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Built across a 523 m 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 - 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' or maintenance. The four large central gates are made of steel and are each 61 metres long, 10.5 metres high and weigh 1,500 tonnes.
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 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 300 people died in flooding in 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, 1984This page is about the year 1984. For other uses of 1984, see 1984 (disambiguation). 1984 is a leap year starting on Sunday (link shows calendar). Events January January 1 Brunei becomes a fully independent state January 1 AT&T is broken up into 22 indepe. 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. Since 1982 the barrier has been raised over 70 times; further, it is always raised every month for testing. The barrier operations was operated by the National Rivers AuthorityThe National Rivers Authority (NRA) was the forerunner of the UK Environment Agency, existing between 1989 and 1996. It was created in 1989 when the 10 Water Authorities in England and Wales were privatised by flotation on the stock market. The NRA took o until April 1996 when it passed to the Environment Agency.