Changes in salinity in the delta area of the rivers Rhine and Meuse resulting from the construction of a number of enclosing Dams

3o

i of Sea Res ,r h

5 (1) : 1-19 (1970)

CHANGES IN SALINITY IN THE DELTA AREA OF THE RIVERS RHINE AND MEUSE RESULTING FROM THE CONSTRUCTION OF A NUMBER OF ENCLOSING

DAMS*

by R. P E E L E N (Delta Institutefor Hydrobiological Research, Terseke, The JVeawrlands) CONTENTS I. II. III. IV. V. VI.

Introduction . . . . . . . . . . . . . . . . . Chloride content as an indicator for the origin of water m a s ~ . Reconstruction of the initial situation . . . . . . . . . . Influence of the construction of the Haringvliet bridge . . . . . Consequences of the construction of the Grevelingen dam . . . . Isohalines and water movement during an entire tidal cycle, after construction of the Grevellngen dam in 1964 . . . . . . . . . VII. Results of the construction of the Volkerak dam . . . . . . . VIII. Summary . . . . . . . . . . . . . . . . . . IX. References . . . . . . . . . . . . . . . . . .

1

3 4 4 6 8 15 18

18

I. I N T R O D U C T I O N I n a previous p a p e r (P~.~LEN, 1967) the m i d t i d e isohalines in the Delta a r e a o f the rivers R h i n e , Meuse a n d Scheldt at high, average a n d low river discharge were discussed in relation to their p a t t e r n before a n d after construction o f the Grevelingen d a m . A survey o f the various closures o f the " D e l t a P l a n " was also given a n d a prognosis m a d e o f the salinity ranges to be expected d u r i n g future phases o f its realization. A description will n o w be given o f the effect o f the construction o f the Haringvliet bridge a n d the Grevelingen d a m on salinity distribution in the Haringvliet a n d o f the changes in the position o f isohalines d u r i n g a n entire tidal cycle. T h e isohalines are d r a w n for average river discharge a n d are depicted o n six maps, b e g i n n i n g at the m o m e n t o f low tide at H o e k v a n H o l l a n d a n d at subsequent t w o - h o u r l y intervals. T h e consequences o f the closure o f the V o l k e r a k will also be discussed. T h e estuarine e n v i r o n m e n t o f the n o r t h e r n Delta area in the southwest o f the N e t h e r l a n d s is c h a r a c t e r i z e d b y "semi-enclosed coastal bodies o f w a t e r having a free c o n n e c t i o n with the o p e n sea, w i t h i n w h i c h r u n - o f f plus direct precipitation exceeds evaporation, a n d hence within * Communication nr. 87 of the Delta Institute for HydrobiologicalResearch, Yerseke, The Netherlands.

2

R. P E E L E N

which seawater is diluted by fresh water". This definition of positive estuaries is given by Parrcm, a~D (1967). The Haringvliet can be classified among the bar-built estuaries with a continuous subsurface bar accross the mouth (Fig. 1). The former Brielse Gat and Brielse Maas and the present-day Grevelingen have a subsurface bar intersected by a two channel system; the flood currents dominate in the southern and the ebb currents in the northern channel. The Keeten-Krammer-Volkerak system has one main channel and sometimes an additional channel and is a partly mixed estuary in the sense used by BOWDEN (1967). The main channel debouches into the Oosterschelde at a depth of 48 m - - N . A . P . (Dutch Ordnance Level). The Oosterschelde itself has an outer subsurface bar with three channels. Thus as regards the connection between the estuarine environment and the sea, there are three different geomorphological types of estuaries in this region (el. POSTMA, 1967; DE VatES KLEIN, 1967). Various rivers enter the estuaries. In the case of the Nieuwe Waterweg, Spui, Dordtse Kil and Nieuwe Merwede fresh water is supplied by branches of the Rijn (Rhine); in the case of the Amer by the river Maas (Meuse), whereas the Krammer-Volkerak receives a very small amount of run-off from a few small rivers (Dintel and Steenbergse Vliet) together with some from brackish polders.

