Some aspects of the Delta project

S O M E A S P E C T S O F T H E D E L T A P R O J E C T

B Y I R . K . F . V A L K E N A N D I R . W . C . B I S G H O F F V A N H E E M S K E R G K SENIOR ENGINEERS STATE PUBLIC WORKS, DELTA WORKS, T H E HAGUE

I . T H E G R E A T F L O O D O F 1953 (i) The storm

T h e first i n d i c a t i o n of the development of the atmospheric disturbance that was to become the disastrous depression of 31st J a n u a r y - 1st February, 1953, was seen on the weather map f o r 12.00 hours G . M . T . , 29th J a n u a r y . I t is thought to have been a split-off depression generated on the w a r m f r o n t o f a depression over the A t l a n t i c Ocean n o r t h of the Azores by an upper-air t r o u g h moving east f r o m the area south of Greenland. I t moved north-east i n the early stages of its development, deepening about 4 m b i n six hours. By l i n k i n g up w i t h an old shallow depression south o f Ice-land, the developing disturbance gradually started transporting colder air southwards f r o m the Iceland area. A t the same time, a ridge o f h i g h pressure south o f Greenland intensified significantly while m o v i n g steadily east. A north-westerly air stream increased i n extent and intensity south o f Iceland between an associated ridge at the 500 mb level and the above-mentioned upper-air trough. T h e depression changed direction under the influence o f this air-stream and began moving south-eastwards along the n o r t h e r n coast o f Scotland and over the N o r t h Sea. As i t continued to deepen, an area of north-westeiiy gales developed to the west of the depression. O n the m o r n i n g of 31st January, a trough of l o w pressure developed near Scotland at sea level causing very steep pressure gradients accompanied by severe gales. T h e y reached their m a x i m u m force as the trough moved south-eastwards along the Scottish coast. A t 12.00 hours G . M . T . on 31st January several stations i n the area reported w i n d veloc-ities o f about 70 knots. A t the same time, the depression itself h a d reached

its greatest depth i n the central N o r t h Sea. Winds attained their m a x i m u m force on the D u t c h coast i n the evening on 31st January and veered n o r t h -west as the low-pressure trough passed.i T h e w i n d reached average veloc-ities of over 45 knots along the coast.

A t 11.00 hours on that day a w a r n i n g of "unusually h i g h tides" had already been sent by telegram to the dyke defence groups i n R o t t e r d a m ,

F I G . 1 Courses followed by troughs of low pressure during some violent storms

Willemstad, Bergen op Z o o m and Gorinchem. A t the end of the after-noon, a f u r t h e r telegram was sent giving a w a r n i n g of "dangerously h i g h tides", b u t only when i t became evident i n the evening that there h a d been no perceptible f a l l i n the level of the water at low tide d i d m a n y

people realize that the next high-water levels i n the South-West w o u l d exceed previously recorded peak levels by more t h a n \ ^ feet and that disaster was i m m i n e n t . M e a n w h i l e , the polder boards and officials of the "Rijkswaterstaat" had already begun to alert the p o p u l a t i o n and re-inforce the dyke defence groups. However, despite the measures taken b y those groups, the water swept over the dykes i n many places, causing serious damage to the dykes f o r hundreds of miles and breaching t h e m i n 67 places. As a result, about 375,000 acres were flooded, more t h a n 1,800 people lost their lives and f r o m 2,500 to 3,000 m i l l i o n guilders' w o r t h of damage was caused.

A c t u a l l y the disaster m i g h t have been m u c h more serious. A breach w h i c h had opened i n the dykes along the HoUandse IJssel was only closed at the eleventh hour as a result o f superhuman efforts. I f these efforts h a d

FIG. 4 Damage to the inner slope. The outer slope is still sound (Schouwen Polder)

not succeeded, the whole of central H o l l a n d w o u l d have been flooded and the entire Netherlands economy w o u l d have been disrupted f o r years to come.

(ii) Damage to dykes

A storm of such intensity had not generally been allowed f o r when the dykes were designed, so that at first reports o f damage to the dykes came as no surprise.

T h e real surprise came later when i t was realized how extensive the damage was. Most o f t h e dykes i n the area i n w h i c h the disaster occurred were seriously damaged, i f not breached. The inner slope and the top were the parts most affected. H a r d l y a single case o f serious damage to an outer slope was reported. I n many places where the inner slope was seriously affected, the outer slope was quite undamaged.

Closer examination of the dykes i n the disaster zone revealed t h a t the outer slope usually varied between 1 i n 3 and 1 i n 4. T h e inner slope was usually f o u n d to be considerably steeper. O n l y i n a few isolated cases was the inner slope less steep t h a n 1 i n 2. The inner slope of the m a j o r i t y o f the dykes, however, was between 1 i n 1.5 and 1 i n 1.75.

Slopes as steep as these are of course extremely vulnerable i f there is any

F I G . 5 Damage to the inner slope. The slope is still intact behind the timber storage shed, where no water broke over the dyke (Oud Herkingen Polder)

seepage of ground-water. But no dangerous seepage of ground-water could have come up t h r o u g h the body o f the dyke or the sub-soil, because the storm had not lasted long enough for that to happen. I t was f o u n d that the dykes were damaged or breached only where large quantities o f water had swept over the top. T h e extensive damage sustained b y the sea dykes i n the South West d u r i n g the storm of 1st February, 1953 must therefore be ascribed p r i n c i p a l l y to water sweeping over the top. T h i s water probably penetrated the dykes t h r o u g h the non-watertight covering of the top and inner slope causing air and water pressures to b u i l d up i n the body of the dyke, i m p a i r i n g the stabiUty of the inner slope.

Once the inner slope has started to slide erosion w i l l cause the body o f the dyke to collapse i n a relatively short time. T h a t was very hkely the manner i n w h i c h most o f t h e 1953 breaches were occasioned.

I I . I N F E R E N C E S (i) Traditional approach

I t is quite evident f r o m the foregoing description o f t h e disaster that water sweeping over the top was the p r i n c i p a l cause of the breaching o f the dykes. I t was n o t h i n g new. I t was the same phenomenon as had man-ifested itself on various occasions d o w n the ages. Andries V I E R L I N G H was m a k i n g no idle statement when he wrote i n the " T r a c t a e t v a n d i j c k a g i e " (Treatise on dyke-building) in about 1577 " o u r greatest welfare depends on the height of a d y k e " . However, the constant problem was how h i g h to b u i l d the dyke. I n f o r m e r times the usual criterion was the highest flood level that could be remembered. T h a t a r b i t r a r y criterion always proved deceptive. People were usually inclined to forget past disasters only too easily when i t was a matter o f raising money f o r heightening the dykes. A d d e d to this was the fact that the comparison of various flood levels was made d i f f i c u l t by the absence of a standard level, b y the rise i n sea level, by the subsidence o f the sub-soil and b y the settling o f the polder l a n d and dykes. Consequently, many floods reached higher levels than had been provided for. So the t r a d i t i o n a l approach was overtaken b y the facts again and again.

(ii) Physical approach

effect" were better understood, i t became possible to o b t a i n a clearer idea o f the type o f floods to be anticipated. I t enabled the engineers to determine how m u c h higher a recent storm level m i g h t have risen i f some of the c o n t r i b u t o r y factors had combined more unfavourably.

Using this method o f reasoning, the conclusion to be d r a w n f r o m the 1953 storm disaster is that, although the peak level of 12^ feet above N . A . P . (New Amsterdam Level = approx. mean sea level) observed at the H o o k o f H o l l a n d was exceptionally high, a level of 1 6 | feet above

N . A . P . might actually have been reached. However i n itself this fact has no real significance, since any of the values of the various factors w h i c h together constituted the "storm effect" m i g h t have been still more u n favourable. T h e studies carried out on this subject b y the R o y a l M e -teorological Institute reveal that appreciably higher flood levels are phys-ically possible. I n fact, i t w o u l d not be possible to predict, even approx-imately, a water level w h i c h could not be exceeded. So an absolute safety l i m i t can never be based on anything b u t pure conjecture.

