, a c c o r d i n g t o Dutch Laws, r e i n f o r c e m e n t works have t o be p e r formed. Because i t was t h e i n t e n t i o n n o t t o e x t e n d t h e s e a d i k e any more, r e i n -f o r c e m e n t had t o be made w i t h sand. I n t h e p a s t ( i n 1974) a r e i n -f o r c e m e n t was a l r e a d y made on l a n d w a r d s i d e o f t h e dune. Such a r e i n f o r c e m e n t does p r o v i d e the r e q u i r e d l e v e l o f s a f e t y b u t does n o t s t o p c o a s t a l e r o s i o n and c o a s t a l r e g r e s s i o n .

P l a n n i n g t h e n o u r i s h m e n t

T h e r e f o r e t h e i d e a was t o make f u t u r e r e i n f o r c e m e n t s b y p l a c i n g sand on t h e seaward s i d e o f t h e dunes. Thus a r t i f i c i a l beach and dune n o u r i s h m e n t . A t e c h -n i c a l p r o b l e m w h i c h arose was t h a t t h e beach i s v e r y -n a r r o w , -n o t a l l o w i -n g p l a c i n g much sand on t h e beach. I t was d e c i d e d t o p l a c e t h e sand as much as p o s s i b l e a g a i n s t t h e e x i s t i n g sea f r o n t o f t h e dunes. The h e i g h t o f t h e dune n o u r i s h m e n t was e q u a l t o t h e h e i g h t o f t h e e x i s t i n g dunes. The s l o p e o f t h e beach was d e t e r m i n e d by t h e n a t u r a l s l o p e o f f r e e f l o w i n g sand f r o m a d i s -c h a r g e - p i p e i n i n t h e s u r f zone. The amount o f sand p l a -c e d i n t h i s way on t h e c o a s t s h o u l d g u a r a n t e e s a f e t y f o r a p e r i o d o f f i v e y e a r s . I t i s i n f a c t a r e -l a t i v e -l y s m a -l -l a r t i f i c i a -l beach n o u r i s h m e n t p r o j e c t b u t t h i s p r o j e c t has seve-r a l i n t e seve-r e s t i n g a s p e c t s f seve-r o m c o a s t a l e n g i n e e seve-r i n g v i e w p o i n t . T h e seve-r e f o seve-r e i t was d e c i d e d t o f o l l o w t h i s n o u r i s h m e n t i n d e t a i l w i t h measurements o f t h e p r o f i l e . The n o u r i s h m e n t was p l a c e d i n 1984. The e x p e c t e d l i f e - t i m e was f i v e y e a r s , i n 1988 a r e n o u r i s h m e n t has been made. T h i s paper d e s c r i b e s t h e changes i n t h e p e r i o d 1984-1988.

Nourishment q u a n t i t y

The t o t a l l o s s o f s t a n d i n t h e p e r i o d 1971-1981 between km 22.550 and km 23.865 i n a s t r i p between d u n e - f o o t (MSL - 4.00 m) was 88000 m . Based upon t h i s f i g u r e t h e r e q u i r e d q u a n t i t y f o r a f i v e y e a r s n o u r i s h m e n t was 44000 m . U s i n g t h i s q u a n t i t y , i t was p o s s i b l e t o improve t h e dune a t t h e seaward s i d e i n o r d e r t o r e a c h t h e r e q u i r e d s a f e t y l e v e l . The w i d t h o f t h e dune was t h e r e f o r e i n -c r e a s e d w i t h 10 m, r e q u i r i n g 22000m^. For l o s s e s d u r i n g e x e -c u t i o n a s u r -c h a r g e o f 10-15% i s r e q u i r e d . I t was d e c i d e d t o p l a c e a q u a n t i t y o f 75.000 m on t h e beach. D u r i n g t h e e x e c u t i o n o f t h e work t h e c o a s t a l manager d e c i d e d t o p l a c e an a d d i t i o n a l 25000 m^ on t h e beach, i n o r d e r t o improve two a d j a c e n t beach s e c t i o n s and i n o r d e r t o g e t a b e t t e r beach l e v e l near t h e s t o n e g r o i n s .