Fig. 1. Survey of the northern Delta area, with the I-Iaringvliet bridge and the dams belonging to it, Grevelingen dam (closure 1964) and the Volkerak dam (closure April 1969).

SALINITY

IN DELTA

AREA

Acknowledgements. In the first place I want to thank my colleagues at the Delta Institute for Hydrobiological Research for their cooperation, especially my colleague F. Vegter and his assistants C. Moison, Miss K. M. Kok and Miss J. C. van Stee. Thanks are also due to my assistants MissJ. M. Verschuure and Miss G. L.J. van Kruiningen and to the crew of the Research vessel C. de Rooy and P. de Koeijer. The cooperation of Messrs. G. Bouw, H. Engel, J. J. Pilon and W.J. van der Ree and their staff, and the criticism of Mr. P. van der Burgh of the Department of Roads and Waterways, is very much appreciated. Furthermore I wish to thank J. A. van den Ende for preparing the diagrams and Miss E. C. Kanaar and Mrs. E. S. Nieuwenhuize for typing the manuscript. I am grateful to Prof. Dr. H. Postma for his advice, and to Dr. K. F. Vaas, director of the Institute for reviewing the manuscript. II. C H L O R I D E

CONTENT AS AN INDICATOR ORIGIN OF WATER MASSES

FOR

THE

T h e non-metabolic chloride ion is a suitable tracer for the origin of

water masses. At average river discharge the chloride content of the Rhine water differs slightly from that of the Meuse. In 1960 the chloride content of the Rhine water near Gorinchem was 130 rag/1 and in 1967 170 rag/1 (ANON. 1961, 1968). The corresponding values for the Meuse were 60 rag/1 and 90 rag/1 (observations Delta Institute). Population growth and industrial development in the drainage area of both rivers were the cause of this increase. The difference between these two water types is reflected in a difference in chloride content on the northern and southern side of the Hollands Diep. The water on the southern bank is usually fresher (Meuse influence via Amer) than the water on the northern bank (Rhine influence via Nieuwe Merwede and Dordtse Kil). Tidal movements and wind action mix the water types as they flow in the direction of the Haringvliet and thus the difference near Willemstad is less pronounced. At average river discharge and at low water slack there is hardly any marine influence on the chloride content of the water up to Willemstad (PEEL~N, 1967). This influence increases with the flood tide. Sea water enters in the first place direct from the Volkerak and then from the Haringvliet as well. Before 1964 the marine influence from the Volkerak originated partly in the Oosterschelde (via the Keeten) and partly in the Grevelingen (via the Krammer). Since the closure of the Grevelingen dam in 1964 the only influence is from the Oosterschelde.

4

R. P E E L E N

III. RECONSTRUCTION

OF THE INITIAL

SITUATION

Before construction of the Haringvliet bridge a narrow channel called the Ventjagers Gaatje existed between the Volkerak and the Haringvliet. (Fig. 1). Along the northern border of the Hollands Diep another channel was found, leading into the Vuile Gat and known as the Noord Hollands Diep. Between this channel and the Volkerak a bar was found at a depth of 4.40 m N.A.P. (Fig. 2, 1952-1ine). The more saline water of the Volkerak was impeded by this bar. The flood current flowed from the Volkerak towards the Haringvliet channel through the Ventjagers Gaatje, but the flow in the direction of the Noord Hollands Diep was checked by the bar. Perusal of the chloride data obtained from the surface at the permanent sample stations operated by "Rijkswaterstaat" (Department of Roads and Waterways) in the Delta region (HARINO, 1960-1967) shows that the chloride content of the water near Den Bommel was normally higher at low tide than at high tide. It is clear that the water with a lower saline content sampled at high tide came from the Haringvliet itself (Table I) and whereas the more saline water sampled at low tide was the water coming in through the Volkerak some hours before (cf. VAN EVDEN & VAN W~ER9~, 1969). IV. I N F L U E N C E O F T H E C O N S T R U C T I O N HARINGVLIET BRIDGE

