Thursday, October 2, 2008

Storm Surge Barrier Rotterdam, the Netherlands

More than half the Netherlands lies below sea level, and the little country is protected from flooding by about 750 miles (1,200 kilometers) of dikes. The process of global warming and the consequent rise in sea levels will challenge their adequacy, and many of them will need to be raised and reinforced. The extensive Deltaworks project, completed in 1986, secured the province of Zeeland by sealing off its sea inlets. Its northern neighbor, South Holland, remained under threat. Responses to disastrous floods in 1953 had included plans to raise the dikes in the region, but by the 1970s there was public resistance to a scheme that entailed demolishing many historic precincts. The alternative was the construction of a movable storm surge barrier in the man-made approach to Rotterdam Europoort. It is the busiest harbor in the world, and an average of ten ships pass through the New Waterway every hour. Technological and economic feasibility studies led to the construction of the Storm Surge Barrier, one of the engineering marvels of the late twentieth century. Otherwise known as the Maeslant Kering, it is located between the Hook of Holland and the town of Maassluis, a little under 4 miles (6 kilometers) from the North Sea. Built at a cost of 1 billion guilders (U.S.$500 million), it was opened on 10 May 1997.

In response to the Dutch government’s call for submissions, the Bouwkombinatie Maeslant Kering consortium’s tender was accepted from among six competitors. Contracts were signed in October 1989, and the first pile for the hinge foundation was driven in November 1991. The barrier has a guaranteed life of 100 years. It consists of a pair of 50-foot-thick (15-meter) hollow, arc-shaped steel gates, each 73 feet (22 meters) high and 700 feet (210 meters) long and weighing 16,500 tons (15,000 tonnes). Each is attached by means of 795-foot-long (238-meter) latticed steel arms to a steel ball joint seated in a massive concrete socket on the riverbank. The 33-foot-diameter (10-meter) ball joints each weigh 760 tons (690 tonnes) and work with a tolerance of 0.04 inch (1 millimeter). The figures are almost meaningless, but in terms of comparative size, each half of the barrier—the gate, the two three-dimensional trusses, and one ball joint—weighs as much as two Eiffel Towers.

Normally, the gates are “parked” in docks in the banks. If a water surge of 10 feet (3.2 meters) above a set acceptable maximum is anticipated, a central computer instructs the automated control system to activate the barrier, and water is pumped into the parking docks. When the hollow gates start to float and the water level in the dock reaches that in the New Waterway, the dock gates are opened. The “locomobiles”—their name explains the function—on top of the gates push them horizontally out of the dock into the Waterway. The gates meet in the middle, not quite touching. They are then flooded and slowly sink to the concrete sill on the bottom of the Waterway, 56 feet (17 meters) down. The ball joints move in differrent directions following the gates’ movements: horizontally (when the gates are floated out) and vertically (upon submersion). The gates must be able to ride with the waves when being closed and opened. In a fierce storm, the water could hit the barrier with up to 33,000 tons (30,000 tonnes) force. The loads on the structure are transmitted to the 57,000-ton (52,000-tonne) triangular concrete foundations of the ball joints. After the storm the gates are floated again and driven back into the dock by the locomobiles. The dock gates are closed, and the dock is pumped dry. From the time the computer registers the need to close the barrier until the gates are in place, the operation takes nine and a half hours. After the storm, it takes two and a half hours to return the gates to their docks.

It is expected that the barrier will have to be closed (on average) once every ten years, but changes in sea levels over the next half-century may double that. Seawater can enter the Europoort area freely through other waterways, and a supplementary dike-reinforcement program is being implemented, with a further defense known as the Europoort Barrier acting to support that in the New Waterway.

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