G r o i n s

Lee e r o s i o n , caused by t h e s t o n e g r o i n s , g i v e s a l o w e r beach and a l o c a l back-wards movement o f t h e dunes, j u s t s o u t h - e a s t o f each g r o i n . I n o r d e r t o main-t a i n a h i g h e r beach l e v e l near main-t o weak dune row a main-t km 23.250, p r e v e n main-t i n g a backwards movement o f t h e dune a t t h a t p o i n t , t h e d i s t a n c e between t h e s t o n e g r o i n s was d e c r e a s e d by c o n s t r u c t i n g t h r e e e x t r a g r o i n s between t h e e x i s t i n g ones. A t t h e l o c a t i o n o f t h e new b u i l d s t o n e g r o i n s o n l y p i l e - r o w s w i t h a s t o n e head were p r e s e n t . These c o n s t r u c t i o n s s t o p p e d t h e l a n d w a r d movement o f t h e t i d a l c h a n n e l (because o f t h e i r s t o n e h e a d ) , b u t had h a r d l y any i n f l u e n c e on e r o s i o n caused by s u r f - i n c l u d e d t r a n s p o r t . I t i s e x p e c t e d t h a t t h e new s t o n e g r o i n s have a p o s i t i v e e f f e c t on t h e beach l e v e l .

A n a l y s i s

I n o r d e r t o e v a l u a t e t h e beach n o u r i s h m e n t , t h e r e g u l a r y e a r l y c o a s t a l measure-ments were used. A l o n g t h e Dutch c o a s t e v e r y y e a r a f u l l p r o f i l e i s measured.

approx. 200 m a p a r t . Because o f t h e p o s i t i o n o f t h e g r o i n s , i n t h i s a r e a p r o f i l e s a r e measured 125 m a p a r t . See f i g . 15. T h i s d a t e a r e w o r k e d o u t i n t h e f o l l o w i n g way.

For n i n e p r o f i l e s , a y e a r l y volume i n t e g r a t i o n has been made f o r t h e p e r i o d 1973 u n t i l 1988. T h i s i n t e g r a t i o n i s done i n h o r i z o n t a l l a y e r s o f 1 m t h i c k -ness, s t a r t i n g f o r m t h e b o t t o m o f t h e c h a n n e l u n t i l t h e t o p o f t h e dune.

- t h r e e s t r i p s have been r e g a r d e d (see f i g . 1 7 ) . - c h a n n e l - s l o p e between b o t t o m and MSL - 4.00 m

- t h e "beach" between t h e c h a n n e l edge (MSL - 4.00 m) and t h e dune f o o t (MSL +3.50 m)

- t h e dune above t h e dune f o o t (MSL = 3.50 m).

The r e s u l t s o f t h e volume i n t e g r a t i o n i s p r e s e n t e d i n f i g . 18, u s i n g 1973 as the b a s e - y e a r .

I n f i g u r e 19 t h e changes i n t h e c r o s s - s e c t i o n a l p r o f i l e a r e p l o t t e d , u s i n g t h e p r o f i l e o f 1984 ( b e f o r e n o u r i s h m e n t ) as base l i n e . I n t h i s f i g u r e a l s o t h e e x p e c t e d 1987 p r o f i l e i s p r e s e n t e d , i f no beach n o u r i s h m e n t was p e r f o r m e d . From f i g u r e 195 can be seen t h a t i n t h e p e r i o d 19841987 t h e e x p e c t e d " n a t u r a l " e r o s i o n i s i n t h e o r d e r o f 4.000 m /m h e i g h t o f p r o f i l e i n t h e zone b e t w e e n MSL -4.00 and MSL + 7.00 m. The beach n o u r i s h m e n t was m a i n l y p l a c e d between MSL and MSL + 2.00 m (17000 m-^/m). I n t h e s t r i p between MSL + 8.00 m t h e p l a c e d quan-t i quan-t y was approx. 5000 m /m. I n quan-t h e s quan-t r i p bequan-tween MSL and MSL - 2.00 m quan-t h e p l a c e d q u a n t i t y was approx. 6000 m /m.

O b s e r v a t i o n s

From t h e a n a l y s i s t h e f o l l o w i n g o b s e r v a t i o n s can be drawn: dune

1. The r e g r e s s i o n - s p e e d o f t h e dune was n o t changed by t h e n o u r i s h m e n t . The r e g r e s s i o n - s p e e d i s now e q u a l t o t h e speed b e f o r e n o u r i s h m e n t . Three y e a r s a f t e r p l a c i n g t h e sand 70% o f t h e n o u r i s h e d sand i s t i l l i n t h e dune-s t r i p .

beach

2. A l s o f o r t h e s t r i p "beach", i t can be seen t h a t d u r i n g t h e f i r s t two y e a r s no sand was moved o u t o f t h i s s t r i p . I n t h e t h i r d y e a r e r o s i o n s t a r t e d . I n 1987 22% o f t h e p l a c e d q u a n t i t y was t r a n s p o r t e d o u t o f t h i s s t r i p .