OF THE

On 20 February 1959 the Ventjagers Gaatje was closed in connection with the access to the Haringvliet bridge. The situation at Den Bommel changed in most cases as much as the chloride content of the water at low tide no longer exceeded values at high tide. TABLE

I

Annual average high tide and low tide chloride content in mg/l at den Bommel and at Nieuwendljk, and chlorinity difference before and after closure of the Ventjagers Gaatje, February 1959. Tear

1956 1957 1958-1959

Den Bommel

Wieuwendijk

Den Bommel minus Wienwendijk

HT

LT

HT

LT

HT

LT

2891 3359 2754

3176 3853 2816

1811 2805 1661

1758 2671 1651

t080 554 1093

1418 1182 t165

6353 3926 2818

6209 3735 2759

5997 3444 2410

4820 2456 1685

356 482 408

1389 1279 1074

(closure) 1959 1960 1961

S A L I N I T Y IN D E L T A A R E A

:f,o. ii

^

~

'°

12

/

~

,4 xx

xx

IX

II

I

"

!

xxx

I

S~

I

16 t

I

18

2O

....

C~.AT[S[ ~

ca.ra[~spLAJ~

t ~ m

1N2 lss?

t~'rf~tvot~

w

~

Fig. 2. Changes in the channel profile of Volkerak a n d Haringvliet, as indicated in Fig. 10. Isohalines ( x X x ) 2 hours after high tide at Willemstad.

Other factors were also instrumental in changing the course of the flood current from the Volkerak to the Haringvliet region. The bar between the Volkerak and the southern side of the Noord Hollands Diep, described earlier, was gradually eroded to a depth o f - 6 . 6 0 m N.A.P. (Fig. 2, 1962-1ine). This meant that the more saline water of the Volkerak subsequently followed this new northern pathway through the Vuile Gat to a greater extent. In the following years, till 1967, a good deal of sand was dredged from the bottom of the Volkerak for building purposes (about 15 x 10e m3). This also influenced the pattern of the currents. In 1967 the bar had reached a depth of --7.80 m N.A.P. with further consequences for the penetration of salt water into the estuary (cf. VAN DER BUROH, 1968). The changes in the depths of the channels are represented in Fig. 2. The profile runs from the Volkerak (Galathese haven) to the Vuile Gat (Nieuwendijk) and is shown on Fig. 10. Comparison between high tide and low tide chloride content at Den Bommel on the Haringvliet channel and at Nieuwendijk on the Vuile Gat, before and after closure of the Ventjagers Gaatje (Table I) shows:

R. PEELEN

(1) At Den Bommel chlorinity before closure was higher at low tide than at high tide, and consequently the values subsequently became lower. (2) At Nieuwendijk chlorinity at low tide was lower than at high tide, and this difference increased after closure. (3) Before closure the difference in chlorinity at high tide between Den Bommel and Nieuwendijk was fairly large; after closure the difference markedly decreased. Closure did not affect the difference between the two stations at low tide. These changes in the differences in chloride content before and after 20 February 1959 are explained by the narrowing of the current profile forcing the ebb current to flow through the Noord HoUands Diep towards the Vuile Gat. Before closure the current profile was wider and better mixing of the water masses took place. V. CONSEQUENCES OF THE C O N S T R U C T I O N OF THE GREVELINGEN DAM The influence of the Grevelingen dam on the salt regime of the Haringvliet region will be discussed here. A comparison is made of the relationship between river discharge at Willemstad and chloride content of the surface water at Middelharnis before and after the Construction of the Grevelingen d a m (Fig. 3). Middelharnis was chosen because it is there that the different water masses are most completely mixed, owing to the 45 m deep channel in this region.

