One of the largest engineering undertakings of recent times has been the reclamation of the Zuider Zee and the recovery of thousands of acres of land from the North Sea, which centuries ago robbed Holland of much valuable territory
BUILDING THE MAIN DAM across the Zuider Zee from the island of Wieringen on the west to Friesland on the east. In the foreground a massive floating grab transporter is depositing boulder clay to form the core of the dam and, in the background, sand is Deing pumped behind the dam to form its inner slope. The boulder clay was brought to the spot in barges.
THE work of the engineer is concerned to an amazing extent with water. He has confined it by dams to make great lakes for the irrigation of thirsty lands, he has made it transport ships from ocean to ocean by canals and he has compelled the headlong river to give up its power in the turbine.
Here and there, too, he has caused the waters to recede and lay bare the earth for the purposes of agriculture, but, with one exception, these last efforts have been undistinguished. The exception is, however, so important that, when the work is completed, even small-scale maps of Europe will need correction. The great landlocked bay of Holland known as the Zuider Zee will have given place to a narrow belt of water; and the salt sea will have become a freshwater lake called the Ijsel Lake, or, more correctly, Ijselmeer.
Long ago the site of the Zuider Zee (in English, the South Sea) was a fertile plain diversified by several lakes, but in the thirteenth century the ocean broke through the sand dunes which protected its northern coast and converted the plain into a sea so deep that in the sixteenth century contending warships blazed away at one another above the drowned fields.
The remains of the original but ineffective north coast of Holland are recognizable to-day in the chain of islands which curves round in nearly a quarter of a circle towards Germany. It might seem that, to recover the lost land, all that was needed was to join the islands to one another again to form an unbroken barrier from behind which the water thus isolated could be removed.
But the problem is not so simple as that. Centuries of restless tides have not only scooped out deep channels between the islands, but have also gnawed away the earth which once lay on the submerged sea bed within them. Some other solution, then, had to be sought for and this was attempted in 1920.
This, however, was not the first attempt to win back the lost land of Holland. Many acres of land had already been restored to her in the course of centuries by the efforts of her engineers. Even now Dutch engineers wage a ceaseless fight against the greed of the sea at nearly every point of the country’s coast.
The result of this warfare is that the Dutch engineer has developed an incomparable skill in this particular form of engineering and, time after time, has been called to instruct other nations in it. For instance, in the seventeenth and eighteenth centuries he taught the Englishman how to drain the fens of Cambridgeshire and Lincolnshire.
On an uncorrected map of Holland, or the Netherlands, as the country is called, there is to be seen a constriction of the Zuider Zee about halfway between the islands and its southern coast. This seems a natural point at which to span its waters by a dam, but the Dutchman was not going to consent to the loss of a valuable area by taking the easiest way out. A bolder scheme made greater appeal.
North of the constriction and on the west coast of the Zuider Zee there is an island called Wieringen. This the Dutch engineer would join to the land on the west by a short dam. He would then stretch out eighteen and a half miles across the sea to Friesland, on the eastern side, with a dam which would cut off the waters of the Zuider Zee from those of the North Sea.
But this cutting off is by no means all that is necessary. In the first place, there flow into the Zuider Zee numerous streams and rivers, notably the River Ijsel, a branch of the Lower Rhine, on the south-east. As it is useless to try to drain a body of water that is continuously being fed from outside, some outlet has to be provided for these rivers. Again, there are many seaports on the coast of the inland sea and they have to be taken into account. One of these ports is busy Amsterdam, whose outlet to the ocean cannot be restricted to the North Sea Canal. Some water, then, must be left in the enclosed area.
Down the centre of what was the salty Zuider Zee there will be an artificial freshwater lake having an area of about 270,000 acres. This will receive the discharge of the River Ijsel and will form a waterway for the ships to and from Amsterdam. The river drainage ultimately escapes from the Ijselmeer through sluices in the great enclosing dam when the tide outside is low. Ships pass the same barrier when the tide is high through enormous locks.
