Policy Analysis of Easternscheldt Alternatives

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Policy Analysis of • Eastemscheldt * Alternatives m May 1976

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rijkswaterstaat dienst getijdewateren bibliotheek greradlersweg 31 -4338 PG middelburg DDRF-77.158 ' "'•^WwwiramnmHuprKW'MT.

I rijkswaterstaat dienst gstijdewateren

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bibliotheek

C0NTENTS

ÏZÏSZ&* *•«.

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ÏZÏSZ&*

1. Introduction

• 2* Description of the alternatives 10 2.1. Description of C3 10 2.2. Description of D4 18 2.3. Description of A3 25

3. Comparison of the alternatives ' 29 3.1. Safety 29 3*2. Environment 5^ 3.3. Fisheries y^ 3»^. Waterinanageraent 36 3.5. Shipping 95

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3.6, Recreation and Town and Country P

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Planning 102

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3•7• Procedures, c o s t s and aspects ofexecution 3.07 3 . 8 . Employnient and Economy 120 * kc S e n s i t i v i t y - a n a l y s i s 125

• 5. Summary 1^8 (1 t o 9)

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Chapter I

INTRODüCTION. . "

The decision about the Easternscheldt is cliaracterized M by tho great importance of factors which cannot be

ex-pressed in terms of jnoney and in some cases are not '• even quantifiable. These factors have not only been • decisive for the decision of the Government in 197^ kut M will also play a decisive role in the decisi,ons in the | f utur e.

The analysis which is now before you intents to offer • a systematic reconnaissance of the comsequences which

will be the resu.lt of certain decisions about the Eastern- I scheldt for all kinds of aspects of society.

Xnterested groups and persons will then be in a position • to express their preference for those decisions which ^ correspond most with their own evsQaations. This Note • will certainly not accomplish that only one specific p alternative will be chosen unanimously. It tends, however, ^ to malte the problem look more or less alike to all parties I concerned. In this way a discussion can be started about i one and the same problem and not, without knowing it, I abOTit a great number of different ones.

This po1icy-ana1ysis, drawn up in cooperation with the • Rand Corporation of Santa Monica (USA) still shows many

deficiencics» Due to the liraited time for study (12 months) it was not possible to apply the techniques

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in fact is ossential. Although effoi7±s have been made suggestdby Rand in an inter-departmental team, which in fact is ossential. Although effoi7±s have been mad to produce an understandable and easy to follow

presen-tation, also in the research-period there was not suffi- • cient time available to organise the information in ,

such a way that it could be incorporated with minimum , • efforts. On the other hand an attempt has been made, J

to reproduce in the Note the problems and the conse- • quences of each of the alternatives as objectively as ™ possible ( 1 ) . *

* Figures between brackets refer to the suminary of literature. j

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The frame-work of the decisions contains a number of elements which will be briefly explained hereafter. About the purpose of the works in the Easternscheldt there is little difference of opinion. These works are primarily directed towards providing Zeeland with

the safèty promised in the Delta Act. At the satne time, however, one must see to it that the costs - and in a broad sense therefore also the consequences for the environment, fishing, shipping, etc., etc,, - remain as low as possible. There are various possibilities, the so-called a11 ernat ivea in the policy-analysis, to achieve this objective. Each alternative for the

protection of the area around the Easternscheldt against floods aonsists of a number of interventions which are of direct importance to the objective in view or in-directly necessary in order to make the newly created system function as well as possible, Due to the fact that each alternative consists of a great number of components quite a lot of alternatives are possible. Por example: storm surge barriers with openings of various dimensions combined with different ways of c ompartmenta t i on.

In view of the fact that for each alternative the conse-quences, the effects in a number of fields wlll have to be reviewed, it is necessary to limit the alternatives to a small number only. The following alternatives have been studied:

- Closure of the Easternscheldt and compartnientation with the Philips dam and Wemeldingedam ( M ) if *i

- Closure with a storrn-surge tiarrier and compartnientation with the Philipsdam and Oesterdam ( C 3 ) .

** The code for the alternatives corresponds with that of the Committee Cteterschelde.

* The last alternative is not based on an elaborated study of Rijkswaterstaat; it has been based on studies

which were made in 1973 by Provinciale Waterstaat \

o f zeeland (6) ,

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- Open Easternscheldt with a hei glit ening of dikes and | compartmentation with the Philipsdam and Oesterdam (A3)*

These alternatives are further discussed in Chapter 2. I The employed method makes ±t possible to study also

other alternatives which deviate on one or more pointe • from the main alternatives.. The ©f fee te of each

alterna-tive are folt in a great nuinber of fields which are I brought togother in the following categories:

- security • « environment - fishing - watermanagement - shipping - recreation

- costs, duration and execution

- economy and environmental planning. I Efforts have been made to establish the effects which

each of the possible alternatives cotild have on these I various fields as clear as possible and to present them

in the most surveyable way. • The used nornis differ per field. For the environment ™ for example the variety of species and the total quantity M of the biomass have been cbosen as standards, The con.se- I quences for professional fishing have been expressed

in the possiblc losses of labour-opportuaities in man/years | and a possible loss in added value in millions of guilders

per year. With all this, one should, however, bear in I mind, that it concerns here consequences of works which

will be executod in the future and of which the results I can never be predicted with certitude. The insight in

this matter has been determined by the oxperience witli • such interventions in the past, which in the best of B cases has been transformed by studies into knowledge ^ in the form of theories or mode Is. ||i

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Experience shows, that an analytical decision-model is indispensable for complex problems, A model portrays the reality. It contains the relevant elements of the reviewed problem and the most real relation of cause and effect between thenu Depending on the kind of problem the model will consist for example of a system

of mathematical equations, a computer-program or a

series of drawings, In the analysis under review mathe-matical models have been used for example to determine the water-levels occurring during a storm and to determine the salinity in the bassins. The expectations about future coast-morphology are based on studies of the results of

tidal-calculations and the depth charts of the reviewed areas during the past decennia.

promising alternatives order of the alternatives

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modoü s effect* ( + ) costs (-)

' ' rorecast tlie criterion

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Pigure 1. Elements of the frame-work of decisions.

The formal structure of the model provides us with a far better opportunity then an intuitive approach, to consider a great number of factors in a balanced manner and to

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One would expect for example that the tide in the mouth

of the Easternscheldt would diminish as a result of the I construction of the compartmentation dams and would show

longer periods of slack-water. Indeed it appears from I the study with the model that the velocity of the flood

flow decreases5 the velocity of the low-tide, however, • decreases much less while the periods of slack-water ™ at the mouth of the Easternscheldt for example in the

case of the alternative with the Wemeldingedam" will rather•then increase.

