SOME ASPECTS OF THE DELTA PROJECT

BY IR. K. F. VALKEN AND IR. W. C. BISGHOFF VAN HEEMSKERGK

SENIOR ENGINEERS STATE PUBLIC WORKS, , THE HAGUE

I. THE GREAT FLOOD OF 1953

(i) The storm The first indication of the development of the atmospheric disturbance that was to become the disastrous depression of 31st January- 1st February, 1953, was seen on the weather map for 12.00 hours G.M.T., 29th January. It is thought to have been a split-off depression generated on the warm front of a depression over the Atlantic Ocean north of the Azores by an upper-air trough moving east from the area south of Greenland. It moved north-east in the early stages of its development, deepening about 4 mb in six hours. By linking up with an old shallow depression south of Ice­ land, the developing disturbance gradually started transporting colder air southwards from the Iceland area. At the same time, a ridge of high pressure south of Greenland intensified significantly while moving steadily east. A north-westerly air stream increased in extent and intensity south of Iceland between an associated ridge at the 500 mb level and the above- mentioned upper-air trough. The depression changed direction under the influence of this air-stream and began moving south-eastwards along the northern coast of Scotland and over the North Sea. As it continued to deepen, an area of north-westeiiy gales developed to the west of the depression. On the morning of 31st January, a trough of low pressure developed near Scotland at sea level causing very steep pressure gradients accompanied by severe gales. They reached their maximum force as the trough moved south-eastwards along the Scottish coast. At 12.00 hours G.M.T. on 31st January several stations in the area reported wind veloc­ ities of about 70 knots. At the same time, the depression itself had reached

259 its greatest depth in the central North Sea. Winds attained their maximum force on the Dutch coast in the evening on 31st January and veered north­ west as the low-pressure trough passed.i The wind reached average veloc­ ities of over 45 knots along the coast. At 11.00 hours on that day a warning of "unusually high tides" had already been sent by telegram to the dyke defence groups in ,

FIG. 1 Courses followed by troughs of low pressure during some violent storms

Willemstad, and Gorinchem. At the end of the after­ noon, a further telegram was sent giving a warning of "dangerously high tides", but only when it became evident in the evening that there had been no perceptible fall in the level of the water at low tide did many

1 Taken from Rapport Deltacommissie, Bijdrage IConinklijk Meteorologisch Instituut.

260 people realize that the next high-water levels in the South-West would exceed previously recorded peak levels by more than \^ feet and that disaster was imminent. Meanwhile, the polder boards and officials of the "Rijkswaterstaat" had already begun to alert the population and re­ inforce the dyke defence groups. However, despite the measures taken by those groups, the water swept over the dykes in many places, causing serious damage to the dykes for hundreds of miles and breaching them in 67 places. As a result, about 375,000 acres were flooded, more than 1,800 people lost their lives and from 2,500 to 3,000 million guilders' worth of damage was caused. Actually the disaster might have been much more serious. A breach which had opened in the dykes along the HoUandse IJssel was only closed at the eleventh hour as a result of superhuman efforts. If these efforts had

261 FIG. 3 Dyke breaches and floods in the south-west as a result of the storm of 1st February, 1953 FIG. 4 Damage to the inner slope. The outer slope is still sound (Schouwen Polder) not succeeded, the whole of central Holland would have been flooded and the entire economy would have been disrupted for years to come.

(ii) Damage to dykes A storm of such intensity had not generally been allowed for when the dykes were designed, so that at first reports of damage to the dykes came as no surprise.

263 The real surprise came later when it was realized how extensive the damage was. Most ofthe dykes in the area in which the disaster occurred were seriously damaged, if not breached. The inner slope and the top were the parts most affected. Hardly a single case of serious damage to an outer slope was reported. In many places where the inner slope was seriously affected, the outer slope was quite undamaged. Closer examination of the dykes in the disaster zone revealed that the outer slope usually varied between 1 in 3 and 1 in 4. The inner slope was usually found to be considerably steeper. Only in a few isolated cases was the inner slope less steep than 1 in 2. The inner slope of the majority of the dykes, however, was between 1 in 1.5 and 1 in 1.75. Slopes as steep as these are of course extremely vulnerable if there is any

FIG. 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)

264 seepage of ground-water. But no dangerous seepage of ground-water could have come up through the body of the dyke or the sub-soil, because the storm had not lasted long enough for that to happen. It was found that the dykes were damaged or breached only where large quantities of water had swept over the top. The extensive damage sustained by the sea dykes in the South West during the storm of 1st February, 1953 must therefore be ascribed principally to water sweeping over the top. This water probably penetrated the dykes through the non-watertight covering of the top and inner slope causing air and water pressures to build up in the body of the dyke, impairing the stabiUty of the inner slope. Once the inner slope has started to slide erosion will cause the body of the dyke to collapse in a relatively short time. That was very hkely the manner in which most ofthe 1953 breaches were occasioned.

II. INFERENCES

(i) Traditional approach It is quite evident from the foregoing description ofthe disaster that water sweeping over the top was the principal cause of the breaching of the dykes. It was nothing new. It was the same phenomenon as had man­ ifested itself on various occasions down the ages. Andries VIERLINGH was making no idle statement when he wrote in the "Tractaet van dijckagie" (Treatise on dyke-building) in about 1577 "our greatest welfare depends on the height of a dyke". However, the constant problem was how high to build the dyke. In former times the usual criterion was the highest flood level that could be remembered. That arbitrary criterion always proved deceptive. People were usually inclined to forget past disasters only too easily when it was a matter of raising money for heightening the dykes. Added to this was the fact that the comparison of various flood levels was made difficult by the absence of a standard level, by the rise in sea level, by the subsidence of the sub-soil and by the settling of the polder land and dykes. Consequently, many floods reached higher levels than had been provided for. So the traditional approach was overtaken by the facts again and again.

(ii) Physical approach Once the physical phenomena which together constituted the "storm

265 effect" were better understood, it became possible to obtain a clearer idea of the type of floods to be anticipated. It enabled the engineers to determine how much higher a recent storm level might have risen if some of the contributory factors had combined more unfavourably. Using this method of reasoning, the conclusion to be drawn from 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 Hook of Holland was exceptionally high, a level of 16| feet above

266 N.A.P. might actually have been reached. However in itself this fact has no real significance, since any of the values of the various factors which together constituted the "storm effect" might have been still more un­ favourable. The studies carried out on this subject by the Royal Me­ teorological Institute reveal that appreciably higher flood levels are phys­ ically possible. In fact, it would not be possible to predict, even approx­ imately, a water level which could not be exceeded. So an absolute safety limit can never be based on anything but pure conjecture.

