5025 x 3363 px | 42,5 x 28,5 cm | 16,8 x 11,2 inches | 300dpi
Altre informazioni:
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. The gates are normally left open to allow ships to pass through, but can be rotated and closed to stop water travelling up the Thames through London. 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[2], 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