Moreover, the use of models in the policy-analysis facili- ' • tates communieation througb unambiguous terminólogy and

it promotes objectivity, because the user is compëlled I I to make clear the hypotheses he is making in those cases

wherö he is lacking in knowledge. The policy-analysis j M makes it impossible to conceal these hypotheses. : It is for escample in.suf ficiently known under what circum- j • stances & dike will collapse. From experience it is known ! • that the dike in general collapses when the waterlevel j _ comes near the crown of the dike or when continuous over- ! | topping of the waves occurs. There are cases known,

however, where dikes collapsed with lower waterlevels j • and with less overtopping of waves. As far as this | collapse-mechamsm is concerned one makes assumptioua \ I which can be checked against the experience with

flood-disasters. •• Of equal importance to the suppositions about the models * to determine the consequences of the alternatives are

the suppositions about the developments in our society. One could assume for example that the present situation

of unemployment will remain as it is during the next ten J years. This and other hypotheses can be laid down in

a scenario, A scenario is a description of a hypothetical I futural situation in the world. It is the intention that

in a scenario only those factors are described, which I could influence to a .considerable extent the costs or

the effects of an alternative. These could be:progress • in science and the scarcity of resources, as well as j • the behaviour-pattern of people, politics and the economie

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8 -situation of the Government.

tnade

Efforts have been to reconnoitre the effects of the wicertainties by examining, wherever possible, vrhat the results of the study would be with different hypotheses or scenarios.

Por the presentation of the results score-charts are being •used, that is a table vith a column for each alternative and as many numbers as there are effects. For ^ase of survey oolour has been added as an extra dimension. The order of an alternative with regards to the effect is dndicated by a colour-code: green for the alternative iwith the highest score and red for the alternative with the lowest score. Pig-ure 2 shows a specimen of such

a. score-chart. alternatives effect S increase jobs (Manyears) " costs^m l n'Bl d^ 1 50 650 2 400

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s :tï,-...^i Pigure 2. Example of a score-chart.

The following outline shows a further corapxlation of

this Note. Chapter 2 contains a description of the alter-natives with the hypotheses. Chapter 3 gives an outline •of the effects which the alternatives will bring about in the selected fields as given in the scenario.

Finally, in chapter k a broad sensitivity-analysis has boen worked out and in chapter 5 the results are being

9

-INLEIDING EN DOELSTELLINGEN

AANNAME BESCHRIJVING ALTERNATIEVEN

SCENARIO ONDERZOEK VAN DE EFFECTEN

GEVOELIGHEIDSANALYSE SAMENVATTING RESULTATEN W

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-10-Chapter II.

DESCRIPTION OF THE ALTERNATIVES.

2.1. DESCRIPTION OF ALTERNATIVE C3 (Storm-surge caisson dam)

Storm-surge barrier sa.lt~wa.le: stagnairü salt-tide Figure 3. Alternative C3<

Storrn-s-ujoge caisson dam.

In C3 the Easternischeldt is being closed with a storm-surge caisson dam. The major part of the basin ( 80%) reraains under normal conditions under the influence of the tide. (fig.3) Based on the conclusions and the

recommendations in the F'inal Report Storm-Surge Caisson dam Easternscheldt (10) it has been assumed in this alternative

-11-tbat the storm-surge caisson dam will be of the type " p i l l a r s fo\mded in pits11 with a wet-cross section of

about 11*500 m2„ The average tide-difference at Yerseke i s 2,3 m.

P a r t i a l heightenlTiff of the dikes.

salt-water load on the Haringvliet-Hollandsch Diep.

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The closure of the E a s t e r n s c h e l d t w i t h a storm-surge ' caisson dam w i l l b e finisbed about 7 y e a r s later than

the closure conform the original p l a n s . For that r e a s o n

i t was decided t o execute a p a r t i a l dike—heightening _ i n order to augraent the safety during the p e r i o d of | c o n s t r u c t i o n of the storm-siirge caisson dam^ A s a basis

for theso dike-improvements a n exceedance-frequency • o f about 1/500 times pei" y e a r was .taken or in other

w o r d s a chance o f about 1 5 % i n .a l i f e - t i m e of 8 0 y e a r s . I C ornp ar tmcnta t i on ,

I f the E a a t e r n s c h e l d t w i l l b e d o s e d w i t h -a storm-surge ] I c a i s s o n dam t h e n there a r e t h r e e areasons w h y i t i s

ixecessary to separate the eastern part of the b a s i n • by dams from the tidal a r e a ;

(l) I n the treaty between Belgium and the Efetherlands • it has heen stipttlated that the Sclieldt-Rhino Canal ! •

w i l l b e tide-free in the f u t u r e . T h e choice of the _ trajectory is also a i m e d a t that f e a t u r e . T h e s i l l - , | depth of the locks and the height of the b r i d g e s

over the Canal have also been designed w i t h that ; I

in v i e w . A t the same time t h e disappearance of t i d a l

currents w i l l Improve the n a v i g a t i o n p o s s i b i l i t i e s • s h i p p i n g . The provisions for the separation of

salt- and fresh-water n e a r the K r e e k r a k locks indicate I the forming of a tide-free fresh-water eastern . section of the E a s t e r n s c h e l d t . ' m (2) For the w a t e r m a n a g e m e n t of the n o r t h e r n D e l t a area , •

it is i m p o r t a n t , that the salt-intrusion via the _ V o l k e r a k locks w i l l come t o a n e n d , I f t h e tide | w o u l d continue to naach u p to the Volkerak l o c k s ,

then there w o u l d b e a continuous and undesirable I

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-12-The fight against salinity and the supply of fresh-water from the northern Delta area will both benefit from the fact that a fresh-water lak e will be formed south of the Volkerakdam.

This fresh-water "Zoommeer" (lake) will guarantee at tVie same time the supply of fresh-water to a part of the southern Delta area in the.future.

(3) With a reduction of the tide to 2, 3 m &t Yers.eke, "the drainage of the West-Brabant rivers and the polders

into the Volkerak would become difficult without compart-mentation; in some cases it would even be impossible

to take place under natural head-loss.

In april 1975 the C.C.O. (2) reported to the Minister of Transport and Public Works about the various possibilities of coinpartmentation. In December, 1975, the Government decided, in conformity with the recommendations of the Board of Waterstaat and the National Committee for Town «md Country Planning that, in case a storm-surge caisson

dam is to be built, the Easternscheldt will be divided into compartments according to alternative C3 - Canal through South-Beveland, from hereon called C3. The eastern part of the Easternscheldt,and the Volkerak, together forming the Zoommeer, will then be separated from the tidal area by two compartmentation dams:

- The Philipsdaro through the Krammer, between St. Philips-land and the Grevelingendam;

- The Oesterdam in the eastern part of the Easternscheldt, between South-Beveland and Tholen, immediately west of the Scheldt-Rhine Canal.