(iii) Statistical approach In recent decades there have also been valuable developments in the field of statistics. The "Rijkswaterstaat" has access to a series of systematically documented records on water levels going back to the end of the last century and has carried out much research into this subject. It soon became clear that the incidence of abnormally high water levels could be represented very well in terms of frequency in accordance with the laws of probability. The higher the flood level, the smaller the hkehhood of it being reached or exceeded. Since, for physical reasons, no maximum level can be given, there is always a certain risk of flooding, however high the "design level", i.e. the flood level selected as a basis when de­ signing the dykes and sluices may be. In order to discover the degree of likelihood there is that a flood will exceed the very high levels hitherto unrecorded, the frequency curves of excess for high-water levels, which are inevitably based on a relatively short-term observation period, had to be extrapolated to points far beyond the field of observation. The Mathematical Centre, in particular, con­ tributed a great deal to these studies. The Royal Meteorological Institute research mentioned previously was also very important in this connec­ tion, since it formed the basis for the selection of the data essential for statistical purposes. Using the extrapolated frequency characteristics, the "design level" can now be selected in such a way that the margin of risk is reduced to an acceptable minimum. The real question is, of course, what risk can be regarded as acceptable. Any decision on this matter is bound to be very subjective. To the uninitiated, for instance, it will differ according to the way in which the probabihty is expressed. Let us take the peak level observed at the Hook of Hofland on 1st February, 1953 as an

267 y y y

y y y y

y y y • y y y y 1953.- y - y . y y .•• y y y .• .• y • y . y .•• ." * .••189 4 4 r' 19)6 1906

EXCESS CURVE CHOSEN POSSIBLE EXTRAPOLATIONS

10° 10' lO^ 10'^ EXCESS FREQUENCY PER YEAR

FIG. 7 Tentative excess curves of high-water levels during storms at the Hook of Hol­ land (1859-1959)

example. The frequency of excess for this level will be judged in two very different ways if we are told, either that the level will be ex­ ceeded on an average once in every 250 to 300 years or that a man has a 20% chance of witnessing a disaster during an average lifetime of 75 years. But both chances are identical. Bearing in mind the growth of the population and the intensive economic development, together with numerous other imponderables, such as human suffering, loss of hfe and the disruption of the routines of life, the Delta Commission regarded a remaining flood risk of 1% per century as being just admissible. That involves a high-water level at the Hook of Holland of 16J feet above N.A.P. The realistic value of this level from a physical standpoint has already been discussed.

268 (iv) Econometric approach As described above, the selection of a "design level" on the basis of 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. To obtain a better understanding of this matter, at­ tempts were made on the suggestion ofthe Delta Commission to approach the problem on a joint economic and statistical basis. This is prompted by the idea that the capital to be invested in flood prevention projects together with the capitalized value of the margin of damage due to flooding should be as small as possible, because that would give us the best value for our money and justify the use of the term "economic optimum".

1^ z 0

COSTS OF HEIGHTENING DYKES AND CAPITALIZED ANTICIPATED DAMAGE

FIG. 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

269 The above principle was worked out both analytically and graphically. The analytical work was done at the Mathematical Centre and the graphical method was developed by "Rijkswaterstaat". Of course, there are many unknown quantities in the calculations. On the one hand, a tentative estimate only can be given of the cost of heightening dykes on an extensive scale, of building new dykes or of carrying out other flood prevention projects and of the capitahzed ex­ penditure that maintenance involves. On the other hand, it is extremely difficult to estimate the amount of damage that flooding might cause. Allowance would have to be made not only for the economic development anticipated, but also for the fact that the extent and consequences of flooding are never the same for any two floods. Moreover, the breach criterion selected, i.e. the exceeding ofthe "design level", is always open to objection, since numerous factors connected with dyke construction, etc. may also be involved. The rate of interest selected is also inevitably an uncertain element in the capitalization of the margin of damage due to flooding. But the ele­ ment of uncertainty involved in the extrapolation of the frequency curves of excess is of much greater consequence. By suitably varying all the above-mentioned elements of uncertainty, it is possible to derive from the calculations two extreme lines, each of which represents the sums of money to be invested in the raising of the dykes, etc., and the capitalized margin of damage due to flooding expressed as a function of the "design level". The true figure will he somewhere be­ tween these two extremes, though it is not known exactly where, so that the actual economic optimum is not known, either. The intermediate level might be chosen in such a way that any deviations from the two extreme economic optima are equal but opposite. That keeps the devia­ tion from the actual economic optimum as low as possible. The above-mentioned economic considerations would certainly be valid from an insurance standpoint if a large number of unrelated risks could be insured on this basis. The fact that this is not apphcable in principle to the present case is one ofthe weakest points in the econometric calcula­ tions. Therefore, the moment when the next disaster will occur has a decisive influence on the value of the calculations. Hence, even the econometric calculations are to a certain extent arbitrary when it comes to determining a "design level". Nevertheless, by giving

270 I

/ 0. ..ECONOMIC RISK" FOR THE VARIOUS / ..DESIGN LEVELS" BECAUSE THE / UPPER LINE MAY BE THE CORRECT / ONE

b. ..ECONOMIC RISK" FOR THE VARIOUS ..DESIGN LEVELS" BECAUSE THE b3 > ^2 |_ LPS LOWER LINE MAY BE THE CORRECT ƒ T~ ONE

02 = b2 1 ba / °x

1 in 0, > 02 '^1 .|. oIII

^ \—^ 1

SUM OF COSTS OF HEIGHTENING DYKES AND CAPITALIZED ANTICIPATED DAMAGE FIG. 9 Diagram showing two extreme econometric estimates of costs and the "design level" interpolated on the basis of the smallest possible economic risk (oj = b.^} a clearer representation of the factors involved, they very clearly support the conclusion which the Delta Commission already felt it was obliged to draw on physical and statistical grounds. In addition, the econometric calculations show conclusively that there is a greater disadvantage in selecting a relatively low "design level" which may lie below the eco­ nometric optimuiTi than there is in selecting a relatively high level which probably hes above it. It is true that if a level below the optimum is chosen, the initial cost of heightening a dyke will be comparatively low, but the flood damage anticipated wiU increase disproportionately. Such a measure would perhaps be penny wise but certainly pound foolish.

(v) Conclusion On the basis of physical and statistical considerations supported by eco­ nometric calculations and bearing in mind the numerous imponderables

271 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 5.65 + 5 Breskens 5.85 + 5 Hansweert 6.15 + 5 Bath 6.60 + 5 Dam Veerse Gat (Vrouwenpolder) 5.45 + 40 Dam Oosterschelde (-Beveland) 5.35 + 40 Dam Oosterschelde (Burghsluis) 5.25 + 35 Top of Schouwen 5.10 + 25 Dam Brouwershavense Gat (Repart) 5.25 + 40 Dam Brouwershavense Gat (Goeree) 5.15 + 30 Top of Goeree 5.05 + 30 Dam (Goederede) 5.20 + 40 Dam Haringvliet (Rockanje) 5.20 + 40 Top of Voorne 5.05 + 30 Dam (Oostvoorne) 5.05 + 25 Hoek van Holland 5.00 + 0 IJmuiden 5.15 Den Helder 5.05 Afsluitdijk 5.85 Harlingen 5.80 Delfzijl 6.40 such as human sufferiitg and loss of liumari lives, the Delta Commission recommended that measures to prevent flooding in the Netherlands should be based on 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 in various parts of the Netherlands, the "design level", which is intended to serve as a basis for the actual design, may be higher or lower than the local basic level, depending on the interests to be safeguarded.

