PROPOSALS to collect rainwater in Oxford to help prevent flooding have been shelved after engineers concluded the idea could cost £140m.

Last October, city councillors ordered an investigation into whether rainwater harvesting on a large scale – collecting water from the roofs of buildings and storing it to use later – was feasible.

Engineers picked the St Mary’s ward, in East Oxford, for the study.

In his report to the council’s executive, engineer Steve Smith said: “Rainwater harvesting would be advantageous for conserving water, which is becoming a scare resource in the South East, and it could also potentially reduce carbon dioxide emissions by 0.4kg for every 1,000 litres of mains water not used.

“Storage tanks could be provided above or below ground level.

“But it could be difficult to find space to accommodate tanks above ground, especially in areas of terraced housing, and installing below ground level, soil excavation and disposal raise costs, and access for machinery would be a problem.

“In most cases, the water could only be usable if pumps were provided.

“Household plumbing would also need to be carefully adapted to enable the use of grey water without risk of cross-contamination.

“The UK Rainwater Harvesting Association estimates it costs between £2,000 and £3,000 to install a system – excluding excavation and disposal costs. Equipping St Mary’s as a pilot scheme would cost about £6m. For the whole of Oxford, it would cost £140m.”

By working out the surface area of roofs in the ward, engineers estimated 9.6 million litres of rain would have been collected in July 2007 – when flooding hit the county – if systems had been installed.

Mr Smith said: “The River Thames in full spate in July 2007 was conveying about 250 cubic metres of water per second through Oxford.

“The 9.6 million litres from St Mary’s would have passed along in 40 seconds. So a pilot scheme in St Mary’s alone would be insufficient to mitigate the current risk in the flood plain.”

The executive agreed to “encourage developers to incorporate harvesting into their current best practices”, but go no further.

Green councillor Matt Morton, who had proposed the investigation, agreed harvesting rainwater for the whole city was prohibitively expensive, but he said he would continue to research the subject, because he believed saving water would become more and more crucial in the future.

He said: “If we were to pilot a scheme in Oxford, I’m sure we would inspire the rest of the region to follow suit.

“As it turns out, the costs are prohibitive – at the moment at least.

“In the short term, it’s up to individuals. If you can find the space in your property, you can get a lot of benefits from collecting your own rainwater.

“Water is a precious commodity in the South East. We have a lot of water, but we have a lot of people and we’re always very close to drought.”