(iii) Statistical approach

I n recent decades there have also been valuable developments i n the field of statistics. T h e "Rijkswaterstaat" has access to a series of systematically documented records on water levels going back to the end o f the last century and has carried out m u c h research into this subject. I t soon became clear that the incidence of a b n o r m a l l y h i g h water levels could be represented very well i n terms of frequency i n accordance w i t h the laws o f probability. The higher the flood level, the smaller the hkehhood of i t being reached or exceeded. Since, f o r physical reasons, no m a x i m u m level can be given, there is always a certain risk of flooding, however h i g h the "design level", i.e. the flood level selected as a basis when de-signing the dykes and sluices may be.

I n order to discover the degree of likelihood there is that a flood w i l l exceed the very h i g h levels hitherto unrecorded, the frequency curves o f excess f o r high-water levels, w h i c h are inevitably based on a relatively short-term observation period, had to be extrapolated to points far beyond the field of observation. T h e M a t h e m a t i c a l Centre, i n particular, con-t r i b u con-t e d a greacon-t deal con-to con-these scon-tudies. T h e R o y a l Mecon-teorological I n s con-t i con-t u con-t e research mentioned previously was also very i m p o r t a n t i n this connec-t i o n , since i connec-t f o r m e d connec-the basis f o r connec-the selecconnec-tion of connec-the daconnec-ta essenconnec-tial f o r statistical purposes.

Using the extrapolated frequency characteristics, the "design level" can now be selected i n such a way that the m a r g i n of risk is reduced to an acceptable m i n i m u m . T h e real question is, of course, what risk can be regarded as acceptable. A n y decision o n this matter is bound to be very subjective. T o the u n i n i t i a t e d , f o r instance, i t w i l l d i f f e r according to the way i n w h i c h the p r o b a b i h t y is expressed. L e t us take the peak level observed at the H o o k of H o f l a n d on 1st February, 1953 as an

y y y y y y y y y y y y y • 1953.-y y y .• y y - y . y y .•• ." * .• y • y . y .•• .••189 4 4 r ' 19)6 1906 E X C E S S CURVE C H O S E N P O S S I B L E EXTRAPOLATIONS 10° 10' lO^ 10'^ E X C E S S FREQUENCY P E R Y E A R

F I G . 7 Tentative excess curves of highwater levels during storms at the Hook of H o l -land (1859-1959)

example. T h e frequency o f excess for this level w i l l be j u d g e d i n t w o very different ways i f we are told, either that the level w i l l be ex-ceeded on an average once i n every 250 to 300 years or that a m a n has a 2 0 % chance of witnessing a disaster d u r i n g an average lifetime o f 75 years. But b o t h chances are identical. Bearing i n m i n d the g r o w t h of the population and the intensive economic development, together w i t h numerous other imponderables, such as h u m a n suffering, loss of hfe and the disruption of the routines of life, the Delta Commission regarded a remaining flood risk o f 1 % per century as being just admissible. T h a t involves a high-water level at the H o o k o f H o l l a n d of 16J feet above N . A . P . The realistic value o f this level f r o m a physical standpoint has already been discussed.

(iv) Econometric approach

As described above, the selection of a "design level" on the basis o f physical and statistical considerations must of necessity be subjective. For instance, we have to consider whether the increased safety obtained war-rants the cost or - to use insurance terminology - whether we are over or under-insured. T o obtain a better understanding of this matter, at-tempts were made o n the suggestion o f t h e Delta Commission to approach the p r o b l e m on a j o i n t economic and statistical basis. T h i s is p r o m p t e d by the idea that the capital to be invested i n flood prevention projects together w i t h the capitalized value of the m a r g i n o f damage due to flooding should be as small as possible, because that w o u l d give us the best value f o r our money and j u s t i f y the use o f the t e r m "economic o p t i m u m " .

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C O S T S O F H E I G H T E N I N G D Y K E S A N D C A P I T A L I Z E D A N T I C I P A T E D D A M A G E F I G . 8 Diagram illustrating an econometric estimate of costs of dyke-heightening and the capitalized anticipated disaster damage expressed as a function of the flood level adopted as the criterion for dyke-heightening

T h e above principle was worked out both analytically and graphically. T h e analytical w o r k was done at the M a t h e m a t i c a l Centre and the graphical method was developed by "Rijkswaterstaat". O f course, there are m a n y u n k n o w n quantities i n the calculations.

O n the one hand, a tentative estimate only can be given of the cost of heightening dykes on an extensive scale, of b u i l d i n g new dykes or o f carrying out other flood prevention projects and of the capitahzed ex-penditure that maintenance involves.

O n the other hand, i t is extremely d i f f i c u l t to estimate the amount o f damage that flooding m i g h t cause. Allowance w o u l d have to be made not only f o r the economic development anticipated, b u t also f o r the fact that the extent and consequences of flooding are never the same f o r any two floods. Moreover, the breach criterion selected, i.e. the exceeding o f t h e "design level", is always open to objection, since numerous factors connected w i t h dyke construction, etc. may also be involved.

T h e rate of interest selected is also inevitably an uncertain element i n the capitalization of the m a r g i n of damage due to flooding. B u t the ele-ment o f uncertainty involved i n the extrapolation of the frequency curves of excess is of m u c h greater consequence.

By suitably varying a l l the above-mentioned elements o f uncertainty, i t is possible to derive f r o m the calculations two extreme lines, each of w h i c h represents the sums o f money to be invested i n the raising o f the dykes, etc., and the capitalized m a r g i n of damage due to flooding expressed as a f u n c t i o n of the "design level". The true figure w i l l he somewhere be-tween these two extremes, though i t is not k n o w n exactly where, so that the actual economic o p t i m u m is not k n o w n , either. T h e intermediate level m i g h t be chosen i n such a way that any deviations f r o m the two extreme economic o p t i m a are equal but opposite. T h a t keeps the devia-t i o n f r o m devia-the acdevia-tual economic o p devia-t i m u m as low as possible.

T h e above-mentioned economic considerations w o u l d certainly be v a l i d f r o m an insurance standpoint i f a large number of unrelated risks could be insured on this basis. The fact that this is not apphcable i n p r i n c i p l e to the present case is one o f t h e weakest points i n the econometric calcula-tions. Therefore, the moment when the next disaster w i l l occur has a decisive influence on the value o f the calculations.

Hence, even the econometric calculations are to a certain extent a r b i t r a r y when i t comes to determining a "design level". Nevertheless, by g i v i n g

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a clearer representation of the factors involved, they very clearly support the conclusion w h i c h the Delta Commission already felt i t was obliged to draw o n physical and statistical grounds. I n addition, the econometric calculations show conclusively that there is a greater disadvantage i n selecting a relatively l o w "design l e v e l " w h i c h may lie below the eco-nometric optimuiTi than there is i n selecting a relatively h i g h level w h i c h p r o b a b l y hes above i t . I t is true that i f a level below the o p t i m u m is chosen, the i n i t i a l cost o f heightening a dyke w i l l be comparatively l o w , b u t the f l o o d damage anticipated wiU increase disproportionately. Such a measure w o u l d perhaps be penny wise b u t certainly p o u n d foolish.