The existing Canal through South-Beveland will be improved on behalf of the through-going shipping-traffie between

the Westernscheldt and the Volkerak. The locks at Wemeldinge will fall into disuse, so that the total number of lock

passages will not be higher than it is now. The reduced tidal basin in the Easternscheldt will now have access to the Canal via a new open mouth at Wemeldinge.

With C3 a number of basins with varying characteristics

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a number of these typical differences.

Table 1. Characteristxcs of basins with C3,

type area on levels ()

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Easternscheldt salt-tide 31*000 raeasured tide-diff e~i rence 2,3m at Yersek Zoommeer freah-water 8.000 , almost constant on ;

NAP | area on levels M

NAP (ha.) •

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Grevelingenmeer salt-water 11.000 almost constant on

NAP - 0,2 m.

Veerse Meer salt-water 2*000 almost constant on I NAP . "

In the plans foi- the creation of the Zoommeer it has been assumed that the Ver dr onleen Land of the Marquisate of Bergen op 25oom will be embariked* This ring-embankment would be desirable during the construction of the Oesterdara in order to avoid a troublesome croos-flow for the navigation in the Scheldt-Rhine Canal north of the Kreekrak locks. Later on this sub-compartmentation has the advantage that within this area which is

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rated frora the shipping-route a better environmental management will be poesible,

Navigation locks and galt-/fresh-water exchange. jI

Xn C3 there are two main shipping routes through the eouthern Delta area;

- the Scheldt-Rhine connection, between the docks of

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Antwcrp and the Volkerak through the eastern part of

the JSasternscheldt;

- the route via the Canal through South-Beveland, between I the Westernscheldt - the region of Gent-Ternouzen,

Sloe-area etc., etc.-and the Volkerak, across the mid-waters Keeten, Mastgat, Zijpe and drammer. Navigation via the Canal through South-Beveland must pass the Philipsdam. This route must be suitable for push-tow and "high" navigation. The Philipsdam needs

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-14-also a yachting-lock. The lock in the Oesterdam is above all important in order to keep the port of

Bergen op Zoom attainable for high (coastal) shipping and yachting- The dimensions of the various navigation-locks in the southern Delta region are in C3 as follows: Table 2« Dimensions of navigation-locks in C3.

Numb er width length ,salt-/fresh water separation

Philipsdam,push-tow locks 2 Philipsdam,yachting-looks 1 Oesterdam locks 1 New locks Hansweert 2 Kreekralc locks 2

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90 280 320 •yes yes yes n o yes

The consixtnption of freah-water of these navigation locks and the inherent remaining salt-loads on the fresh-water Zoommeer are broadly indicated in table 3» These data are based on the asstunption that all locks are always working at full capacity* These values are based on

model research; they have not yet been tested in practic^e. The values will increase, if the Philipsdam locks and

[ the Kreekrak locks due to enhanced shipping-traffic will

have to be expanded vrith a third push-tow navigation lock.

Table 3» Loss of fresh-water and jsalt-loads on the Zoommeer. fresh-water loss from Zoommeer (m /s) Salt-load on Zoommeer (kg C 1 " / B ) Philipsdam locks Oesterdam locks Kreekrak locks 20 1 to 2 20 5 to 8 0,1 to O,5 1 to 5 Discharge-medium Zoommeer•

At the southern side of the Zoommeer a discharge-medium onto the Westernscheldt will be necessary; it {the lake) will have to be desalted immediately after closure of the com^artmentation dams and after that it can be

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flushed from north to south. The Zoommeer will be per- ! inanently burdened by salt-loads via the navigation- I locks and drainage of the polders. The discharge-medium

offers at the same "time the possibility to control the contents of nutrients in the water. Also for control

of the level a discharge-medium in the Zoommeer will be » necessary. In order to be able to maintain an almost | fixed level even in extreme wet periode a total

discharge-3 'tt

capacity of about 100 m /sec. will be required. • G r e v e 1 i n g e nin e e r . i In it's "Red Note" the National Committee for Town and • Country Planning expresses it's preference for a salt- j_ water Grevelingenmeer, this in the interest of a proper | aquatic environment. Moreover, a salt-water circular

flow from the Grevelingenmeer in C3 is favourable in • order to be ab].e to increase the salinity on the stretch Zijpe-Mastgat-Keeten. In this case we also assume that I in C3 the Grevelingenmeer will remain salt in the near ; future. " ~ S In order to be able to maintain the grade of salinity ™ high enough - at least at 15,5 g Cl"/l flushing with

seawater will be necessary. Sluices in the Brouwersdam

and in the Grevelingendam will mako this possible. The ' duct-capacity of these sluices are designed for approx, g| 100 nr/sec,

Dased on the recomraendations of the Board of Waterstaat • about the report of the C.C.O. (3, 11) the Philipsdam

will be link ed with the Grevelingendam in such a way B that the possibility remains open to de-salt the

Grevelingenmeer completely or partially if for example • that would become necessary in order to create a storage- P basin for the supply oft drinking-water.

Veerse Meer.^

From the point of view of watermanagement the now rather brackish Veerse Meer is less atractive. An increase of

the salinity in the Veerse Meer can be achieved with ^ a flushing of approx. 20 mJ/sec. from the Easternscheldt p via the Canal through Walcheren to the Westernscheldt.

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-16-Because the water-level in the Canal through Walcheren averages NAP + 0,9 m., a pumping-station at Veere will' be required. Level-control on the Veerse Meer novr takes place under natural head-loss via the navigation-locks

at Kats. However, with a reduced tide on the Eastern-scheldt the flushing-capacity of this navigation-lock will probably be too small in wet periods. The relatively ; small Veerse Meer namely functions as a basin for a

rather large drainage area. Via the new pumping-station at Veere the discharge of superfluous water into the Westernscheldt can also take place.

For environmental reasons it is desirable to maintain during the whole year an almost constant level on the Veerse Meer, contrary to the prcsently used summer-level of NAP - 0,7 m. Further research will have to show whether duration of polder-discharges from the Veerse Meer to the Eastern- and Westernscheldt will be necessary. T i m e - s c h o rl xi .1. e r"i

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P R I M A R Y DAM Storin-sur"ge b a x r i e r C O M P A R T M E N T A T I O N P h i l i p s d a m N a v i g a t i o n - l o c k s O y s t c r d a m Navigation-loclc R i n g - e m b a n k m o n t B e r g e n op Zoom D i s c h a r g e - m e d i u m Zoom l a k e CANAL T H R O U G H S O U T H -B E V E L A N D E N L A R Ü E M E N T OF D Ï K E S P a r t i a l h e i g h t e n i n g of dakes WORKS i . b . o , W A T E R -M A N A G E -M E N T Sluice Brouworsdani Sluice G r e v e l i n g e n d a Pxnnp inga t n t i on V o ar c /U3JUSTMENT C O N -3TRUCTI0NS

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-Costs C3.