III. THE DELTA PROJECT

(i) The choice On the basis of the "design levels" discussed above, all the dams and

272 Highest recorded in level Design level in meters above N.A.P. in 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 in the south-west would have to be raised by from 5 to 6J feet at least for many hundreds of miles. The reinforcement work such a plan would entail would be extremely difficult to carry out owing to the fact that there are many houses, industries, harbours, plant, etc., alongside and even on the dykes themselves. Added to this is the fact that the nature of the soil by no means everywhere admits of the reinforcement of the body of a dyke, whilst several breaches revealed that various sec­ tions of a dyke may have other defects that cannot all be detected, so cannot be removed. On the other hand, damming off the will result in a short coastal defence-line of dunes and dams, which can be given both the height and the strength, regarded as necessary for the future without running into the difficulties mentioned above. Moreover, if the rise in the average sea level in relation to the land should continue or if in the distant future the

273 basic levels should have to be raised for any other reason, an additional, even considerable, heightening of the dams wiU be comparatively simple to carry out. Furthermore, when it is borne in mind that the existing dykes behind the main dams will constitute a valuable second line of defence, it will be clear that closing-off the estuaries will result in a much greater degree of safety being obtained than could ever be achieved by heightening the existing dykes. Only by closing off the estuaries can a defence line be obtained that can be regarded as effective and reliable, even for the distant future. If the fact is also taken into consideration that the cost of an all-round heightening of the dykes and that of damming-off the estuaries will not differ appreciably whilst the latter alternative will offer great advantages in matters of fresh water management and the provision of road links, it is not difhcult to understand why the choice fell on damming off the estuaries, on what is now known as the "Delta Project". However, the problem was by no means as simple to solve as the above statements would lead one to suppose, because it had first to be ascer­ tained whether the Delta Project was feasible from a hydrologieal point of view. It was reahzed that the waters of the estuaries that would have to be held in check while the estuaries were being dammed off would be deeper, more powerful and more capricious than any previously sub­ jugated. The Delta Commission's conhdence that they could made a

aVUUia.UiC i CCUiliiliCliUaLiUii VViUll ICgClXUl LO L±HÖ IIIO-LLI^I \V U.J k/tiJ^V-L u»VvJ closely interrelated factors. In the hrst place, they cited the experience gained in the planning and execution of extensive and difficult damming operations for undertakings such as the Zuyder Zee Project, the closing of the breaches in the dykes on after the war and the repairing of the damage resulting from the floods in 1953. However, the Delta Commission added a rider to the effect that it would be necessary to gain more experience by conducting intensified research on a scientific basis before it could be applied to still larger projects; they drew attention to the splendid opportunities modern methods of research offer in this respect. Before a final decision could be taken with regard to the Delta Project and apart from any consideration as to whether the closing-off of the estuaries would in itself be feasible, data had to be collected on such matters as the changes in the movement ofthe water that such an opera­ tion would cause at other places. In the interests of shipping and to

274 FIG. 11 Tidal data of various closures Gross sec­ Mean flood Mean tion below volume in tidal range N.A.P. in millions of Time in meters sq. meters cubic meters

Zuyder Zee Barrier Dam 1925 0.9 120,000 575 Brielse Maas Barrier Dam 1948 1.8 2,700 17 Braakman 1952 4.0 850 17 Dyke repair 1953: Kruiningen 1953 4.4 550 26 Schelphoek 1953 2.8 8,000 130 Ouwerkerk 1953 3.0 1,500 36 Delta works: Veerse Gat 1955 2.9 7,500 70 Haringvliet 1955 1.9 18,000 260 Brouwershavense Gat 1955 2.4 30,000 325 Eastern 1955 2.8 90,000 1100

prevent damage to the river banks, there may be no dangerous currents either during or subsequent to the execution ofthe Delta Project. In view of this problem, which had a decisive influence on the general planning ofthe project, an extensive systematic investigation was undertaken about which more will be said in the following sections of this chapter.

(ii) Planning The studies undertaken by the "Rijkswaterstaat" with regard to the increase in safety and the improvement of fresh water management in the delta area began in 1938, at about the same time as the first fuU-scale statistical investigation mentioned in the previous chapter. The hydro- logical tests could not get under way until a hydrologieal model of the lower reaches of the rivers was completed in 1948. A number of very difi'erent plans, the Two, Three, Four and Five plans with adjust­ able flood barriers and/or dams in the lower reaches of the rivers, were examined, either individually or in conjunction with plans for the protec­ tion of the Brabantse Biesbos and the mainland of from floods. A dam originally projected in the near Klundert, combined with large sluices (Weir X) to drain off" the water and ice from the and the Maas, was moved farther and farther out to sea and was finally sited at the mouth ofthe Haringvliet near Hellevoetsluis. Thus,

275 276 the main framework of what was later to grow into the Delta Project had already been fixed before disaster overtook the country on 1st February, 1953. The following are the principal elements ofthe Delta Project.

The adjustable flood barrier in the Hollandse IJssel for the protection of a large part of central Holland Under normal conditions the barrier is raised, so that inland shipping between Amsterdam and Rotterdam can pass unhindered. A lock with a movable bridge is planned in addition to the flood barrier for the benefit of shipyards situated behind the barrier, so that ships with high 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 will be a lock for the passage of the vessels needed for the main­ tenance ofthe engineering works and the approaches, and for the passage of fishing vessels. There will also be an inner and outer harbour.

The main dams across the Brouwershavense Gat, the and the Veerse Gat The incorporation of locks in these dams has been advised against because of the risk of salination that would entail. Drainage facihties will of course be needed so that the water behind., the barrier can be flushed.

The secondary dam across the Zandkreek This dam is required to enable work on the dams across the Veerse Gat and the Eastern Scheldt to be carried out independently; it will prevent current velocities in the Zandkreek from rising to dangerous levels. A lock will be incorporated in the dam to allow the passage of shipping to and from the canal crossing Walcheren. This lock will also serve as a sluice with which the water in the basin formed by damming off the Zandkreek and the Veerse Gat can be controlled.

The secondary dam across the upper reaches of the The dam is required to enable work on the dams in the Brouwershavense Gat and the Eastern Scheldt to be carried out independently; it wifl prevent current velocities in the Zijpe from rising to dangerous levels.

277 A lock-cum-sluice will be ineorporated in the dam. When completed, the Grevelingen dam, like the Zandkreek dam, will continue to have a practical hinction, providing a land link for road traffic and serving for water control purposes.

