Agency plans London’s future flood defences

London’s flood defences should suffice until 2030 but the Environment Agency is already considering the options for its fortification.

The Thames Barrier was first raised 25 years ago and should continue to protect London for another 22. But it is unlikely to withstand the combined impact of climate change and the sinking landscape over the rest of the century, prompting the government to consider alternative options.

The Environment Agency, which is in charge of the barrier, will not make final recommendations to government until 2010, but its initial findings were released in March as part of a press offensive to pre-empt a disaster film being shown on ITV in the spring.

Flood shows the barrier being overtopped by a tidal wave. The Agency is adamant this could not happen but regards the film as a useful opportunity to get flooding messages across to the public.

The flood risk facing London is not new; the diarist Samuel Pepys wrote in 1663 of a high tide that swamped Parliament and the walls along the river have been raised throughout the city’s history. More than 1.25 million people now live and work in areas below the high water mark and there are assets worth an estimated £80 billion protected by a series of walls and barriers.

South-east England has been sinking since the end of the last ice age and climate change is expected to bring a further rise in sea level. The size of tidal surges is also expected to increase due to more intense low pressure events in the North Sea and higher river flows. London is particularly vulnerable as tidal waters get funnelled up the Thames Estuary. Risks are increased by development shifting eastwards into the Thames Gateway.

The current defences were developed in the 1970s after the devastating storm surge that hit the east coast in 1953. This killed several hundred people and would have been even more devastating had it reached London. At the time, the government decided the flood walls in central London could not be raised indefinitely and opted for a barrier that would keep out the highest tides and storm surges. Sea walls were heightened below the barrier at Woolwich, and secondary barriers put in place on the Dartford creek, at Barking and around Canvey Island.

The main barrier was finished in 1982 and has been closed 107 times since - used with increasing frequency over the past 25 years. The Agency believes it will still be able to meet its original design brief in providing protection from a 1-in-1,000 year event until 2030, despite the challenge of climate change. The original designers, who calculated future peak sea level rise by extrapolating from past events, allowed for annual increases of up to 8 millimetres a year. The current rate is around 3mm/year.

The Agency expects the barrier to be raised 30 times a year by 2030. Technical fixes plus the restoration of central London’s walls could extend the barrier’s life and protect against another metre of sea level rise. But its effectiveness will decline once it is used more than 70 times a year as there will be insufficient time for maintenance.

With modifications, the barrier should be able to cope with a 2.6 metre increase in peak sea levels. But the Agency is working on the assumption that peak storm surges will be one to four metres higher than today’s levels by 2100. It has set the upper limit above the latest predictions of the UN’s Intergovernmental Panel on Climate Change to take the uncertainties of polar ice melt into account.

An additional barrier alongside the existing one might protect against a rise of 3m, but beyond that the only option would be a fixed barrage in the outer estuary with locks to allow shipping through. The original barrier cost £1.3 billion at current prices but a fixed barrage would cost £20 billion, said the Agency.

A decision on a new barrier or barrage is not expected until at least 2030.

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