Monday, July 28, 2008

Deltaworks , The Netherlands

The Deltaworks comprises a series of audacious engineering projects that effectively shorten the coastline of the southwest Netherlands by about 440 miles (700 kilometers), seal outlets to the sea, and reinforce the country’s water defenses. Taking more than forty years to complete, the works involved the construction of huge primary dams totaling 20 miles (30 kilometers) in length, in four sea inlets between the Western Scheldt and the New Waterway, Rotterdam.

The Netherlands is located in the broad deltas of the Rhine, Maas, and Scheldt, and the small country’s history and geography have been greatly influenced by a continuous struggle against the rivers and the sea. Through the coincidence of several events in 1953, the southwestern provinces suffered huge floods in which nearly 2,000 people died and thousands of homes were destroyed. The central government quickly reacted, and the Ministry of Transport, Public Works, and Water Management set up the Delta Committee to devise measures to avert a future disaster. The plan informed the Delta Act of 1958, but its implementation, placed in the hands of a complex instrumentality known as Delta Service, took over four decades to complete.

The major elements of the plan were achieved in the following order: the Hollandse IJssel storm flood barrier (1954–1958), the Zandkreekdam (1957–1960); the Veerse Gatdam (1958–1961); the Grevelingendam (1958–1965); the Volkerakdam (1955–1977); the Haringvlietdam (1956–1972); the Brouwersdam (1963–1972); and the Oosterschelde storm flood barrier (1967–1986). The vast scope of the Deltaworks cannot be fully described here, but it may be measured by a brief overview of the largest, most difficult, and most expensive phase: the Oosterschelde (Eastern Scheldt) storm, flood barrier, immodestly referred to by its builders as “the eighth world wonder.”

It was originally intended to close off the Oosterschelde with a permanent dam, and work started in 1967. By 1973 joining das between parts of the coast had closed 3 miles (4.8 kilometers)—more than half—of the river mouth, and three sluices had been built. Then, in response to public protests, it was decided to construct a storm flood barrier instead of completely closing the estuary. Huge concrete pylons standing on the river bottom would support gates that could close to resist storm surges; a concrete roadway would cross the structure. The government signed a contract with the consortium De Oosterschelde Stormvloedkering Bouwkombinatie in 1977. A 3,000-yard-long (2.78-kilometer) access bridge was built to the 50-foot-deep (15-meter) construction docks needed to fabricate the massive pylons. Commenced in April 1979, the first was finished early in 1983. In the meantime, work began on the sliding gates. Fifty-foot-deep foundations were prepared to support the pylons, and a special dredge was designed to secure the estuary floor against uneven scouring. By the end of 1982, the river bottom was secured by vast mats laid by purpose-designed vessels. All was ready for placing the pylons.

The construction docks were flooded and the pylons, each weighing 21,600 tons (18,300 tonnes) and between 100 and 135 feet (30 and 40 meters) high, were floated into position, then sunk to the prepared floor. Sixty-five pylons formed the spine of the barrier: sixteen in the northern opening, seventeen in the central, and thirty-two in the southern. They were connected by prefabricated elements, and the sliding gates, each 150 feet (45 meters) long and weighing 1,440 tons (1,220 tonnes), were then installed, a task that took a little under two years to complete. Then followed the fixing of each of the sixty-two 3,000-ton (2,270-tonne) precast concrete elements that carried the roadway across the barrier. The Stormvloedkering Oosterschelde was officially opened on 4 October 1986. It cost about a sixth of the 11 billion guilder (U.S.$5.5 billion) total of the Deltaworks.

The danger of overflowing rivers in the winter and early spring also threatens large parts of the Netherlands. Several inland engineering works—the Philipsdam (1976–1987); the Oesterdam (1977–1988); the Markiezaatskade (1980–1983); and the Bathse Spuikanaal and Spuisluis (1980–1987)—were adjuncts to the primary dams of the Deltaworks.

Holland’s struggle against the water continues. Despite the pleas of regional and local water authorities for river dike reinforcement, the national government concentrated its funding for forty years upon the Deltaworks. Moreover, conservationists oppose any dike improvements that would spoil the landscape. The Boertien Commission was established early in the 1990s to address potential problems, and it produced the Great Rivers Delta Plan, which involved reinforcing nearly 190 miles (300 kilometers) of river dikes and embankments. The first phase was completed by the end of 1996; the second, covering another 280 miles (450 kilometers), was finished by 2001. But that will not solve the problem; if nothing else is done, the next generation of Hollanders will have to raise the dikes again. Climate changes, deforestation, urbanization, and drainage in their upper reaches mean that the river systems will carry increasingly large peak volumes. Cooperative policy and water management must be integrated internationally, from the sources to the deltas.

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