(v) Conclusion

O n the basis o f physical and statistical considerations supported b y eco-nometric calculations and bearing i n m i n d the numerous imponderables

FIG. 10 "Basic" and "design" levels along the Dutch coast

Basic level

in meters Anticipated above N . A . P . rise due

(excess frequency to damming Station lO-i) (in cm)

1 2

Vlissingen 5.65 + 5

Breskens 5.85 + 5

Hansweert 6.15 + 5

Bath 6.60 + 5

D a m Veerse Gat (Vrouwenpolder) 5.45 + 40

D a m Oosterschelde (Noord-Beveland) 5.35 + 40

Dam Oosterschelde (Burghsluis) 5.25 + 35

Top of Schouwen 5.10 + 25

D a m Brouwershavense Gat (Repart) 5.25 + 40

Dam Brouwershavense Gat (Goeree) 5.15 + 30

Top of Goeree 5.05 + 30

D a m Haringvliet (Goederede) 5.20 + 40

Dam Haringvliet (Rockanje) 5.20 + 40

Top of Voorne 5.05 + 30

D a m Brielse Maas (Oostvoorne) 5.05 + 25

Hoek van Holland 5.00 + 0

IJmuiden 5.15 + 0 Den Helder 5.05 Afsluitdijk 5.85 Harlingen 5.80 Delfzijl 6.40

such as h u m a n sufferiitg and loss o f liumari lives, the D e l t a Commission recommended that measures to prevent flooding i n the Netherlands should be based o n levels that are likely to be reached or exceeded on an average 10-4 times per year or 1 % per century. Such levels are called "basic levels".

Since the interests safeguarded by dykes or dams differ widely i n various parts o f the Netherlands, the "design l e v e l " , w h i c h is intended to serve as a basis f o r the actual design, may be higher or lower t h a n the local basic level, depending on the interests to be safeguarded.

I I I . T H E D E L T A P R O J E C T (i) The choice

Highest recorded i n level Design level i n meters above N.A.P. i n meters above N.A.P. for permanent for temporary

situation purposes

(1 + 2 - 3 ) (15 cm below 4) year level 4 5 6 7 5.40 1953 4.55 5.60 1953 4.80 5.90 1953 5.07 6.35 1953 5.60 5.55 5.45 5.30 1953 4.20 5.05 1953 4.10 5.35 1953 4.18 5.15 5.05 1953 4.00 5.30 1953 4.05 5.30 1953 4.10 5.05 1953 3.95 5.00 5.00 • 1953 3.85 5.15 1953 3.85 5.05 1953 3.25 5.25 5.10 1953 3.70 5.50 5.35 1954 3.69 6.20 1825 4.60

dykes i n the south-west w o u l d have to be raised by f r o m 5 to 6J feet at least f o r m a n y hundreds of miles. The reinforcement w o r k such a p l a n w o u l d entail w o u l d be extremely d i f f i c u l t to carry out owing to the fact that there are m a n y houses, industries, harbours, plant, etc., alongside and even on the dykes themselves. A d d e d to this is the fact that the nature o f the soil by no means everywhere admits o f the reinforcement of the body o f a dyke, whilst several breaches revealed that various sec-tions o f a dyke may have other defects that cannot a l l be detected, so cannot be removed.

O n the other h a n d , d a m m i n g o f f the estuaries w i l l result i n a short coastal defence-line o f dunes and dams, w h i c h can be given b o t h the height and the strength, regarded as necessary f o r the f u t u r e w i t h o u t r u n n i n g i n t o the difficulties mentioned above. Moreover, i f the rise i n the average sea level i n relation to the land should continue or i f i n the distant f u t u r e the

basic levels should have to be raised for any other reason, an a d d i t i o n a l , even considerable, heightening of the dams wiU be comparatively simple to carry out. Furthermore, when i t is borne i n m i n d that the existing dykes behind the m a i n dams w i l l constitute a valuable second line o f defence, i t w i l l be clear that closing-off the estuaries w i l l result i n a m u c h greater degree of safety being obtained t h a n could ever be achieved by heightening the existing dykes. O n l y by closing o f f the estuaries can a defence line be obtained that can be regarded as effective and reliable, even for the distant f u t u r e .

I f the fact is also taken i n t o consideration that the cost of an all-round heightening o f the dykes and that o f d a m m i n g - o f f the estuaries w i l l not d i f f e r appreciably whilst the latter alternative w i l l offer great advantages i n matters o f fresh water management and the provision of road links, i t is not d i f h c u l t to understand w h y the choice f e l l on d a m m i n g o f f the estuaries, on what is now k n o w n as the " D e l t a Project".

However, the problem was by no means as simple to solve as the above statements w o u l d lead one to suppose, because i t had first to be ascer-tained whether the Delta Project was feasible f r o m a hydrologieal point of view. I t was reahzed that the waters of the estuaries that w o u l d have to be held i n check while the estuaries were being dammed o f f w o u l d be deeper, more p o w e r f u l and more capricious than any previously sub-j u g a t e d . T h e Delta Commission's conhdence that they could made a

a V U U i a . U i C i C C U i l i i l i C l i U a L i U i i V V i U l l I C g C l X U l L O L ± H Ö I I I O - L L I ^ I \V U.J k / t i J^ V - L u » V v J closely interrelated factors. I n the hrst place, they cited the experience gained i n the p l a n n i n g and execution of extensive and d i f f i c u l t d a m m i n g operations for undertakings such as the Zuyder Zee Project, the closing of the breaches i n the dykes on Walcheren after the war and the repairing of the damage resulting f r o m the floods i n 1953. However, the D e l t a Commission added a rider to the effect that i t w o u l d be necessary to gain more experience b y conducting intensified research on a scientific basis before i t could be applied to still larger projects; they drew attention to the splendid opportunities modern methods of research offer i n this respect. Before a final decision could be taken w i t h regard to the Delta Project and apart f r o m any consideration as to whether the closing-off of the estuaries w o u l d i n itself be feasible, data had to be collected o n such matters as the changes i n the movement o f t h e water that such an opera-t i o n w o u l d cause aopera-t oopera-ther places. I n opera-the inopera-teresopera-ts of shipping and opera-to

F I G . 11 T i d a l data of various closures

Gross sec- Mean flood Mean tion below volume i n tidal range N . A . P . i n millions of Time in meters sq. meters cubic meters

Zuyder Zee Barrier Dam 1 9 2 5 0.9 1 2 0 , 0 0 0 5 7 5 Brielse Maas Barrier Dam 1 9 4 8 1.8 2 , 7 0 0 1 7

Braakman 1 9 5 2 4 . 0 850 17 Dyke repair 1 9 5 3 : Kruiningen 1953 4 . 4 5 5 0 2 6 Schelphoek 1953 2 . 8 8 , 0 0 0 1 3 0 Ouwerkerk 1 9 5 3 3.0 1,500 3 6 Delta works: 1,500 Veerse Gat 1 9 5 5 2 . 9 7 , 5 0 0 7 0 Haringvliet 1 9 5 5 1.9 1 8 , 0 0 0 2 6 0 Brouwershavense Gat 1 9 5 5 2 . 4 3 0 , 0 0 0 3 2 5 Eastern Scheldt 1 9 5 5 2 . 8 9 0 , 0 0 0 1 1 0 0

prevent damage to the river banks, there may be no dangerous currents either d u r i n g or subsequent to the execution o f t h e Delta Project. I n view of this p r o b l e m , w h i c h had a decisive influence on the general p l a n n i n g o f t h e project, an extensive systematic investigation was undertaken about w h i c h more w i l l be said i n the f o l l o w i n g sections of this chapter.