3-

=3/t~

Fig. 3. Relation between river discharge at W'dlctnstad and chlorinity content of surface water at Middelharnis; 1960--1964 betide construction of the G r i n dam (©), and 1964-1968 after construction ofthe dam (O).

S A L I N I T Y IN D E L T A A R E A

It is clear that discharges between 1500 mS/see and 750 mS[see at WiUemstad there is a slight increase in chloride content, and above and below these discharges a slight decrease. However, the difference between the two situations is not large. T h e respective cMoride contents (1) at WiUemstad at low tide, (2) at Middelharnis, where the fluctuation in chlorinity over a tidal cycle is very small, (3) at coastlight D, on the junction of Volkerak and Hollands Diep, two hours after high tide, plotted on a logarithmic scale, are shown in relation to the river discharge at Willemstad (Fig. 4). T h e data form straight lines, all crossing the abscissa at the 18½~oo CI' level, approximately the chlorinity of undiluted seawater. T h e line for Willemstad shows that above 1125 mS/see there is no marine influence of the sea. Between 1125 and 750 mS/see there is a little influence of the brackish water, as chlorinity rises to about 0.6~oo CI'. Below 750 mS/see there is an increasing marine influence. !

50 c~ I 20

10

5

2

I

0.5

0.2

0.1

i

I

i

I 500

i

I

i

~ 1000

I

i

I 1500

i

I

'

I 2000

i

i

'

I

i

2~=00

I '

I 3000

'

I

'

I

i

3500

1

i

i 4000

iD

I

i

I 4500

i

I

i

I

-~,

5000 m T s E c

Fig. 4. Relation between river discharge at Willemstad and chlorinity content of surface water at stations along Hollands Diep and Haringvliet: (C)) Willemstad at low tide, (I-1) Haringvliet near Middelharnis, and (Q) .coastlight D.

At a river discharge above 3375 mS/see the water of the Haringvliet is fresh as far as Middelharnis. T h e water of coasflight D has then a cMorinity of 1.1 ~oo CI' as it enters the Haringvliet. However, this influx is not capable of raising the salinity of the entire water-body of the

8

R. P E E L E N

Haringvliet. Below 3375 mS/sec there is increased marine influence. The water from the Volkerak, flowing into the Haringvliet at coastlight D, is seen to be still brackish at a discharge of about 5000 mS/sec. It was possible to assess the volume of this brackish influx in the river water, because its bluer and more transparant quality made it clearly visible at the surface, while tiny waves also formed around the edges. The downward extention of the influx was measured with a conductivity meter. Using data on the various depths of the estuary, contained in the hydrographical charts, in 1966 the total volume was calculated at 10 × 108 m s at average discharge. VI. ISOHALINES AND WATER MOVEMENT DURING AN ENTIRE TIDAL CYCLE, AFTER C O N S T R U C T I O N OF THE GREVELINGEN DAM IN 1964 In cooperation with the Department of Roads and Waterways (Rijkswaterstaat) salinity measurements were carried out in the channels with the aid of several research vessels and also at suitable locations along the banks and on bridges. In each channel, measurements were carried out during an entire tidal cycle, as a rule at half-hourly intervals, when river discharge was about average. These simultaneous measurements (Table II), together with data gathered in the course of five years during some 100 expeditions on board the Institute's research vessel formed the basis for drawing isohalines for the area. The resulting maps (Figs. 5-10), show the situation at the surface at two-hourly intervals throughout the tidal cycle. The fact that fresh water is less heavy than sea water often gave rise to a complicated pattern. Near the bottom, water movement is less and the pattern is therefore not so complicated. For this reason the situation near the bottom is depicted only in charts showing the chlorinities at local low tide slack and local high tide slack or, respectively, the local minimum and maximum chlorinities (Figs. 11-12). In the neighbourhood of Willemstad slack water is reached one hour and a half after low water and high water. Paucity of suitable data meant that it was not possible to make a similar series of maps for the situation before the Grevelingen dam was constructed. As far as possible, measurements were made during calm weather, when the influence of meteorological conditions could be left out of consideration. Fig. 5 represents the situation at low tide at Hoek van Holland, which is 1 hour 15 min before low tide at Willemstad. Eastward of the island of Tiengemeten a local increase above 3~oo CI' was found, owing to the previously mentioned influence of the Volkerak. Two hours later (Fig. 6) the fresh and brackish waters have their

