A set of large hydraulically-operated steel gates set between nine concrete piers on the River Thames, that protect London from flooding by surge tides. Construction began in 1974 and completed in 1982, when the barrier became operational (since then it has closed to protect London from risk of flooding almost 100 times) with an intended service life of about 55 years. It is located in Woolwich (south east London), close to London City Airport and the Millennium Dome. The barrier's Visitor Centre is also there, about a quarter mile from the barrier itself.
It's no secret that Londoners live under quite high risk from flooding compared to other parts of Great Britain, due to how low the south-eastern end of the country lies compared to sea level. In fact, Britain is sinking towards Europe at roughly 3 mm per year. London has been susceptible to flooding for most of its existence and is known to have been flooded in the past; there are reports of London flooding to various degrees from the ninth century right through to the twentieth. One of the more recent floods in 1928 resulted in four people drowning in the basement of Westminster Abbey.
By the 1950s, the possibility of building flood defences on the Thames had been discussed several times (London is very close to the Thames' sea outlet), but nothing had ever been done. 1953 was the British government's wakeup call. Just north of the Thames Estuary in south-east England there is a small island about 5 miles long, called Canvey Island. It was settled in the 17th century and now has roughly 37,000 inhabitants. Several place names on the island have changed since it first got a civilian population, in an effort to erase part of the island's past; Great Britain's worst peacetime disaster.
At the end of January a storm made its way down the North Sea, with central pressures so low that they effected a rise in the sea level of up to 30cm (each millibar drop in atmospheric pressure allows the sea level in that area to rise 1 cm). Blown by the storm winds, this dome of water arrived at East Anglia (consisting of the counties of Norfolk, Suffolk and Essex) early on January 31st, continuing on to reach Canvey Island the following day. By this point over 100 people in coastal East Anglia had already been killed either by drowning or exposure to the freezing water. Canvey's defensive dykes were flattened by the 90 billion gallons of extra seawater and the island was completely submerged by the sea.
56 people died in this flooding. At this time there were no flood defences at all along the River Thames and further inland flooding killed another 150 people and ruined 160,000 acres of farmland. Almost 2000 people died along the British and Dutch North Sea coasts in total. This prompted an urgent inquiry by the British government into methods of flood prevention. The committee that conducted the investigation returned with a recommendation that a flood barrier be built on the River Thames to combat the risk of flooding from surge tides, one of which was responsible for the 1953 flooding.
Surge tides are "humps" of water up to 30cm higher than average sea level, caused by areas of low atmospheric pressure. They originate in the North Atlantic off the coast of Canada and usually pass north of the United Kingdom, but strong northerly winds can force them down through the North Sea, causing coastal flooding in Britain and Denmark.
The main problem with the idea of building a barrier on the Thames was the volume of shipping using the London Docklands. Ships were being built bigger and bigger at this time and it was felt that any barrier design would have to allow at least 1400ft clearance when open in order to not impinge on river traffic. This factor limited the points in the river at which the barrier could be built without widening the river itself. However, once the new container docks at Tilbury were built, river traffic diminished significantly in both size and quantity, and only a 200ft gap was necessary - the same as Tower Bridge. This made the design of the barrier system easier f