rom 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**
** A |CCCC**
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
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,