The new land, however, is not absolutely continuous with the old. The space inside the great dam is being divided into four polders, that is, into four enclosed areas of drained land protected by dikes. These enclosing dikes are banks of clay dumped into the water and having their tops rising well above its surface. They run in great angular loops from the old coastline and the water enclosed in these loops is, after their completion, to be pumped out and discharged into the Ijselmeer.
The smallest of the four polders, which is south of what was once the island of Wieringen, has already been drained and is now under cultivation, having added seventy-five square miles of land to crowded Holland. It was enclosed while the great main dam was still under construction, as the land to be recovered was urgently needed. This plan involved making the enclosing dam or
dike heavier and stronger than those for the other three polders will need to be. This dam had to withstand the tides and currents of what was then the open sea, whereas the enclosing dikes for the remaining polders will be built in a tideless lake now that the main dam has been successfully completed. A brief account of the construction of the first polder will give a good idea of what is entailed in the whole scheme. The first step was to connect the island of Wieringen with the mainland. This was done by a straight dam about one and a half miles in length, carrying a road and a railway. To the south of this straight dam was built a curved one, the space between the two forming a storage lake. The next step was to connect the eastern tip of Wieringen to the mainland south of the island, the junction of the dike with the mainland being at the ancient town of Medemblik.
FOUR POLDERS, or areas of new land, are being reclaimed from the Zuider Zee by Dutch engineers. The main dam was built from the island of Wieringen across the sea to Friesland. Further dikes will cut off the polders from what remains of the Zuider Zee, now a freshwater lake called Ijselmeer, fed by the Ijsel and other rivers. The polders are surrounded by drainage canals, and a ship canal through the largest two will serve as an alternative route between Amsterdam and the open sea.
The dike between the two points is eleven miles long and is formed, in plan, of two straight sections with an “elbow” at their junction. Imagine the engineer standing on the coast at Medemblik. Before him are eleven miles of sea varying in depth from 11 ft. 6 in. to 16 feet, and across this stretch of water he has to build a bank to a height of about 13 feet above the water level. The sea bottom consisted, in the main, of firm sand and clay in alternating layers, but it would not do merely to dump down the bank on this, as there were patches of soft clay. An interlocking construction had therefore to be adopted.
The first step was to dredge an enormous trench in the sea bottom from shore to shore; the next step was to fill the trench with sand up to the level of the bottom. Construction then began. Dredgers went as far afield as the area between the island of Tessel and the Friesland coast and loaded hopper-bottomed barges with the boulder clay that forms the sea bed there. Boulder clay is a dull greyish-blue stuff, extremely hard and dense. It was laid down ages ago by glacial streams. Nothing will grow on it, but it makes an excellent core for a dam.
The barges were towed into position above the sand-filled trench, and for long months their contents were dropped into the water until a vast ridge with a base over 100 feet in width was formed, its top being about 4 ft. 6 in. below low water, that is, so high that the barges could no longer work above it. This core was next carried up above the water level by floating grab cranes or gigantic floating belt conveyers, which deposited the clay on the ridge from barges floating besides them. In this way the ridge grew up from both shores above the water. A gap, however, was left until the whole was nearly finished, to balance the water pressure on either side of the dike.
When the core was in place, it was backed by an enormous bank of sand pumped behind it, that is, on the side eventually to face the dry land. The sand was deposited by suction dredgers, whose long discharge pipes passed over the core. The finished dike is rather more than 200 feet wide at the base and its highest point is a ridge 6 ft. 6 in. wide. On the side next to the lake the ridge slopes down to the bottom of the water and the face is protected by mattresses of brushwood below the water and by stone pitching above it. On the polder side of the dam the ridge slopes down to a flat 32 feet wide and about 6 feet lower down. This flat carries a road, and thence the dike slopes down to the ground.