The [Hellegat) dam The dam across the Volkerak will make it easier to carry out other sec­ tions of the Delta Project. Together with the Haringvliet dam it will speed up the completion of work in the Northern sector ofthe delta; this part of the project can be finished long before the Eastern Scheldt is closed off. A series of locks will be incorporated in the Volkerak dam allowing for the passage of ships from and the water­ ways to central Holland or the . There will also be a large sluice in the Volkerak dam for the water management ofthe "Zeeland Lake" formed by damming off the waters round Zeeland.

The possible construction of an adjustable harrier across the lower reaches of the for fresh water management purposes.

The heightening and strengthening of dykes for increased safety along such waterways as the Rotterdam Waterway and the , which have to remain open to shipping to and from Rotterdam and Antwerp.

Ancillary projects Included under this heading are such things as hydrologieal improve­ ments in rivers for the efficient discharge of water and ice, measures to prevent erosion of the bed and banks, facilities for shipping, facilities in harbours and unloading quays, etc., which will make for smooth adapta­ tion to the changed conditions; improved facilities for the discharge of polder and sewage water and provisions for the protection of fishing interests.

(iii) Order of operations The order of Delta Project operations is determined by various factors. Priority will be given to the projects that ensure maximum safety. At the same time care must be taken to ensure that harmful hydrologieal

278 situations do not arise in the intermediate stage, i.e. before all the projects have been completed. It is also important that the operations be carried out in such an order that the experience gained in previous small-scale projects of such matters as the use of materials and equipment and meth­ ods of research and operation can be put to good use in the larger and most difhcult projects. Therefore there is a strong case for proceeding from "small" or, rather, "less large" to "large". The more so, since the execution of the largest projects, the damming of the Brouwershavense Gat and the Eastern Scheldt, is hkely to be gi'eatly hindered by the vagaries of tide and weather. Therefore it will be of paramount impor­ tance that a deeper understanding of Nature's whims be gained in advance. Some ofthe Delta Projects, such as the construction ofthe flood barrier in the HoUandse IJssel and the execution of the Three-Island Project (Veerse Gat and Zandkreek dams), can be carried out more or less in­ dependently. In view of the much greater safety of central Holland to be secured by the elimination of the risk of flooding along Schieland's Hoge Zeedijk (HoUandse IJssel) and in view of the experience to be gained from build­ ing a dam as close to the sea as the one across the Veerse Gat, work on those projects was given as much priority as possible. Consequently, the HoUandse IJssel project was finished in 1959, the Zandkreek dam in 1960 and the Veerse Gat dam in 1961. The other dam projects, with the exception of the adjustable barrier in the Oude Maas, are very interdependent. For hydrologieal reasons, it would not be advisable to put the Haringvliet sluices into operation before the Volkerak has been closed off. As long 4

HOLLANDSE 'JSSEL

ZANDKREEK-VEERSE SAT

GREVELINGEN

VOLKERAK

HARINGVLIET

BROUWERSHAVENSE GAT

EASTERN SCHELDT

FIG. 13 Time table for the execution ofthe Delta Project

279 as the Brouwershavense Gat and the Eastern Scheldt remain open, dam­ ming of the Volkerak would cause a rise in the high-water levels on the south side, which would be a source of danger at high water during storms. However, the rise in flood levels will be reduced by the timely damming of the Grevelingen, which, in its turn, must be completed fairly soon to enable the large Brouwershavense Gat and Eastern Scheldt dams to be completed independently. Therefore the major delta projects are inseparably linked. This is particularly so, because all the work in the coastal area will be exposed to the force of tidal currents and waves, while the ground is very unstable. So, a project once begun must be carried out and completed at a set rate. Although the rate will, of course, have to be consistent not only with what is possible technically but also with what is economically justified, it does not alter the fact that laying the first section of wihow "mattress" in the Haringvliet, as it were, determines the moment at which the dam in the Eastern Scheldt will be completed and vice versa. The other projects must be fitted in between those two operations in the correct manner, thus covering a period of approximately 25 years from the 1953 storm disaster. Of all the large dams, the one in the Haringvliet will be the most im­ portant from the point of view of safety, for the greatest length of existing dykes is involved. Moreover, the building of the dam itself will present the least serious problems once the huge sluices to be incorporated in it are finished and can be opened. So of course priority was given to opera­ tions in the Haringvliet. The operations have been timed so that advan­ tage can be taken ofthe experience gained in the damming ofthe Veerse Gat and the Grevelingen. When the project has been completed in 1968, one year after the damming of the Volkerak, not only will the northern delta area be protected against floods entirely according to plan, but it will also be possible to take full advantage of the benefits the Delta Project offers this area in the sphere of fresh water control. If the time-table is adhered to, the Brouwershavense Gat and the Eastern Scheldt dams will be completed in 1970 and 1978, respec­ tively. These projects are so huge that much preparatory work is already being done. In particular a series of experiments is being carried out in the field and on models, which are essential if a sound design is to be produced.

280 (iv) Safety When the Delta project has been completed, the only open link the closed-off lower reaches of the rivers and estuaries will have with the sea will be the one via the Rotterdam Waterway. So storm-floods will still be able to intrude along this route. However, the height of any sueh flood wifl fall rapidly on its way inland because of the strong suction effect of the Haringvliet basin, whilst the water levels in the closed-off Haringvliet and associated waters will rise but shghtly, at all events when the Waal and Maas discharges are normal. When an abnormally high discharge is combined with storm floods, the water levels in the Haringvliet basin and associated waters wUl, of course, rise much higher. Statistical research has shown that the simultaneous occurrence of storm floods and abnormally high discharge is purely a matter of chance. The two phenomena are hardly related, if at all. Hence, there are a number of different ways in which large or small storm floods may combine with abnormally high river discharges, each of which would produce different water levels in the delta area. The "Rijkswaterstaat" and the Hydrolog­ ieal Laboratory determined the water levels in the delta area for a large number of combinations, whilst the frequency of these combinations was deduced from the frequency data available on the discharge of the upper Rhine, and for the high tides recorded at the Hook of Hofland. By adding up the entire range of possibilities, the number of times abnormally high water levels are likely to be exceeded was finally determined for a number of places in the delta area. Not untfl that was done could basic water levels be established for the situation as it would be after eompletion of the Delta Project. It was found that as one moves inland 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 Hook of Holland or at Rotterdam is most hkely to be reached or exceeded when a very high storm tide and a normal river discharge occur simultaneously. Further inland the water levels will then remain low. On the other hand, the basic level in the Haringvliet basin or in waters further upstream is most likely to be reached when a relatively low storm tide coincides with a very high river discharge. That combination will present no problems in the Rotterdam Waterway. The same phenomenon can be observed in the lower reaches of the rivers now, but is very much less marked. The virtually complete exclusion of

281 -BASIC LEVELS

•MAXIMUM LEVELS DURING STORM WITH AVERAGE RIVER DISCHARGES (RHINE DISCHARGE 3000 CUB. MTRS. PER SEC.)

