A map released by its operator, the Environment Agency, in December showed how London would look if sea levels continued to rise and there was no barrier. The Houses of Parliament, the O2 arena, Tower Bridge, and areas including Southwark, the Isle of Dogs, Whitechapel and West Ham were shown to be flooded.
The barrier, built in 1982 on the Thames on the eastern side of the capital at Woolwich, was designed to protect 48 sq miles (125 sq km) of central London from flooding caused by tidal surges.
At the moment, with so much rainfall travelling down the Thames, there is a danger during high tide that the extra water will be pushed back up river by the sea and cause flooding in the capital and to the west.
To prevent this, the barrier has been used at record levels, says Eamonn Forde, one of its controllers. It has closed 28 times since 6 December. This represents one fifth of all the closures – about 150 – since it was inaugurated.
Some years it hasn’t been used at all. When it shut in December 2012, it was reported to be the first closure since March 2010.
The barrier, made up of 10 steel gates, reaches 520m (1,700ft) across the river. When open, the gates lie flat on the river floor and close by being rotated upwards until they block the river. The four main gates span 61.5m (200ft) and weigh more than 3,000 tonnes each. The barrier is closed just after low tide to create an empty “reservoir” for the river flow to fill up. It takes 75-90 minutes to close it, starting with the gates on the outside until the middle gates are shut.