On one side, then, the ridge, which is now grass-grown, being finished with ordinary clay, runs at a height of rather more than 13 feet above the water level of the lake. On the other side it is above the newly made land by amounts ranging from 25 feet to 30 feet.
The closing of the gap between the north and south sections of the dike was the most difficult part of the work. The polder just before the closure resembled a lake from which a narrow river flowed. When the tide in the Zuider Zee was ebbing, the water ran out from, and when it was flowing ran into, the lake. The difference in water level on the opposite sides of the dike was as much as 16 in. at certain states of the tide and velocities of over 9 feet a second were recorded.
Such a rush of water made it difficult to keep the floating plant in place. The grab cranes, one on either side of the gap, had to be strongly moored, but moorings were out of the question for the barges, as they were emptied in a few minutes ; so they had to be kept in position by tugs pushing against them. The work as a whole did not proceed without unexpected difficulties cropping up, but the Dutch engineer accepts such troubles merely as obstacles to be overcome. The eleven miles of dike to-day stand firmly from shore to shore and they withstood successfully the tides and storms of the Zuider Zee until the great main dam was completed.
The new land in the polder, 48,000 acres of it, that is, more than half the area of the Isle of Wight, is not continuous with the old coastline, but the division amounts to no more than a canal. This, as its technical name, catchwater drain, implies, is for the purpose of taking away the water which formerly flowed into the Zuider Zee. The channel runs all along the coast to the storage lake at the west end of Wieringen; the storage lake communicates with the North Sea by another canal and sluices. In stormy weather it may be impossible for days together to open the sluices, and as such weather is generally accompanied by heavy rainfall, the flow into the catchwater drain is increased. The storage lake receives this water until such time as it is possible for the sluices to be opened again.
BACKING THE MAIN DAM with sand and facing it with stone. The narrow part near the top of the picture shows a point of recent closure. The North Sea is on the left and the former Zuider Zee, now the Ijselmeer, on the right.
While the dike of the north-west polder was growing up from the sea bottom, work was steadily going on with the great main dam between the east end of Wieringen and Friesland. This eighteen and a half miles of dam had to cross a sea bottom over 33 feet deep in places. Fortunately, however, near the centre of the span there was a shallower area, the Breezand Shoal, over which the water was only about 16 feet deep. Here was built an island about half a mile long and having harbours on its North Sea and Zuider Zee sides, for the protection of the floating plant and to accommodate the vessels bringing the stone and brushwood which were stored in immense quantities on the island.
The island formed part of the dam, and was extended simultaneously in westerly and easterly directions to meet the lengths which were being built out from the shores. The shore ends of the dam were not, however, plain, straightforward work. A good deal of subsidiary diking had to be built to contain the great sluices and locks by which the dam is pierced at these points. These dikes were, in effect, gigantic cofferdams from which the water had to be pumped out to enable the locks and sluices to be built. This alone was a remarkable feat and would have been looked upon in many other countries as something worth advertising, but it was taken as a matter of course in the Netherlands. The dikes were levelled and dredged away as soon as the sluices and locks had been finished.
The cross-section of the dam is on a much larger scale than that of the dike for the north-west polder. The part appearing above the water is about 285 feet wide. On the side of the North Sea a sloping bank rises to nearly 24 feet above the mean water level; behind this bank, on the Ijselmeer side, there is a flat, or berm, to give it its technical name, about 13 feet above water level. The berm carries a railway, a road, a footpath and a cycle track. The method of construction was generally the same as for the dike, namely, a core of boulder clay backed with sand covered with ordinary clay. The seaward side is pitched with basalt blocks in the vicinity of the tide range, and the toe is protected by brushwood mattresses held down by stone.
884,500,000 Gallons a Day
Special precautions had, however, to be taken where the dam crossed the deeps, the initial layers of boulder clay being protected by brushwood mattresses, each 350 feet long and 80 feet wide, pegged down by wooden stakes and weighted with stone. Many difficulties were encountered in building this immense barrier, but all were surmounted. Three gaps were left and were closed at different times. It had been prophesied that the last gap would defy all efforts, but it was filled in to the accompaniment of a triumphant blowing of all the ships’ sirens, on May 28, 1932.