A survey from January l s t , 1976 onwards of the costs of the most important works i s «ho\m in figure k.

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Pigure 4. Snrvey Costs C3,

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-, ©ESCRIPIXON €F ALTEIRWATIFE _{EASTERNSCHELDT)}

•NOOHTHBC VIXANDVF ^~ri™

5* Alternative D4 Easternscheldt-dam.

lai D'i tiie Easternscheldt Is closed according to the

ori-glnal plans. In the Easternscheldt at the place of the

construction-island a sluice lias been projected.On the ïloggenplaat, immedia-tely behind the Easternscheldt-dam a revitallsation- and Btorage-basin for raxissels can be made, in which the tide is raaintained via a sluice to

the sea« ( 5 ) .

JPartial

It is assximed that with D 4 the execu,tion of the partial whicii started in 1975, will be finished

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-19-b e c a u s e the definite safety w i l l already -19-be reached in

1 9 8 0 . Other ill-effects of t h e «Like-heightening on the jl s c e n e r y and the environment raight t h e n n o t occur..

C o m p a r t m e n t a t i o n . | T h e argumentation for compartmentation. of a closed :• Easternscheldt according t o alternative D 4 is being ™ s h o w n in appendix I. I n v i e w of t h e w a t e r - q u a l i t y one ,_ prefers rather a salt-water lake iratnediately b e h i n d the !|

E a s t e r n s c h e l d t - d a m instead o f a c o m p l e t e l y fresh-water

b a s i n . C o m p a r t m e n t a t i o n w i l l t h e n b e c o m e n e c e s s a r y in • order to be able to create a fresh-water basin in the

e a s t e r n part on b e h a l f óf the w a t e r m a n a g e m e n t . T h e salinity jB in a closed salt-water lake is consideirably m o r e sensitive ' to fresh-water loads than in a salt-water tidal a r e a . In !• a closed salt-water lake a sufficiently h i g h salinity • c a n only b e a c h i e v e d if the fresh—water load is m i n i m a l . M ¥ i t h compartmentation of D 3 w i ± h the Philipsdam and |

o/e

Öesterdam and with a big fresh-water load of approx. 20 m * via the ï"hilipsdam locks the r^quired chloride—content I

of 15»5 g Cl /l caraiot be met» It is probahly feasible

with compartmentation Bk with the Philipssdam and the I Wemeldingedam and a minimal fresh—water input at the

locks in these dams. Moreover D4, could be finished by I 1980 and D3, due to the construction of the huge lock- * complex in the Philipsdam not' before 1984. • In Dk is reckoned with the creation of a stagnant salt- m water lake behind the Easternscheldt-dam., The Zoommeer _ will then be separated from this salt-water area by two |

compartmentation dams:

- the Philipsdam through the Krammer, between St. Philips- • land and the Grevelingendam;

- the Wemeldingedarn, between Tholen and South-Beveland; I the connection in South-Bevel*md is located between the

existing mouth of the Canal through South-Beveland at • : Wemeldinge and the projected new — eastern - mouth of ™

this Canal.

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-20-The Canal through South-Beveland will be improved on behalf of the through-going navigation which will then be led via the Eendracht-section of the Scheldt-Rhine connection. This Canal will at the seune time fulfil a function for the discharge of fresh-water frora the Zoommeer•

The southern Delta area in D4 is subdivided into a

number of basins of a different nature; the characteristics of these basins are shown in table h.

Table 4. Characteristics of basins in Bk.

Type _{area on} NAP (ha) levels

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STasternscheldt Zoommeer Grevelingenmeer Veerse Meer salt-water fresh-water salt-water salt-water

21.000 altnost constant on NAP 18.000 almost constant on NAP 11.000 almost constant on NAP

-0,20 m

2,000 almost constant on NAP

In order to be able to maintain in 04 a sufficiently high salinity in the salt-water lake behind the Easternscheldt-dam, flushing with seawater will be necessary. The best

results will be obtained by a circular flow in the direction North Sea- Easternscheldt-Grevelingenmeer5 less effectiye

is a flow-direction Easternscheldt-Veerse Meer - Canal through Walcheren — Westernscheldt• In this Note it is assumed that in D^ sections of the Zoommeer will not be separated by subcomparttnentation.

Nayjgation-locks and salt-/fresh—water exchan^e.

In model D4 the through-going navigation is being led through the Canal through South-Beveland and further via the new eastern - mouth at Wemeldinge through the Scheldt-t Rhine connecScheldt-tion. Due Scheldt-to Scheldt-the facScheldt-t Scheldt-thaScheldt-t Scheldt-this main shipping route is situated outside the salt-water lakes, relatively «mail locks in the Philipsdam and Wentel ding e dam will suffice

-21-The dimensions of the various locks are shown in table 5. Table 5. Dimensions of ttavigation-locks in Uk*

numiber width length (m) salt~/fresh-water separation

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Philipsdam lock 1 Philipsdam,yachting-lock 1 Wemeldingedam lock 1 New locks at Hansweert2 Kreekrak locks 2 21 120 _yes 9 12 Uk 2 4 7 5 90 280 320 y e s y e s y e s y<js

In table 6 the consumption of fresh-water and the inhaerent remaining salt-load in the fresh-water Zoommeer are roughly indicated. It is assumed that all locks are always working at full capacity. These values will augtnent if the new lock-complex at Hansweert and the Kreekrak locks will have to be expanded^ due to increased shipping-trafficjwith a third push-tow navigation lock .,

Table 6. Loes of fresh-water and salt-loads in. the Zoommeer.

Loss of fresh- Salt-load on the_ water from the Zoommeer (kg Cl /s) Zoommeer (mJ/s.)

Philipsdam locks 3

Wemeldingedam locks 1 to 2 New locks at Hansweert 20 Kreekrak locks 20

0,1 to 0,5 1 to 5 1 to 5

Pischarge-medium Zoommeer.

For the de-salting, fltishing and level-control of the Zoom-meer a discharge-medium onto the Westernscheldt will be necessary*

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When the whole improvement-plan for the Canal through South-Beveland will be completed - this will be around I985 - the superfluous water can be discharged via the

scouring-sluice near the new navigation-locks at Hansweert, In order to be able to have a discharge—medium available for the discharge of superfluous water immediately after the closure of the compartmentation dams , the following projects must be finished by mid-1980:

« the new open canal-mouth at Wemeldinge andthè profile-constriction near the Postbrug south of Wemeldinge must be eleminated \

— the small West-lock at Hansweert, which is in disuse

after the opening of the Scheldt-Rhine connection must v. be remodelled to function as a scouring-sluice,

It is assumed in this Note that these works will be finished in time. That means that while selecting model Bk a decision about these works will have to be made simultaneously.

