Thames Barrier

Flooding in London has been a problem since the city was first established in Roman times.[1] In 1954 the Waverley Committee, established to investigate the serious North Sea flood of 1953 which affected parts of the Thames Estuary and parts of London, recommended that "as an alternative to raising the banks, the possibility and cost of erecting a structure across the Thames which could be closed in a surge should be urgently investigated".[2] A large number of designs were put forward, ranging from a huge road viaduct with two 500 foot (150 m) sluice gates crossing the Thames at Crayfordness to flap gates lying on the river bed and floated up by compressed air.[3] By 1965, when the Greater London Council (GLC) took over responsibility, two major schemes were under consideration, costed at £24 million and £41 million respectively[3] (£500 million and £800 million at 2020 prices).

In 1966 Sir Hermann Bondi was asked to take an independent view of the situation. He considered the estimated construction costs and the probability of a flood and of damage if the barrier was not built. He strongly recommended that a barrier should be built in order to avoid the catastrophe of flooding central London, and a site was agreed at Woolwich.[3]

The barrier protects central London against a storm surge, caused when a deep depression forms to the north of Scotland and progresses across the North Sea and south-easterly towards southern Scandinavia. When such a surge coincides with a high spring tide, the high winds associated with the depression can funnel the water up the Thames Estuary and cause surges of up to 3.5 metres. The planners assessed that in the absence of a barrier, such a surge could inundate 45 square miles (117 km²) of land, put hospitals, powers stations and the London Underground out of action and cause damage estimated in 1966 at £2.0 billion[4] (about £50 billion at 2020 prices). The barrier was designed to provide a flood defence capable of resisting a once in 1000 year surge tide at a base date of 2030.

Diagram showing how the gates work, though the barrier actually rises further than this to allow water to "underspill" under the barrier in a controlled fashion

The concept of the rising sector gates was devised by (Reginald) Charles Draper. In 1969, from his parents' house in Pellatt Grove, Wood Green, London, he constructed a working model. The novel rotating cylinders were based on the design of the taps on his gas cooker. The barrier was designed by Rendel, Palmer and Tritton for the Greater London Council and the concept tested at the Hydraulics Research Station, Wallingford[1]. The site at New Charlton was chosen because of the relative straightness of the banks, and because the underlying river chalk was strong enough to support the barrier.

The Thames Barrier and Flood Prevention Act, authorising construction, was passed in 1972. In 1974 the GLC placed the two major construction contracts. Civil construction was undertaken by a Costain/Hollandsche Beton Maatschappij/Tarmac Construction consortium.[3] A separate contract for the gates and operating machinery was placed with the Davy Cleveland Barrier Consortium[3], formed by Davy McKee Ltd[5] of Sheffield and Cleveland Bridge UK Ltd.[6].

Work began at the barrier site in 1974 and progressed in two phases. The southern piers (9 to 6) were built first, with river traffic diverted to the north side, then traffic routed through the completed southern spans whilst the north side piers (1 to 5) were built. During construction of the piers, precast concrete sills were built in a cofferdam on the north side of the river and floated out and sunk between the piers to form the gate recesses, with access tunnels at the upstream and downstream ends.[7]

The gates of the barrier were fabricated in sections at Cleveland Bridge's Darlington works and assembled at Port Clarence on the River Tees.[8] The gates, gate arms and rocking beams were transported from the Tees to the Thames by barge and lifted into position by two very large floating cranes operated by Neptun of Hamburg (now part of SMIT).[9] The mechanical and hydraulic machinery was built by Davy Loewy, Henry Berry and Vickers and trial assembled in Davy's Darnall works. Delays to the civil works required changes to the construction and installation sequence, but commissioning was relatively uneventful and the first trial operation of all the gates together was carried out on 31 October 1982.[10]

In addition to the barrier, the flood defences 18 kilometres (11 mi) down river were raised and strengthened. The barrier was officially opened on 8 May 1984 by Queen Elizabeth II.[11] The barrier cost £461 million (£1.16 billion now).[11][12] Total construction cost was around £534 million (£1.6 billion at 2016 prices) with an additional £100 million for river defences.

Built across a 520-metre (1,710 ft) wide stretch of the river, the barrier divides the river into four 61-metre (200 ft) and two approximately 30-metre (100 ft) navigable spans. There are also 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 40 millimetres (1.6 in) thick. The gates are filled with water when submerged and empty as they emerge from the river. The four large central gates are 20.1 metres (66 ft) high and weigh 3,700 tonnes each.[13] Four radial gates by the river banks, also about 30 metres (100 ft) wide, can be lowered. These gate openings, unlike the main six, are non-navigable.

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 metres (16.0 ft) in central London. Though Teddington marks the Normal Tidal Limit, in periods of very high fluvial flow the tidal influence can be seen as far upstream as East Molesey on the Thames.[14]

The barrier from Silvertown on the north bank of the river during normal operation looking across to New Charlton.

During the barrier's entire history up to March 2020, there have been 192 flood defence closures. 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.[16] 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.[17]

On 20 August 1989, hours after the Marchioness disaster, the barrier was closed against a spring tide for 16 hours "to assist the diving and salvage operations".[18]

The barrier has survived 15 boat collisions without serious damage.[19]

On 27 October 1997, the barrier was damaged when the dredger MV Sand Kite, operating in thick fog, hit 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. The vessel was refloated in mid-November 1997.[20]

The annual full test closure in 2012 was scheduled for 3 June to coincide with the Thames pageant celebrating Queen Elizabeth II's Diamond Jubilee. Flood risk manager Andy Batchelor said the pageant gave the Environment Agency "a unique opportunity to test its design for a longer period than we would normally be able to", and that the more stable tidal conditions in central London that resulted would help the vessels taking part.[21]

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.