The sluices themselves are another example of successful large-scale work. There are fifteen of them at the Wieringen end of the dam close to the land, and ten in the dam about two and a half miles from the Frisian coast, where the water is shallow near
the land. At this end there are two locks, one for vessels up to 465 feet long, and the other for vessels up to 350 feet long. There is only one lock at the Wieringen end, but it is divided transversely by gates so that it can be used for vessels of small size in one or other of the compartments or can be used as a whole, in which event it will accommodate ships of the same size as the large lock at the other end. The railway and road cross the locks by swing bridges. The main large-sized traffic through the locks will be from and to Amsterdam.
THE NORTH-WEST POLDER BEING PUMPED DRY. The town in the background is Medemblik, once engaged in a flourishing sea trade with the East Indies. The dam in the centre runs for eleven miles to the east end of Wieringen. To the left is the Zuider Zee, to the right the reclaimed north-west polder, which the pumping station on the dam is rapidly uncovering.
The north-west polder lies far below the level of the water in the Ijselmeer. When, therefore, the dike had been completed the water could not drain away. This immense volume had thus to be pumped out. Two pumping stations were erected, in cofferdam enclosures, one at Medemblik and the other at Wieringen. The first has electrically driven centrifugal pumps and the second has the same type of pump driven by diesel engines. Both stations are permanent structures and are normally used for keeping the reclaimed polder free from rainwater accumulations and the like; but for the emptying process special impellers were fitted to the pumps to cope with the greater volume to be handled. This pumping was carried out at the rate of 884,500,000 gallons a day, which reduced the water level by rather more than three-quarters of an inch every twenty-four hours. It was a long job, but was carried out without a hitch. Two years afterwards good crops of rye and other grains were being reaped from what ancient proverb stigmatizes as worthless — sea sand.
The question arose as to whether the reclamation scheme was not something of a gamble, as the land, before reclamation, was invisible. But the Dutchman takes no chances. An experimental polder of about 100 acres had been formed at the village of Andijk, on the coast a few miles south-east of Medemblik, and here the plan was tried out. One of the problems to be solved was the most economical and most effective way of getting the salt out of the land. Then there were experiments to determine the best methods of drainage and cultivation and the kinds of crops that would thrive best at the start.
By the time the dike of the northwest polder had been completed there were growing on the Andijk polder flowers, grass, cabbages, beans, rye, maize and many other plants. By 1937 the north-west polder was already growing, in parts, flourishing crops. It is now being divided into economic holdings. The land, which consists of 80 per cent of rich clay, 10 per cent of light clay and 10 per cent of sand, is gradually being worked into condition by drainage and by the use of artificial fertilizers, and it is estimated that by 1940 it will be in full production.
The second polder, that to the northeast, is being enclosed, and geologists have ascertained the nature of the land that will be available, which is stated to be even better suited for cultivation than that of the north-west polder.
Inland Seaport
The area of the north-east polder is more than twice that of the northwest polder. The north-east polder embraces the island of Urk, on which a fishing community has for centuries lived in a great measure of isolation. This island will become a hill. Then will come the turn of the two remaining polders, that to the south-east being nearly five times as big as the northwest polder, and that to the south-west being rather larger than the north-east polder.
The south-west polder also will include an island, the island of Marken, and a famous old seaport, Hoorn, will be far inland. Cape Horn, at the tip of the South American continent, was named after this town by the doughty navigator Willem Schouten, who discovered the passage to the Pacific round the cape (1616). In Hoorn, too, was born J. P. Coen, a pioneer in the Netherlands East Indies. Again, within sight of the town, on waters beneath which now lies the future south west polder, the Spanish fleet was roundly beaten by a small collection of Dutch vessels in 1573, when Holland was struggling to attain her independence.
