Zuiderzee Becomes IJsselmeer
A new study, commissioned after doubts arose over the financial feasibility of the project, recommended that work should not only continue, but should be accelerated. It was therefore decided to start the next two major projects at the same time, in 1927. The most important of these would be the main dam, the Afsluitdijk (Enclosure Dam), running from Den Oever on Wieringen to the village of Zurich in Friesland. It was to be 32 km long and 90 meters wide, rising to 7.25 meters above sea-level, with an incline of 25% on each side. Incidentally, the word "dijk" is inappropriate. A "dijk" (dike) is to secure land. A "dam" is to separate water levels.
Experience showed that till (boulder clay), rather than just sand or clay, was the best primary material for a structure like the Afsluitdijk. An added benefit was that it was easily available; it could be retrieved in large quantities by simply dredging it from the bottom of the Zuiderzee. Work started at four points: on both sides of the mainland and on two purpose-made construction-islands (Kornwerderzand and Breezanddijk) along the line of the future dike.
From these points the dike grew as ships deposited till into the open sea in two parallel lines. Sand was then poured between the two dams and as this emerged above the surface it was covered by another layer of till. The nascent dike was then strengthened with basalt rocks and mats of willow switch at its base. The dike was finished by raising it further with sand and finally clay for the upper surface of the dike, which was planted with grass.
Construction progressed better than expected. At three points along the line of the dike there were deeper underwater trenches where the tidal current was much stronger than elsewhere. These had been considered major obstacles to completing the dike, but proved not to be so. On May 28, 1932, two years earlier than forecast, the Zuiderzee went out of existence when the last tidal trench of the Vlieter was closed by a bucket of till. The IJsselmeer was born, even though it was still salty.
The dike was not finished. It still needed to be brought up to its full height, and a road linking Friesland and North Holland remained to be built. Beside the dam itself, there was also the construction of shipping locks and discharge sluices at the ends of the dike. The complex at Den Oever includes the Stevin lock and three series of five sluices for discharging the IJsselmeer into the Wadden Sea; the other complex at Kornwerderzand is composed of the Lorentz locks and two series of five sluices, making a total of 25 discharge sluices. Periodically discharging the lake is necessary since it is continually fed by rivers and streams (most notably the IJssel river that gives its name to the lake) and polders draining their water into the IJsselmeer.
The Afsluitdijk was opened September 25, 1933, a monument marking the spot where the dike was finished. It used 23 million m³ of sand and 13.5 million m³ of till. An average of 4,000 to 5,000 workers were employed in the dike's construction, relieving unemployment in the Great Depression. Total cost of the dam was about €700 million (2004 equivalent).
Read more about this topic: Zuiderzee Works