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
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):
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
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 c