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Storing the problem

Banbury has suffered from two devastating floods in the past 13 years. Declan Lynch reports on a new flood alleviation scheme for the town.

Memories of 1998’s devastating flooding in Banbury are still fresh. The floods closed the railway station, shut local roads and caused more than £12.5M of damage to properties.

Serious flooding nine years later again highlighted the need for flood defences.

As result, the Environment Agency is constructing a flood alleviation scheme in Banbury by creating a storage reservoir north east of the town to protect it from flooding. Main contractor Galliford Try is building the 2.85km earth embankment which forms the protective dam wall.

1998 flood

“The genesis of the scheme was the 1998 event,” explains Environment Agency area flood risk manager Ian Tomes.

Between 1999 and 2001 the Environment Agency and its designer Black & Veatch developed a flood alleviation scheme to protect Banbury from a 1 in 200 year flooding event.

The geology around Banbury - a clay-lined valley which reacts quickly to heavy rain - means the area is prone to flooding.

“Having raised expectations locally that the scheme would be built, we weren’t in a position to walk away because the funding had changed”

Ian Tomes, Environment Agency

Engineers decided the best way to protect the town was to create a flood storage reservoir to the north east of the town beyond the nearby M40 motorway.

“With the available space north of the M40, an upstream storage scheme was the most obvious option,” explains Environment Agency project manager Richard Harding who has been working on the project since its inception.

Engineers are creating a flood storage area to the north of the M40 to collect rainwater from the Cherwell Valley, which would normally swamp the River Cherwell which runs through Banbury.

Difficult inception

However, bringing the project to life has been difficult for the Environment Agency.

“In many respects the design has been the easy bit,” says Harding.

Once the design was in place, the Environment Agency began discussions with the farm owners whose land would be used as part of the flood storage reservoir. It took longer than expected, but the Agency reached agreement with most of the landowners. Funding changes also delayed the scheme.

Originally the £15M scheme was to be funded by the Department for the Environment Food and Rural Affairs (Defra) via the Environment Agency - typical of other major capital spend schemes.

Funding withdrawn

But in 2006 the way Defra withdrew funding for the project after it changed its funding criteria increasing its focus on coastal flood defences.

“Having raised expectations locally that the scheme would be built, we weren’t in a position to walk away because the funding had changed,” explains Tomes. Severe flooding in 2007 further highlighted the need to protect Banbury.

With funding unavailable from central government, the Environment Agency set about looking at other options.

The only option was to use local funding raised via levies on property owners collected by the Thames Regional Flood Defence Committee.

Other funding sources

However, with an annual budget of £10M per year for the whole region, spending £15M on one scheme meant other funding had to be found. “We persuaded other stakeholders to help fund it,” adds Tomes.

Cherwell District Council is contributing £2M, Network Rail £300,000 and Thames Water a further £430,000.

With funding in place, the scheme won planning permission in May 2010, and Galliford Try was selected as main contractor.

There are four main elements of work for the scheme, the flood storage reservoir, localised defences in Banbury at Wildmere and Tramways Industrial Estate and a pumping station at Moorfield Brook constructed back in 2004.

Earth embankment

Main contractor Galliford Try is constructing a 2.8km earth embankment with an average height of 2.5m, to form the dam wall. The embankment runs from north to south between the Oxford Canal to the west and the River Cherwell to the east.

After completion, the storage area will be classed a “reservoir” under the Reservoir Act and undergo regular inspection.

An 860m stretch of the A361, which runs through the reservoir area, is being raised by 750mm. Culverts under it will allow water to flow through the storage area . During major flooding, the road will be closed and a diversion put in place.

The embankment contains two identical flow control structures which regulate the water flow during a flooding event. The two concrete structures have 30m long tapering slipways, which are 2m wide at the upstream end and 10m wide at the downstream end.

Throttled water flow

During flooding events, the upstream openings throttle the water flow, holding back floodwaters. “It’s much cheaper than a proprietary system,” explains Black & Veatch site supervisor John Hopkins. “Because it’s a passive system there’s no need to connect it to a power system, or the maintenance which goes with it.”

Black & Veatch and the Environment Agency developed the concept and it underwent full-scale testing at HR Wallingford. It was developed for this scheme but has already been used in Scotland.

Building the southernmost flow structure was the most challenging according to Galliford Try project manager Peter Caulfield.

“It was a tight site - we had a 132kV overhead line limiting crane height, a public footpath on the southern side and utility diversions,” explains Caulfield.

Borrow pit

The 2,850m embankment requires over 100,000m3 of earth, so key to ensuring the scheme was cost effective was being able to use a borrow pit locally.

“We looked at a borrow pit in the storage area but the archaeology meant that we choose a bit between the embankment wall and the M40,” adds Irving.

Engineers used a borrow pit between the Oxford Canal and the M40 to create the embankment walls.

“Because it was soft ground, we needed to allow for settlement,” adds Caulfield.

Engineers have created three river diversions to allow the embankment to be built safely, one at the northern end and two around the flow control structures.

For extreme flooding events, there are three defences downstream of the reservoir in Banbury Town centre.

First, at the Wildmere industrial estate, engineers are constructing a 400m, rising to 2m high earth embankment.

Sheet pile walls

For the second, engineers are installing 210m long sheet pile walls and a 100m long, 2m high earth embankment wall at the Tramways Industrial estate. It protects the industrial estate and Banbury Football Club from flooding.

The final piece of the scheme is Moorfield Brook pumping station. This was constructed in 2004, under the original funding arrangements. It protects over 400 homes in the Grimsbury area of Banbury and was effective during the 2007 flooding. The pump station collects local flood water and pumps it downstream into the river Cherwell.
Once construction is complete, towards the end of the year, the borrow pit will be transformed into a country park for use by Cherwell District Council. Over 12ha of Biodiversity Action Plan (BAP) habitat will be created from the scheme, and engineers are seeding the embankments and planting new hedgerows.

“After one growing season, you won’t even know we’ve been here,” adds Caulfield.

The project is due for completion in Spring 2012.

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