Delta Works
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In the North Sea flood of 1953, breaks in the dikes and seawalls in the Netherlands killed 1,835 people and forced the evacuation of 70,000 more. 10,000 animals drowned, and 4,500 buildings were destroyed. To prevent such a tragedy from happening again, an ambitious flood defense system was conceived and deployed, called the Delta Works (Dutch: Deltawerken).
The Dutch government often cites the Delta Works project as the world's largest flood protection project. With over 10,250 miles of levees (1,500 miles designated as primary levees and 8,750 miles as secondary levees) and 300 structures, the project is one of the most extensive engineering projects in the world. The project isolated 13 estuaries from the ocean. Approximately 1,650 sq. km. of land were reclaimed from the sea by the project.
This project was intended to improve the safety of the lower areas of the Netherlands against severe storms and flooding; since more than one third of the nation's land lies below sea level, this is no simple task. Dunes along the entire seashore were raised by as much as 5 metres, while the islands in Zeeland province were joined together by dams and other large scale constructions to shorten the coastline. The most sophisticated and famous of these dams is the Oosterscheldekering (left of the leftmost "i" on the satellite image), which can be opened and closed to keep the sea at bay while preserving the saltwater river delta for wildlife and the fishing industry. Less known, but no less impressive, is the Maeslantkering storm surge barrier near the port of Rotterdam.
[edit] The Maeslant Barrier (1997)
The construction of the Maeslantkering was a part of the "Europoortkering"-project which, in turn, was the final stage of the Delta Works. The main objective of this Europoortkering-project was improving the safety against flooding of the Rotterdam harbour, of which the Europoort is an important part, and the surrounding towns and agricultural areas. This had to by carried out by the reinforcement of existing dikes as far as 50 kilometers inland. During the 1980s it became clear that this project would take at least 30 years and would cost a huge amount of money. It would also mean that historic town centres, sometimes built more than four centuries ago, had to be broken down and rebuilt behind renewed, larger dikes. This storm surge barrier had to be located in the Nieuwe Waterweg waterway. This played an important role in the planning stage of the construction, as this waterway is the main route to the port of Rotterdam, the world's second largest port. Therefore a barrier like the Dutch Oosterscheldekering and the Thames Barrier could not be constructed, as such a barrier would block the shipping route. The construction of the barrier started in 1991. First the dry docks were constructed on both shores and a sill was constructed at the bottom of the Nieuwe Waterweg. Then the two 22 meter high and 210 meter long steel gates were built. After this, 237 meter long steel trusses were welded to the gates. Standing upright, these arms would be as high as the Paris Eiffel Tower, but each one weighs four times more than the Eiffel Tower. The main purpose of the arms is transmitting the immense forces, exerted on the gates while closed, to one single joint at the rear of each gate. During the closing or opening process, this ball shaped joint gives the gate the opportunity to move freely under the influences of water, wind and waves. It acts like the ball shaped joint between a human’s arm and shoulder. After six years of construction the storm surge barrier was finished. At May 10, 1997, Queen Beatrix opened the Maeslantkering. From that moment the barrier was in full operation. The barrier is connected to a self-operating computer system which is linked to weather and sea level data. When a storm surge of 3.00 meter above normal sea level is anticipated in Rotterdam the barrier will be closed automatically. Four hours before the actual closing procedure will begin, in and out going ships are warned. Two hours before closing the traffic at the Nieuwe Waterweg will come to a stand still. Thirty minutes before closing the dry docks that bear the gates are inundated. After this the gates start to float and two so called "locomobiles" will move both the gates towards each other. When the gap between the gates is about 1.5 meter wide, the gates are submerged to the bottom of the waterway, thus protecting the inland. Under normal weather conditions the two doors themselves are well protected in their dry docks and a 360 meter wide gap in the waterway gives ships enough space to pass without any inconvenience. The Maeslantkering is expected to be closed once every ten years due to a storm surge. With the rise in sea levels the storm surge barrier will need to close more frequently in 50 years time, namely once every five years. Until this date the barrier had never been closed due to a storm. The barrier is closed for testing once a year, usually in September or October, just before the beginning of the storm season in November.
[edit] Oosterscheldekering (1987)
The Oosterscheldekering (Eastern Scheldt storm surge barrier), between the islands Schouwen-Duiveland and Noord-Beveland, is the largest of 13 ambitious Delta works series of dams, designed to protect a large part of the Netherlands from flooding. The construction of the Delta Works was a response to the North Sea Flood of 1953.
Image:Oosterscheldekering, Netherlands.JPG Image:Oosterscheldekering, detail, Netherlands.JPG The nine kilometer-long Oosterscheldekering (kering meaning barrier) was initially designed, and partly built, as a closed dam, but after public protest huge sluice-gate-type doors were installed in the remaining four kilometers. These doors are normally open, but can be closed under adverse weather conditions. In this way the saltwater marine life behind the dam is preserved and fishing can continue, while the land behind the dam is safe from the water.
On 4 October 1986 Queen Beatrix officially opened the dam for use.
At the artificial island Neeltje-Jans, at one end of the barrier, a plaque is installed with the words: "Hier gaan over het tij, de wind, de maan en wij" ("here the tide is ruled, by the wind, the moon and us").
The Oosterscheldekering was the biggest, most difficult to build and the most expensive part of the Delta works. Work on the dam itself started in April 1976 and was completed in June 1986, though the road over the dam was completed only in November 1987. To facilitate the building, an artificial island Neeltje-Jans was first created in the middle of the estuary.
Four ships were custom designed and built for this project:
- Mytilus, a ship equipped with various groundworking tools, such as needles to make the seabed more dense and stable.
- Cardium, a ship to transport and lay a special foil carpet on the seabed for the pillars to rest on.
- Ostrea, a ship capable of lifting a concrete pillar from the dry dock and placing the pillar within a tiny margin on a special foil on the seabed. The ship is 85 meters long and has a portal of 50 meters high. The ship can lift 10000 tonnes, the pillars are under water for a large part and require less lifting power than the 18000 tonnes. This ship is considered the flagship of the construction fleet mainly because of its impressive size and power.
- Macoma, a ship that works closely with the Ostrea, cleaning the foil and helping in navigating the pillar to the final place.
The ships are named after various types of shellfish.
The dam is constructed out of 65 concrete pillars with 62 steel doors of 42 meters wide. The parts were constructed in a dry dock. The area was flooded and a small fleet of special construction ships would lift the pillars and place them on their final place. Each pillar is between 35 and 38,75 meter high and weighs 18000 tonnes.
The Oosterscheldekering is sometimes referred to as the eighth Wonder of the World, and has been declared one of the modern Seven Wonders of the World by the American Society of Civil Engineers.
The dam was designed to last 200 years.
The dam is operated by humans but if human controls fail an electronic security system acts as a backup. A Dutch law regulates the conditions under which the dam is allowed to close. The water levels must be at least 3 meters above regular sea level before the doors can be completely shut. For testing, all doors are shut once in a while (each door once per month). Also emergency procedures are tested on prescheduled dates. Once the test is passed, the shutters are immediately opened again to create a minimum impact on tidal movements and the marine ecosystem. It takes approximately one hour to close one of the doors.
The dam has been closed 23 times since 1986, due to water levels exceeding or being predicted to exceed the 3.00 meters.
The cost of operation is 17 million Euro per year.
[edit] External links
[edit] See also
de:Deltawerke es:Plan Delta fi:Deltawerken fr:Plan Delta ka:დელტაპროექტი nl:Deltawerken no:Deltaprosjektet sv:Deltaprojektet