Salt-water lakes Easternscheldt., Grevelingenmeer and Veer se Meer. ^

In order to be able to maintain a sufficiently high salinity of at least 15i5 g Cl /l in these salt-water lakes, the

de-salting as a result of precipitation and brackish polder-discharges must be fought by flushing with salt seawater. In D4 an almost complete flushing of the salt-water lake in the Easternscheldt and the Grevelingenmeer can be achioved by a salt-water circular flow around Schouwen-Duiveland. To this end seawater must be taken in at the sluice Noordland in the Easternscheldt—dam and must be discharged into the sea again via the ducts in the Grevelingedam and Brouwersdanu Rough calculations show

that the salinity in the Easternscheldt and the Grevelinge-meer would meet the requirements with a circular flow of

o

about 100 m-Ysec. The salinity of the Veerse Meer could,

o

as in C3, be augmented by a flushing of about 20 m /sec. from the Easternscheldt into the Westernscheldt. For this flushing and for the 1evel-control of the Veerse Meer after closure of the Easternscheldt, a pumping-station at Veere would be required.

-23-Further research will have to show whether deviation of polder-discharges frora the V^erse Meer into the Eastern-and VTesternscheldt will be necessary.

Time-schod-ulo Tik» PRIMARY DAM Ea s t e m s c h o l d t dam S l u i i c e N o o r d l n n d COMPARTMENTATION Philipsdam Navigati on-loclc Wemeldingcdam Navj.gation-lock CANAL THROUGH. S0UTH-13EVELAND

New mouth Wemel-dingi a n d P o s t l i r u g T o t a l invprovement WORIvS i c b . o , WATER MANAGEMENT

Sluice Brouwers da: Sluice Grevelin-gendam , Ptiraping-station ADJUSTMENT WORKS MUSSEL-"REVITALI-SATION(IFACÏLITY 76 ₇₇ • 70 .79 8 0 81 82 ₈₃ 8^ 85 86

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Costs D4.

A survey from January lstf 1976» onwards of the costs of

the most important works is given in figure 6.

-25-BESCRIPTION OF ALTERNATIVE A3 (OPEN EASTERNSCHELDT)

\m

Figure 7- Alternative A3.

Mouth of the Easternscheldt.

With model A3 the mouth of the Easternscheldt remains open* The works already executed, that is, the three construction-islands and the dara-section Geul with behind it the con-struct ion- doek s for caissons, have already closed off 15% of the orxginal vret-cross section. In connection with the displacement of the channel near the construction-islands and the safety of the banks of North- Beveland and

Schouwen-Duiveland, it might be necessary to remove the vorks at the mouth of the Easternscheldt; it is also quite posèible that an additional layer of rubble will prove to be sufficient.

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Further studies w i l l h a v e to p r o v e t h i s . For the time b e i n g it is a s s u m e d in this N o t e that the w o r k s w i l l h a v e t,o b e reraoved,

D ik e -h e i Rh t en ing.

For the improvement of the f l o o d - r e t a i n i n g structures along the E a s t e r n s c h e l d t to " D e l t a - s t a n d a r d s " a few studies h a v e b e e n m a d e , b a s e d on experience w i t h dikeheightening e l s e -w h e r e . A n o r i e n t a t i o n - p l a n for dlke-hoifhtoa&ngsvas m a d e i n the N o t e of the P r o v i n c i a l W a t e r s t a a t of Zeeland on b e h a l f of the C o m m i t t e e O o s t e r s c h e l d e ( 6 ) . T h i s N o t e is

b a s e d in b r o a d lines on the s u g g e s t i o n s about d i k e - h e ± g h t e n i n g m a d e in that p l a n . As a s t a r t i n g - p o i n t a e x c e e d a n c e - f r e q u e n c y of 1/^000 time per year has b e e n a p p l i e d . I n the area east of the c o m p a r t m e n t a t i o n d a m s , p a r t i a l dike-heightening

w i l l b e executed, b e c a u s e those dams w i l l only be c o m p l e t e d a r o u n d

Coftipartmentation.

E v e n w h e n the Easternscheldt stays open, c o m p a r t m e n t a t i o n , a s i n C 3 , r e m a i n s necessaryt b e c a u s e of the screening off of the Scheldt*Rhine c o n n e c t i o n and b e c a u s e of the w a t e r -m a n a g e -m e n t . M o r e o v e r , w i t h the c o n s t r u c t i o n of the

.Philipedam and the O e s t e r d a m the total length of the dikes w h i c h are to b e improved, w i l l b e shortened from a p p r o x ,

2^0 kn to about 1^5 knu r

Tlie c h o i c e of c o m p a r t m e n t a t i o n w i t h A 3 is b a s e d on the same c o n s i d e r a t i o n s as w i t h C 3 . A n u m b e r of a s p e c t s , h o w -e v -e r , a r -e diff-er-ent: i.-e,

- the C a n a l through S o u t h - B e v e l a n d c a n n o t - for safety r e a s o n s - simply r e m a i n in o p e n c o n n e c t i o n at W e m e l d i n g e w i t h a n open E a s t e r n s c h e l d t .

M o r e o v e r , the n e w V l a k e t u n n e l in N a t i o n a l Route 58 is not designed for high w a t e r - l e v e l s w h i c h could occur in an o p e n E a s t e r n s c h e l d t . I n order to p r e v e n t an i n c r e a s e of the n u m b e r of lock-passages b e t w e e n the W e s t e r n s c h e l d t and the H a r i n g v l i e t , a m o v a b l e storm-surge c a i s s o n dam s h o u l d b e built in the n e w c a n a l - m o u t h at W e m e l d i n g e , w i t h nexfe to i t , a n a v i g a t i o n l o c k for the "high" n a v i

27 the construction of the Thilipsdam and the Oesterdam 27 -to be completed in 1985 ~ causes an increase « f the tidal range west of these dams and consequently a rise -of the storra-suige levels;

- due to greater tidal rang$, th« sills of the navigation-locks will have to be about 1 m JLower tha:n in model C 3 ;

the exchange of salt-/fresh-wat«cr will, as a result thereof increase with 15 to 20%;

- the compartmentation dams and the locks thex-eiir sho-uld be designed as a primary water-r«taining structxire*

Tim o - gched-ulo A3 o PRIMARY DAM C l c a r a r t c e WOÏ'IÏS a t mouth E a s t o r n s c h c 3 . c COMPARTMENTATION Philipsdain Navxgation-locks Oysterdam Navigation-lock Ring" etnbanlcm ent Marq.IÏergon op Zoon

Discharge-medium Zoom lake

CANAL TÏ-IROUGI-I SOUTH-BEVELAND RE-INFORCEMENT D1KEÏ D i k e - i m p r o v e m e n t s WORKS i c b . o . WATER-MANAGEMENT S l u i c e B r o u w e r d a m Sluice Grevelingen-dam Pximping s t a t i o n Vee ADJUSTMENT WORItS 7S t r e 77 • 78 79 80 81 82 ₈₃ 8'i ₈₅ • 86

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-28-Costs A3.

