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 from an engineering standpoint (easier hold back 200ft of water than 1400ft of it), and gave more possibilities as to where to put it. The further downstream the barrier could be built, the more land it would be able to protect.

41 design proposals were submitted for flood prevention/control measures for the Thames. The design selected was created by Charles Draper, chosen for the minimal interference it would make to the normal flowing of the river and its traffic, its compactness, its relative simplicity and its looks (such as they are). Construction of the barrier began in 1974 and took eight years, at a cost of £535 million. It is worth noting that today it has an estimated value of over £10 billion, taking into consideration the cost of repairs after any significant flooding of the 45 km2 area it protects - including the now-redeveloped docklands, which is home to Canary Wharf, amongst others - which is estimated at £20 billion.

The design, the world's largest movable flood barrier, consists of nine concrete piers built into the solid chalk bed of the Thames. Between these piers are ten individual barriers. The piers contain rocker beams operated by push-pull hydraulic rams, which rotate the four main barriers between positions. There are two of these mechanisms on each pier - one for the barrier section on either side - meaning each barrier is operated by a movement beams at either end.

The simplest way to think of each of the main barriers is as a hollow cylinder with two-thirds of its sides removed. Flat rotating discs are at each end to support the barrier of which one side is flat and the other flush with the discs, between which it is fitted. Viewed in cross section, when raised each barrier looks something like this:

         *****
     ****  B  |C**
   ***        |CCC**
  **          |CCCC**
  **       A  |CCCC**
  **          |CCCC**
   ***        |CCC**
     ****     |C***
         *****
A - Central pivot point
B - Pivot point for rocker beam
C - Barrier section

Each of the four main barrier sections are 20 m high - about the height of a 5-storey building - and weigh about 3,700 tonnes each including their movement arms. Each section is capable of withstanding a load of over 9,000 tonnes. These four barrier sections are near the centre of the river; the other six smaller sections are distributed on either side, with four off the north bank and two off the south. These are regular sluice gates.

Rocker beam isn't noded so the simplest way to illustrate it is...to illustrate it. Very crudely, the system that moves the barriers works roughly thus (this is obviously not to scale):

                                                    xxxxx
                                                xx    P x
 _______________________                   xx         xyyyy
|                       |_________    xx           xx    yyyy
|o    Hydraulic Ram     |________xxP             xx        yyyy
|_______________________|          xx          xx            yyyy
 _______________________              CP     xx                yyyy  
|                       |______________xx  xx                    yyyy
|o    Hydraulic Ram     |________________xP                        yyyy
|_______________________|                                            yyyy
                                                                       yyyy

Two pivoted hydraulic rams, one pushing and one pulling, pivot a triangular frame (xx) around the centre of its base (CP). This action raises or lowers the apex of the frame, which translates to motion in the rocker beam (y) which is connected at one end to a pivot point on the apex of the frame. The other end of the rocker beam is connected to the pivot point on the gate (B). The action of the beam "raises" or "lowers" the barrier section (it doesn't actually do either - it just rotates around its centre). The main benefit of the actuator system being built like this is it allows almost all moving parts to be clear of the water at all times, extending the life of the mechanics and making maintenance much easier. The machinery that drives the hydraulics is housed inside the distinctive stainless steel silver shells on each of the piers. Electrical power supplied via the riverbanks is doubly redundant and all backed up by 3 generators housed on the piers themselves.

It takes about half an hour to move the barrier from one position to another and presumably this is affected by the volume of water. The barrier is always raised at low tide before any surge tide can reach it, which is usually about 5 hours after the risk has been identified. To cause the least interference possible to the flow of the river, the gates are closed in a pre-programmed sequence, 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.


Sources:

  • Palmer, Alan; "The Thames Barrier";
    http://greenwichengland.com/tourism/barrier.htm
  • (Author unknown); "Thames Barrier";
    http://www.corrosion-doctors.org/Landmarks/Thames.htm
  • Waugh, Priscilla; "The Thames Barrier";
    http://www.thames-search.com/barrier.html
  • (Author unknown); "Thames Barrier";
    http://www.places-to-go.org.uk/Thames_Barrier.htm
  • 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=
    thames+barrier&topic=&area=&month=
  • mark.funnell@environment-agency.gov.uk; "Sharp rise in Thames Barrier closures";
    http://www.environment-agency.gov.uk/yourenv/environmentactionissues/139861/140035/?lang=_e®ion=
  • Foden, Heidi; "Londoners facing imminent risk of tidal flooding";
    http://www.ucl.ac.uk/development/experts/pressreleases/thamesbarrier.html
  • (Author unknown); "Thames Barrier closures against tidal surges";
    http://www.environment-agency.gov.uk/yourenv/indicators/NaturalForces/v5-4_thamesbarrier/?version=1&lang=_e
  • 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";
    http://www.esemag.com/0998/barrier.html
  • Meek, James; "50 years on, new menace of fatal flooding";
    http://www.guardian.co.uk/weather/Story/0,2763,868469,00.html

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