TABLE

II

Goeree C,revelingen Haringvlietmouth Haringvliet-Hollands Diep Keeten-I(rammer-Volkerak Oosterschelde-C, revelingen I(eeten-lframmerVolkerak

Region

1400 1500 1200 1200 1300 1250 1000

Discharge WiUemstad mS]sec

-

-

--0.65 -- 1.09 --0.58 --0.58 1.02 -- 1.74 - - 0.98

At beginning LT

* Conductivity m e a s u r e m e n t s of the D e p a r t m e n t of R o a d s a n d Waterways.

26-8-1965 28-9-1965 15-6-1966" 15-6-1966 6/7-7-1966 27-6-1967 26-7-1967

Date

--0.76 -- 1.36

--0.75 --0.75 --0.75 -- 1.34 --0.68

1.12 1.41

1.30 1.30 1.22 1.26 1.22

--0.79 --0.83 --0.83 - - 1.68 --0.83

-- 1.24

--0.79

1.06 1.11 1.29 1.69 1.29

1.35

1.06

L T and H T deviationsfrom Ar.A.P. At end Long-term average HT LT LT HT

General d a t a for days on which salinity m e a s u r e m e n t s were carried o u t d u r i n g a n entire cycle in the channels m e n t i o n e d . Average river discharge a t W i l l e m s t a d is 1210 mS/sec.

¢,D

~" ~" t~

Z

r~

10

R. P E E L E N

Fig. 5. Surface isohalincs in the northern Delta area before closure of the Volkerak dam; low tide at Hock Van Holland.

Fig. 6. Surface isohalines in the northern Delta area before and after (inset) closure oft_he Volkerak dam, 4 hra before H T at Hock van Holland.

SALINITY

IN DELTA

AREA

11

greatest extension on the Nieuwe Waterweg. At the same time a patch of water with a chlorinity above 15~oo CI', probably caused by the onset of the flood, suddenly occurs in the coastal water near the Maasvlakte at the mouth of the estuary. At this moment of minimum waterlevel, at Moerdijk the difference between the "Rhine water" on the northern shore and the "Meuse water" on the southern shore is most pronouced and the isohaline of 0. I ~ooe l ' extends maximally into the Hollands Diep. Two hours before high tide at Hoek van HoUand (Fig. 7), there is a flood current on the Nieuwe Waterweg and in the Haringvliet, but in the Volkerak the ebb is still in process, while there is also a slight flood current in the Keeten-Krammer region, coming from the Oosterschelde. Around the island of Goeree, a body of water with a chlorinity of less than 15~ooC1' moves in the direction of Ouddorp on the Grevelingen. At high tide at Hoek van Holland (Fig. 8) flood currents occur in the whole area. The isohaline of 15~oo CI' is forced into the Grevelingen basin. The fresh water influence around the island of Goeree has now reached a point near Ouddorp. In the Volkerak there is a difference between chlorinities in the central channel and in the shallow parts. The brackish water, pushed onward by the flood and flowing through the central channel of the Volkerak, bifurcates the river water from the Hollands Diep into two parts, one part of which flows into the shallow Ventjager and the other towards the Hollands Diep (Fig. 8 shows the isohalines of0.3~0o CI' and 1~ooCI', on both sides). The water with a chlorinity below 0.1~oo CI' is forced back into the Biesbosch and the Amer. Two hours after high tide at Hoek van Holland (Fig. 9) proves to be the most saline phase for the Nieuwe Waterweg and the HaringvlietHollands Diep region. In a corner of the Grevelingen near the dam, the isohaline of 15~oo CI' suddenly appears. This fresh water influence originates from discharge of polders, as investigations in 1968 confirmed. In the Krammer-Volkerak region there is still a flood current with isohalines of irregular form. Four hours after high tide at Hoek van Holland (Fig. 10) there is no longer any local decrease near Ouddorp. Everywhere except the Volkerak region the isohalines are shifting towards the sea. Near Willemstad the marine influence from the Volkerak is evident in the Hollands Diep. It has already been shown how this influence is spread by the ebb current over the Hollands Diep-Haringvliet region (Fig. 5). At every phase of the tidal cycle chlorinities near the bottom exceed those at the surface, owing to higher density. During low tide slack the seaward shift of the isohalines is greater at the surface than near the bottom; correspondingly the landward shift at the surface during high tide slack exceeds the landward shift of the isohalines near the