(ii) Planning

T h e studies undertaken by the "Rijkswaterstaat" w i t h regard to the increase i n safety and the improvement of fresh water management i n the delta area began i n 1938, at about the same time as the first fuU-scale statistical investigation mentioned i n the previous chapter. T h e h y d r o -logical tests could not get under way u n t i l a hydrologieal model of the lower reaches of the rivers was completed i n 1948. A n u m b e r of very difi'erent plans, the T w o , Three, Four and Five Island plans w i t h adjust-able flood barriers and/or dams i n the lower reaches of the rivers, were examined, either i n d i v i d u a l l y or i n conjunction w i t h plans f o r the protec-t i o n o f protec-the Brabanprotec-tse Biesbos and protec-the m a i n l a n d o f N o r protec-t h Brabanprotec-t f r o m floods. A d a m originally projected i n the Hollands Diep near K l u n d e r t , combined w i t h large sluices (Weir X ) to d r a i n off" the water and ice f r o m the W a a l and the Maas, was moved farther and farther out to sea a n d was finally sited at the m o u t h o f t h e H a r i n g v l i e t near Hellevoetsluis. Thus,

the m a i n f r a m e w o r k of w h a t was later to grow into the Delta Project h a d already been fixed before disaster overtook the country on 1st February, 1953. T h e f o l l o w i n g are the p r i n c i p a l elements o f t h e Delta Project.

The adjustable flood barrier in the Hollandse IJssel for the protection of a large part of central Holland

U n d e r n o r m a l conditions the barrier is raised, so that i n l a n d shipping between Amsterdam and R o t t e r d a m can pass unhindered. A lock w i t h a movable bridge is planned i n a d d i t i o n to the flood barrier f o r the benefit of shipyards situated behind the barrier, so that ships w i t h h i g h super-structures can also pass.

Tlie main Haringvliet dam, incorporating a row of sluices through which the excess water and ice coming down the rivers Waal and Adaas can be discharged There w i l l be a lock for the passage o f the vessels needed f o r the m a i n -tenance o f t h e engineering works and the approaches, and for the passage of fishing vessels. There w i l l also be an inner and outer harbour.

The main dams across the Brouwershavense Gat, the Eastern Scheldt and the Veerse Gat

T h e incorporation of locks i n these dams has been advised against because of the risk of salination that w o u l d entail. Drainage facihties w i l l o f course be needed so that the water behind., the barrier can be flushed.

The secondary dam across the Zandkreek

T h i s d a m is required to enable work on the dams across the Veerse G a t and the Eastern Scheldt to be carried out independently; i t w i l l prevent current velocities i n the Zandkreek f r o m rising to dangerous levels. A lock w i l l be incorporated i n the d a m to allow the passage o f shipping to a n d f r o m the canal crossing Walcheren. This lock w i l l also serve as a sluice w i t h w h i c h the water i n the basin formed by d a m m i n g o f f the Zandkreek and the Veerse Gat can be controlled.

The secondary dam across the upper reaches of the Grevelingen

T h e d a m is required to enable w o r k o n the dams i n the Brouwershavense Gat and the Eastern Scheldt to be carried out independently; i t w i f l prevent current velocities i n the Zijpe f r o m rising to dangerous levels.

A lock-cum-sluice w i l l be ineorporated i n the dam. W h e n completed, the Grevelingen d a m , like the Zandkreek dam, w i l l continue to have a practical h i n c t i o n , p r o v i d i n g a l a n d l i n k for road traffic and serving for water control purposes.

The Volkerak [Hellegat) dam

T h e d a m across the V o l k e r a k w i l l make i t easier to carry out other sec-tions o f the Delta Project. Together w i t h the H a r i n g v l i e t d a m i t w i l l speed up the completion o f w o r k i n the N o r t h e r n sector o f t h e delta; this part of the project can be finished l o n g before the Eastern Scheldt is closed off. A series o f locks w i l l be incorporated i n the V o l k e r a k d a m allowing f o r the passage o f ships f r o m A n t w e r p and the Zeeland water-ways to central H o l l a n d or the Rhine. There w i l l also be a large sluice i n the V o l k e r a k d a m for the water management o f t h e "Zeeland L a k e " f o r m e d b y d a m m i n g o f f the waters r o u n d Zeeland.

T h e possible construction of an adjustable harrier across the lower reaches of the Oude Maas for fresh water management purposes.

The heightening and strengthening of dykes for increased safety along such waterways as the R o t t e r d a m W a t e r w a y and the Western Scheldt, w h i c h have to remain open to shipping to and f r o m R o t t e r d a m and A n t w e r p .

Ancillary projects

I n c l u d e d under this heading are such things as hydrologieal i m p r o v e -ments i n rivers for the efficient discharge of water and ice, measures to prevent erosion of the bed and banks, facilities for shipping, facilities i n harbours and unloading quays, etc., w h i c h w i l l make for smooth adapta-t i o n adapta-to adapta-the changed condiadapta-tions; i m p r o v e d faciliadapta-ties for adapta-the discharge o f polder and sewage water and provisions for the protection o f fishing interests.

(iii) Order of operations

T h e order o f Delta Project operations is determined by various factors. P r i o r i t y w i l l be given to the projects that ensure m a x i m u m safety. A t the same time care must be taken to ensure that h a r m f u l hydrologieal

situations do not arise i n the intermediate stage, i.e. before a l l the projects have been completed. I t is also i m p o r t a n t that the operations be carried out i n such an order that the experience gained i n previous small-scale projects o f such matters as the use of materials and equipment and meth-ods of research and operation can be p u t to good use i n the larger and most d i f h c u l t projects. Therefore there is a strong case f o r proceeding f r o m " s m a l l " or, rather, "less large" to " l a r g e " . T h e more so, since the execution o f the largest projects, the d a m m i n g o f the Brouwershavense Gat and the Eastern Scheldt, is hkely to be gi'eatly hindered by the vagaries o f tide and weather. Therefore i t w i l l be of p a r a m o u n t i m p o r -tance that a deeper understanding o f Nature's whims be gained i n advance.

Some o f t h e Delta Projects, such as the construction o f t h e f l o o d barrier i n the HoUandse IJssel and the execution of the Three-Island Project (Veerse Gat and Zandkreek dams), can be carried out more or less i n -dependently.

I n view of the m u c h greater safety o f central H o l l a n d to be secured b y the elimination o f the risk of flooding along Schieland's Hoge Zeedijk (HoUandse IJssel) and i n view of the experience to be gained f r o m b u i l d -ing a d a m as close to the sea as the one across the Veerse Gat, w o r k on those projects was given as m u c h p r i o r i t y as possible. Consequently, the HoUandse IJssel project was finished i n 1959, the Zandkreek d a m i n 1960 and the Veerse Gat d a m i n 1961.

T h e other d a m projects, w i t h the exception of the adjustable barrier i n the Oude Maas, are very interdependent.

For hydrologieal reasons, i t w o u l d not be advisable to p u t the H a r i n g v l i e t sluices into operation before the Volkerak has been closed o f f . As long

4 H O L L A N D S E ' J S S E L Z A N D K R E E K - V E E R S E S A T G R E V E L I N G E N V O L K E R A K H A R I N G V L I E T B R O U W E R S H A V E N S E G A T E A S T E R N S C H E L D T

as the Brouwershavense Gat and the Eastern Scheldt remain open, d a m -m i n g o f the Volkerak w o u l d cause a rise i n the high-water levels on the south side, w h i c h w o u l d be a source of danger at h i g h water d u r i n g storms. However, the rise i n flood levels w i l l be reduced by the t i m e l y d a m m i n g o f the Grevelingen, w h i c h , i n its t u r n , must be completed f a i r l y soon to enable the large Brouwershavense Gat and Eastern Scheldt dams to be completed independently. Therefore the m a j o r delta projects are inseparably l i n k e d . This is p a r t i c u l a r l y so, because a l l the work i n the coastal area w i l l be exposed to the force of t i d a l currents and waves, w h i l e the g r o u n d is very unstable. So, a project once begun must be carried out and completed at a set rate.