-MAXIMUM LEVELS DURING MEAN TIDES AND VERY HIGH RIVER DISCHARGES (RHl NE DI SCHARGE 13000 CUB. MTRS. PER SEC.)

FIG. 14 Excessive high-water level curves in 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 will move the characteristics of the upper rivers downstream as it were, i.e. from Gorinchem and Werkendam to the neighbourhood of . How greatly the closing-off of the estuaries will enhance the safety of the region can be seen by comparing present basic levels in the area of the lower reaches with the situation after the eompletion of the Delta Project.

282 The basic levels are practically identical for both situations at the Hook of Holland, so that strengthening the dykes along the Waterway will be essential. With the completion of the Delta Project, the basic level will fall steeply as one moves inland as a result of the suction effect of the Haringvliet basin. High levels such as those caused by very high tides will have been practically eliminated in that basin. The basic level will drop by 6^ to 8 feet compared with present conditions (Moerdijk). Further upstream (Werkendam), where the influence ofthe upland dis­ charge already outweighs the influence of the tides, the completion of the will of course have Utde or no effect. Here, the basic levels will be almost the same for present and future conditions. Tidal movement in Lake Zeeland wih be completely ehminated or very nearly so. What a difference this will 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 19| feet above N.A.P.! If it had not been decided to close off the estuaries, the dykes here too would have had to be heightened considerably and strengthened. Even then a degree of safety equal to that offered by the completed Delta Project would never have been obtained.

(v) Water control The Netherlands depends for its supply of fresh water mainly on the Rhine and the Maas. The supply is amply sufficient to meet the country's water requirements for domestic, industrial and agricultural purposes. There is bound to be a shortage, however, since too large a proportion of the river water is needed to drive back the salt in the coastal areas. Sea water penetrates into the open estuaries. Lock and sluice leakage, seepage and lockage water all increase the salt content of polder water. In addition, there has been an alarming rise in the chloride content of the Rhine water itself, due to the discharge of more and more saline waste water from the Ruhr area and the Alsace. Consequently, the quantity of 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 in the delta area is often considerable. More­ over, harm is being done to some agricultural areas by desiccation. The closing of the estuaries constitutes the most effective means of check-

283 PRESENT CONDITIONS AFTER COMPLETION OF DELTA PROJECT AND CANALIZATION OF RHINE

CHLORIDE CONTENT CHLORIDE CONTENT IN MG. PER LITRE IN MG. PER LITRE UPPER RHINE 150 è 200 UPPER RHINE 150 a 200 PARKHAVEN 400 S 800 PARKHAVEN 200a 300 WILLEMSTAD 5000 a 10.000 HARINGVLIET SLUICES 200 a 300 ZEELAND ESTUARIES lOOOOè 18000 LAKE ZEELAND 20O a 300

FIG. 15 Distribution over the major rivers and estuaries of a normally low discharge from the Rhine and the Maas ing the penetration of sea water. It will then be possible to desalt the waters farther inland. Enough fresh water wih become generally available for agricultural purposes. The distribution of fresh water in the tidal reaches of the Rhine and the Maas will be partially controllable, because it will be possible to regulate the flow by means of large sluices in the Haringvliet and the Volkerak. By reducing the discharge, more water wifl be carried off via the Noord through the Rotterdam Waterway than flows down it at present, with the result that the quantities of sea water penetrating this channel will be reduced to reasonable proportions. For the fresh water supply of the south-western part of the Netherlands, and particularly for those areas which receive their water from the upper reaches of the Rotterdam Waterway, it would be most advantageous if, within the frame-work of the Delta Project, the Oude Maas could also be shut oflF, at least during periods in which the volume of water coming down the Rhine is small.

284 However, in spite ofthe execution ofthe Deha Project and the closing of the Oude Maas, an adequate supply of fresh water from the upper reaches of the Rotterdam Waterway stiU cannot be guaranteed under all circumstances. This is mainly due to the fact that Rhine water usually has an excessively high chloride content when the river is very low. The dykes shutting off the Brouwershavense Gat, the Eastern Scheldt and the Veerse Gat, together with the dam across the Volkerak, will enclose "Lake Zeeland". This lake is to be fed chiefly with water from the Rhine and the Maas, which will be led to it via the Haringvhet basin and the sluice in the Volkerak dam already mentioned. The water of Lake Zeeland is likely to have become desalted in a few years, provided the discharge facihties are properly located and equipped and are of sufficient capacity. Even when Lake Zeeland has been desalted, large quantities of river water will still be required to keep it fresh, because for a long time to come brackish drainage water from the surrounding polders will continue to be discharged into it. The quantities needed will be particularly 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 will not always be available, the contingency must be reckoned with that it may not be possible to maintain an average chloride content of less than 300 mg Cl/htre under all circumstances. It is therefore essential to take the necessary measures to restrict as much as possible the penetration of salt seepage, lockage and leakage water into the lake.

When the estuaries have been closed, the levels and rates of flow in the waters farther inland will undergo changes. The water levels wifl not alter appreciably in the open Rotterdam Water­ way, but the rates of flow in the lower reaches wifl decrease considerably. This will be an advantage to shipping. In the rivers Noord and Dordtse KU, which connect the Rotterdam Waterway with the Haringvhet basin, the rates of flow wifl become higher than they are now. Any modifications to these rivers wifl cafl for special study. In the Haringvhet basin the rates of flow wifl generally become much lower than they are at present. The high-water levels wfll be lower, but

285 the ebb-tide levels will be appreciably higher. So provision will have to be made for the drainage of polders and higher ground. The effect of tidal movements will disappear entirely or almost entirely in Zeeland. The shape of the lake to be formed there and the control of its water level will require further study. It is not only considerations of safety and the interests of those who will beneht by the creation of a large fresh water reservoir that are important, but also matters involving drain­ age, land reclamation, recreation, shipping and fisheries.

