London's last line of defence against rising sea levels is the Thames Barrier.
A combination of climate change and geological tilting means that sea levels around Britain are rising, and an increase of 200mm over the past century has had a dramatic effect on the safety of the UK's capital.
There has been human settlement on the river Thames since at least Roman times and the historic record is littered with dramatic floods and their associated loss of life, due to surge tides funneling down the North Sea and forcing their way up the Thames. When these floods combine with spring tides there is a serious risk to the capital.
Fourteen people died in one such occurrence in 1928. But it wasn't until after the loss of 307 lives in the 1953 North Sea Flood that serious attempts were made to mitigate the effects of future incidents. Sir Herman Bondi's report on the disaster put in motion a sequence of events that would result in the Thames Barrier, as we know it today.
The barrier consists of 10 moveable gates of differing sizes strung across the river, six of which are navigable for the capital's water traffic. They are supported between concrete piers containing the operating and control equipment necessary to create a continuous steel wall facing downriver to hold back surges. The Environment Agency, which is responsible for the defence system, describes how it works: "When we close the barrier we seal off the upper Thames from the sea. When we are not using it, the six rising sector gates rest out of sight in curved recessed concrete sills in the riverbed. This means that boats and other river traffic can get through the piers most of the time."
Central to the design are the four main rotating rising gates, each with a span of 61m. These are built to withstand a load of more that 9,000 tonnes and are moved into position by hydraulic cylinders which move a rocking beam that pushes and pulls the gate linkages in opposite directions. Although there is a mechanism at both ends of each gate, individual gates can be controlled from one pier, meaning that there is always a back-up mechanism in the event of failure.
The concept for the rotating barrier is based on one of the most simple of mechanical engineering designs – the tap – and was originally conceived by Charles Draper, who built a working model in his house in London in the 1950s. The design was taken up by Rendel, Palmer and Tritton and tested by the Hydraulics Research Station in Wallingford. Construction work began in 1974 at a site in New Charlton that had been selected based on the relative straightness of the Thames's banks at this point and the stability of the chalk bed that was to support the barrier.
The role of the barrier in protecting the city is becoming ever more important. In the 1980s there were four closures, while in the 1990s there were 35. In the first decade of the 21st Century there were 75 closures with the 119th shutdown on 2 March 2010.
Due to projected increases in frequency and severity of tidal surges the barrier will only offer full protection until 2030, after which the protection will decrease, but remain within acceptable limits. Currently there are proposals to build a further barrier downstream, including a 16-mile construction at Sheerness.
The idea of one of the world's most important commercial hubs being washed off the face of the earth is so apocalyptic that it found its way into the plot of the 2006 'Doctor Who' Christmas special. Not to be outdone, Take That included the landmark in the video for their 2010 hit 'The Flood'.