3. The beach a r e a between MSL and t h e dune f o o t ( t h i s i s an i m p o r t a n t v a l u e f o r beach r e c r e a t i o n ) was i n c r e a s e d by d9% due t o t h e n o u r i s h m e n t . How-e v How-e r , bHow-ecausHow-e o f t h How-e o f f s h o r How-e t r a n s p o r t t h How-e bHow-each a r How-e a i n c r How-e a s How-e d u n t i l 23% more o f t h e o r i g i n a l s u r f a c e i n t h e 2nd y e a r . I n t h e 3 r d y e a r t h e beach s u r f a c e was s t i l l 19% more t h a n b e f o r e n o u r i s h m e n t .

dune and heach

4. I n t h e t o t a l p r o f i l e t h e r e was n o t any l o s s i n t h e f i r s t and second y e a r a f t e r n o u r i s h m e n t . I n t h e t h i r d y e a r was a l o s s o f 24%.

5. There was some l o n g s h o r e r e d i s t r i b u t i o n o f t h e beach sand, b u t t h i s quan-t i quan-t y was n e g l i g i b l e w i quan-t r e s p e c quan-t quan-t o quan-t h e o f f s h o r e quan-t r a n s p o r quan-t .

6. Because o f t h e n o u r i s h m e n t a n o n e - n a t u r a l , s t e e p beach s l o p e was f o r m e d . T h i s caused a o f f s h o r e t r a n s p o r t , i n t r o d u c i n g e r o s i o n o f t h e dune f r o n t . Approx. 9000 m was t r a n s p o r t e d f r o m t h e dune t o w a r d s t h e beach.

As m e n t i o n e d b e f o r e , a f t e r t h r e e y e a r s 25% o f t h e m a t e r i a l was removed f r o m t h e s e c t i o n (above MSL - 4.00 m) . I t may be assumed t h a t t h i s i s m a i n l y caused by o f f s h o r e t r a n s p o r t . The a c c u r a c y o f s o u n d i n g s i n t h e t i d a l c h a n n e l i s t o o low f o r p r o v i n g t h i s . The q u a n t i t y o f sand n o u r i s h e d above MSL g i v e s a d i r e c t c o n t r i b u t i o n t o t h e s a f e t y o f t h e dunes. D u r i n g t h e f i r s t 3 y e a r s 53% o f t h e sand n o u r i s h e d above MSL was s t i l l above MSL and c o n t r i b u t e d t o t h e s a f e t y .

D i s c u s s i o n

I n t h i s case t h e r e was n o t much space f o r a beach n o u r i s h m e n t . T h i s gave t h e need t o p l a c e t h e sand v e r y h i g h on t h e beach and n e a r t h e dune f r o n t . The r e s u l t i n g beach p r o f i l e i s t o o s t e e p and n o t s t a b l e . From t h e d a t a p r e s e n t e d f o l l o w s t h a t t h e s t e e p p r o f i l e w i l l a d a p t t o a more s t a b l e p r o f i l e , b u t t h a t t h i s a d a p t a t i o n i s a s low p r o c e s s , I n a case l i k e t h i s i t i s d o u b t f u l i f even a s t a b l e p r o f i l e w i l l be r e a c h e d .

However, h a v i n g an u n s t a b l e , t o o s t e e p p r o f i l e g i v e s some a d v a n t a g e s . Because r e l a t i v e much sand was p l a c e d above MSL t h e dune became more s a f e . A l s o t h e beach i s w i d e r t h a n i t w o u l d be a t a s t a b l e beach. I t i s n o t known how l o n g t h e a d a p t a t i o n p e r i o d i s , b u t t h i s p e r i o d i s i n any case l o n g e r t h a n f i v e y e a r s , w h i c h i s t h e l i f e t i m e o f t h e n o u r i s h m e n t . So d u r i n g t h e w h o l e l i f e - t i m e the p r o f i l e i s u n s t a b l e , g i v i n g t h e advantages o f e x t r a s a f e t y and a w i d e r beach.

C o n c l u s i o n

D e s i g n i n g a beach n o u r i s h m e n t , t h e main d e s i g n q u a n t i t y i s t h e r e q u i r e d quan-t i quan-t y o f sand. A s e c o n d a r y p r o b l e m i s where quan-t o p l a c e quan-t h e sand i n quan-t h e p r o f i l e . I quan-t i s advantageous t o p l a c e t h e sand as h i g h as p o s s i b l e on t h e beaoh. T r y i n g t o make a s t a b l e p r o f i l e i s i n many cases n o t u s e f u l .

