Environment Agency engineers want to add up to £248M of flood defence channels to a two year old £110M Berkshire stormwater channel amid growing fears about flooding of the Thames.
The plans would effectively extend the 11.6km Jubilee River scheme, which diverts flood water away from the Thames as it passes through Maidenhead, Windsor and Eton, by around 16km.
The Agency is working up plans following the January 2003 floods when the Thames and its tributaries flooded 500 properties between Oxford and London.
It estimates that in January 2003, 1,000 properties were saved from flooding by the Jubilee River. But there was serious flooding downstream, with 128 homes flooded internally.
In all 10,000 properties remain at risk of flooding downstream.
The Agency is still in dispute with residents flood committee Community Support Group South (CSGS), which claims that mismanagement of the flood channel made downstream flooding worse.
This is despite an independent investigation into the cause of flooding produced by Arup associate Clive Onions which absolved Jubilee River of blame.
It has also given the Agency confidence to proceed with plans to extend the scheme.
The plans are a resurrection of a study carried out in 1992, which was thrown out because of the high cost.
Construction of the proposed new flood defence channels is complicated by the density and location of buildings and infrastructure in the flood plain downstream of Windsor.
Instead of a single channel, a series of three flood relief channels would have to be built, along with hard defences, through the centre of Staines.
Five options providing different levels of protection are under consideration. All remain technically and physically viable.
The construction costs of all five options have risen since original proposals were drawn up in 1992. But so too has the cost of damage.
This, coupled with new guidelines from the Treasury and the Department for Food, Environment & Rural Affairs (DEFRA), has encouraged the Agency to revisit the idea.
The cost of the work at 2004 prices would range from 92M for 1 in 18 year protection to 248M for 1 in 100 year protection. The 110M Jubilee River is designed to provide 1 in 67 year protection.
River reprofiling remains the cheapest option and offers a benefit to cost ratio of 2.6. But this is likely to be considered environmentally unacceptable by DEFRA.
The other four options offer different levels of protection through construction of different lengths of diversion channel.
In comparison with 1992, whole life costs have increased by up to three times. This is because of increases in construction costs, and a 185% increase in maintenance and running costs.
Consultants Babtie and KBR have been brought in to identify the best option before the Agency develops a detailed funding case.