The northern relief road proposals in Shrewsbury, put forward by the River Severn Partnership, includes designing a road embankment that could help contain and store floodwater away and upstream of the town.
But campaigners from the action group Save Our Severn commissioned an independent researcher to find out out how much water the proposed dam would hold in the event of a flood, and found it to be less than three days.
Save Our Severn says the research, based on Environment Agency data from the 2000 floods, has revealed "worrying flaws".
Could flood new developments
The group claims the dam would overflow within two-and-a-half days in a full flood, and to keep the river level below Shrewsbury’s floodplain, it would have to hold at least another 100 million cubic meters of water and that to achieve this, the depth of the dam wall would need to increase from 9.2m to 11.8m – 2.6 metres higher.
Estimations show once the dam overflows, it would have to submerge nearly 14,000 acres of agricultural land – almost double what the council is currently predicting – or else it could flood new developments and houses in the area.
A spokesperson from Save Our Severn said: “This multi-million pound mega dam is going to be a very expensive waste of taxpayers’ money.
“These are all issues we have raised with Daniel Kawczynski MP and the Environment Agency months ago and we have had no reply.
"It’s completely unacceptable but really does show how ill-thought-out these proposals are.”
The River Severn Partnership is made up of parties including Shropshire Council and the Environment Agency.