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Afsluitdijk

The Afsluitdijk (Closure-dike) is a major dam in the Netherlands, constructed between 1927 and 1933 and running from Den Oever on Wieringen in North Holland to the village of Zurich (mun. Wûnseradiel) in Friesland over a length of 32 km and a width of 90 m, at an initial height of 7.25 m above sea-level and an incline of 25%. It is a fundamental part of the larger Zuiderzee Works, damming off the Zuiderzee, a salt water inlet of the North Sea and turning it into the fresh water lake of the IJsselmeer.

Previous experiences had demonstrated that till (boulder clay), rather than just sand or clay, was the best primary material for a structure like the Afsluitdijk, with the added benefit that till was in plentiful supply in the area; 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 specially made construction-islands (Kornwerderzand and Breezand) along the line of the future dike.

From these points the dike slowly grew by ships depositing till into the open sea in two parallel lines. Sand was then poured in between the two dams and as it emerged above the surface was then covered by another layer of till. The nascent dike was then strengthened from land by basalt rocks and mats of willow switch at its base. The dike could then be finished off by raising it further with sand and finally clay for the surface of the dike, on which grass was planted.

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 to be major obstacles to completing the dike, but all of them proved to be relatively straightforward. On May 28, 1932, two years earlier than initially thought, the Zuiderzee ceased to be as the last tidal trench of the Vlieter was closed by a final bucket of till. The IJsselmeer was born, even though it was still salty at the time.

The dike itself however wasn't finished yet as it still needed to be brought up to its required height and a road linking Friesland and North Holland (the current A7/E22 motorway) also remained to be built. It wouldn't be until September 25, 1933 that the Afsluitdijk was officially opened, with a monument marking the spot where the dike had been closed. The amount of material used is estimated at 23 million m³ of sand and 13.5 million m³ of till and over the years an average of around 4 to 5000 workers were involved with the construction every day, relieving some of the unemployment following the Great Depression.

Beside the dam itself there was also the necessary construction of two complexes of shipping locks and discharge sluices[?] at both 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.