12

R. P E E L E N

Fig. 7. Surface isohalines in the northern Delta area before closure of the Volkerak dam, 2 hrs before H T at Hoek van Holland.

Fig. 8. Surface isohalines in the northern Delta area before closure of the Volkerak dam, H T at Hoek van Holland.

S A L I N I T Y IN D E L T A A R E A

13

Fig. 9. Surface isohalines in the northern Delta area before dosure of the Volkerak dam, 2 hrs after H T at Hoek van Holland.

Fig. 10. Surface isohalines in the northern Delta area before and after (inset) closure of the Volkerak dam, 4 hrs after H T at Hoek van Holland. Dots indicate the profile given in Fig. 2.

R. PEELEN

14

i/

¢

, " 1

"

~

"--Lj

Fig. 11. Bottom i ~ l i n e s in the northern Delta area; c h t ~ i n i t i ~ at l~,al low tide slack or, respectively, local rn;n;mum chlorinities.

Fig. 12. Bottom iso~llnes m the northern Ddta area; c b J o ~ c s at I o ~ high tide slack or, respectively, local ~ u m chiorinities~

S A L I N I T Y IN D E L T A A R E A

15

bottom, because during the entire tidal cycle friction acts as a curb on the movement of the water in the lower layers. Nevertheless at every phase of the tidal cycle, chlorinities at the bottom exceed those at the surface. In connection with the stream difference mentioned earlier chlorinities near the bottom are not given for a certain phase of the tide, but in relation to the local tide as m i n i m u m and m a x i m u m values (Figs. 11 and 12). The object is to represent the extreme values less dynamically than by drawing the isohalines at the surface. For organisms living in the lower water layers these maxima and minima are critical values. The situation around the island of Tiengemeten merits special attention. As shown in Fig. 5 an isolated patch more saline water is formed on the surface, so obviously curious phenomena are bound to occur in the lower strata. At the eastern end of Tiengemeten an isolated patch is seen on the bottom in exactly the same place (Fig. 11) as on the surface (Fig. 5). A second patch is found on the bottom at the western end of the island. Both originate from previous periods of high tide in the Volkerak. At the following situation of high tide chlorinities (Fig. 12) the westernmost of these patches is swallowed up in the 45 m deep channel of Middelharnis, the other is halfway up the Vuile Gat, and a new one is being formed near the Haringvliet bridge. At average discharge a third isolation of more saline water is never found. Isohalines 2 hours after high tide at Willemstad (Fig. 10) are also given along the vertical profile of the transection Volkerak-Haringvliet (Fig. 2). The penetration of brackish water from the Volkerak into the Haringvliet and the isolated patch near Nieuwendijk are clearly visible. VII. RESULTS OF THE C O N S T R U C T I O N OF THE VOLKERAK DAM O n 28 April 1969 the Volkerak was closed by lowering the slide-valves in 12 caissons forming part of the dam. Closure took place at lowwater slack, at a period when river discharge was double normal values so that the salinity of the Keeten-Krammer-Volkerak region was greatly reduced. After closure this fresh water could no longer flow off, but was slowly absorbed in the salt water from the Oosterschelde. Before closure this typical salt wedge estuary was characterized by oblique isohalines. However, in the first phase of increasing salinity, fresh and brackish water flowed into the upper layers leaving water of greater salinity below. Thus the isohalines ran more horizontally or rather became saucer-shaped, and the water of the upper layers flowed off in a westward direction.

