The Afsluitdijk (Dutch: [ˈɑfslœydɛik] ; West Frisian: Ofslútdyk [ˈɔfslyːdik]; "closure dyke") is a major dam and causeway in the Netherlands. It was constructed between 1927 and 1932 and runs from Den Oever 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 metres (300 ft), at an initial height above Amsterdam Ordnance Datum of between 6.7 metres (22 ft) along the section at Friesland, and 7.4 metres (24 ft) where it crosses the deep channel of the Vlieter. The height at the greater sea depths west of Friesland was required to be a minimum of 7 metres everywhere when originally constructed.
Increases to the height of the Afsluitdijk have been made several times since 1958 as part of regular maintenance since the deadly North Sea Flood of 1953. The section between the Stevinsluizen and Lorentzsluizen sluice complexes was raised to 7.8 metres. Major upgrade works commenced in 2019, with one of the design conditions being that only limited wave overtopping during the design storm condition be permitted. The design proposed by the successful contractor to meet this criterion will see a further increase in the height of the dam, by approximately 2 metres.
The Afsluitdijk 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. It is a major land reclamation project and provides a road connection between the North and West of the Netherlands.
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.
Wieringen was connected to the mainland with the short Amsteldiepdijk in 1925; the Afsluitdijk would be 32 kilometres (20 miles) 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 23 million m3 (810 million cu ft) of sand and 13.5 million m3 (480 million cu ft) 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
The height of the crest of the Afsluitdijk was originally determined based on insufficient data about wave run-up. This became apparent soon after the first significant storm surge following the completion of the works, which occurred in December 1936. During the storm, the water in the Wadden Sea reached to around half a metre below the dike. For the crown height, storm surge heights indicated by the Staatscommissie Lorentz (SCL) (English: Lorentz State Commission) were primarily used, based on storm surges that had occurred in the period 1825–1926.
According to committee's predictions, if the Zuiderzee had been closed during the 1825–1926 period, the highest water level reached would have been 3.1 metres above Amsterdam Ordnance Datum at Den Oever, and about 3.4 metres on the Friesland coast. However, the commission drew attention to the fact that in a very rare and unfavourable set of circumstances, even higher storm surge heights were possible.
All values in centimetres. Z and A are in centimetres above NAP (Normaal Amsterdams Peil/Amsterdam Ordnance Datum).
The lower value was based on the assumption that wave run-up in the shallow area near the Frisian coast would be reduced. However, it became evident as early as 1934 that this assumption was incorrect due to the narrowness of the shallow zone. To address this issue, a section of the dike body was removed. Despite these adjustments, a 1937 storm revealed that they were insufficient, leading to an increase in the crest height to 7.4 metres.