The Thames Barrier. © NMM

The Thames Barrier is a unique structure built to protect London against flooding caused by tidal surges from the North Sea.

It is the world's largest movable flood barrier, spanning 520 metres (a third of a mile) across the Thames at Woolwich Reach.

Ship passing through the Thames Barrier. © NMM

The Thames Barrier is a set of 10 separate movable gates positioned end-to-end across the river. Between the gates are the concrete piers housing the operating machinery.

In the event of a surge warning, the gates are closed to form a steel wall. This effectively closes off the Upper Thames from the sea. When not in use the main gates lie in special recesses on the riverbed to allow ships to pass through. The Thames Barrier is maintained and operated by the Environment Agency.

Tilbury under water in 1953. © NMM

Much of London lies in the floodplain of the Thames and its tributaries - the Brent, Fleet, Lea, Roding and many others. Because of this, the city has always been vulnerable to flooding.

On 31 January 1953 Britain and the Netherlands suffered one of the worst floods in their history. Huge waves battered the east coast, and a devastating tidal surge rushed up the Thames, with sea levels 3 metres above normal in some places.

London was very fortunate to escape the worst of the flooding. The Thames and the Lea burst their banks and damaged more than 1100 houses in Silvertown and Canning Town, but the floods did not reach central London.  Although London escaped disaster in 1953, the floods showed just how vulnerable the city could be.

One of the many failed proposals for a Thames Barrier. © NMM

Had the tidal surge reached central London, the outcome would have been horrendous. Over a million people would have been in danger. The likely damage to London's infrastructure - water and sewage systems, and power, gas and phone lines - would have disrupted life in the capital for months and cost a fortune to repair.

After the floods, an enquiry was set up to work out ways of protecting London in the future. As the old system of embankments was clearly no longer adequate, the enquiry recommended the construction of a flood barrier across the Thames.

• The width of the Barrier from bank to bank is about 520 metres (a third of a mile) with the four main openings each having a clear span of 61 metres (200 feet).
• The four main gates are constructed as a hollow steel-plated structure over 20 metres (65 feet) high and weighing about 3700 tonnes. Each is capable of withstanding an overall load of more than 9000 tonnes.
• There are two further gates with 31-metre (101 feet) navigation openings.
• The four falling radial gates have non-navigable openings adjacent to the riverbanks.
• 4000 men and women were involved in building the Thames Barrier.
• It cost nearly £500 million to build.
• In addition, 18.5 kilometres (11.5 miles) of riverbank east of the barrier were protected by new walls, to a new defence level of 7 metres (23 feet).
• The Thames Barrier was not the only defence against flooding built in this period. There are more than 30 other barriers.
• There are also more than 150 kilometres (94 miles) of embankments and defensive walls along the Thames.