Most of the time the River Cole in north Warwickshire does not look like a river that can surge into the heart of a town’s manufacturing centre, paralysing industry and causing havoc for residents. But first impressions can be wrong. The river has burst its banks in the market town of Coleshill on numerous occasions through history. The river has also flooded a nearby quarry.
In a £2M three phase project that stretches back to the late 1990s, engineers at Cemex UK, working in partnership with the Environment Agency, have been working on a flood alleviation scheme that reduces the flood risk to 1 in 100 in any given year.
Cemex central region development manager Tony Rowley says: "The River Cole is tight alongside the western boundary of our Coleshill Quarry. Most of the flooding was a result of water pouring down the channel and over spilling into the industrial estate, which was part of the flood plain. There was also an issue with stormwater run-off from the town."
The Cole flows into the River Blythe to the north of the site and along much of its length had been canalised – straightened with a consistent cross-section installed – to improve the water flow. But studies carried out with the University of Birmingham helped show that by designing meanders back into the river, engineers could boost its carrying capacity as part of a flood management scheme. "reviously it had all been about getting the water away as quickly as possible but by slowing down the flow [with meanders], the river water fills more of the cross-sectional area of the channel and as a consequence increases its capacity,"says Rowley.
Holding more water in the Cole means that it no longer surges down into the Blythe, causing it to burst its banks and create a backlog of floodwater with nowhere else to flow but into the floodplain. It was also decided that a 540m section of the river would be diverted away from the town and placed behind a flood bund.
The flood bund is made of Keuper Marl – a layer of red clay deposited during the Triassic period, which underlies the entire site. "During that particular phase of work we built three bunds using the marl," says Rowley. "Two helped form the river channel while the third became the flood protection bund." This section of work would be split into two phases.
The first leaves a natural sand and gravel bed and side slopes to the river channel while the second comprised a channel formed from the Keuper Marl arisings. To construct the sand and gravel channel the centre line of the proposed river course was laid out and offsets plotted from each side of this line to sterilise the deposits along its length. Overburden material lying above the sand and gravel helped create the channel banks while Keuper Marl from a borrow pit on the adjoining quarry site formed the flood bunds.
A similar construction method was used for the final stage of the scheme to the north of the site except that here the channel bed too was lined using the marl arisings. The site has been far from easy to work on in part thanks to the clayey, tacky nature of the sand gravel and overburden and also the timing of the work. Mineral extraction also took more than two years in the first phase of the river diversion.
Engineer: Halcrow and the Environment Agency
Main contractor/project manager: Cemex UK
Earthworks contractors: Greyhound Plant and Jones Plant Services.