IV. THE TECHNIQUE OF CLOSING SEA INLETS

(i) Methods employed There are sandbanks and more or less deep gullies in the inlets to be closed off. The bulk of the water moves along the gulhes and but little water passes across the sandbanks. Consequently, the building of a section of dyke on the banks will not affect the tidal movement very much and will only cause a slight increase in the current velocities in the remaining cross section. Therefore it is natural that work on a dyke should start with the sections on the banks, steps being taken to protect the bottom of the gullies against erosion as soon as it appears necessary to do so. Damming off the gullies is a much more difhcult job. The engineers have a choice of several methods. The most obvious to the layman would be to build the dyke outwards from the sides of the gully. But the current velocities would increase greatly while that was being done. It has been calculated that current velocities of from 5 to 6 metres per sec would occur during the final stages of the work in the wide inlets if that method were applied. Such high current velocities present serious problems, espe­ cially in the Netherlands, where the soil is sandy and much of it is there­ fore liable to be washed away. Moreover, the lateral narrowing would cause considerable contraction of the water mass and powerful eddies near the projecting dyke heads. Because of the serious risk of scouring by the eddies the method outlined above was only employed to close some of the secondary inlets (Zand­ kreek, Grevelingen - southern gap), where tidal 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 turn of the tide on a sill constructed for the

286 purpose so that the work should be completed within the period set. However, most ofthe inlets to be dammed off as part ofthe Delta Project are too wide to be closed in this way: the current velocities would be too great and the operation would take too long. A better method is to con­ struct the dam in horizontal layers, the layers being added one at a time along the entire length of the dam. Again, the gap wih be narrowed gradually and as a result the current velocities will increase at hrst. Since the dam is buih gradually along its

FIG. 16 Closing of southern gap in Grevelingen dam by means of caissons

entire length, critical overflow will be reached at a certain moment. From that time on not only will the current velocities cease to increase as the dam is raised, they wifl even decrease. Nevertheless, current velocities of approximately 4 metres per sec will have to be reckoned with if the wide inlets are closed off by this method.

287 CROSS-SECTION OF GAP IN THOUSANDS OF SQUARE METRES

BUILT OUTWARDS FROM EITHER BANK BUILT UPWARDS FROM BOTTOM SLUICE CAISSONS FIG. 17 Current velocities during closure ofthe Brouwershavense Gat for the various methods contemplated

This method, too, will cause eddies along the sides of the gully. Though they will be less powerful than the eddies that would occur if the lateral method were employed they may still cause trouble by scouring on ac­ count of the current velocities obtaining in the final gap. Moreover, building the dam layer by layer wiU entail the risk of the water dipping down along the downstream slope. It would eah for special devices, which are not always easy to construct. Though the layer-by-layer method would seem to be easier to carry out than a lateral narrowing ofthe gap, all the "gradual" methods of closing off a sea inlet are risky. That is why a method of closing off the wide inlets instantaneously is also being considered. This would be achieved by means of sluice caissons. They are "open" caissons with sluice gates through which the tides can move freely, even after the caissons have been moved into position. Then all the gates are lowered simultaneously at slack water, thus closing off the gap instantaneously. However, sluice caissons are restricted as regards height and they require

288 FIG. 18 Sluice caissons.

a liorizoiUal foundation. In practice this means that they must be placed on a sill constructed during the year preceding the closure. The presence ofthe sill itself win narrow the gap through which the current must flow and high current velocities may occur, especially when the gully being closed is deep. Again, it is obvious that even sluice caissons cannot be entirely "open"; parts ofthe structure wfll always impede the flow of water to a certain extent. So in actual practice the method is a compromise between gradual and instantaneous closing off.

(ii) Scouring The problem of current velocities was emphasized in the foregoing review of the advantages and drawbacks of the various methods of closing the inlets. It was tacitly assumed that they would provide a direct indication

289 ofthe scouring to be expected and ofthe stability ofthe material of which the sill is made. However, that assumption was not fully justihed, for in addition 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 for the sill and the problem of erosion. Moreover, the height of the sill is also an important factor governing erosion. It is true that as the dam becomes higher, the current velocities will in­ crease but the depth of the water above the sifl will decrease. The latter factor predominates in coastal areas in that the volume of water discharged across the dam per unit of width diminishes as the dam becomes higher.

Consequently, the current velocities at the extremities of the soil protec­ tion devices, where the current is again spread over the full depth of the gully, wfll decrease as the dam is raised. Nevertheless, turbulence is ex­ pected to increase. It is difficult to predict how much scouring wifl occur as a result of afl these factors. Experiments with scale models have shown that erosion beyond the soil protection devices wifl increase while the inlets are being dammed off until the dam has reached a height equal to approximately three quarters of the depth of the water.

290 O 10 20 30

EROSION IN METRES AFTER A CERTAIN NUMBER OF DAYS OF FLOW

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

Apparently, the decrease in the volume of water discharged predominates over the increase in velocities and turbulence as the height becomes greater; consequently, the risk of scouring diminishes. When the Veerse Gat was closed with sluice caissons the water was 20 metres (66 feet) deep. The sill on which the caissons were placed was 10 metres (33 feet) high. So the ratio between the two figures was 1 : 2. Therefore, the maximum scouring point had not nearly been reached. It was avoided entirely by closing the remaining gap at the right moment by lowering all the sluice gates at once. If the gully had been much deeper, 40 metres (132 feet) for instance, the height of the sill would have had to be three quarters ofthe depth of the water for caissons of the same height. Maximum scouring would then

291 have oecurrecl for a longer period and much soil would have been washed away. However, fewer diffieulties would have been encountered if the dam had been built by the gradual method. There would be no caissons, the restricted height of which is decisive for the height of the sill and the top of the latter would not necessarily have to be horizontal. There­ fore, especially in winter it would be possible to adapt the height of the sih to the depth of the gully, thus preventing serious scouring for any appreciable time. In view of those considerations it might be concluded that where the gulhes are deep building the dam by the gradual method is preferable to closing the gap by means of sluice caissons, but that the latter method would be better for shallower gullies. However, that does not hold for every set of conditions and it certainly does not always hold for the Dutch inlets in which loosely packed sand may occur.

FIG. 21 Closmg the Veerse Gat by means of sluice caissons

292 Ill I I SAND OC ÜI

0.

0 100 200 kg per sq. cm

CONE RESISTANCE FIG. 22 Results of penetration test and boring in the Brouwershavense Gat

(iii) The Siihsoil Samples obtained by boring have sltown that the ground on which the dams are to be built consists largely of comparatively fine sand (0.2 mm). In certain places, however, and at varying depths, layers of sand occur in which the "cone resistance" is far below what one would expect to find for sand. Exactly what causes this is not known, but it is presumed to be due to the looseness of the sand. If further research should prove the assumption to be correct, it may greatly influence the method to be employed for damming ofl^ the wide sea-inlets. Loosely packed sand constitutes a great danger since the frictional stress in the subsoil will of course be altered by various factors during the damming-off process. A dry sand base would readily stand sueh change in frictional stresses. But the sand wfll undergo a change in volume. In tightly packed sand there is an increase in volume; in loosely packed sand there is a decrease.

293 In principle, the sand retains this property of its grain structure even when it is saturated with water. Since the water between the grains is not very compressible, the changes in volume are compensated for in loosely packed sand by extra water-pressure and in tightly packed sand by a depression. In consequence of these extra water-pressures, the grain structure is omnilaterally placed under stress or reheved of stress. This, too, causes a change in volume which compensates the previously mentioned change in volume necessary for absorbing the frictional stress. So if the frictional stress in loosely packed sand is altered as a result of the building of the dams, there will be extra water pressures m it. Extra pressures will obviously bring about not only a compensatory change in

FIG. 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 in decimetres water column volume but also a decrease in the frictional resistance to sliding, which could constitute a serious threat to the stabihty of the construction. If the changes in frictional stress take place slowly, the extra water- pressure will remain slight, for the water between the grains would have time to run away and so lose its extra pressure.

