The Thames Flood Barrier is one of the largest movable flood barriers in the world, and celebrated its 25th anniversary in 2009. It protects London against surge tides from the Atlantic, and rising sea levels generally.

The need for a barrier was recognised in 1954 following disastrous storm surge floods in 1953, when 300 people died and 24,000 properties were destroyed. The then Greater London Council, statutory body responsible for flood protection from the 1960s, presided over an area of approximately 115 sq km below the 1953 river level.

Consultant Rendel, Palmer & Tritton first suggested a drop-gate barrier with two large navigable openings in 1958, which was rejected. The government appointed mathematician Sir Herman Bondi to review the situation. Following his recommendations, in 1968 the council commissioned physical model studies of the River Thames between Teddington Weir and Southend.

By autumn 1969, it was decided to site the barrier at Woolwich Reach and to improve the downstream flood defences as far as the outer estuary. Charles Draper devised the final design, using the shape of gas taps as his inspiration for the gates. Additional design work was by Rendel, Palmer & Tritton. Construction began in 1974.

The barrier has nine concrete piers and 10 gates, which together span some 520m from north to south bank. The piers are founded on steel cofferdams built on solid chalk 15.2m below river level. The iconic boat-shaped pier roofs are timber clad in stainless steel, containing machinery and lift shafts.

There are six openings for navigation, each with rising sector gates, and four non-navigable tidal openings, each with falling radial gates. Individual gates can be closed in 10-15 minutes, but closing the entire barrier takes one and a half hours, allowing for dissipation of reflected waves and equipment checks. Gates are closed in pairs from bank to centre. The optimum time to close the barrier is just after low tide.

The four central rising sector gates span 61.5m and weigh 3,300 tonnes each. The two flanking rising sector gates each span 31.5m. The straight back of each gate is at least 20m high, and just the paint can weigh up to 200 tonnes per gate.

Gates are recessed 4m into the river bed when the barrier is open to river traffic. When the barrier is closed, they are rotated up into the vertical position by pairs of yellow-painted hydraulically powered steel rocking beams weighing 49 tonnes. The gates can resist a maximum differential head of 9.1m, at which time each one transfers a thrust of 9,100 tonnes to the piers.

Construction work on the barrier was completed in October 1982, at an estimated cost of £535 million, with the first closure in February 1983. It was officially opened by HM Queen Elizabeth II on 8th May 1984.

The barrier was out of use temporarily in 1997, after the MV Sand Kite collided with it in thick fog on 27th October and sank. The vessel was refloated and most of her cargo salvaged, and the barrier resumed normal operations.

Although futuristic in appearance, the barrier relies on proven engineering technology rather than innovation. Resilience and reliability, together with a rigorous maintenance programme —each component has its own schedule — should ensure that the barrier is operational into the 22nd century, well beyond its design life of 2030.

The barrier has been closed against flooding 114 times, although closures occur monthly for maintenance too. It protects some 125 sq km of central London from flooding, notably during the storm surges of November 2007.