16

R.

P~.ELEN

Changes in chlorinity with the passage of time were studied at Ooltgemplaat, at the mouth of the Steenbergse Vliet, and at Zijpe (Fig. 13). In the beginning the water at Ooltgensplaat was totally fresh, reaching a value of 13.75%o CI' after 120 days. The difference between the surface layer and the layer near the bottom amounted to 3~oo CI' after 2 days, to 1~ooCI' after 5 days, to 0.5%0 CI' after 10 days, later achieving homogeneity. Near the mouth of the Steenbergse Vliet salinity was seen to decrease from a value of 7.8~oo CI' to 3.8~oo CI' during a period of 4 days after closure, owing to the westward flow of the river water of low salinity mentioned before. Later on, the renewed inflow of water from the Oosterschelde caused a constant rise in salinity to the values also found at Ooltgensplaat (14.4~oo CI' after 120 days). After closure, the chlorinity in the deeper layers near the bottom remained at about 10~oo CI' for a month, and rose slightly to 14.5~oo CI' after 120 days. At Zijpe the water was homogeneous at high tide and at low tide salinity was about 1~oo CI' less in the surface layer, which was a few metres in depth and flowed off towards the Oosterschelde.

DAYS

100 50

20

10 5

2

CLOSURE 2e/~/'69

PE

I

FRESH [OLIBO- {)ME.SO-I

~ MESO-

I

POLyHALINE

161 18 %~Ci I I MARINE

Fig. 13. Progressive increase of chlorinity at three statiom in the Volkerak after closure of the dam on 28 April 1969.

S A L I N I T Y IN D E L T A A R E A

17

In June, when there was hardly any discharge from the small rivers Dintel and Steenbergse Vliet, or from the polders, the influence of the summer was clearly apparent at the Steenbergse Vliet and at Zijpe. The situation a t Ooltgensplaat is characterized by a nearly-constant supply of fresh water via the locks in the Volkerak dam, and differs from the two other situations described, where no such constant supply is found. Later in the year, the chlorinity of the entire Keeten-KrammerVolkerak area fluctuates between 11 and 15~oo CI' and thus the whole region must be classified as polyhaline with the exception of a small area south of the Volkerak d a m where water from the Hollands Diep enters via the locks, bringing the salinity into the mesohaline traject. In the same way the saline water from the Volkerak penetrates into the Hollands Diep via the locks where the water layer near the bottom is most saline, thus increasing the salinity north of the locks from values below 0.3~oo CI' at the surface to about 0.5~oo CI' and at the bottom to 1.5~oo CI'. This area now belongs in the oligohaline traject. Three consecutive tidal cycles after closure sufficed to change the whole of the Haringvliet down to the newly-constructed sluices into a fresh water area. This is evident from the position of the isohalines on 2 7 J u n e 1969 at minimum salinity (Fig. 6) and at m a x i m u m salinity (Fig. 10). T h a t day simultaneons measurements were made at several stations. River discharge had fallen to average. At m i n i m u m salinity the 10~oo CI' isohaline is seen to be in a similar position as before but the 3~oo CI' isohaline is now far out to sea, while the 1~oo CI' line is near the westernmost bend in the coastline of the isle of Voorne and the 0.3~oo CI' isohaline, formerly situated near the Haringvliet bridge, has shifted 27 km westward and is now near the sluices. At m a x i m u m salinity the position of the 10~oo CI' isohaline is almost the same, but the 3~oo CI' line is near Hellevoetsluis, where formerly the 5~oo CI' isohaline could be found and the 1~oo CI' isohaline is now situated where the 3~oo CI' line used to be. The 0.3~oo CI' isohaline can be seen near Middelharnis, 20 km westward of its former position near Willemstad. The comparisons discussed above pertain to situations of average discharge. Towards the end of October 1969 river flow reached a minimum value of ½ normal. The main flow of the rivers towards the sea is through t h e Vuile Gat north of the island of Tiengemeten, because the eastern end of the Haringvliet channel is blocked by a bar extending from "coastlight D" to the island. The m a x i m u m extension of marine influence, conspicuous in a rise in chlorinity to 0.5~oo CI', was found at the western end of Tiengemeten in the Vuile Gat and near the eastern end in the Haringvliet channel.

