Thames Barrier

51°29′52″N 0°02′12″E / 51.497744°N 0.036679°E / 51.497744; 0.036679

The Thames Barrier is the world's second largest movable flood barrier. It is a flood control structure located downstream of central London. The barrier's purpose is to prevent London from being flooded by an exceptionally high tide moving up from the sea, often exacerbated by a storm surge. It only needs to be raised for the duration of the high tide; at ebb tide it can be lowered to release upstream water that backs up behind it. On the northern bank it lies in the area of Silvertown in the London Borough of Newham. On the southern bank it lies in the New Charlton area of Charlton in the London Borough of Greenwich.

Historical needs for a barrier

London is quite vulnerable to flooding. A storm surge generated by low pressure in the Atlantic Ocean sometimes tracks eastwards past the north of Scotland and may then be driven into the shallow waters of the North Sea. The storm surge is funnelled down the North Sea which narrows towards the English Channel and the Thames Estuary. If the storm surge coincides with a spring tide then dangerously high water levels can occur in the Thames Estuary. This situation combined with downstream flows in the Thames provides the triggers for flood defence operations.

The threat has increased over time due to the slow but continuous rise in high water level over the centuries (20 cm / 100 years) and the slow "tilting" of Britain (up in the north and down in the south) caused by post-glacial rebound.

Fourteen people died in the 1928 Thames flood, and after 307 people died in the UK in the North Sea Flood of 1953 the issue gained new prominence.

Early proposals for a flood control system were stymied by the need for a large opening in the barrier to allow for vessels from London Docks to pass through. When containerization came in and a new port was opened at Tilbury, a smaller barrier became feasible with each of the four main navigation spans being the same width as the opening of Tower Bridge.

Design and construction

The concept of the rotating gates was devised by Charles Draper. The barrier was designed by Rendel, Palmer and Tritton for the Greater London Council and tested at HR Wallingford Ltd. The site at Woolwich was chosen because of the relative straightness of the banks, and because the underlying river chalk was strong enough to support the barrier. Work began at the barrier site in 1974 and construction, which had been undertaken by a Costain/Hollandsche Beton Maatschappij/Tarmac Construction consortium,^[1] was largely complete by 1982. In addition to the barrier itself the flood defences for 11 miles down river were raised and strengthened. The barrier was officially opened on 8 May 1984 by Her Majesty Queen Elizabeth II. Total construction cost was around £534 m (£1.3 billion at 2001 prices) with an additional £100 m for river defences.

Built across a 572 yard (523 m) wide stretch of the river, the barrier divides the river into four 200 feet (60 m) and two 34 yd (33 m) navigable spans and four smaller non-navigable channels between nine concrete piers and two abutments. The flood gates across the openings are circular segments in cross section, and they operate by rotating, raised to allow "underspill" to allow operators to control upstream levels and a complete 180 degree rotation for maintenance. All the gates are hollow and made of steel up to 1½ inches (3.8 cm) thick. The gates fill with water when submerged and empty as they emerge from the river. The four large central gates are 220 ft (67 m) long, 35 ft (10 m) high (above local ground level) and weigh 3,500 tonnes; the outer two gates are 100 ft (30 m). Additionally, four radial gates by the riverbanks, also 100 ft long, can be lowered. These gate openings, unlike the main six, are non-navigable.

The view from the northern side of the Thames.

Prediction for operation

A Thames Barrier flood defence closure is triggered when a combination of high tides forecast in the North Sea and high river flows at the tidal limit at Teddington weir indicate that water levels would exceed 4.87 m in central London. Forecast sea levels at the mouth of the Thames Estuary are generated by Met Office computers and also by models run on the Thames Barrier's own forecasting and telemetry computer systems. About 9 hours before the high tide reaches the barrier a flood defence closure begins with messages to stop river traffic, close subsidiary gates and alert other river users. As well as the Thames Barrier, the smaller gates along the Thames Tideway include Barking Barrier, King George V Lock gate, Dartford Barrier and gates at Tilbury Docks and Canvey Island. Once river navigation has been stopped and all subsidiary gates closed, then the Thames Barrier itself can be closed. The smaller gates are closed first, then the main navigable spans in succession. The gates remain closed until the tide downstream of the barrier falls to the same level as the water level upstream.

After periods of heavy rain west of London, floodwater can also flow down the Thames upstream from London. Because the river is tidal from Teddington weir all the way through London, this is only a problem at high tide, which prevents the floodwater from escaping out to sea. From Teddington the river is opening out into its estuary, and at low tide it can take much greater flow rates the further one goes downstream. In periods when the river is in flood upstream, if the gates are closed shortly after low tide, a huge empty volume is created behind the barrier which can act as a reservoir to hold the floodwater coming over Teddington weir. Most river floods will not fill this volume in the few hours of the high tide cycle during which the barrier needs to be closed. If the barrier was not there, the high tide would fill up this volume instead, and the floodwater could then spill over the river banks in London.

Barrier closures and incidents

Before 1990, the number of barrier closures was one to two per year on average. Since 1990, the number of barrier closures has increased to an average of about four per year. In 2003 the Barrier was closed on 14 consecutive tides.

An incident which had the potential to be catastrophic for London occurred on 27 October 1997. The dredger, MV Sand Kite, sailing in thick fog, collided with one of the Thames Barrier's piers. As the ship started to sink she dumped her 3,300 tonne load of aggregate, finally sinking by the bow on top of one of the barrier's gates where she lay for several days. Initially the gate could not be closed as it was covered in a thick layer of gravel. A longer term problem was the premature loss of paint on the flat side of the gate caused by abrasion. One estimate of the cost of flooding damage, had it occurred, was around £13 billion.^[2] The vessel was refloated in mid-November 1997.

The barrier was closed twice on 9 November 2007 after a storm surge in the North Sea which was compared to the one in 1953.^[3] The main danger of flooding from the surge was on the coast above the Thames Barrier, where evacuations took place, but the winds abated a little and, at the Thames Barrier, the 9 November 2007 storm surge did not completely coincide with high tide.^[4]

Ownership / operating authority

The barrier was originally commissioned by the Greater London Council under the guidance of Ray Horner. After the 1986 abolition of the GLC it was operated successively by Thames Water Authority and then the National Rivers Authority until April 1996 when it passed to the Environment Agency.

Future

The barrier was originally designed to protect London against a flood level with a return period of 1000 years in the year 2030 after which the protection would decrease but be within acceptable limits ^[5]. This defence level included long term changes in sea and land levels as understood at that time (c. 1970). Since then sea level rise due to global warming has been identified. Based on current estimates^[6] the barrier will be able to cope with projected sea level rises until around 2060–2070 and is expected to serve its full term. Since 1982 (up to 2007) the barrier has been raised over 100^[7] times; further, it is raised every month for testing.

In 2005, a suggestion was made public that it might become necessary to supersede the Thames Barrier with a much more ambitious 16 km (10 mi) long barrier across the Thames Estuary from Sheerness in Kent to Southend in Essex.^[8]