A l t h o u g h the rate w i l l , of course, have to be consistent not only w i t h w h a t is possible technically b u t also w i t h what is economically justified, i t does not alter the fact that laying the first section of w i h o w "mattress" i n the H a r i n g v l i e t , as i t were, determines the moment at w h i c h the d a m i n the Eastern Scheldt w i l l be completed and vice versa. T h e other projects must be fitted i n between those two operations i n the correct manner, thus covering a period of approximately 25 years f r o m the 1953 storm disaster.

O f a l l the large dams, the one i n the H a r i n g v l i e t w i l l be the most i m -portant f r o m the point of view of safety, f o r the greatest length of existing dykes is involved. Moreover, the b u i l d i n g of the d a m itself w i l l present the least serious problems once the huge sluices to be incorporated i n i t are finished and can be opened. So o f course p r i o r i t y was given to opera-tions i n the H a r i n g v l i e t . The operaopera-tions have been timed so that advan-tage can be taken o f t h e experience gained i n the d a m m i n g o f t h e Veerse Gat and the Grevelingen. W h e n the project has been completed i n 1968, one year after the d a m m i n g of the Volkerak, not only w i l l the n o r t h e r n delta area be protected against floods entirely according to plan, b u t i t w i l l also be possible to take f u l l advantage o f the benefits the D e l t a Project offers this area i n the sphere of fresh water control.

I f the time-table is adhered to, the Brouwershavense Gat and the Eastern Scheldt dams w i l l be completed i n 1970 and 1978, respec-tively. These projects are so huge that m u c h preparatory w o r k is already being done. I n particular a series o f experiments is being carried out i n the field and on models, w h i c h are essential i f a sound design is to be produced.

(iv) Safety

W h e n the Delta project has been completed, the only open l i n k the closed-off lower reaches of the rivers and estuaries w i l l have w i t h the sea w i l l be the one via the R o t t e r d a m W a t e r w a y . So storm-floods w i l l still be able to i n t r u d e along this route. However, the height o f any sueh flood w i f l f a l l r a p i d l y on its way i n l a n d because of the strong suction effect of the H a r i n g v l i e t basin, whilst the water levels i n the closed-off H a r i n g v l i e t and associated waters w i l l rise but shghtly, at a l l events when the W a a l and Maas discharges are normal. W h e n an abnormally h i g h discharge is combined w i t h storm floods, the water levels i n the H a r i n g v l i e t basin and associated waters wUl, of course, rise m u c h higher.

Statistical research has shown that the simultaneous occurrence of storm floods and abnormally h i g h discharge is purely a matter of chance. T h e two phenomena are hardly related, i f at a l l . Hence, there are a number of different ways i n w h i c h large or small storm floods may combine w i t h abnormally h i g h river discharges, each of w h i c h w o u l d produce d i f f e r e n t water levels i n the delta area. T h e "Rijkswaterstaat" and the H y d r o l o g -ieal L a b o r a t o r y determined the water levels i n the delta area f o r a large number of combinations, whilst the frequency of these combinations was deduced f r o m the frequency data available o n the discharge o f the upper R h i n e , and for the h i g h tides recorded at the H o o k of H o f l a n d . By adding up the entire range of possibilities, the number of times abnormally h i g h water levels are likely to be exceeded was finally determined for a number of places i n the delta area. N o t u n t f l that was done could basic water levels be established for the situation as i t w o u l d be after eompletion o f the Delta Project. I t was f o u n d that as one moves i n l a n d these basic levels are influenced to a decreasing extent by storm tides and to an increasing extent by surface water discharges. So the basic level at the H o o k of H o l l a n d or at R o t t e r d a m is most hkely to be reached or exceeded w h e n a very h i g h storm tide and a n o r m a l river discharge occur simultaneously. F u r t h e r i n l a n d the water levels w i l l then remain low. O n the other h a n d , the basic level i n the H a r i n g v l i e t basin or i n waters f u r t h e r upstream is most likely to be reached when a relatively low storm tide coincides w i t h a very h i g h river discharge. T h a t c o m b i n a t i o n w i l l present no problems i n the R o t t e r d a m Waterway.

T h e same phenomenon can be observed i n the lower reaches o f the rivers now, b u t is very m u c h less marked. T h e v i r t u a l l y complete exclusion o f

- B A S I C L E V E L S

• M A X I M U M L E V E L S DURING S T O R M WITH A V E R A G E RIVER D I S C H A R G E S (RHINE D I S C H A R G E 3 0 0 0 CUB. MTRS. PER S E C . )

- M A X I M U M L E V E L S D U R I N G MEAN T I D E S AND VERY H I G H RIVER D I S C H A R G E S (RHl NE DI S C H A R G E 1 3 0 0 0 CUB. MTRS. PER SEC.)

F I G . 1 4 Excessive high-water level curves i n the lower reaches of the rivers under pre-sent conditions and under the conditions anticipated after the completion of the Delta Project

the sea and its storm effect w i l l move the characteristics of the upper rivers downstream as i t were, i.e. f r o m Gorinchem and W e r k e n d a m to the neighbourhood of Dordrecht.

H o w greatly the closing-off o f the estuaries w i l l enhance the safety of the region can be seen by comparing present basic levels i n the area o f the lower reaches w i t h the situation after the eompletion of the D e l t a Project.

T h e basic levels are practically identical f o r b o t h situations at the H o o k of H o l l a n d , so that strengthening the dykes along the W a t e r w a y w i l l be essential. W i t h the completion of the Delta Project, the basic level w i l l f a l l steeply as one moves i n l a n d as a result of the suction effect of the H a r i n g v l i e t basin. H i g h levels such as those caused by very h i g h tides w i l l have been practically eliminated i n that basin. T h e basic level w i l l drop b y 6^ to 8 feet compared w i t h present conditions ( M o e r d i j k ) . Further upstream ( W e r k e n d a m ) , where the influence o f t h e u p l a n d dis-charge already outweighs the influence of the tides, the completion of the w i l l of course have Utde or no effect. Here, the basic levels w i l l be almost the same f o r present and f u t u r e conditions.

T i d a l movement i n Lake Zeeland w i h be completely ehminated or very nearly so. W h a t a difference this w i l l make to the safety of an area where the river banks are stih so prone to slide and where the basic levels under present conditions are 18 to 1 9 | feet above N . A . P . ! I f i t h a d not been decided to close o f f the estuaries, the dykes here too w o u l d have had to be heightened considerably and strengthened. Even then a degree o f safety equal to that offered by the completed Delta Project w o u l d never have been obtained.

(v) Water control

T h e Netherlands depends f o r its supply of fresh water m a i n l y o n the R h i n e and the Maas. T h e supply is amply sufficient to meet the country's water requirements f o r domestic, industrial and a g r i c u l t u r a l purposes. There is b o u n d to be a shortage, however, since too large a p r o p o r t i o n of the river water is needed to drive back the salt i n the coastal areas. Sea water penetrates into the open estuaries. Lock and sluice leakage, seepage and lockage water a l l increase the salt content of polder water. I n a d d i t i o n , there has been an alarming rise i n the chloride content of the Rhine water itself, due to the discharge of more and more saline waste water f r o m the R u h r area and the Alsace. Consequently, the q u a n t i t y o f river water needed to drive out the salt is becoming disproportionately large.