A survey front January lst, 1$76 onyards of the costs of the most important works has been given in figure 8.

IIHIIIJin» \ ,

Figure 8. Survey of costs A3,

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-29-

;

' L

SAFETY»

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Introduc tion.

The works at tb e mouth «f «the Easternscheldt have as I an ultimate goal to provide sa£ety to a laa-ge part of

Zeeland as promised in the Del~ta Act» Safety, however, jl has rather a relative meaning, One can only approach it \ by speculating Vhich safety measures have to be taken; I in otber words by making a mental picture of a possible i™ calamity. Then it appears to what extent one undergoes • a calamity as a disaster* This ïidghly kepende upon the I personal involvement, Experiences from the past play a j role: has one personally been snbjected once upon a time || to siich a calamity or seen it from nearby'? Also the

economie interest might tip the scales: the inundation I of a sraall polder might stir the inhabitant of higher

grounds less than he wbo sees His immediate surroxindings I being distroyed. Expressed ±n xisk—analytical tertnsj

we must determine whetber the xisk is to be considered •

SUB a micro-meso or macro-risk. In general one can say j'

that in case of major flood—disasters our population \m is involved in an almost emotional wa.y even inundations | without loss of huraan life malce «• <ïeep impact. A

clear-i • cut example were the f loods of Tuindorp-Oostzaan. Though |

there were no victims the incident -was seen and

expe-rienced as a national disaster* Some important risks >| of floods are: ; - loss of human life I - gr'oups of people losing hearth and home: the "sans-abris" - loss of cultural and environmental values.

- (social)-economie chaos

- material loss both direct and indirectly. _ Apart from those there are other less noticeable immediate | effects: for example a lack of xïonfidence of safe-feeling.

The latter could be regarded as a significant lack of I well-being.

In our country great importance is being attached to I the safe-guarding against floods*

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-30-For thls protection, however, certain standards have to be taken into consideration: for exainple, vrhat

degree of safety should be pursued and what is society prepared to pay for it?

"Safety of the Delta-area",

In former times in our country the dimensions of the water-retaining structures vere based on éxpèrience: the standard was the highest water-level known from the past. With the report of the Delta Committee a completely different approach vas made: the height of the water-retaining structures of the North Sea was based on a theoretically established and not yet occurred water-level at Hoek van Holland: N.A.P. + 5 m. This figure is called the basic level. Water-levels higher than the basic level may occur natnely with a chance of 1/10000 time per year, Colloquially now one says that the basic-level has a chance of being exceeded 1/10000 time a year.

The difces are designed to retain the so-called "design-level". This theoretical level equals the basic-level for central-Holland and thus the chance of it being exceeded is 1/10000 time a year. For the Delta-area a theoretical level is being maintained

is

which 30 cm lower than the basic-level and which has a chance of being exceeded 1/4000 time per year, There seems little enough chance of it. One has to bear

in mind, however, that this chance of exceeding could present itself every year, a person has therefore during his life-time - say 80 years - a chance of 80 x 1/4000 times or 1 in 50 that he may experience a storm which can cause such a high-water-levelï The Delta Committee has investigated by means of econometrieal calculations in how far it is economi-cally justified to take the theoretical level as a

basis for improveraent of the water-retaining structures. Notvithstanding the many uncertainties in the calcu-lation it appeared that the design-levels could be

-31-considered as economically justified. The method of cal- — culating (24) developped by the Committee is stil! being | applied, recently by the Committee Oosterschelde. The , chances of exceeding indicated by the Delta Committee '• I are being called for convenience's sake the "Delta-standard".

According to the Delta-Act our country is protected either I hy the enclosing dams in the Delta-area or by heightening

of the existing primary water-retaining structures in • accordance with the Delta-standard, • • • The measure of safeguarding of a certain area does not ; m» depend solely upon the height of the primary water-retaining 8 structure. Often this problem is unjustly simplified to

this aspect albne. In reality it is a combination of a j | great number of factors of which the most important are:

- the height of the primary water-retaining structure I .- the quality of the primary water-retaining structure

- the condition of the sub-soil. \M

- the length of the primary water-retaining structure

- the alignment of the primary water-retaining structure - the foreland

- the secondary water-retaining structures . - the s±ze of the polder involved • , •• | - the depth of the particular polder

- the possibility of heightening at a later date. il These factors will be further discussed in the next

para-graph. I

Furthermore, one has to bear in mind that also other causes ! than high water-levels alone can lead to a breaching of M the dikes. For example the indirect cause of the inundation * of Tuindorp-Oostzaan probably was a b u s t e d water-pipe. One may

jno^ therefore accept without further thought that two areaa which are both protected by a "Delta-dike" have an equal chance of being flooded and also not that this chance coincide "! with the Delta-standard.

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* Page 30. This is a simplification, based on statistics; • the chance is in fact somewhat smaller. •

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-32- ,

The Delta Committee has signalled this already. The

Committee highly values the presence of secondary retaining structures. The Committee based it's advice for the Delta standard for the south-western part of the country unequi-vocally on the presence of secondary water retaining

structures:"For the south-vre stern part of the country which ie divided into small areas, each one individually protected by pritnary water retaining structures, design-levels have been fixed which have a 2,5 time's b'igger chance of being exceeded than the locally accepted basic

levels" (25), The Committee also offered some recommendations for the construction and maintenance of a systetn of secondary retaining structures although the primary

water-retaining structure needs attention in the first place. A similar reeomniendation is made by the Technical Advisory Committee for the water-retaining structures in it's report

"The Secondary Water-retaining structures in the Netherlandst'26) Thus in fact the Delta Act regulates only one aspect of

the safeguarding against floodsï the construction of adequate primary water-retaiuing structures, by building enclosing

dams in the south-western part of the country and by heigh-tening dikes elsewhere. The chance of floods in our country is-on the one hand higher than could be expected with

regards to the theoretical height of primary water-retaining structures - because other causes than the decisive storm-surge may lead to a breach of dxkes - on the other hand the chance of floods might be lower than expected owing to the existance of secondary retaining structures. How effective the safeguarding might be is difficult to say.

