 where the six smaller barriers towards the river bank close first followed by the four main barriers in the centre, which close from the outside in. The central two close at the same time. Before they are lowered again, water levels are checked for equality on either side to ensure no tidal wave goes in either direction.
When not in use, each of the main barriers are at a position that puts their flat edge flush with a concrete sill on the river bed. This sill has a curved depression in it that matches the curved edge of the barrier, so when the barrier is "open" it forms a flat surface on the river bed and is no obstruction to passing craft. The barrier can be moved to positions other than "open" and "closed": it can be moved slightly beyond the "closed" position so that more water is allowed to flow through (the "closed" position still allows a small amount of water through). The main purpose of this is to allow some of the pressure of any incoming tidal wave through the barrier rather than just reflecting it all away. Apparently canoeists often enjoy surfing on the rapids generated by this. Finally, the barrier can be rotated clear of the water, in a mirror of its "open" position, for cleaning and maintenance.
The barrier has been closed several hundred times since it became operational in 1982 (although it was not officially opened by the Queen until 1984). The majority of these have been monthly tests, but every so often the barrier is raised as a precaution against flooding. The most recent reports (January 2003) state the barrier has now been used 82 times as a protective measure. After construction completed, the barrier was closed, on average, a maximum of four times a year. This has increased during the last decade; during October 2000 (a period of localised flooding all over Britain) it was closed seven times, but between January 1, 2003 and January 8th the barrier had to be closed fourteen times. In fact almost half of all protective barrier closures to date have taken place since mid-2000.
This of course raises questions about climate change affecting sea levels. When the barrier was completed in 1984, the water level on the Thames at Tower Bridge was recorded as increasing by about 2ft every century. It is now estimated that this could have risen to as much as 1 metre per century. As well as the overall sinking of Britain, the rate of the melting of polar ice caps has increased over the last twenty years, which could go toward explaining this difference. However according to Britain's Environment Agency, the planet as a whole has experienced a period of relative drought over the last decade and water levels were expected to rise significantly once it concluded. However, questions still remain about whether the Thames Barrier will be able to protect London until 2030 as it was originally designed.
As for Canvey Island, after the 1953 floods it had its flood defences rebuilt bigger and better. A solid concrete wall over six metres high now completely surrounds the island. This is similar to the improved defences which were installed along the banks of the River Thames while the barrier was being built. After it was completed, these were heightened further and the banks of the Thames from shortly after the river passes the barrier are now eight metres above the average water level.
- Palmer, Alan; "The Thames Barrier";
- (Author unknown); "Thames Barrier";
- Waugh, Priscilla; "The Thames Barrier";
- (Author unknown); "Thames Barrier";
- Henshall, Natalie; "Barrier closes a record 14 times to protect London from flooding";http://www.environment-agency.gov.uk/news/429013?lang=_e®ion=&projectstatus=&theme=&subject=&searchfor=
- firstname.lastname@example.org; "Sharp rise in Thames Barrier closures";
- Foden, Heidi; "Londoners facing imminent risk of tidal flooding";
- (Author unknown); "Thames Barrier closures against tidal surges";
- Beyer, Elke; "The Thames Barrier";
http://www.biw.fh-deggendorf.de/alumni/1999/beyer/thames-en.htm (and accompanying pages)
- (Author unknown); "Thames Barrier investment pays off"