A consequence of the insufficieney of fresh water is that the chloride content of the polder water i n the delta area is often considerable. M o r e -over, h a r m is being done to some a g r i c u l t u r a l areas by desiccation. T h e closing of the estuaries constitutes the most effective means o f

check-PRESENT C O N D I T I O N S A F T E R C O M P L E T I O N O F D E L T A P R O J E C T A N D C A N A L I Z A T I O N O F R H I N E C H L O R I D E C O N T E N T IN MG. P E R L I T R E UPPER RHINE 150 è 2 0 0 P A R K H A V E N 4 0 0 S 8 0 0 WILLEMSTAD 5 0 0 0 a 10.000 ZEELAND ESTUARIES lOOOOè 1 8 0 0 0

F I G . 1 5 Distribution over the major rivers f r o m the Rhine and the Maas

C H L O R I D E C O N T E N T IN MG. P E R L I T R E UPPER RHINE 1 5 0 a 2 0 0 PARKHAVEN 2 0 0 a 3 0 0 HARINGVLIET S L U I C E S 2 0 0 a 3 0 0 LAKE ZEELAND 2 0 O a 3 0 0

and estuaries of a normally low discharge

ing the penetration o f sea water. I t w i l l then be possible to desalt the waters farther i n l a n d . Enough fresh water w i h become generally available for a g r i c u l t u r a l purposes.

T h e distribution of fresh water i n the tidal reaches o f the R h i n e and the Maas w i l l be partially controllable, because i t w i l l be possible to regulate the flow b y means o f large sluices i n the H a r i n g v l i e t and the V o l k e r a k . By reducing the discharge, more water w i f l be carried o f f v i a the N o o r d t h r o u g h the R o t t e r d a m W a t e r w a y t h a n flows d o w n i t at present, w i t h the result that the quantities o f sea water penetrating this channel w i l l be reduced to reasonable proportions.

For the fresh water supply o f the south-western p a r t o f the Netherlands, and p a r t i c u l a r l y f o r those areas w h i c h receive their water f r o m the upper reaches o f the R o t t e r d a m Waterway, i t w o u l d be most advantageous i f , w i t h i n the f r a m e - w o r k o f the Delta Project, the Oude Maas could also be shut oflF, at least d u r i n g periods i n w h i c h the volume o f water c o m i n g d o w n the Rhine is small.

However, i n spite o f t h e execution o f t h e Deha Project and the closing o f the Oude Maas, an adequate supply of fresh water f r o m the upper reaches of the R o t t e r d a m Waterway stiU cannot be guaranteed under a l l circumstances. T h i s is m a i n l y due to the fact that R h i n e water usually has an excessively h i g h chloride content when the river is very low. T h e dykes shutting o f f the Brouwershavense Gat, the Eastern Scheldt and the Veerse Gat, together w i t h the d a m across the Volkerak, w i l l enclose " L a k e Zeeland". This lake is to be fed chiefly w i t h water f r o m the Rhine and the Maas, w h i c h w i l l be led to i t v i a the H a r i n g v h e t basin and the sluice i n the V o l k e r a k d a m already mentioned.

T h e water of Lake Zeeland is likely to have become desalted i n a few years, provided the discharge facihties are properly located and equipped and are o f sufficient capacity.

Even when Lake Zeeland has been desalted, large quantities of river water w i l l still be required to keep i t fresh, because for a long time to come brackish drainage water f r o m the surrounding polders w i l l continue to be discharged i n t o i t . T h e quantities needed w i l l be p a r t i c u l a r l y large when the river water admitted is not sufficiently fresh.

As there are limits to the discharge capacity of sluices and since sufficient quantities of water for flushing purposes w i l l not always be available, the contingency must be reckoned w i t h that i t may not be possible to m a i n t a i n an average chloride content of less t h a n 300 m g Cl/htre under all circumstances. I t is therefore essential to take the necessary measures to restrict as m u c h as possible the penetration o f salt seepage, lockage and leakage water i n t o the lake.

W h e n the estuaries have been closed, the levels and rates o f flow i n the waters farther i n l a n d w i l l undergo changes.

T h e water levels w i f l not alter appreciably i n the open R o t t e r d a m Water-way, b u t the rates of flow i n the lower reaches w i f l decrease considerably. This w i l l be an advantage to shipping.

I n the rivers N o o r d and Dordtse K U , w h i c h connect the R o t t e r d a m W a t e r w a y w i t h the H a r i n g v h e t basin, the rates of flow w i f l become higher than they are now. A n y modifications to these rivers w i f l cafl f o r special study.

I n the H a r i n g v h e t basin the rates of flow w i f l generally become m u c h lower than they are at present. T h e high-water levels w f l l be lower, b u t

the ebb-tide levels w i l l be appreciably higher. So provision w i l l have to be made f o r the drainage of polders and higher ground.

T h e effect of tidal movements w i l l disappear entirely or almost entirely i n Zeeland. T h e shape of the lake to be f o r m e d there and the control of its water level w i l l require f u r t h e r study. I t is not only considerations of safety and the interests of those who w i l l beneht by the creation of a large fresh water reservoir that are i m p o r t a n t , b u t also matters i n v o l v i n g d r a i n -age, land reclamation, recreation, shipping and fisheries.

I V . T H E T E C H N I Q U E O F C L O S I N G S E A I N L E T S (i) Methods employed

There are sandbanks and more or less deep gullies i n the inlets to be closed off. T h e bulk of the water moves along the gulhes and b u t little water passes across the sandbanks. Consequently, the b u i l d i n g o f a section of dyke on the banks w i l l not affect the t i d a l movement very m u c h and w i l l only cause a slight increase i n the current velocities i n the r e m a i n i n g cross section. Therefore i t is n a t u r a l that w o r k on a dyke should start w i t h the sections on the banks, steps being taken to protect the b o t t o m of the gullies against erosion as soon as i t appears necessary to do so. D a m m i n g o f f the gullies is a m u c h more d i f h c u l t j o b . T h e engineers have a choice of several methods. T h e most obvious to the l a y m a n w o u l d be to b u i l d the dyke outwards f r o m the sides o f the gully. B u t the current velocities w o u l d increase greatly while that was being done. I t has been calculated that current velocities o f f r o m 5 to 6 metres per sec w o u l d occur d u r i n g the final stages of the work i n the wide inlets i f that method were applied. Such h i g h current velocities present serious problems, espe-cially i n the Netherlands, where the soil is sandy and m u c h o f i t is there-fore liable to be washed away. Moreover, the lateral n a r r o w i n g w o u l d cause considerable contraction o f the water mass and p o w e r f u l eddies near the projecting dyke heads.

Because o f the serious risk o f scouring by the eddies the method outlined above was only employed to close some o f the secondary inlets ( Z a n d -kreek, Grevelingen - southern gap), where t i d a l movements were slight. Even there everything was done to avoid the most unfavourable con-ditions by closing the final gap as quickly as possible at slack water. Caissons were placed at the t u r n of the tide on a sill constructed f o r the

purpose so that the work should be completed w i t h i n the period set. However, most o f t h e inlets to be dammed o f f as part o f t h e Delta Project are too wide to be closed i n this w a y : the current velocities w o u l d be too great and the operation w o u l d take too long. A better method is to con-struct the d a m i n horizontal layers, the layers being added one at a time along the entire length of the dam.

A g a i n , the gap w i h be narrowed gradually and as a result the current velocities w i l l increase at hrst. Since the d a m is b u i h gradually along its

F I G . 16 Closing of southern gap in Grevelingen dam by means of caissons

entire length, critical overflow w i l l be reached at a certain moment. F r o m that time on not only w i l l the current velocities cease to increase as the d a m is raised, they w i f l even decrease. Nevertheless, current velocities o f approximately 4 metres per sec w i l l have to be reckoned w i t h i f the wide inlets are closed o f f by this method.

C R O S S - S E C T I O N O F GAP IN T H O U S A N D S O F S Q U A R E M E T R E S

BUILT O U T W A R D S FROM E I T H E R B A N K BUILT UPWARDS FROM BOTTOM S L U I C E C A I S S O N S

F I G . 1 7 Current velocities during closure o f t h e Brouwershavense Gat for the various methods contemplated

This method, too, w i l l cause eddies along the sides o f the gully. T h o u g h they w i l l be less p o w e r f u l than the eddies that w o u l d occur i f the lateral method were employed they may still cause trouble by scouring o n ac-count o f the current velocities obtaining i n the final gap. Moreover, b u i l d i n g the d a m layer by layer wiU entail the risk o f the water d i p p i n g d o w n along the downstream slope. I t w o u l d eah for special devices, w h i c h are not always easy to construct.

