thames barrier

The Thames Barrier is a flood control structure on the River Thames at Woolwich Reach in London. It is the world's largest movable flood barrier. Built across a 523 m wide stretch of the river the barrier divides the river into six channels between nine large concrete piers. The flood gates across the openings are radial i.e. half-cylindrical, and they operate by rotating, rising by hydraulics out of a horizontal sill below the water to form the barrier, they can rotate further to allow for 'underspill' for maintenance. All the gates are made of steel, and the four large central gates are 61 metres long, 10.5 metres high and weigh 1,500 tonnes. The outer two gates are ???. London is vulnerable to flooding. The threat has increased over time due to the slow but continuous rise in high water level over the centuries (75 cm / 100 years) and the slow 'tilting' of Britain (up in the north and down in the south) caused by post-glacial rebound. This general rise in potential water levels combined with the tidal conditions of the Thames and with particularly severe weather conditions can create serious flood conditions - surge tides. After 307 people died in the UK in the North Sea Flood of 1953 the issue gained new prominence. Early proposals for a flood control system were stymied by the need for a large opening in the barrier to allow for vessels from London Docks to pass through. When containerization came in and a new port was opened at Tilbury a smaller barrier became feasible. The barrier was designed by Rendel, Palmer and Tritton for the GLC, the concept of the rotating gates was devised by Charles Draper. The site at Woolwich was chosen because of the relative straightness of the banks, and because the underlying river rock was strong enough to support the barrier. Work began at the barrier site in 1974 and construction was largely complete by 1982. In addition to the barrier itself the flood defences for 11 miles down river were raised and strengthened. The barrier was officially opened on May 8, 1984. Total construction cost was around 500 m with an additional 100 m for river defenses. The barrier was designed to cope with sea level rises until around 2030. Based on current estimates, it is expected to serve its full term. Since 1982 the barrier has been raised over 70 times; further, it is always raised every month for testing. The barrier was operated by the National Rivers Authority until April 1996 when it passed to the Environment Agency. In 2005, a suggestion that it might become necessary to supersede the Thames Barrier with a much more ambitous ten mile long barrier across the Thames Estuary from Sheerness in Kent to Southend in Essex was made public.

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