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People project: Godmanchester's award winning flood defences

Sensitive stakeholder management, won the Godmanchester flood alleviation project the 2014 British Construction Industry award for outstanding contribution to the UK construction industry.

Godmanchester is a pretty, quintessentially English town north west of Cambridge. The Great Ouse river splits, and winds its way through the area creating picturesque views from the homes situated along its banks. Across the river from the town lies Portholme Meadow, the largest ancient water meadow in the UK and a designated site of scientific interest and special area of conservation.

Unfortunately, however, the town has a long history of flooding, with the most recent flood event in Easter 1998. After this, informal defences provided by garden walls and low earth embankments were strengthened by the deployment of sandbags at low points to prevent more serious flooding.

In 2003, flood water started to back up through the drains onto a nearby road during what was considered to be a 1 in 20 year event. Fearing a 1 in 100 year event would cause even more devastating consequences, the Environment Agency started to look at a business case for the provision of formal flood defences.

Godmanchester completed work

Completed work: Flood wall built along the causeway and around the Town Hall

It was estimated that the devastating floods in the summer of 2007 cost the country a total of £3.2bn, of which more than £2bn fell on homeowners and businesses. Estimated to have cost the average home between £23,000 and £30,000 and having forced 30% of households to relocate to temporary accommodation, the maths for improved defences at Godmanchester stacked up.

Plotting the 1 in 100 year event flood line on a map of the area revealed that over 524 residential and 42 commercial properties were at risk of flooding. Damages predicted to result from such an event were estimated to be around £115M and the costs of preventing this, including future maintenance over the life of the scheme was calculated at £13M.

Godmanchester work underway

Work underway: Mill Stream flood wall being constructed

Despite the obvious cost benefits of the project, it was a bumpy ride to getting the required funding. It was stalled twice when the Flood Defence Grant in Aid (FDGiA) repeatedly cut funding in 2007 and 2010. The project team’s determination to pursue funding was one of the reasons the project won the 2014 British Construction Industry Awards (BCIA) Outstanding Contribution to UK Industry Award.

However, with funding secured in December 2011, construction of the £9.3M scheme started in April 2012.

Godmanchester completed work

Mill Stream wall after work had finished

Consultant Atkins was appointed principal designer and worked hand in hand with contractor Jackson Civil Engineering. The teams were co-located and used the Projects In Controlled Environments (PRINCE2) form of project management which is used extensively by the government.

“[The approach] provided clear governance, with the right people undertaking the key roles with the right supply chain partners and allowed them to undertake their jobs affectively,” explains Anglian Environment Agency project team manager Andrew Rouse.

Project team

Client: the Environment Agency
Principal designer: Atkins
Principal contractor: Jackson civil engineering

The team also took inspiration for management of health, safety and the environment from the Olympic cycling team using its philosophy of “marginal gains”.

“The principle being that if you break everything down, and then improve it by at least 1%, you will get a significant increase in overall performance when you put them all together,” says Rouse.

Overall, the project worked to provide flood defences to all the properties at risk. It comprised three main elements:

  • 1.5km of engineered defences consisting of earth embankment, sheet piles and masonry clad reinforced concrete walls
  • river widening and dredging to improve conveyance
  • two pumping stations which would control seepage through the underlying soils.

Godmanchester completed work

Ladnscaped: Flood wall built within landscaping to reduce visual impact

As far as possible the defences were standardised. However, where necessary, bespoke solutions were used. Defences for 80 listed structures had to be sympathetically incorporated into the design and a tree surgeon was employed to minimise the impact of the scheme on the local trees, many of which were well established.

One of the challenges of the scheme was to thread the new defences through the back gardens of 31 private properties which were closest to the river.

“Those works really benefited the properties that were located some way away from the river. They [the homeowners] were supportive of protecting the historic town of Godmanchester,” says Rouse.

The project also included work on Cook’s Stream and on The Avenue, a picturesque grassy area next to the river lined with mature trees, restricting the working area available. Located close to the centreline of the valley floodplain, this area is underlain by seams of course gravel providing a seepage path for flood waters to flow under the new defences. Sheet piles were installed within the new embankments to cut-off seepage flows and to retain and raise the embankments by the required heights, ranging between 800mm and 1.8m. River banks were raised up to the top of many of the flood walls, giving a softer appearance to retain the unique landscape character and setting, while providing a hard defence against rising water levels.

Godmanchester before work

Before: Arches were silted up, impeding water flow

Another part of the project involved removing silt that had built up over time within four of the arches of the Grade II* Listed masonry arch bridge over Cook’s Stream. Four of the arches were silted up,significantly reducing the flow capacity under the bridge, causing higher flood levels upstream. To combat this, the silt was cleared out from between the arches, the river banks and existing embankments were realigned and a berm was included to facilitate future maintenance. The soft engineering of the reinstatement included coir rolls (to provide erosion control and rapid vegetation establishment) and reed planting at the waterline to enhance the rich ecology of the watercourse.

“This was an opportunity to redefine this section of the watercourse, to enable it to be maintained safely and reinstate the amenity for canoeists and boaters to access Cook’s Stream into the future,” says Chubb.

“The sensitivity of working around such an old and important structure made the works pretty awkward, only using small and specialist plant in and around the narrow arches to avoid damaging the bridge.”

Godmanchester completed work

After: Channel widened and arches cleared, improving flow

A key part of the scheme was the installation of two pumping stations in Rectory Gardens and at Cook’s Stream.

BCIA judges comments

“This was a high quality flood protection scheme delivered by a passionate project team.

The judges appreciated the commitment from the client in gaining funding for the project together with the extensive measures put in place to engage with the community to deliver the project over seven

years. An excellent example of outreach and long term team commitment to achieving a result that was bespoke for each resident but beneficial to many communities.”

“The design concept was to capture and remove increased surface and ground water flows during a flooding incident and to alleviate any potential flooding to the rear of the new defence alignment,” says Rouse.

Atkins principle engineer Richard Chubb adds: “The pumps are located in the low spots behind the defences to control the seepage through the underlying gravel layers. Although a cut-off was provided below some of the flood defences this was not possible in some sensitive and constrained areas such as private gardens. This also provided the opportunity for linking the Rectory Gardens pumping station to the town’s surface water drainage network, providing a huge improvement as it cannot operate under gravity during times of high river levels.”

The capacity of the Rectory Gardens pumping station is 300l/s. It discharges through a single 400mm diameter rising main, installed by directional drilling under an embankment which carries the A14, into a receiving watercourse downstream.

The scheme has already proved its worth as elements were tested during construction over the wettest summer on record in 2013. 

Godmanchester completed work

Sensitive: New floodwall incorporated into boundary wall and existing floodwall in residential garden

“The elements of the scheme completed by this time stood up extremely well and prevented properties from being flooded,” says Rouse.

He adds that he is extremely pleased with the way the local community facilitated the work.

“My favourite element of the scheme was interacting with friendly residents, who, although they were going through a time of disruption extremely close to their homes, had the faith and vision to see what we were going to achieve, he says.”

“This was only possible through working as an integrated team [with our contractor Jackson Civil Engineering] setting and agreeing the project objectives and being clear of the high standard required and resolving issues quickly,” he says.

Working closely with local residents was also a key factor which helped the project team secure its BCIA award.

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