T h o u g h the layer-by-layer method w o u l d seem to be easier to carry out than a lateral n a r r o w i n g o f t h e gap, a l l the " g r a d u a l " methods o f closing o f f a sea inlet are risky.

T h a t is w h y a method of closing o f f the wide inlets instantaneously is also being considered. This w o u l d be achieved by means of sluice caissons. T h e y are " o p e n " caissons w i t h sluice gates through w h i c h the tides can move freely, even after the caissons have been moved i n t o position. T h e n a l l the gates are lowered simultaneously at slack water, thus closing o f f the gap instantaneously.

F I G . 18 Sluice caissons.

a liorizoiUal foundation. I n practice this means that they must be placed on a sill constructed d u r i n g the year preceding the closure. T h e presence o f t h e sill itself w i n narrow the gap t h r o u g h w h i c h the current must flow and h i g h current velocities may occur, especially w h e n the g u l l y being closed is deep. A g a i n , i t is obvious that even sluice caissons cannot be entirely " o p e n " ; parts o f t h e structure w f l l always impede the flow o f water to a certain extent.

So i n actual practice the method is a compromise between g r a d u a l and instantaneous closing off.

(ii) Scouring

T h e problem of current velocities was emphasized i n the foregoing review of the advantages and drawbacks of the various methods o f closing the inlets. I t was tacitly assumed that they w o u l d provide a direct i n d i c a t i o n

o f t h e scouring to be expected and o f t h e stability o f t h e material of w h i c h the sill is made.

However, that assumption was not f u l l y justihed, for i n a d d i t i o n to the current velocity the depth of the water flowing over the sill has to be taken into account when considering the stability of the material f o r the sill and the problem of erosion. Moreover, the height o f the sill is also an i m p o r t a n t factor governing erosion.

I t is true that as the d a m becomes higher, the current velocities w i l l i n -crease b u t the depth of the water above the sifl w i l l de-crease. T h e latter factor predominates i n coastal areas i n that the volume of water discharged across the d a m per u n i t of w i d t h diminishes as the d a m becomes higher.

Consequently, the current velocities at the extremities o f the soil protec-t i o n devices, where protec-the currenprotec-t is again spread over protec-the f u l l depprotec-th o f protec-the gully, w f l l decrease as the dam is raised. Nevertheless, turbulence is ex-pected to increase. I t is d i f f i c u l t to predict how m u c h scouring w i f l occur as a result o f a f l these factors.

Experiments w i t h scale models have shown that erosion beyond the soil protection devices w i f l increase while the inlets are being dammed o f f u n t i l the d a m has reached a height equal to approximately three quarters o f the depth of the water.

O 10 2 0 3 0

E R O S I O N IN M E T R E S A F T E R A C E R T A I N N U M B E R O F DAYS O F F L O W

F I G . 20 Scouring as a function of tlie ratio between the height of a sill and the depth Qf Q rr.,1K,

A p p a r e n t l y , the decrease i n the volume of water discharged predominates over the increase i n velocities and turbulence as the height becomes greater; consequently, the risk o f scouring diminishes.

W h e n the Veerse Gat was closed w i t h sluice caissons the water was 20 metres (66 feet) deep. The sill on w h i c h the caissons were placed was 10 metres (33 feet) h i g h . So the ratio between the two figures was 1 : 2. Therefore, the m a x i m u m scouring point had not nearly been reached. I t was avoided entirely by closing the remaining gap at the right moment by lowering a l l the sluice gates at once.

I f the gully had been m u c h deeper, 40 metres (132 feet) for instance, the height of the sill w o u l d have had to be three quarters o f t h e depth o f the water for caissons of the same height. M a x i m u m scouring w o u l d then

have oecurrecl for a longer period and m u c h soil w o u l d have been washed away.

However, fewer diffieulties w o u l d have been encountered i f the d a m had been b u i l t by the gradual method. There w o u l d be no caissons, the restricted height of w h i c h is decisive f o r the height of the sill a n d the top o f the latter w o u l d not necessarily have to be horizontal. T h e r e -fore, especially i n winter i t w o u l d be possible to adapt the height o f the sih to the depth of the gully, thus preventing serious scouring f o r any appreciable time.

I n view o f those considerations i t m i g h t be concluded that where the gulhes are deep b u i l d i n g the d a m by the gradual method is preferable to closing the gap by means of sluice caissons, b u t that the latter m e t h o d w o u l d be better for shallower gullies.

However, that does not h o l d for every set of conditions and i t certainly does not always hold for the D u t c h inlets i n w h i c h loosely packed sand may occur.

Ill OC ÜI 0. I I SAND 0 100 2 0 0 kg per sq. cm CONE RESISTANCE

F I G . 22 Results of penetration test and boring i n the Brouwershavense Gat

(iii) The Siihsoil

Samples obtained by boring have sltown that the ground on w h i c h the dams are to be b u i l t consists largely of comparatively fine sand (0.2 m m ) . I n certain places, however, and at varying depths, layers of sand occur i n w h i c h the "cone resistance" is far below w h a t one w o u l d expect to find for sand. Exactly w h a t causes this is not k n o w n , b u t i t is presumed to be due to the looseness of the sand. I f f u r t h e r research should prove the assumption to be correct, i t may greatly influence the m e t h o d to be employed f o r d a m m i n g ofl^ the wide sea-inlets.

Loosely packed sand constitutes a great danger since the f r i c t i o n a l stress i n the subsoil w i l l of course be altered by various factors d u r i n g the d a m m i n g - o f f process. A d r y sand base w o u l d readily stand sueh change i n f r i c t i o n a l stresses. But the sand w f l l undergo a change i n volume. I n t i g h t l y packed sand there is an increase i n v o l u m e ; i n loosely packed sand there is a decrease.

I n principle, the sand retains this property of its grain structure even when i t is saturated w i t h water. Since the water between the grains is not very compressible, the changes i n volume are compensated f o r i n loosely packed sand by extra water-pressure and i n t i g h t l y packed sand by a depression.

I n consequence o f these extra water-pressures, the g r a i n structure is omnilaterally placed under stress or reheved of stress. This, too, causes a change i n volume w h i c h compensates the previously mentioned change i n volume necessary f o r absorbing the f r i c t i o n a l stress.

So i f the f r i c t i o n a l stress i n loosely packed sand is altered as a result o f the b u i l d i n g of the dams, there w i l l be extra water pressures m i t . E x t r a pressures w i l l obviously b r i n g about not only a compensatory change i n

F I G . 23 Lines of equal extra water pressure under the foundation of a telpher pier when loads are moved along the telpher. Heavy arrows show the greatest load that may occur. The extra pressure is expressed i n decimetres water column

volume b u t also a decrease i n the f r i c t i o n a l resistance to sliding, w h i c h could constitute a serious threat to the stabihty o f the construction. I f the changes i n f r i c t i o n a l stress take place slowly, the extra water-pressure w i l l remain slight, f o r the water between the grains w o u l d have time to r u n away and so lose its extra pressure.

However, d u r i n g d a m m i n g operadons stresses may also b u i l d up r a p i d l y , as happens when caissons are p u t i n position, when the caissons are sub-jected to pressure from the w i n d and the waves, and w h e n loads are

moved along a telpher, causing stresses i n the pylons. The process o f scouring is i n itself a slow one, b u t i n the slopes i t creates sliding can occur that causes a sudden drop i n the load on the subsoil.

