Keeping the town dry

The convergence of three spate streams in Ross-on-Wye presented Edmund Nuttall with a difficult flood alleviation scheme

It takes a while for things to happen in Herefordshire. From the maturing of its beef cattle to the ripening of its cider apples, things are not to be rushed.

Except, that is, when it rains and millions of gallons of water surge down from the foothills of the Welsh mountains into the River Wye, causing it to bulge and threatening local communities.

The market town of Ross on Wye is one such community. Its outskirts are often threatened by the rising waters of the river but it is the waters of three streams, tributaries to the Wye, that have brought misery to the town for more than a century.

Now though, an ingenious flood defence system being installed by main contractor Edmund Nuttall should dry out the town for good.

But as Martin Jackson, project manager for client Herefordshire Council, points out, the £7 million Ross on Wye Flood Alleviation Scheme will concentrate on negating the threat of flooding from the streams which run into the town rather than the River Wye itself.

"Rudhall Brook, Chatterley Brook and Srnallbrook have caused flooding problems in Ross-on-Wye for a century and flood alleviation schemes have been talked about for most of that time," he says. "But it was the December 2000 floods which focused the mind."

Back then heavy downpours saw torrents of rainwater gushing down the three brooks, helping them burst their banks and sending floodwater through the town centre.

Bottle neck

'”We had huge volumes of water converging on the Kings Acre car park area of the town which acts as a bit of a pinch point The existing culverts were not big enough to deal with the flash floods and the town centre was inundated," he says.

Kings Acre has always acted as something of a sacrificial lamb with flood water ponding in the area, helping avoid flooding in the town centre. But the December 2000 downpours proved far too much initially for the aging Victorian culvert that distributes the Chatterley Brook and then the Kings Acre floodplain itself.

The flooding forced a rethink and within weeks tentative designs had been drawn up that would help stop the problem.

These included the simple step of enlarging the existing culvert dismissed as impractical and serving merely to shift the problem further downstream - to much grander schemes that just did not work.

"There were approximately five designs that were discussed and we started to whittle these down," says Mr Jackson.

But as stated, things mature slowly in Herefordshire and the process of getting the criteria in place to ensure the council could apply for central government funding from the Department for Environment, Food and Rural Affairs took some time. Eventually in 2004 plans were submitted to Defra but even then securing the cash proved difficult (see box, right).

"As part of the scheme there were several compulsory purchase orders we had to go through as well as winning planning permission and getting land drainage consents. Each area presented problems," says Mr Jackson.

Funding was confirmed in early May 2007 and focus could move to work on site, placing the innovative design that will ease the likelihood of flooding from the three streams.

"The most unusual section of the project is the inverted siphon," says Chris Gilmartin, project manager for consulting engineer Amey. It’s an ingenious solution."

In simple terms the inverteded siphon is an underground chamber bored along the lower reaches of the Rudhall Brook from its confluence with the Chatterley Brook down to its emergence to the north west of the town centre.

Underground relief

During floods, water will be diverted through this chamber underneath the town before flowing back out into the enlarged channel of the Rudhall Brook shortly before it flows into the River Wye.

But like most outwardly simple systems, beneath the face of the concrete lies a brilliantly engineered design.

Rain water sloshing down the Chatterley Brook and Smallbrook will converge in a new, enlarged, 3.6 m-wide by 1.5 m-tall precast concrete box culvert. During normal flow conditions this will carry water down the 100m or so length of the culvert, through the cast in situ reinforced concrete bifurcation chamber and on into the existing culverted section of the Rudhall Brook.

During storm flows, though, the design of the bifurcation chamber will see excess water diverted through into the falling shaft of the siphon, helping reduce pressure on the streams.

The falling shaft itself is a twin diameter chamber cornprising a 10.3m internal diameter outer ring with a 6m internal diameter main shaft slightly off centre to the outer ring.

A 900 mm high, 350 mm wide upstand helps ensure water only flows into the 12.5 m deep main chamber when the three streams are reaching capacity.

Up and down

At the foot of the chamber a 2.1m internal diameter tunnel will run approximately 400m from the falling shaft, linking it to the 6m diameter rising shaft located at the western end of the Rudhall Brook, close to the banks of the River Wye.

"The tunnel itself is being bored through the sandstone and mudstone typical of the area at a depth to invert of between 12 and 13 m," says Edmund Nuttall sub agent Lee Davies. "It is programmed to be a 50 day drive so the tunnelling should be finished toward the end of May."

Along most of its length the tunnel will be bored underneath Greytree Road, one of the reasons why it is so deep.

"It means the tunnel is being bored directly through the rock and so there shouldn't be any settlement. Obviously we have to monitor these things carefully," says Mr Gilmartin.

Some 10,000 cu m of spoil will be excavated from the tunnel during the drive and although the majority of the arisings are to be dumped, the site team has managed to minimise the environmental impact as much as possible.

"Much of the material is wet sand and clay and is just not suitable for reuse in any of the work. We have managed to find a source of material for the earth embankment in Hereford not far from where we tip the tunnel arisings so we will be trucking material back as well," says Mr Davies.

Trapezoidal precast concrete is grouted into position behind the tunnel boring machine. The cementitious grout mix itself is an accelerated mix engineered for rapid strength gain so that there is no chance of the material polluting the aquifer.

With the tunnel due to be completed in May and final project completion expected in July, Mr Jackson is confident the residents of Ross on Wye will not be plagued by flooding from the three brooks again.

Further Info

For further information, contact Peter Bishop, Head of Public Relations at:
Edmund Nuttall Limited
St James House, Knoll Road, Camberley,
Surrey GU15 3XW
Tel: 01276 63484
Fax:01276 66060
E-Mail: peter.bishop@edmund-nuttall.co.uk