The Thames Barrier is located downstream of central London, on the river Thames on the eastern side of the capital at Woolwich. It is the world's second largest after the Oosterscheldekering barrier in the Netherlands. The barrier was built in 1982 to prevent folding of all but the easternmost boroughs of Greater London by exceptionally high tides and storm surges moving up from the North Sea. The barrier is made up of 9 steel gates reaching 520m across the river. When open, the gates lie flat on the river floor allowing the Thames to flow freely and ships to pass through the gates. When closed, the gates are rotated upwards until they block the river. The gates, which are hollow and made of 1.6 inch-thick steel, span 61 meters and each weigh 3,200 tonnes. The gates are filled with water when submerged and empty as they emerge from the river.

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