R o u g h calculations show that a disturbance of e q u i l i b r i u m can easily occur, locally at any rate, i n loosely packed sand. Local disturbances o f e q u i l i b r i u m of this nature can produce new water-pressures elsewhere, causing a sort o f chain reaction that may assume serious proportions. This phenomenon is called a f l o w slide.

A flow slide d u r i n g the closing of an inlet w i t h caissons m i g h t have serious consequences. The caissons m i g h t move or topple over. T h e water w o u l d then be free to flow t h r o u g h the resulting gap and cause sdll more destruction. I t w o u l d take months i f not years to repair the damage.

However, i f the closing process is carried out gradually, the material for b u i l d i n g the d a m being deposited, for instance, by means o f a telpher, i t is always possible to ensure that the few telpher supports remain stable under a l l circumstances. Should a flow slide then occur and the d a m damaged, n o t h i n g need be done b u t to continue depositing material. N a t u r a l l y , the telpher w o u l d then be set to work solely at the damaged spot, resulting m a quick repair.

So the gradual closing method is better t h a n the sluice-caisson method i n some cases, i f the gullies to be dammed o f f are not too deep, because of the greater ease w i t h w h i c h a flow slide can be repaired.

There are o f course many other factors to be considered before deciding u p o n the methods to be employed for d a m m i n g o f f the wide sea-inlets. For instance, the engineers hesitate to use u n t r i e d methods for b u i l d i n g the large dams. Sufflcient experience has already been gained o f several methods i n v o l v i n g the use o f caissons. As an experiment, the n o r t h e r n gully o f the Grevelingen is being closed by b u i l d i n g up the d a m grad-ually. T h e material w i f l be deposited by means of a telpher.

M e a n w h i l e , theoretical and experimental research w i l l continue. O n l y w h e n the research has been completed and more experience has been gained w i l l i t be possible to decide on the methods to be employed f o r d a m m i n g o f f the wide sea-inlets.

V . T H E B A R R I E R D A M S

(i) Cross-section

As has already been stated, i t was realized after the disaster o f 1953 that the m a i n dykes needed to be higher than had previously been assumed adequate. T h e "design l e v e l " was raised by 1.5 metres. T h e logical con-sequence o f this step was that higher waves, therefore greater wave r u n up, had to be reckoned on. Moreover, i t has been discovered i n the meantime that local oscillations due to meteorological conditions m i g h t f o r short periods cause the water level to rise higher i n places t h a n had been thought possible i n the l i g h t o f previous considerations.

I n view of all these faetors together, i t has been decided that, i f the barrier dams were designed w i t h an outer-slope incline of 1 : 5 or 1 : 6, the crests w o u l d i n some cases have to rise to as m u c h as 15 metres above mean sea level.

Another lesson learned f r o m the disaster was that m u e h greater safety could be obtained i f the inner-slope incline was made a good deal less steep t h a n had previously been the case. Accordingly, as a rule, no inner-slope incline w i h be steeper t h a n 1 : 3 i n f u t u r e .

•

F I G . 24 Size of barrier dam to be built across the Eastern Scheldt compared with that of the Zuyderzee dam

Consequently, the new barrier dams w i l l not only be m u c h higher t h a n the o l d dykes b u t their slopes w i l l also be m u c h gentler. Since the gullies to be d a m m e d o f f are between 20 and 30 metres deep, i t is clear that the new barrier dams w i l l d w a r f a l l previous structures of this k i n d . W h i l e the dams are being constructed, large quantities o f material w i l l have to be brought up and deposited i n a comparatively short time. Sand, of w h i c h there is no shortage i n the inlets, was the only material that could be considered f o r this purpose; i t w i l l f o r m the core o f all the dams.

(ii) The Revetment

I n the Netherlands the conventional type of revetment f o r this k i n d o f dyke b u i l t of sand is a layer of clay covered w i t h brick-rubble and faced w i t h basalt blocks.

However, there is not enough clay i n the Delta area and i t w o u l d prove very expensive to b r i n g i t i n f r o m elsewhere. Basalt is i n l i m i t e d supply and has become very expensive. Moreover, there w o u l d not be enough skiUed stone-setters to tackle such large-scale projects.

This being so, i t w o u l d have been impossible to revet the dams w i t h the desired r a p i d i t y by the conventional method. Fortunately, another k i n d of revetment has been developed since the Second W o r l d W a r made o f bituminous material produced d u r i n g the oil-refining process.

T h e asphalt facing can be applied direct to the sand quite quickly. I t is therefore endrely suited to the present-day technique of d y k e - b u i l d i n g , whereby p o w e r f u l suction-dredgers r a p i d l y b u i l d up a sand base. O f course, these arguments are not i n themselves sufficient reason f o r adopting the asphalt method. The m a i n desideratum is that the d a m shall be a reliable and permanent construction. I n October 1956 a w o r k i n g group was f o r m e d to investigate the strength and permanence o f asphalt construction. T h e group has published a provisional report, i n w h i c h i t is stated that asphalt revetments, provided they are expertly designed a n d constructed, can give the dykes rehable protection.

I n the light of the conclusinns arrived at through these and other studies, i t was decided to provide the barrier dams w i t h asphalt revetments. As a result, the dams w i l l have the simplest f o r m of crosssection i m a g -inable, viz. a core of sand covered w i t h a layer of asphah. Nevertheless, designing such a d a m is not as simple as i t may seem. I t is precisely because the revetment is comparatively t h i n that a number o f problems arise w h i c h , despite modern resources, are not a l l easy to solve.

For instance, little is k n o w n about its permanence; however, j u d g i n g f r o m experience w i t h 15yearold asphalt revetments, i t is reasonable to a n t i -cipate that i t w i l l be satisfactory i n this respect. A l l the same, the com-position of the asphalt mixture and its applicadon w i l l have to meet certain requirements.

A g a i n , insufficient research has been conducted into the force o f t h e waves, so i t is not yet k n o w n how strong the asphalt revetment w i l l have to be to withstand them. I t is thought at present that a coating 30 c m t h i c k on a sand base w i l l be sufficiently strong to stand up to the waves, at a l l events under D u t c h conditions. But the matter is still uncertain.

Another factor that the engineers designing the d a m must bear i n m i n d is the water pressures likely to b u i l d up behind the asphalt revetment. Since asphaltic concrete is m u c h more impervious to water t h a n the underlying materials, hydraulic stresses may b u i l d up under the f a c i n g , w h i c h w ü l reduce the m a x i m u m f r i c t i o n between this facing and the material to w h i c h i t has been applied. Consequently, after a certain moment, as the water pressure rises an ever-increasing p a r t o f the com-ponent of the weight of the facing itself parahel to the slope wiU be absorbed by the facing itself T h i s may give rise to inadmissible deforma-tions i n the long r u n because of the viscosity o f asphahic concrete. T o discover the extent of these deformations i t w o u l d be necessary to know the stress and deformation conditions i n the facing under the i n -fluence of the weight o f the facing itself and the ever-changing h y d r a u l i c stresses. So far, no satisfactory method o f o b t a i n i n g this i n f o r m a t i o n has been devised, so that f o r the time being a comparatively a r b i t r a r y stand a r stand has to be astandoptestand w h i c h must be met to cope w i t h viscous stand e f o r m a -tion, viz., that under frequently recurring circumstances such as n o r m a l spring tides the component o f the weight o f the facing itself parallel to the slope shall never exceed the m a x i m u m f r i c t i o n between the f a c i n g and the underlying material.

Moreover, a different standard must be observed f o r rarely o c c u r r i n g circumstances. T h e n the requirement is that nowhere shall the excess pressure be greater t h a n the component of the weight of the facing itself perpendicular to the slope.

So i n an extreme case the facing w o u l d " f l o a t " on the slope i n places. The component of the weight o f the facing parallel to the slope is t h e n entirely absorbed by the facing itself This is considered acceptable under