Turning back the tide10 January 2014
The 1953 east coast floods in England were the catalyst for construction of the Thames Flood Barrier. Charged with the responsibility of protecting Central London from flooding, the barrier is one of the world’s largest moveable tidal surge flood defences. Thirty years after the structure first swung into operation its future role is now being analysed in an effort to respond to the challenge of climate change. Suzanne Pritchard reports.
The devastating east coast floods which inundated areas of England on 31 January 1953, have been described as the worst national peacetime disaster to hit the UK. A high spring tide, combined with a tidal storm surge, saw many low lying areas of East Anglia and the Thames Estuary suffer from severe flooding. Exceptional weather conditions coupled with an inability to warn people meant that whole communities were unaware of the imminent threat of up to a 3m increase in water levels.
“Over 300 people lost their lives," Nathan Ellis from the Environment Agency explains, "with the east coast of England and the Thames Estuary feeling the full force of a disastrous flood. If the flood had got all the way to the highly populated, low lying areas of central London," he adds, "the results would have been even more horrific."
Lessons were learnt quickly though. The flood, the loss of life, and the prospect of the damage it could have caused led to a dramatic rethink in the way flood defences were built to protect London.
“Historically," according to Ellis, "the solution to flooding was to build higher and stronger river walls and embankments. This form of flood defence became popular following the Thames Flood Act of 1879 and remained an accepted method until the early 1970s. As a solution to flood risk raising walls had many advantages. Walls are permanent, easy to maintain and not likely to fail through human error. On the other hand though building walls higher and higher would eventually block the River Thames from view."
After the 1953 flood event the Waverley Committee was appointed by the government of the day to examine the danger and make recommendations for the future. Published in 1954, the committee addressed the issue of a surge tide combining with a high spring tide to create extreme water levels. The report favoured construction of a barrier, as opposed to a barrage, because it would not only stop the passage of water but could be left open to permit the passage of shipping: an important consideration for the River Thames.
In 1966 Sir Herman Bondi reviewed the investigations which had been carried out into the potential flood barrier. The best solution was considered to be bank raising in conjunction with a flood barrier to incorporate movable gates built across the river. Thus construction of the Thames Barrier and its associated defences began in 1974. The structure was completed in October 1982 and was first used in February 1983 to protect London from flooding. It was formally opened by Queen Elizabeth II on 8 May 1984.
One of the largest in the world, the Thames Barrier is a 500m wide moveable tidal surge flood defence which spans the River Thames at Charlton. Its ten steel gates protect more than 125km2 of Central London from flooding caused by tidal surges. Falling radial gates sit above the river making non-navigable sections, with rising sector gates resting on the river bed to allow traffic to pass through the barrier. In the open position the gates lie flat with the river bed.
The four largest rising sector gates span across 61.5m and weigh 3700 tonnes each, while the two smaller rising sector gates span 31m. The straight back of each gate measures 20m, 4m of which are below the riverbed and can hold back loads of 9000tonnes.
Each central pier of the barrier penetrates 15m into the river bed and is 50m from top to bottom, 11m wide and 65m long. They all house machinery rooms, service routes, stairways, a lift, local control room, crane for machinery maintenance and support to the adjoining cills.
The pier roofs consist of wooden structures covered by stainless steel plates, which are maintenance free. Looking like two ends of a boat tilted to 90 degrees, the pier roofs are ideally shaped for the machinery they house. They are also aerodynamic and reduce wind disturbance: an importance consideration for navigation purposes.
An individual gate can be closed in 10 to 15 minutes but to close the whole barrier for a flood defence closure usually takes one and a half hours. This provides enough time to check equipment and stops the potential of a reflective wave. The gates are closed in pairs from the outside into the centre and the ideal situation is for them to be closed just after low tide. When closing the barrier the total energy demand is in the region of 1.1MW. There are 11,000 volt power supplies across the barrier with three back-up generators available if the mains supply fails.
The main method of moving the gates is by using one of two pairs of yellow 'rocking beams', which are moved using hydraulic oil pressure. High pressure 190hp electric pumps in the power packs send oil to the main operating cylinders on the floors below. Substantial parts of the hydraulic system are duplicated so that each gate can be closed by any one of its two duty and two standby power packs.
A connecting link between the tip of the rocking beam and the gate arm transfers the beam movement and makes the gate rotate. A dual purpose shift and latch mechanism holds the gate into position - either the underspill or maintenance position. As an extra safety measure it can also be used to move the gate to the normal defence position.
Desilting at regular intervals ensures that the gates do not become jammed with built up silt from the river. When not in use the gates rest in concrete sills under the riverbed, the largest of these weighs 10,000 tonnes. There is a gap between each sill and gate (approximately 10cm) to allow the natural tidal flow to keep the sills clean. The gap is narrower at each edge of the gate, preventing large debris getting into the slot.
The Thames Barrier and its associated tidal flood risk management infrastructure are of vital importance. They protect 1.25M people, £200B worth of property, the UK government, major infrastructure and businesses. In the event of extreme flooding, without this level of protection, the Environment Agency says that many of London's famous and historic landmarks would be affected. These include:
• Canary Warf.