THE LAST GAP in the main dam which separates the North Sea from what was formerly the Zuider Zee. This photograph was taken two days before the gap was closed. A strong current flowing through the gap with the ebb and flow of tides caused grave difficulties, particularly with barges and floating cranes. The cranes were strongly moored, but the barges had to be kept in place by tugs.
When the whole scheme is complete Holland will have added to her land an area approximately equal to the county of Warwickshire. From this new land it is likely that a population of 300,000 will be able to gain a comfortable living. But this is not all. Great Britain spends millions in creating reservoirs, but Holland, in the Ijselmeer, has one ready made. The great freshwater lake will provide an inexhaustible supply of drinking water for cattle in those districts which formerly suffered from shortage in times of drought. Again, the lake being tideless, there will be no risk of flooding in those areas from heavy rainfall or from the melting snows of winter.
It is considered, too, that the upkeep of the dikes, of which there have always had to be many on the several coasts, will be less than it is now, as the tideless fresh water will have a less destructive action than the ebbing and flowing sea it displaces. Another advantage of the scheme is that the railway running across the main dam improves the access to Friesland and shortens the route to Northern Germany.
The reclamation has, however, given rise to certain problems which, though ultimately certain of satisfactory solution, are not without drawbacks. These problems are chiefly connected with the fishing industry. The water of the enclosed lake has now become almost wholly fresh and this change is not appreciated by all its occupants. The anchovy has virtually disappeared, the herring nearly so, and the eel is changing its habits. The eel fry comes from the Atlantic deeps and finds its way to fresh water to mature. The canals of Holland provided it with a home in which it could grow and from which it was easily removed when wanted for food. Now, finding the main body of water fresh it is content to stay there and is more elusive.
Another change not welcome is the increase of the mosquito, which needs fresh water in which to breed. It is possible, however, that this will prove to be only a temporary nuisance, which will be mitigated when the lake becomes populated with the small fish which feed on the larvae. Altogether the changes in aquatic life are of great interest, especially to the naturalist, who has never before been witness of such a large-scale experiment in acclimatization. As fresh water freezes more readily than salt, the facilities for skating are likely to be greatly extended, and the icebreaker may have to be called in for the traffic between Amsterdam and the open sea.
Enhanced Economic Value
During the winter of 1928, even before the closing of the main dam, the Ijselmeer was frozen sufficiently hard to bear motor cars: one Sunday there were 300 cars outside the harbour of Urk. No car had ever been seen before on this solitary island.
Into the question of the cost of this gigantic work it is impossible to go at the present time, but one thing is certain, the money will not be wasted. A large proportion will be expended in wages and, in any event, the investment is almost certain to prove well worth the making.
Figures give some idea of the amount of employment created. It is estimated that work on the north-east polder will take about four years, and that the number of men employed, directly and indirectly, will be 6,500 for the first year, 8,500 for the second, 10,000 for the third and 8,000 for the fourth.
Another eleven years will be required for the completion of the south-east and south-west polders. During the last year of this work some 15,000 men will be needed.
The economic value of the reclaimed area will be enormous. Holland has to import most of her raw material and is, therefore, not suitable for general industrialization. On the other hand, by increasing agricultural production she can better support her dense population. The total area of the country will be increased by 7 per cent and the cultivated area by 10 per cent; for there are in the country over a million acres of waste land unsuitable for treatment. The reclaimed land, as far as can be seen, is likely to be all cultivable, and it is therefore a valuable asset.
The engineer, in thus adding to his country’s resources, has destroyed nothing and his work in this historic land will be well worth travelling far to see.
BUILDING THE SLUICES AND LOCKS which afford outlet for navigation and for excess water from the Ijselmeer to the North Sea. For purposes of construction subsidiary dikes, arranged in the same way as a vast cofferdam, were built to enclose the area, which could then be pumped dry to allow work to proceed unhindered. The dredger in the foreground is beginning to cut away these temporary dikes.
Click here to view the photogravure supplement to this chapter.
