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Defending the Gateway

Thames Barrage - Support for a massive civil engineering project to protect London from a massive storm surge is gathering momentum. Damian Arnold reports.

Britain's highest tides for 20 years this autumn have focused the minds of civil engineers on a spectacular proposal to build a 9km flood barrage across the mouth of the Thames Estuary.

Should the high tides set for 7 and 10 of October come at the same time as stormy weather and wreak havoc along the Thames Gateway, then the clamour for the proposed barrage from Southend to Sheerness is likely to get stronger.

The barrage is a key component of an overall vision for London's Thames Gateway worked up by world famous architect Sir Terry Farrell.

Farrell was so disconcerted by the government's unstructured plans to build up to 200,000 new homes in the Thames Gateway that he assembled a multi-disciplinary team to draw up a blueprint for a community centred around Britain's first national park for 150 years.

Top of the agenda is one piece of ood defence infrastructure that would render the whole of the vast development area safe from the dreaded 1:200 year storm surge.

With sea levels rising by an estimated 3mm a year and the Thames Barrier expected to be redundant by 2030, consultant Scott Wilson was asked to come up with a grand project that would effectively solve the problem in the long term.

Appointed by Farrell in March 2006 and told to 'think big', the consultant worked up a scheme for the barrage, which in June was presented to mandarins from Department for Environment Food & Rural Affairs, the Department for Communities & Local Government at Westminster and to potential investors in the City.

The case was made for one of the biggest pieces of civil engineering infrastructure in Britain. A barrage that would be topped by a cable-stayed road and rail bridge to create a transport link between north Kent and the east of England.

'There's been a fair amount of discussion in the public and private sectors, and it's fair to say there is enthusiasm for what we are putting forward.

We will continue to see people in government and the private sector, ' says Eugene Dryer, urban design director at Terry Farrell & Partners.

Three man-made islands behind the barrage would be developed for housing and leisure uses and the money raised could help pay for the barrage project. The uplift in land values from the improved transport links in the area could also be used to contribute to the cost of the barrage.

'The banks are interested conceptually and it's now up to us to show them a reasonable business case, ' says Ian Marsden, regional director at Scott Wilson. 'It's do-able'.

'No price has yet been put on the scheme, but it is likely to be similar in cost to the Severn Barrage proposal, currently estimated at £14bn.

Scott Wilson has already engaged with the Environment Agency's flood-busting 20202100 team that is assessing long-term ood protection solutions in London's Thames Gateway.

'As far as they are concerned it is one option of many, ' says Scott Wilson principal consultant Jon Robinson.

Most engineering ood experts are agreed that a radical solution is needed for the long term.

'If the Dutch can build a 30 mile barrage to hold back the North Sea so can we, ' says Peter Brett Associates partner and ood defence specialist Ben Mitchell.

'Sea levels are rising at not insignicant levels so we've got to do something. Future generations will thank us for it.'

The barrage Solutions considered by Scott Wilson included upgrading the Thames Barrier and building a new barrier near Tilbury, where the Thames is still quite narrow.

The solution of a barrage between Shoeburyness, Southend, and Sheerness in north Kent would protect the vast area of land behind it slated development.

Further work would focus on whether the barrage would have gaps or locks to give access to shipping, or whether the barrier would rise hydraulically from the bed of the estuary during times of flood risk.

Construction would be made easier by the fact that the channel is relatively shallow with a maximum depth of 18m and average depth of 9m, says Mike Pauley, Scott Wilson's technical director for structures.

Three man-made islands built around the barrage would also help to 'throttle' the flow of water into London and lower the water level by an estimated 100mm.

'They would take up a volume of water and make it more difcult for the storm surge to come through, ' says Robinson.

The land reclamation would create two islands of about 8km 2 and one smaller island. Land would also be added to the Isle of Grain at the south end of the barrage.

But the plan does have drawbacks, says Mitchell.

'If you reduce the amount of water coming into the Thames Gateway the tidal range would be much less and you would reduce the salt marshes and mud ats that the wildlife relies on.' Mitchell says a better solution might be a rubber dam that is inated at high tides - a solution common in Japan.

He adds that the current ow of water is needed for the majority of the time to ensure that the Thames does not get silted up.

'Another danger is that the water not going to the Thames will go somewhere else along the coast and that could create problems, Mitchell says.'

The bridge A barrage across the Thames estuary would also create an opportunity for a road and rail bridge directly above and integral to the structure of the barrage.

The bridge would comprise approach viaducts on either side and a cable-stayed central span of 450m with 60m aerial clearance for shipping. The 150m high pylons would create an iconic image for the Gateway, says Scott Wilson's technical director for structures, Mike Pauley.

'We believe it would be something not dissimilar to the Øresund Crossing between Sweden and Denmark, ' says Pauley.

The bridge would open up a strategic route between north Kent and south Essex. It would be a key road and rail freight route from the Channel Tunnel and Dover, and up the east side of England and the East Coast Main Line. This would also free up commuter space for rail lines coming into London.

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