That's Lincolnshire green
Alleviation and defence schemes are expensive, particularly where built defences are required. But an integrated approach - and innovative re-use of existing defences - can prove cost-effective and environmentally sensitive too.The Lincolnshire town of Gainsborough is protected from flooding from the tidal Trent by nearly 4km of defences in the form of sheet pile walls, mass gravity walls and earth embankments. The town is particularly vulnerable to flooding due to the meeting of tidal and fluvial floodwaters and the low-lying nature of the landscape, with a number of buildings located close to the river.
Following major flooding in the town in 1947, a flood relief scheme was implemented, providing the rigid flood defence along the river. This was raised after another significant tidal surge in 1954.
In the late 1990s, 800m of the defences were replaced when a National Rivers Authority (NRA) report revealed that the defences – having been in place for more than four decades and in most places founded on existing structures, some of which were more than 100 years old – were likely to fail within five years.
This project, completed in 2000, addressed those assets in urgent need of repair and included traditional sheet piled walls along with a piled free-standing structure in the bed of the river.
In late 2003, the Environment Agency (EA) and Atkins took the initial steps in developing a programme of asset replacement for those remaining assets. For each flood defence section, the team considered options to replace, improve or continue to monitor the assets. Ken Ford, principal engineer at Atkins, explains: “Our focus was on maximising value while making sure that the scheme was sustainable in both design and construction. The project team compared the whole life cost and present value of new walls against carrying out remedial works. “With the exception of one failed wall, the existing defences did not show signs of major distress and we concluded that it would be more cost effective to extend the life of these existing assets through a programme of strengthening and improvement works rather than replacing them with new defences.
“Our design to strengthen and re-use existing structures, which included large areas of the sheet piled wall, saved £7M compared to the cost of a new-build defence scheme! There were also clear carbon benefits, saving approximately 1,100 tonnes of carbon compared to the carbon spend of constructing new defences.”
Throughout design and construction, the project team identified many opportunities to refine the scheme and its implementation so that costs could be reduced and to manage and mitigate risks. The team took an integrated risk-based approach, drawing together a range of framework partners and specialists together from the outset.
“By developing a highly-performing, integrated team with all the appropriate skills immediately available, we were able to successfully manage risks, identify opportunities to improve the value of the works, and work successfully with the local community to deliver the programme safely,” says Ford.
“Overcoming problems and risks jointly as a team enabled us to identify better solutions more quickly and we could avoid all manner of unnecessary and abortive costs.”
The team also had a rigorous culture of challenging proposed changes and risk mitigation options.
This informal approach to value engineering and openness to innovating ideas ensured value for money and helped keep a check on scope creep and additional costs enabling a further saving of £3M during delivery of the scheme.
Construction works, undertaken by Birse Civils, began in June 2006 and completed in September 2010. More than 440 ground anchors were installed over the tidal river to strengthen 700m of existing sheet piled flood defence walls.
Recycled plastic sheet piles were also used to strengthen 1.75km of existing earth flood defences. This has subsequently been used as an example for other schemes.Community support of any major project with substantial impacts on local residents and owners of the private land on which much of the defences are built is also critical to keeping costs down. A dedicated public liaison representative was appointed to work with local stakeholders and schools and the project was designed to embrace local community schemes, engaging residents with the works through an artwork scheme and other area enhancements.
Ford says: “The Considerate Constructors Scheme inspector was particularly impressed with our community engagement programme. “We held consultations with the public to discuss the benefits of the works when complete and presented in local schools to increase awareness. “The community responded in a very supportive way and the design solution included many enhancements to the area. It now boasts a seasonal wetland for wildlife and an artistic public footpath along the flood defence embankment made of more than 8,000 coloured squares!” The flood alleviation scheme at Gainsborough has greatly improved the condition of the flood defences and significantly reduced annual flood risk to more than 2,600 properties, with risk dropping from a 1-in-70 to a 1-in-200 annual chance event.