In any case one is striving to make the protection as adequate as possible. The Delegated States of North-Holland, South-Holland and Utrecht have formulated this

effort in such a way that a good safe-guarding has to meet at least one of the three following conditions:

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a. Flooding raust, according to human standards be impossible because of a high formation of dunes with sufficient

width or if a fortified narrow formation of dunes is protijeting the country;

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b) A possible breach should not immediately cause a I flooding of the area itself because a secondary

retaining structure of sufficient strength and at a • proper distance is situated frora the primary water- • retaining structure. IM c) In case of a possible breach no spill-tTny in the dike may M

develop: "the dike should therefore be concealed on

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the outer-side with a high foreland of sufficient width g or with a strong low over-flow structure behind it.

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A proteci;ion that meets one of those criteria is regarded

as an optimal protection. Behind the enclosing dams in the south-west of the Netherlands, as laid down in the Delta Act, there are water-areas of a large size. The low-lying polder- :

land is being protected by forraer priraary water-retaining • structures. It is therefore highly improbable that the ! polders lying along-side the Delta-lakes woüld be flooded , M in case of a breach of a Delta-dam. The Delta Act provides ]• these polders therefore with an"optimal protection."

Sxunmarizing, the following can be established:

a. In our country floods are considered disastrous. In their disrupting results this kind of disasters are difficult to compare with other social risks. A comparisbn with ' earthquakes or violence of war has the; closest

resem-blance, Great significance therefore is being attached to an adequate safeguarding against floods.

b. The Delta Act provides the construction or the improve-ment of a system of primary water-retaining structures

with quantifiable chances of breaching: the Delta Standard. c. The actual safeguarding of a certain area cannot without _

further ado be deducted frora the Delta-standard. For this | the area has to considered in its toteility.

d. The endeavour is aimed at "optimal safety". | e. The Delta Act holds in prospect "optimal safety" for the

areas around the Eastemscheldt. I ïf one would want to introducé for the Easternscheldt-area

the somewhat confusing term "Delta-Security" then the safe- • guarding for this area would imply the construction of a * dam (prc*Kded with or without a storm-surge barrier) at m the mouth of the Eastemscheldt.

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The System of

In the former paragraph it has bsen explained that the of a certain area is not «xclusively determined Tay the height of the primary water-retaining structure, bnt that ones has to talk a sy^stem of safeguarding composed of a great nvunber of elements. The elements that play an important role in the security of the Easternscheldt-area will discussed successively now, Also attention will be paid to the specifie:problem of dike slides.

a, The hejglit of the primary water-retaining structure, In many cases a breaching of the dike starts with over-flowing or overtopping water. Due to this the landward

slope of the dike soaks through which may result in slides. Thus the height of the crown of the dike is the principal criterion in designing a dilte. It is not true that a dike cannot bear any vrat er on the landward slope; some wave overtopping need not be dangerous. More or less due to the ïack of anything better a so-called 2% criterion is used; a criterion of the design which stipulates that 2%

of the decisive v a v e s m a y overtop the dike. Thiis criterion is somewhat gross; one dike may well be able to withstand a higher wave-overtopping than another. An exact and opti-mal deterraination for the height of the dike is as yet not possible. No more is it possible to determine exactly the remainder of the design; one has to start from great extrapolations. Ultiraately the decisive wave-overtopping which establishesf the design-level is a factor which hides many unknowns. All together the determination of the

height of a dike remains a diffictilt task, which in itself militates against attribution of an absolute value to

the numerical value of the safety provided by the dike,

b« The quality of the primary water-retaining structure,

The quality of the retaining structure depends on many

factors; such aa the prof.ile, i.e. the width and the gradiënt of the slopes, and the width of the banquettes, the revetment and the tttrf, tlqe material with which the dike has been built,

-35-absolute certainty about the retaining-power of a retaining structure; inaccuracy in the theories and weak spots in

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and the presence of alien components such as sluices,

^buildings and retaining walls. The foundation is also

important; a dike is never better than the foundation ' ' on which it rests. Th© modern techniques enable us to I rebuild already existing and qualitatively not very !

good dikes and turn them into water-retaining structures • of full value again* However, a completely new dike j which need not be a coraproraise betveen existing interests H -and deraands for retaining water will harbour fewer uncer- i

tainties. m

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c, The length of the primary vater-retaining structure. J_

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The foregoing has shown that there is no . such thing as m

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the water-retaining structures still entail certain risks. ; It is evident that these risks -are fjewer with a short I water-retaining structure than with a long one; the

extent of the risk is directly proportional to thé length j I of the watör-retaining structure. Por that reason one

has endeavoured in our country since a long time already ; I to shorten the coast-line to be protected. Another impor- j

factor

tant in favour of a short dike is the maintenance;a

water-retaining structure requires continuous inspection,

maintenance and surveillance. ! •

d. Alignment of the primary watenr-r_etaining structure»

A dike is being attacked most strongly when the wind hits I the dike perpendicularly. A straight dike therefore offers m a better protection than a tortuous one, because with a | tortuous dike the chance of the wind hitting it somewhere

perpendicularly is greater. In this respect a dike around ,' I a bowl-shaped basin is the most unfavourableï practically

every wind-direction might be aimed unfavourably at some I1 section of the dike. An alignment with many sharp angles

could also be called unfavourable; especially at re-entrant , • angles the water will be tilted extra high. The tortuous • coast-li*ie of North-Beveland is a fair exatnple of an »

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e, Secondary water-retaininy gtructures,

Secondary water-retaining structures can add cpnsiderably

to the safety of an area. To this end the retaining strxictures have to meet with certain demands. The most important require-rnent is that the secondary water-retaining structure

is stiuated at a certain distance from the primary water-retaining structure, Behind the primary water-water-retaining structure there should be an area that would need a certain period of time to fill up when a dike breach occurs, so that the secondary structure only has to retain the water when the high tide is already subsiding again. .IrtersoctjL.ij; clikes cannot be regarded as secondary water-retaining structure; they are not meant to b e . As a ruXe one tries to construct a secondary water-retaining structure which can retain water at a level that eorresponds with the so-called

"border"-level: that is a water-level that has a chance of being exceeded 1/2 time per year.

fc The possibility of heightenin^dikes later on.

