Thames Barrier
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The Thames Barrier

 

(by Melissa Soh and Pei-Chea Tran)

INTRODUCTION

 

The Thames Barrier is a flood defense mechanism that protects London from tidal floods. It is the largest movable flood barrier and represents a great achievement in engineering. Thames Barrier is located at Woolwich Reach, where it spans 520 metres. The long straight section at this site allows for plenty of time for negotiation of the gaps.

 

The barrier can prevent London from floods that could cover all land up to 7.2m above sea level, and is designed to give total protection against the worst floods that might occur once in 1000 years.

 

 

 

HISTORY

 

The first plans for a flood barrier were put forward in 1907 but no decisions were taken until 1953, when a serious flood drowned over 300 people and caused 65,000 hectares of farmland to be covered by salt water. A committee was appointed by the Government to look at the flood problem.

 

Many proposals were put forward and rejected. It wasn’t until 1965, after the formation of the Greater London Council, that government consent was given to build a barrier. The design had to allow cargo ships to pass through while holding back surge tides in emergencies. Charles Draper was the engineer that came up with the winning design. The barrier works on the same principle as a domestic gas tap.

 

The design was chosen because it minimises interference with the natural flow of the river does not pose a headroom restriction for shipping, is attractive and practical. Work on the barrier finally began in 1972 and it was completed in 1982.

 

 

 

 

 

SURGE TIDES

 

The high water levels are caused by tidal surges, the melting of the polar ice caps (greenhouse effect) and narrowing of the river (drudging). But the major causes of flood threats to London are tidal surges.

 

A surge tide is caused by a sequence of meteorological events:

 

  1. A trough of low pressure travels across the Atlantic Ocean towards the British Isles, creating a great surge of seawater.
  2. The depression passes around the North of Scotland, down the east coast, carrying the surge to the mouth of the Thames estuary.
  3. As the mass of water from the deep seas hits shallower water, a great wave is formed.
  4. Strong northerly winds drive the mass of water down the east coast.
  5. The wave coincides with a high ‘spring’ tide at the mouth.
  6. The combined mass of water rushes up the River Thames to London – a surge tide.

 

Another contributing factor is the tilting of the British Isles towards Europe. The south of England is sinking at the rate of 30 cm per century!

 

 

CONSTRUCTION

The Final Design

During the design process, the following constraints were identified:

  1. The barrier must let cargo ships pass through when not in operation.
  2. The barrier must be able to hold back surge tides to protect London from flooding (see Fig. 1).

The final solution utilised a moveable flood barrier with selective bank raising. Six raising sector gates across the Thames are based on this idea. In addition, four falling radial gates of a different design are located near the banks of the river. These gates are kept permanently upright and do not let river traffic pass through.

The design for the Thames Barrier was chosen to let ships through and also allow construction without disturbing river traffic. Woolwich Reach was chosen to be the site of construction because it had solid chalk foundation ideal for building on. The river is also comparatively straight and narrow here. The Thames Barrier took 8 years to build and was inaugurated by The Queen in 1984. It is valued at over £1,000 million.

 

 

 

 

 

 

 

 

Fig. 1: The Thames Barrier with all main sector gates closed. Note that when the gates are open, there is enough space between the piers for ships to pass through.

 

Foundation Construction

Chalk was first removed from the riverbed to make room for the foundation. Piles were then driven into the riverbed and interlocking steel beams were used to form cofferdams. Horizontal steel joists were located within the foundation to withstand pressure. The pile driving was followed by the placement of 250,000 tons of rock on the riverbed to counter the tidal flow effect. The foundation was completed by pumping concrete into the riverbed.

 

Barrier Construction

Elements of barrier were constructed simultaneously – either via on-site construction or pre-fabrication. A total of half a million tons of concrete was used to build the piers and sills.

The piers were constructed in the river first. These support the gates and house the machinery. All that is visible of the piers above the water level are the stainless steel domes. These domes house the electric supplies required to drive the gate arms.

The sills are assembled next. These are located on the riverbed to support the gates when the gates are not in operation. The sills are prefabricated and are concrete with steel reinforcing bars for added strength. Cross-sectional hollow steel tubes are located in the sills, providing access, service and power to the piers. After the piers and sills have been constructed, the sills are positioned accurate to a few millimetres in between the piers. The sills are flooded, then lowered into the river.

Each main sector (moveable) gate has a semi-cylindrical shape and is constructed out of 4000 tons of steel. Computer controlled cranes maneuver each gate into place during construction. Each main sector gate is as high as a five-storey building and as wide as the opening of Tower Bridge (61 m). The gates are raised by hydraulic machinery, also known as hydraulic power packs. The reciprocating gate arms used to raise and lower the gates weigh 8700 tons. Power is supplied from 3 alternative sites, and 3 on-site power generators are on hand in case of emergency.

 

Fig.2: A scale model of one pier of the Thames Barrier, with the main sector gate closed. Note the sill below the gate, and the gate arm used to raise and lower the gate.

 

 

OPERATIONS

More than 50 staff operate and maintain the Barrier. During operation, computers control electric powered hydraulic machinery. The controls and machinery are located in the piers. Computers are also used for fault detection and backup systems are in place for the control of the Thames Barrier. The gates are closed very slowly to prevent reflective waves travelling back to London. Such reflective waves may cause their own mini-floods, and hence must be prevented. The Thames Barrier is also designed so that it is still capable of preventing a flood even if one main sector gate is open.

 

Fig. 3a: Main sector gate in closed Fig. 3b: Main sector gate in open position.

position.

 

Usage

The Thames Barrier has been raised 27 times since its completion, mainly as a precaution to protect London from flooding. It is also raised monthly for 2 ½ hour test at low tide and annually for a full day to test the effects of high tide.

Public access to the Thames Barrier is available via the Thames Barrier Visitors Centre. Scale models and various information on the Thames Barrier are available here. A public viewing platform is also situated on the banks of the Thames.

 

 

 

 

Fig. 4: A picture of the Thames Barrier taken from the viewing platform. Note that all main sector gates are open except for the middle one.

 

 

COMMUNICATIONS

A potential flood can be identified within 36 hours of high tide. The following is a breakdown of events.

  1. 36 hours: satellite pictures and meteorological data show the development of a low pressure trough.
  2. 24 hours: The Meteorological Office shows a deepening low pressure area approaching the Thames.
  3. 12 hours: With the sea levels are higher than normal, the barrier closure team is called out.
  4. 9 hours: The floodgate inspection team is called out.
  5. 8 hours: As the surge continues moving up the Thames, call-out teams assemble at Thames Barrier.

The early flood warning is made possible with the use of sensors and satellites. The tides from North Scotland to Holland are monitored by computers in Thames Barrier Control Room and at the East Coast Storm Tide Warning Service at Bracknell.

 

CONCLUSION

The Thames Barrier is an excellent showcase of civil, mechanical, electrical, electronic and information technology engineering. The construction of the Thames Barrier highlighted the importance of project management, while its continued successful operation is due to the expertise of the engineers and operators involved.