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The Afsluitdijk (Dutch pronunciation: [ˈɑf.slœyd.ˌdɛik], Frisian: Ofslútdyk; English: Enclosure Dam) is a major causeway in the Netherlands, constructed between 1927 and 1933 and running from Den Oever on Wieringen in North Holland province, to the village of Zurich in Friesland province, over a length of 32 kilometres (20 mi) and a width of 90 m, at an initial height of 7.25 m above sea-level.
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. The Afsluitdijk was the initial demonstration site for a 130 km/h speed limit in the Netherlands.
Previous experiences had demonstrated that 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 Breezanddijk) 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 dikes 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 28 May 1932, two years earlier than initially thought, the Zuiderzee ceased to be, as the last tidal trench, 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 was not 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. On 25 September 1933, the Afsluitdijk was officially opened, with a monument designed by architect Dudok 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 four to five thousand workers were involved with the construction every day, relieving some of the unemployment following the Great Depression.
Beside the dike 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 (named after Hendrik Stevin, a son of mathematician and engineer Simon Stevin) 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 (named after Hendrik Lorentz, the famous physicist, who personally did the calculations of the tides that were crucial to the construction of the Afsluitdijk) and two series of five sluices, making a total of 25 discharge sluices. It is necessary to routinely discharge water from the lake 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.
In 2012, the State Secretary for Infrastructure and the Environment, Joop Atsma, issued a statement detailing the factors influencing the decision to replace the locks at Kornwerderzand. A naviduct would have a high construction cost compared with standard locks and bridges, but would benefit the economy by reducing delays. After the cost–benefit analysis of all potential solutions was considered, a decision was made to strengthen the existing locks. However, it is quite possible that a naviduct will be built to replace the locks when they reach the end of their useful life in 2050.
Provision for a railway line linking North Holland and Friesland (between Anna Paulowna and Harlingen) had been included on the Afsluitdijk, in the form of a linear reservation, along with extra drawbridge abutments at the locks. However, construction of the line was never undertaken by the NS, for reasons of cost and relative lack of benefits. The reservation and abutments for the rail line were utilized instead for a second carriageway for the dike's two-lane highway in the 1970s, transforming the latter into today's four-lane A7 motorway.
During the period 12–13 May 1940, it was the site of the Battle of the Afsluitdijk.
- "Holland's first 130 kph road is the Afsluitdijk". DutchNews.nl. Tuesday 1 March 2011. Retrieved 2012-02-20. "The speed limit on the Afsluitdijk, the stretch of the A7 on the dyke between Wognum in Noord-Holland and Zürich in Friesland, has been increased to 130 kph from Tuesday."
- Atsma, Joop (3 July 2012). "Toezegging naviduct Afsluitdijk" (in Dutch). Ministerie van Infrastructuur en Milieu. Retrieved 2 August 2013.
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