HUNDREDS OF homeowners close to the River Thames upstream of London face a serious risk of flooding this winter after engineers confirmed that a two year old $196M flood channel can carry just two thirds of its design capacity.
The capacity is limited by a catalogue of design and construction defects on the 11.6km long Jubilee River, which diverts water from the River Thames around Maidenhead, Windsor and Eton.
The errors are exposed in a damning report produced for the UK Environment Agency by consultant Atkins. They include fundamental mistakes such as building banks too low, using inappropriate materials and failing to follow standard design criteria (see box).
The findings have outraged The Royal Borough of Windsor & Maidenhead Council.
It now has to tell homeowners and developers that properties built or given planning permission since the scheme's opening in November 2001 are not safe from flooding.
'The news has shattered our confidence in the entire scheme, ' said cabinet leader Mary-Rose Gliksten.
'This gives us enormous problems. After they completed the Jubilee River our flood plain maps changed and permissions have been granted.
'Homes have been sold to people who thought they were protected against flood, ' added Gliksten. 'This is a huge issue for hundreds of people.'
The council was to hold crisis talks with the Environment Agency as NCEI went to press.
The Agency has recently issued new planning advice for the area. It confirmed that this was 'more precautionary' than that previously issued but refused to say how many people could be affected.
'What we have said to the council is that some areas that we have previously said are protected to a higher standard are actually protected to a lower standard, ' said Thames Region regional director Chris Birks.
'But this is by definition a flood plain. The Jubilee River does reduce risk, but the current reduction in risk is not as great as it was.'
Atkins was brought in by the Environment Agency after doubts were raised over the scheme's performance during floods in January 2003.
A peak flow of 144m 3/s was diverted down the channel, effectively saving around 1,000 homes in east and west Maidenhead from flooding.
But there was widespread flooding to homes downstream and the channel suffered major damage, even though the peak flow was just 67% of the channel's 215m 3/s design flow.
Residents' groups still dispute the findings of an official independent report into the effects of the Jubilee River on flooding downstream (see box). It concluded that the Jubilee River could have raised downstream water levels by a maximum of 3mm.
But Atkins' study has since revealed significant errors in the Lower Thames model that was used to design the Jubilee River and formed the basis of the official report.
Atkins has concluded that flood water levels should have been assumed to be up to 300mm higher at the downstream boundary, during modelling.
This throws into question the findings of the official independent report.
It also believes that modelling should have anticipated that more water would have been held back by key structures like weirs and bridges contributing to higher flood levels.
These factors, plus the fact that the banks were too low, mean that in its current condition the channel and the Thames can convey a maximum flow of 415m 3/s, 81% of the original 515m 3/s combined design capacity.
If operated according to original design procedures agreed at public inquiry this flow capacity reduces to 325m 3/s - just 63% of original design specification.
The Agency said that the banks will be raised to their design level, but this work will not be complete until 2005/06 at the earliest.
The Agency has already committed to spending $3.5M on emergency repairs to a 300m section of structurally unsound bank near the southern end of the scheme.
'Restoring the banks to their original design height is relatively simple and most of them will be raised as they are.
Some will require more work and are currently being looked at, but most will be completed by the end of next year.'
But residents remain unsatisfied. Residents' group Community Support Group (South) is pushing for a meeting with Windsor & Maidenhead council ahead of its meeting with the Agency.
'We want to know what can be done to safeguard us in the coming winter, ' said Windsor resident Gillie Bolton. 'This is the most extraordinary horror story.'