• Tower Bridge.
• Tower of London.
• Globe Theatre.
• London Eye.
• Houses of Parliament.
• Battersea Power station.
In London it is estimated that for every £1 invested in protecting communities from flooding, a saving of £38 in damage repair is made. And this is proving to be a worthwhile investment. As of 25 September 2013 the Thames Barrier had closed 124 times to protect London. "The barrier has been closed more often in recent years," Nathan Ellis from the Environment Agency states. And the big question is whether more closures will be necessary in the future.
The Thames Estuary's vulnerability to flooding is due to tidal surges occurring at high astronomical tides, and this is further compounded by the fact that the south eastern corner of the UK is slowly tilting downwards. Sea levels are rising and as a result the high tide in Central London is estimated to be rising at a possible rate of 75cm per century. Climate change is also an important consideration. The effects of thermal expansion of oceans and melting polar ice are all contributing to rising sea levels.
“We are examining the barrier for its potential design life under climate change," Ellis says. "It was originally designed to protect London from a 1 in 1000 year return flood on the year 2030. Early indications are that, subject to appropriate maintenance, the barrier will be able to maintain this level of protection to London against rising sea levels until at least 2070."
The system of existing flood defences has been described as of a higher standard than anywhere else in the country. Indeed the barrier provides an even greater degree of protection to London and the Thames estuary than originally anticipated: engineers planned for an 8mm/yr sea level rise but sea levels are currently rising by about 3mm/yr. However the current system must continue to be maintained to ensure reliability and reduce major costs in the future.
Wide ranging studies and investigations have been carried out to look at how tidal flood risk is increasing in the Thames estuary due to ageing flood defence structures; climate change and rising sea levels; as well as more people living and working in the floodplain.
The Thames Estuary 2100 (TE2100) project was formed in 2002 and led by the Environment Agency to develop a comprehensive action plan to manage tidal flood risk for the Tidal Thames from Teddington in West London, through to Sheerness and Shoeburyness in Kent and Essex.
This has been described as the first major flood risk management project in the UK to have climate change at its core. The results of the six-year TE2100 Plan were published in November 2012 and have been recognised internationally as a leading example of climate change adaptation. Recommendations included maintaining, refurbishing, and replacing the tidal flood risk management structures and assets up to the end of the century.
According to the report: "Investigations have shown that we are unlikely to need major changes to the existing system for the next 25 years. But from 2035 we may need to upgrade the current tidal defence system which will give opportunities to landscape the riverside so it's ready for the impacts of climate change.
“The plan is based on current guidance on climate change, but is adaptable to changes in predictions for sea level rise and climate change over the century. It was based on a relative sea level rise estimate of 90cm by 2100 but is adaptable to differing rates of sea level rise up to 2.7m, and an increase of 40% in peak river flood flows."
Peter Quarmby is Executive Manager of the EA's Thames Estuary Programme. "Working with the Met Office and other key organisations, we have used the latest science and improved our understanding of future climate change impacts in the Thames estuary," he says about the TE2100 plan. "Flooding, from any source, can cause great distress and disturbance to those who experience it. The primary risk of flooding to the Thames estuary communities however is from the sea. It is this tidal flood risk that the plan seeks to manage. The potential impacts of a tidal flood could be far more catastrophic than those from rivers or surface drains...Our plan is needed to provide confidence to those who live and work in London and the Thames estuary area that flood risk is understood and is manageable. Planners and investors will be reassured that there is an effective plan to manage flood risk today and for future generations."
The programme is now well underway with the introduction of Thames Estuary Phase 1. This flagship programme was launched with an industry event on 18 September 2013 at the Thames Barrier and interested parties have been invited to bid for a multi-million pound contract. Described as the UK's largest flood risk management contract, it will be made up of capital works including:
• Major refurbishment works of fixed assets, such as tidal walls and embankment.
• Refurbishing works of active assets (including major flood barriers).
• New assets such as pumping stations.
• Capital renewals and replacements.
• Packages of major or complex maintenance works, such as repairing long lengths or fixed flood defence walls or subsidence of earth embankments.
As Peter Quarmby, Thames Estuary Flood Risk Programme Director, summarises: "This is a major investment which will create jobs and protect future generations of Londoners from tidal flooding and the impacts of a changing climate."
2013 marks the 60th anniversary of the devastating floods which were the catalyst for the creation and improvement of London's flood defences. It has also been 30 years since the Thames Flood Barrier was closed for the first time to protect London from potentially damaging flood water.
Looking to the next 30 years and beyond, the Environment Agency is determined that the barrier and its associated defences will continue to play an important role and respond to whatever challenges lie ahead. As Peter Quarmby, from the EA's Thames Estuary Programme says: "A primary purpose of the TE2100 project has been to plan proactively for the future, rather than waiting for the next flood catastrophe to provoke society into action."
The Thames Estuary 2100 plan can be downloaded from www.environment-agency.gov.uk/homeandleisure/floods/125045.aspx
Information for this article was obtained from the Met office and the Environment Agency.
All images used courtesy of the Environment Agency.