The Oosterscheldekering, or Stormvloedkering Oosterschelde (Flood Barrier Oosterschelde), is the largest dam in the Delta Works
. It also serves as a nine kilometer long bridge between two of the islands of Zeeland
. Its contruction was started in 1978 and in 1986 Queen Beatrix
opened the dam with the words: "The Delta Works are completed. Zeeland is safe."
Nature or safety?
The Oosterscheldekering was started as a closed dam in 1969, and was to completely shut off the Oosterschelde. Three artificial islands were constructed in the Oosterschelde for that plan: Roggenplaat (1969), Neeltje Jans (1970) and Noordland (1971). While construction had already started, in 1973 a discussion began on whether the dam should be open, so as to leave all nature behind it intact, or closed, for maximum safety. The construction was halted. In true Dutch fashion, eventually a compromise was reached: the dam would be half-open. In that way, in normal situations the dam would be open, allowing the sea in. In harsh weather the dam would be closed, keeping the land behind it safe.
An engineer's wet dream
The building of the Stormvloedkering started again in 1976. In three channels in the Oosterschelde, Hammen, Schaar and Roompot, a total of 65 columns was placed, each 60 meters high. Between these, 62 slides or doors were placed, that can be lowered when high water is expected.
For the construction of the dam, many innovative technologies had to be developed. For a dam like this, that is placed in fast flowing water, stability is very important. Because the sea bottom itself was not stable enough to place the pillars on, a foundation had to be created. A ship, equipped with huge vibtrating needles, compacted the sand to create more stability. Under each pillar two mats were placed, made of cloth filled with gravel. A factory was built on Neeltje Jans to make these mats, and a special ship was constructed to roll out and position the mats.
The pillars themselves were built on Neetje Jans, on dry docks that were filled with water when the pillars were finished. The pillars then stood in 15 meters of water. Special ships were constructed to move them to their place and position them with an accuracy of less than a centimeter on the sea bottom, taking care that no sand came between the pillars and the mats. Finally the foundation was finished with 15 million tons of stone, placed over the feet of the pillars and the mats, that was transported over Neeltje Jans over a period of four years.
The original plans provided for 63 doors and 66 pillars. To reduce costs, one door was removed from the plans. The 66th pillar however was built anyway. It became the Universeel Inzetbare Pijler (Universal Pillar): when anything went wrong with the placing of one of the other pillars, there would be one left to use. It has not been necessary and the UIP is now in use as a climbing object.
The doors of the dam are made of steel. They are each 42 meters long. Their height depends on the local depth of the water and varies between 6 and 12 meters. The doors are moved with a hydraulic system that is controlled from the ir. J.W. Topshuis (ir. J.W. Tops House) on Neeltje Jans.
As mentioned, the doors of the Oosterscheldekering are closed only when necessary. This means that they are closed when the sea level comes above +3.00 NAP, or three meters above Normaal Amsterdams Peil (Amsterdam Ordnance Datum), a measure for water level. The doors are closed from the ir. J.W. Topshuis, where there is also a system to automatically close them at +3.00 NAP when no people are present. Up till the time of writing this, the doors have been closed 22 times.
On Neeltje Jans you can find "Waterland Neeltje Jans", with exhibitions on the Delta Works, the Oosterscheldekering, marine animals and more. From there you can also walk along the Oosterscheldekering and wonder about the sheer size of the thing...
Sources: www.neeltjejans.nl, nl.wikipedia.com.