Being the world’s second-largest movable flood barrier (after the Oosterscheldekering in the Netherlands), the Thames Barrier prevents London from being flooded by exceptionally high tides and storm surges moving up from the sea. It needs to be raised (closed) only during high tide; at ebb tide it can be lowered to release the water that backs up behind it.
Its northern bank is in Silvertown in the London Borough of Newham and its southern bank is in the New Charlton area of Charlton in the Royal Borough of Greenwich.
A Thames Barrier flood defence closure is triggered when a combination of high tides forecast in the North Sea and high river flows at the tidal limit at Teddington weir indicate that water levels would exceed 4.87 metres in central London.
About 9 hours before the high tide reaches the barrier a flood defence closure begins with messages to stop river traffic, close subsidiary gates and alert other river users. Once river navigation has been stopped and all subsidiary gates closed, then the Thames Barrier itself can be closed. The smaller gates are closed first, then the main navigable spans in succession. The gates remain closed until the tide downstream of the barrier falls to the same level as the water level upstream.
London is vulnerable to flooding. Fourteen people died in the 1928 Thames flood, and after 307 people died in the UK in the North Sea Flood of 1953, the issue gained in importance. Early proposals for a flood control system were stymied by the need for a large opening in the barrier to allow for vessels from the London docks to pass through.
When containerisation replaced older forms of shipping and the Port of Tilbury was expanded, a smaller barrier became feasible with each of the four main navigation spans being the same width as the opening of Tower Bridge.
Work began at the barrier site in 1974 and construction was largely complete 8 years later. In addition to the barrier itself the flood defences for over 17 kilometres down the river were raised and strengthened. The barrier was officially opened on 8 May 1984 by Queen Elizabeth II.
An incident that was potentially catastrophic for London occurred on 27 October 1997. The dredger MV Sand Kite operating in thick fog, collided with one of the Thames Barrier’s piers. As the ship started to sink she dumped her 3,300-tonne load of aggregate, finally sinking by the bow on top of one of the barrier’s gates where she lay for several days. Initially the gate could not be closed as it was covered in a thick layer of gravel. A longer term problem was the premature loss of paint on the flat side of the gate caused by abrasion. One estimate of the cost of flooding damage, had it occurred, was around £13 billion. The vessel was refloated in mid-November 1997.
About a third of the closures up to 2009 were to alleviate fluvial flooding. From 1982 until 19 March 2007, the barrier was raised one-hundred times to prevent flooding. It is also raised monthly for testing. The barrier was closed twice on 9 November 2007 after a storm surge in the North Sea which was compared to the one that caused the great flood in 1953.
In the 1980s there were four closures, 35 closures in the 1990s, and 75 closures in the first decade of this century. All told, there have been 119 flood defence closures up to the closing on March 2, 2010.
The barrier was closed on 3 June 2012 to reduce the river flow whilst the Thames Diamond Jubilee Pageant took place as part of the celebrations for the Queen Elizabeth II’s Diamond Jubilee celebrations.
In November 2011 a new Thames Barrier, further downstream at Lower Hope between East Tilbury in Essex and Cliffe in Kent, was proposed as part of the Thames Hub integrated infrastructure development. The barrier would incorporate hydropower turbines to generate renewable energy and include road and rail tunnels, providing connections from Essex to a major new hub airport on the Isle of Grain.
Built across a 520-metre wide stretch of the river, the barrier divides the river into four 61-metre and two about 30-metre navigable spans. There are also four smaller non-navigable channels between nine concrete piers and two abutments. All the gates are hollow and made of steel up to 40 millimetres thick.
The flood gates across the openings are circular segments in cross section, and they operate by rotating, raised to allow “underspill” to allow operators to control upstream levels and a complete 180 degree rotation for maintenance. The gates fill with water when submerged and empty as they emerge from the river. The four large central gates are 20.1 metres high and weigh 3,700 tonnes. Four radial gates by the riverbanks, about 30 metres wide, can be lowered. These gate openings, unlike the main six, are non-navigable.
The barrier was originally designed to protect London against a very high flood level (with an estimated return period of one thousand years) up to the year 2030, after which the protection would decrease, whilst remaining within acceptable limits. Despite consequently greater predicted rate of sea level rise, recent analysis extended the working life of the barrier until around 2060–2070.