Reasons for construction The Afsluitdijk (literally translated: shut-off-dyke) was completed in 1932, thereby shutting off the
Zuiderzee (lit: Southern Sea) from the
North Sea. Until then, the Zuiderzee had been a large bay south of the North Sea which gave maritime access to five provinces of The Netherlands, and particularly during the
Dutch Golden Age provided a protected entrance and exit for the harbour of
Amsterdam and several other important Dutch sea harbours. Furthermore, the Zuiderzee provided relatively shallow and calm fishing grounds for the many towns bordering the bay. However, the opening of the
North Sea Canal (Noordzeekanaal) in 1876 gave a much shorter direct entrance to the Amsterdam harbour, and overfishing had depleted the shallow bay. In the second half of the 19th century, the Dutch population was exploding, and there was an increasing need for land for agriculture and animal husbandry. The Dutch already had centuries of experience of building dykes around lakes, emptying them and converting them to fertile
polderland. The next large project was to convert the Zuiderzee into polder. In 1886, a few notables established the Zuiderzee Society to investigate whether reclamation was feasible. One of the most prominent members of the society was
Cornelis Lely, a
civil engineer, prominent member and later chairman of the society. In 1891 he designed the first plan for the closure and reclamation of the Zuiderzee. In 1913, Lely was Minister of Water Management, and land reclamation was included in the government program. His plan was initially opposed, particularly for its huge costs and also by the fishing industry. But after the
flood of 1916 and the
famine of 1918, opinions were ready for this mega project and the parliament agreed, but it took another 9 years, until 1927, before the works actually began.
Building the dyke Wieringen was connected to the mainland with the short
Amsteldiepdijk in 1925; the '''' would be in length. The inland side is heavy stone; the seaward side is
boulder clay with
brushwood mattresses above, weighed down by
basalt boulders and old concrete. Previous experience had shown that boulder clay was superior to just sand or clay 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
dyke. From these points, the dyke slowly grew by ships depositing till into the open sea until it breached the surface. The nascent dyke was then strengthened from land by basalt rocks and mats of
willow switch at its base. The dyke could then be finished off by raising it further with sand and finally clay for the surface of the dyke, on which grass was planted. As the dyke grew, physicist
Hendrik Antoon Lorentz calculated the force of the tide as the smaller gap made it stronger. Ten thousand workers, 27 large dredges, 13 floating cranes, 132 barges, and 88 tugs worked on the project at the end, timed to close the dyke at low tide; it was finished on 28 May 1932. Construction progressed better than expected; at three points along the line of the dyke 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 dyke, but all of them proved to be relatively straightforward. 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 dyke itself however was not finished 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
Willem Marinus Dudok marking the spot where the dyke had been closed. The amount of material used is estimated at of sand and 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. Besides the dyke itself, there was also the necessary construction of two complexes of shipping
locks and discharge
sluices at both ends of the dyke. The complex at Den Oever includes the
Stevin lock (named after the son of
mathematician and
engineer Simon Stevin, Hendrik Stevin, who was the first making a plan to close the Zuiderzee in 1667) 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 the physicist) 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. == Post-construction developments ==