— — t i m e (year)

1960 1970 1980

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liF'i-LiiaFRD.ï^IEL 'ZDHDER TDE.-LAG iHF.-Lf|i3FF;aFIEL HET l O E J L i i G i3REH:'FRDFIEL A F S L A G H O E V E E L H E D E N A a n z a n d i n g ( e x c l . i n v l o e d l a n g s t r a n s p o r t ) A f s l a g ( e x c l . i n v l o e d l a n g s t r a n s p o r t ) A f s l a g b o v e n r e k e n p e i l ( e x c l . i n v l o e d l a n g s t r . ) T o e s l a g op a f s l a g b o v e n r e k e n p e i l AFSTANDEN A a n z a n d a f s t a n d A f s i a g a f s t a n d E x t r a v e r s c h u i f a f s t a n d V e r s c h u i f a f s t a n d i n c l V e r s c h u i f a f s t a n d v o o r V e r s c h u i f a f s t a n d i n c l X c o o r d . X c o o r d . ( X C A 3 - X C B 3 ) ( X C B3 - X [ E ] ) t g v l a n g s t r a n s p o r t ( i n B C ) l a n g s t r a n s p o r t ( X [ B 3 - X C C ] ) d e t o e s l a g ( X C C ] - X [ D ] ) l a n g s t r a n s p o r t + t o e s l a g ( X C B ] - X [ D ] ) v a n a f s l a g p r o f i e l op r e k e n p e i l ( X C C ] ) a f s l a g + t o e s l a g p r o f i e l op r e k e n p e i l ( X C O j ) 4 8 2 . 4 9 ( m 3 ) - 4 8 2 . 4 9 ( m 3 ) - 3 8 9 . 8 8 ( m 3 ) - 1 1 7 . 4 7 ( m 3 ) 2 9 1 . 8 6 7 4 . 3 3 ü. 0 0 7 0 . 2 5 3 3 , 5 6 (m) (m) (m) (m) (m) 1 0 3 . 8 2 (m) • 1 0 3 . 0 5 (m) • 1 3 6 . 6 2 (m) O e f e n i n g Z e e b u r g e r b a d a l l e r a a i e n s t o r m v l o e d RWS-DWW D U I N A F S L A G P G M D U I N A F - P C O O l 3

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oQsnt Location quq!?i:i' iy (10^ bP)' periodi 1 toeland 2,2 1980 2 flnwland 0,31 1979 3 Texel, E l e r l c i M 3,05 1979 4 Texel, E l e r l a r x l ' 2,85 1985 5 Texel, de Koog 3,02 1984 6 Callantsoog 0,35 1976-1977 7 Callantsoog 0,47 1979-1980 8 Schevenii^en 0,045 1969 9 Schevenin^in 0,7 1975 10 ScheveniiKjen 0,33 1985

11a Hoek V. Holland 18,94 1971-1972 11b Hoek V. Holland 1,50 1976-1977 12 HoeJ^ V. Holland 0,87 1977-1978 13 B r i e l s e Gat Dam 0,15 1979-1980 14 Voorr» 0,11 1974 15 Voorna 0,15 1974 16 Voorna 1,10 1977 17 Voorna 0,44 1983 18 Voorna 3,40 1984-1985 19 Goerea 0,40 1969-1970 20 Goeree 0,61 1971 21 Goeree 3,64 1973-1974 22 Goeree 1,27 1977 23 Goeree 0,33 1984 24 Goeree 0,53 1985 25 Sciiouwen 0,112 1975 26 Noord Beveland 0,21 1973 27 Walcheren 0,10 1984 28 Sfelcheren 0,775 1952-1959 29 V l i s s i n g e n 0,05 1952 30 Vlissingen 0,032 1966 31 Vlissingen 0,045 ,1975 32 Breskens 0,206 1971 33 Callantsoog 1,3 1986 34 Terheijda 3,0 1986 35 Voome 1,0 1986 36 Dacburg 0,23 1986 37 Kop V. Schouwen 1,83 1987 38 Westkapelle 0,025 1988 39 Westkapelle 0,41 1988 40 Zoutelande 0,15 1988 41 Cadzand 0,85 1988 42 Zwanenwater 1,85 1987 43 Hoek V . Holland 0,2 1988 Totaal 59,14

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erosion

erosion profile s h i f t s in landward direction until erosion = sand deposit

Fig. 1. Principle o f t h e computational model for dune erosion.

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1960

time (year)

SEASIDE

LANDSIDE

1970 ₁₉₈₀

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- the influence of the gradient in longshore transport (5)

regression line extrapolation

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