294 However, during damming operadons stresses may also build up rapidly, as happens when caissons are put in position, when the caissons are sub­ jected to pressure from the wind and the waves, and when loads are moved along a telpher, causing stresses in the pylons. The process of scouring is in itself a slow one, but in the slopes it creates sliding can occur that causes a sudden drop in the load on the subsoil. Rough calculations show that a disturbance of equilibrium can easily occur, locally at any rate, in loosely packed sand. Local disturbances of equilibrium of this nature can produce new water-pressures elsewhere, causing a sort of chain reaction that may assume serious proportions. This phenomenon is called a flow slide. A flow slide during the closing of an inlet with caissons might have serious consequences. The caissons might move or topple over. The water would then be free to flow through the resulting gap and cause sdll more destruction. It would take months if not years to repair the damage. However, if the closing process is carried out gradually, the material for building the dam being deposited, for instance, by means of a telpher, it is always possible to ensure that the few telpher supports remain stable under all circumstances. Should a flow slide then occur and the dam damaged, nothing need be done but to continue depositing material. Naturally, the telpher would then be set to work solely at the damaged spot, resulting m a quick repair. So the gradual closing method is better than the sluice-caisson method in some cases, if the gullies to be dammed off are not too deep, because of the greater ease with which a flow slide can be repaired. There are of course many other factors to be considered before deciding upon the methods to be employed for damming off the wide sea-inlets. For instance, the engineers hesitate to use untried methods for building the large dams. Sufflcient experience has already been gained of several methods involving the use of caissons. As an experiment, the northern gully of the Grevelingen is being closed by building up the dam grad­ ually. The material wifl be deposited by means of a telpher. Meanwhile, theoretical and experimental research will continue. Only when the research has been completed and more experience has been gained will it be possible to decide on the methods to be employed for damming off the wide sea-inlets.

295 V. THE BARRIER DAMS

(i) Cross-section As has already been stated, it was realized after the disaster of 1953 that the main dykes needed to be higher than had previously been assumed adequate. The "design level" was raised by 1.5 metres. The logical con­ sequence of this step was that higher waves, therefore greater wave run up, had to be reckoned on. Moreover, it has been discovered in the meantime that local oscillations due to meteorological conditions might for short periods cause the water level to rise higher in places than had been thought possible in the light of previous considerations. In view of all these faetors together, it has been decided that, if the barrier dams were designed with an outer-slope incline of 1 : 5 or 1 : 6, the crests would in some cases have to rise to as much as 15 metres above mean sea level. Another lesson learned from the disaster was that mueh greater safety could be obtained if the inner-slope incline was made a good deal less steep than had previously been the case. Accordingly, as a rule, no inner- slope incline wih be steeper than 1 : 3 in future.

EASTERN SCHELDT BARRIER DAM 1978 • ZUYDER SEA BARRIER DAM 1932

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

Consequently, the new barrier dams will not only be much higher than the old dykes but their slopes will also be much gentler. Since the gullies to be dammed off are between 20 and 30 metres deep, it is clear that the new barrier dams will dwarf all previous structures of this kind. While the dams are being constructed, large quantities of material will have to be brought up and deposited in a comparatively short time. Sand, of which there is no shortage in the inlets, was the only material that could be considered for this purpose; it will form the core of all the dams.

296 (ii) The Revetment In the Netherlands the conventional type of revetment for this kind of dyke built of sand is a layer of clay covered with brick-rubble and faced with basalt blocks. However, there is not enough clay in the Delta area and it would prove very expensive to bring it in from elsewhere. Basalt is in limited supply and has become very expensive. Moreover, there would not be enough skiUed stone-setters to tackle such large-scale projects. This being so, it would have been impossible to revet the dams with the desired rapidity by the conventional method. Fortunately, another kind of revetment has been developed since the Second World War made of bituminous material produced during the oil-refining process. The asphalt facing can be applied direct to the sand quite quickly. It is therefore endrely suited to the present-day technique of dyke-building, whereby powerful suction-dredgers rapidly build up a sand base. Of course, these arguments are not in themselves sufficient reason for adopting the asphalt method. The main desideratum is that the dam shall be a reliable and permanent construction. In October 1956 a working group was formed to investigate the strength and permanence of asphalt construction. The group has published a provisional report, in which it is stated that asphalt revetments, provided they are expertly designed and constructed, can give the dykes rehable protection. In the light of the conclusinns arrived at through these and other studies, it was decided to provide the barrier dams with asphalt revetments. As a result, the dams will have the simplest form of cross-section imag­ inable, viz. a core of sand covered with a layer of asphah. Nevertheless, designing such a dam is not as simple as it may seem. It is precisely because the revetment is comparatively thin that a number of problems arise which, despite modern resources, are not all easy to solve.

FIG. 25 Cross section of the Veerse Gat dam

297 For instance, little is known about its permanence; however, judging from experience with 15-year-old asphalt revetments, it is reasonable to anti­ cipate that it will be satisfactory in this respect. All the same, the com­ position of the asphalt mixture and its applicadon will have to meet certain requirements. Again, insufficient research has been conducted into the force ofthe waves, so it is not yet known how strong the asphalt revetment will have to be to withstand them. It is thought at present that a coating 30 cm thick on a sand base will be sufficiently strong to stand up to the waves, at all events under Dutch conditions. But the matter is still uncertain. Another factor that the engineers designing the dam must bear in mind is the water pressures likely to build up behind the asphalt revetment. Since asphaltic concrete is much more impervious to water than the underlying materials, hydraulic stresses may build up under the facing, which wül reduce the maximum friction between this facing and the material to which it has been applied. Consequently, after a certain moment, as the water pressure rises an ever-increasing part of the com­ ponent of the weight of the facing itself parahel to the slope wiU be absorbed by the facing itself This may give rise to inadmissible deforma­ tions in the long run because of the viscosity of asphahic concrete. To discover the extent of these deformations it would be necessary to know the stress and deformation conditions in the facing under the in­ fluence of the weight of the facing itself and the ever-changing hydraulic stresses. So far, no satisfactory method of obtaining this information has been devised, so that for the time being a comparatively arbitrary stan­ dard has to be adopted which must be met to cope with viscous deforma­ tion, viz., that under frequently recurring circumstances such as normal spring tides the component of the weight of the facing itself parallel to the slope shall never exceed the maximum friction between the facing and the underlying material. Moreover, a different standard must be observed for rarely occurring circumstances. Then the requirement is that nowhere shall the excess pressure be greater than the component of the weight of the facing itself perpendicular to the slope. So in an extreme case the facing would "float" on the slope in places. The component of the weight of the facing parallel to the slope is then entirely absorbed by the facing itself This is considered acceptable under

298 extreme conditions, because the viscous deformations are expected to remain within reasonable limits owing to the short duration of those conditions. It must be pointed out, however, that in such cases the grain pressures in the material immediately underneath the facing wih be reduced to zero, thus increasing the risk of deformation ofthe slope underneath the facing. In order to determine the thickness of the asphalt by means of the stan­ dards mentioned above the potential curves must be known, both for frequently recurring circumstances and for extreme situations.