In the cours© of the years the height of the dike decreases due to natural wear and tear, settletnent of the dike's body and settlement of the foundation. In the long run the fact that the sea-level rises with respect to our country also plays a role. For this so-called relative rise of the sea-level a standard of 20 cm/per century is tnaintained as a rule. Furthermore one should bear in mind that the chance of obcurrence of the design-levels has been based on the storms that took place in the past. If climatie changes would occur which would manifest themselves by a higher frequency of storm-floodö in the North Sea area then the design-levels would have to be adjusted. Though new dilces are being constructed with a certain excess-height one has to take into account that in the future, this need for heightening of dikes might present itself again. Dikes that can easily be heightened in the future

offer an additional safety in the Zong run,

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,g« Bank and dike slides. • • . • I Various areas along the Ea stern scheldt are "sensitive to

slides". Two conditions have to Tje with if slides. occur: j • the bottom must consist of loosely packed layers of sand j and the bank has to be steep. One can distinguish three • kinds of slides: slab-slides, bank-slides and dike-slides. • Though slab- and bank-slides indirectly might also endanger

the water-retaining structure, the dike-slides are of course the most dangerous. Most of the times slides will occur

at low water-levels; this ia why it is often assumed that I this phenomenon is not so dangerous,

This is not wholly corr«ct1 h o w v e r . Especially dike-slides 11 during the stormy season (October through March) are \ exceedingly disagreeable* It is hxghly conceivable that • a storm-flood occurs even before the datnaged dike can be * repaired. Then a breach of the dike is xmavoidable. By the • Provincial Waterstaat of Zeeland has been calculated that I there is a chance that such a c^lamity can happen 1/60 time _ per year* Thus, this is considerably higher than the chance | •of occurrence of the storm-flood of 1953»

That the phenomenon of bank- and dike—slides happens much I more frequently along the Easternscheldt than along the

Westernscheldt is probably due to morphological develop- I ments in the Easternscheldt, Since a long period of time '

the basin has had the tendency to deepen and w i ^ n . (it is • a so-called erosive basin). Time and time again man had |" to give up land here; a process that is still continuing M up to this day. I

Comparison of the alternatives ±n the final stage.

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In. the foregoing a qualitative view has been given about

the various components which determine the safety of the country. With the three altematives the security can be

obtained in a different way; this is shown in fig. 9 • regarding the area in front of -the corapartmentation dams

and in fig, 10 with regard to the area behind them. The • alternatives will be corapared as far as the aspect of jj safety is concerned on the following points:

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— the quality of the primary water-retaining structure — the length and the alignment of the primary

water-retaining structure

— subsequent dike-heightening — dike-slides

A. closed off Easternscheldt» (D4)

— Measure of safety

The closing dam causes all present primary water-retaining structures to become secondary ones* Between the secondary water-retaining structures and the nevr primary water-retaining

structure a big lake is situated which acts as a buffer-zone. The measure of safety complies with the fixed Standard; the

Secondary dams Non heigbtened d POLDER

Ib4

Secondary p^ r tial.lv

dtiras heightoned c etora-surgo b a r r i e r North S POUPER

.fC3

Totally heightened

Secimda.ry dik e e Ooaterschelde-toasin North

/T T"\ POLDER _,./ l"*^~*""~*""""

dams 39 dams -C-ompart ment «dam

Sea •*>OLOER Compartmcnt-dam Storm-surge barrioi Nortb. Sea POLDER r ÉC3. ZOOMMGEH 1' Compartment—dam \ ZOOMMEER Oosterschelde-basiï A3 Sea

F-r£We~TÖ.~Area behind cotnpartraentation dams.

Quality of the primary water-retaining structure

Without any restriction the .quality of the Delta-dams can be said to be of a high staridard. There is nb need for

fear of hidden deficiencies. . \ The length and the alignment of the primary water-retaining : structure. :

With the closing dam the present coaat-line will be shortenedi

from 2k5 km to 9 km, extremely short in comparison with the ! protected area. ' Subseqnent heightening of dikes. ; Future heigthening of the Delta-dam* can be done without i great interventions.

Dike-slides

The danger of bank- and dike-slides has been warded off.

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Storm-surp;e barrier, C3. - Measure of safety

The situation in C3 reminds one of the storm-surge barrier in the Hollandsche IJssel, Assuming that the storm-surge barrier in case of a storm-surge will be closed in time, the measure of safety equals that of Dk. The existing dikes, however, will require a little more care than with D4, but on the other hand with C3 we may start with

•'1/500 dikes".

- Quality of the primary water-retaining structure. The quality of the storra-surge barrier will correspond with that of other great hydrológical structures,

- Subsequent dike-heightening.

Th© retaining height of the storm-surge barrier is exclu-sively based on the design-level.The wave-overtoppping is not detremental to the stability of the water-retaining

structure. Future adaptations of the construction to the relative rise of the sea-level for example will not be

simple to realise, but in view of the foregoing it will not be so urgent either. One has to assutne that possible necessary adaptations will be executed as part of a total overhaul or replacement of the by then old structures. - Dike-slides,

How the morphological development of the Easternscheldt basin will be is as yet difficult to predict. Though a further erosion of the banks will go more slowly as a result of decreasing flow-velocity, the danger of dike-slides, though considerably less, will not disappear completely, because the shifting of channels has to be taken into account. Calamities, however, can be avoided, because one has the possibility to close the storm-surge barrier.

Open Easternscheldt, A3« - Measure of safety

Over a length of 1^5 ktn the height of the primary water-retaining structure will have to b© adjusted to the design-level wdfeth a chance of exceeding of 1/4000 time a year.

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The plan (6) herefore developed by the Province of _ ^!e«land has been sufficiently illustrated in all lts ' | «spects t<i allow for a fundamental policy-decision,

^which the Cahinet did in November, 197^* However, a • number of important data for an «laboration of the plan

«re still laclcing, especially the design-level for the I

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Very recent studies have shown that the exceeding of ; • high storm-sTirge levels in an estuary as the Easternscheldt ! *

may occur more often th*an was assumed ±n the past. More insight in particular has been obtained in the behaviour of the wind under extreme circumstances and its influence

on the water-levels of the basin of the Easternscheldt, jg This affects the design-levels to be used. The estimates

concerning the wind used until now appear to be on the I low side, both with regard to the duration and the velocity. As a result both higher water-levels and higher .waves M jnight occur under "Delta-storm" conditiorts. According

io the most recent views this study could lead to a •

not inconsiderable rise of the design-levels, especially '• in the eastern part of the basin, where the order of _

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jnagnitude might easily reacb a few meters of dike-height.

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In appendix II this problem will be further elaborated. With great certainty, however, it can be said already that the height of the dikes in the "plan Zeeland" is

too low especially in the eastern part of the basin. | The Provincial Waterstaat stated this already in a recently

published actualisation of the Plan (6). I In order to offer security to as many areas as possible

around the Easternscheldt, a safety that at least is I comparable to what is being required by the Delta Act, :

the forming of a system of secondary water-retaining :•' structures will be necessary. However, this cannot be 'j jrealised everywhere. For the area in front of the cora- H partmentation dams no "optimal safety" as in Dk will be I possible* The purpose of the- Delta Act will not be tnet#