18

R°PEELEN

The consequences of the closure of the Volkerak can be summarized as follows: From an area showing a salt gradient the Keeten-KrammerVolkerak region became a polyhaline one throughout. The Haringvliet became nearly totally fresh up to Middelharnis. From there down to the sand bar in the North Sea a gradient was formed. Beyond the sand bar, more or less coinciding with the isohaline of 10~oo CI', no changes took place. VIII. SUMMARY

Salinity in the northern part of the Delta area of Rhine and Meuse is discussed. The initial situation, prior to realization of the Delta Plan, the successive influences of the construction of the d a m forming part of the bridge over the Haringvliet and the construction of the Greve, lingen dam are described. Isohalines have been drawn depicting the situation in the surface water during 6 phases of the tidal cycle. The isohalines of the water near the bottom at low and high tide, are shown on two further charts. The consequences of the closure of the Volkerak dam are discussed.

IX. REFERENCES

ANON,, 1953-1967. Internationale Kommission zum Schutze des Rheim gegen Verunreinigung, Zahlentafeln. Bownm¢, K. F., 1967. Circulation and diffusion. In: G. H. LAUFF. Estuaries.--Am. Ass. Advmt Sci. Publ. 83- 15-36, Buaon, P. VANDER, 1968. Prediction of the extent of saltwater intrusion into estuaries and seas.--Jnl hydraul. Res. 6 (4) : 267-288. E ~ E N , W. VAN & J. VAN WSSRI~N, 1969. Hydrologische aspecten van de waterstaatkundige toestand op her Haringvliet-HoUands Diep voor en na de uitvoering der Deltawerken. Rijkswaterstaat nota 13-69. H_xzn~o, J., 1960-1967. Uitkormten van chloride gehalte metingen verricht door de waterloopkundige afdeling van de Deltadienst op de vaste meetplaatsen, met losse gegevens 1950-1959. Rijkswaterstaat (unpublished). POST~, H., 1967. Sediment transport and sedimentation in the estuarine environment. In: G. H. LAud. Estuaries.--Am. Ass. Advmt Sei. Publ. 83- 158-179. P~T~N, R., 1967. Isohalines in the Delta area of the rivers Rhine, Meuse and Scheldt.--Neth. J. Sea ges. 3 (4) : 575-597. - - , 1969. Morfometrisch en hydrometrisch overzicht van het Deltagebied van Rijn, Maas en Sehelde. Delta Imtituut, Yerseke (unpublished). PRrr~, D. W., 1967. What is an estuary: Physical view point. In: G. H. I~uFv. Estuaxies.--Am. Ass. Advmt Sci. Publ. 83: 3-5.

S A L I N I T Y IN D E L T A AREA

19

Rijkswaterstaat, 1966a. Veranderingen van de getijbeweging westelljk van de Grevelingendam.--Driemaand. Ber. Deltawerken 36: 283--293. - - , 1966b. De zout- en zoetwaterbeweglng in het mondingsgebied van het Haringvllet en Nieuwe Waterweg.--Driemaand. Bet. Deltawerken 38: 424--435. Vz~s KLEIN, G. DE, 1967. Comparison of recent and ancient tidal fiat and ¢stuarine sediments. In: G. H. LAUFF.--Am. Ass. Advmt Sci. Pub1. 83: 207-218.