(iii) Calculation of water-stresses The distribution of potential in the body of a dyke can be found by calculation if the boundary conditions are known and the mass of soil through which the water runs can be reduced to simple terms. In this particular problem the latter has turned out to be impossible to achieve as a rule without seriously sacrihcing accuracy. In view ofthe high expense of the asphalt facing on the one hand and the sensitivity of the thickness of facing required on the other hand, such inaccuracy was regarded as unacceptable and other methods of investigation were considered. Water stresses in the body of a dyke are determined by a great number of factors. A number of these factors are known to be liable to great variations, for example, the permeability may differ considerably at one spot com^pared with another. Neither can the tidal boundary conditions and the wind effect to be superimposed on them be regarded as constant quantities. In most cases it wiU moreover be desirable to investigate a great number of alternative designs. It must therefore be possible to vary these and other factors during the experiments. In order to fulfil these requirements it seemed to be advisable to choose a method in which use is made of the wellknown analogy between a stream of liquid passing through porous substances and an electric current passing through resistances and capacities. In principle this can be done in different ways. An analogous model composed of a network of resist­ ances and capacities would in many respects have been preferable for the solution of the present problem. Such a model, however, would have to contain a very great number of sections, all the resistances and capac­ ities of which would have to be adjustable in view of the variations re­ ferred to above.

299 i j.^-iT^.i i i i

'X^ SOURCE OF ALTERNATING CURRENT

i CONDENSER

FIG. 26 Electrical circuit diagram for an analogous model

In order to gain the insight necessary for designing such a comprehensive model, preliminary experiments were carried out with plate-shaped elec­ trical conductors. These conformed to the shape of the water-bearing soil-mass, the upper edge of the model being located at a height equal to the capillary elevation above the phreatic plane formed if the water level on either side of the dyke are held stationary at their average level. Along this edge short electrodes are hxed, all of which are connected to an earthed condenser. These condensers represent analogously the water- retaining capacity. The boundary conditions are applied as alternating current. In order to ntake the latter follow the desired shape of curve use is made of a special boundary condition apparatus. By this means a great number of periodic tidal curves can be obtained from a few harmonics, while a flood tide can be imitated by superimposing a wind effect on the tidal curve thus obtained. This wind effect, the curve of which can also be varied at will, occurs in the model every 64 tides. The effect of the previous flood tide has then largely disappeared by the time the next flood tide appears. If there is a horizontal or practically horizontal beach in front of the dyke, the part of the tidal curve below beach level will not be expressed in the ground water potential in front of the dyke. This is represented in the model by means of a special device which prevents the current applied from decreasing below a certain predetermined value correspond­ ing to the level of the beach. Part of the tidal curve is thus lopped off, as it were. Another device switches the condensers off wherever the free water level reaches the under surface of the asphalt facing.

30Q NORMAL TIDAL MOVEMENT

NAP RISE IN SEA LEVEL DUE TO STORM time

COMPOSED BOUNDARY CONDITION

FIG. 27 Analysis of boundary condition used in research for the Veerse Gat dam

The frequency of the boundary condirion of the tide can be varied be­ tween 200 Hz, and .5.000 Hz. Since a clmnge in frequency has the same effect as a change in permeabihty or as a change in the water-retaining capacity of the soil, variations in these can easily be investigated. Alterations in the shape of the water-bearing soil-mass can quickly be made, while anisotropy and inhomogeneity ofthe permeability can easily be reproduced. It also takes very little time to measure the distribution of potential in the model, so that the influence of a great number of variables can be investigated with little delay. That is the method by which the water pressures likely to occur in the barrier dams are calculated. Both measurement on the spot and laboratory experiments revealed that the permeability factor in DARCY'S formula for sand found in the coastal areas of the Netherlands might vary from 5x10-5 meters per sec to 5x10-4 meters per sec. Both the dyke body and the subsoil are composed of such sand. This mass of sand reaches down to between 30 and 40 metres

301 below average sea level practically everywhere; it rests on an unbroken stratum of clay. Clay lenses may occur in higher deposits locally, but these, too, are often found at such great depths as hardly to affect the current pattern on the site ofthe dyke. But the variation in permeability referred to above is a factor that should always be given careful consid­ eration. Most dams will have a retaining embankment of rubble. The permeabil­ ity of rubble is far greater than that of sand. However, the possibihty of the sand impregnating the rubble during the pouring operation must be reckoned with. Accordingly, experiments are being carried out with re­ taining embankments, the rubble in which is made less permeable than, equally permeable as and more permeable than the sand in the dam.

K ( sand ) K ( rubble ) 10 - ^ mtrs. per sec. lO'^to 10'^ mlrs.per sec. . \0-4 mtrs. persec. 10"^ mtrs.per sec. 10-4 mtrs. persec. 0

metres

1 • H 1 1 1 1 1 1 1 1 1 1 \ h 0 I 2 3 4 5 5 7 S 0 10 11 12

FIG. 28 Enveloping lines of maximum potentials for the toe of the Veerse Gat dam (the maximum values are reached at different moments)

It was found that the greatest excess pressures occur in retaining embank­ ments of rubble, which is very permeable compared with sand. When the dam in the Veerse Gat was under construction it was by no means certain that the embankment would actually be impregnated with sand. Consequently dimensioning was based on the assumption that the most unfavourable conditions would obtain. This and several other factors have led to the toe of the dyke being constructed in a manner altogether different from what had been planned at the outset. In the first place the toe was placed higher up. Then, instead of a 50 centimetre coat of

302 -05 NAP .'••••.'••I'j.v.

03

NAP

-280

Fjo. 29 Veerse Gat dam. Original design with asphalt concrete and modified structure with basalt and liquid asphalt. (Levels in metres relative to N.A.P.) asphaltic concrete, a coat of ballast stones impregnated with bitumen was applied tapering from 90 to 60 centimetres thick. A method is being sought for the construction of dams on which work has not yet been started that guarantees that the rubble will be impreg­ nated with sand. This will very likely result in, an altogether different and much cheaper revetment. As stated before many other factors must also be varied, when computing the water pressures. Consequently, some hundred computations will have to be made for the design of each dam. Now if we take into account that this is actually only the basis ofthe design of a comparatively minor detail of the dam, it is only natural that designing an apparently simple cross- section takes a long time and calls for great knowledge and experience.

303