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The Thames Barrier is a flood control structure on the River Thames, constructed between 1974 and 1984 at Woolwich Reach, London, and first used defensively in 1983. It is the world's second largest movable flood barrier (the largest is the Maeslantkering in The Netherlands).

Located downstream of central London, the barrier's purpose is to prevent London being flooded by an exceptionally high tide moving up from the sea, exacerbated by a storm surge. It only needs to be raised for the duration of the high tide; at low tide it can be opened to release water flowing down the Thames which backs up behind it.

Built across a 523 metre wide stretch of the river, the barrier divides the river into four 61m and two 31m navigable spans and four smaller non-navigable channels between nine concrete piers and two abutments. The flood gates across the openings are circular segments in cross section, and they operate by rotating, raised by hydraulics from a horizontal cill on the riverbed to form a barrier of steel and concrete. They can rotate further to allow "underspill" to allow operators to control upstream levels and a complete 180 degree rotation for maintenance. All the gates are hollow and made of steel up to 40mm thick. The gates fill with water when submerged and empty as they emerge from the river. The four large central gates are 61 metres long, 10.5 metres high (above local ground level) and weigh 3,500 tonnes; the outer two gates are 31.5 metres. Additionally, four radial gates by the riverbanks, also 31.5 metres long, can be lowered. These gate openings, unlike the main six, are non-navigable.

Before 1990, the number of barrier closures was one to two per year on average. Since 1990, the number of barrier closures has increased to an average of about four per year. In 2003 the Barrier was closed on 14 consecutive tides. The barrier was closed twice on November 9, 2007 after a storm surge in the North Sea which was compared to the one in 1953.

The concept of the rotating gates was devised by Charles Draper. The barrier was designed by Rendel, Palmer and Tritton for the Greater London Council and tested at HR Wallingford. The site at Woolwich was chosen because of the relative straightness of the banks, and because the underlying river chalk was strong enough to support the barrier. Work began at the barrier site in 1974 and construction, which had been undertaken by a Costain/Hollandsche Beton Maatschappij/Tarmac Construction consortium, 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 £534 m (£1.3 billion at 2001 prices) with an additional £100 m for river defences. The barrier was originally designed to protect London against a flood level with a return period of 1000 years in the year 2030 after which the protection would decrease but be within acceptable limits. This defence level included long term changes in sea and land levels as understood at that time (c. 1970). Since then sea level rise due to global warming has been identified. Based on current estimates the barrier will be able to cope with projected sea level rises until around 2030–2050 and is expected to serve its full term.

Since 1982 (up to 2007) the barrier has been raised over 100 times; further, it is raised every month for testing. The barrier was originally commissioned by the Greater London Council under the guidance of Ray Horner. After the 1986 abolition of the GLC it was operated successively by Thames Water Authority and then the National Rivers Authority until April 1996 when it passed to the Environment Agency.

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