Managed realignment at Alkborough Flats, Lincolnshire

The Alkborough Flats managed realignment scheme is a major part of the Environment Agency’s flood risk management strategy on the Humber Estuary. Completed in 2006, the scheme covers 440ha and cost £10.2m to deliver. It is situated at the confluence of the River Trent and River Ouse. It features a newly created wildlife habitat and has been engineered to act as a storage site for floodwater.

The Alkborough Flats realignment site sits at the confluence of the River Trent and River Ouse
The Alkborough Flats realignment site sits at the confluence of the River Trent and River Ouse

The Alkborough Flats managed realignment scheme is a major part of the Environment Agency’s flood risk management strategy on the Humber Estuary.

Completed in 2006, the scheme covers 440ha and cost £10.2m to deliver. It is situated at the confluence of the River Trent and River Ouse. It features a newly created wildlife habitat and has been engineered to act as a storage site for floodwater.

What is Managed Realignment?

Managed realignment is an environmental management approach that involves altering the location of the line of defence, working to provide a more sustainable position from which to manage flood and erosion risks. It can involve advancement (moving forward), set back, or breach of the existing defence line. Most commonly, it involves establishing a new set back line of defence on the coast or within an estuary.

The need for managed realignment is driven by a number of factors, including historic and proposed development, climate change and increasing costs of maintaining fixed, linear coastal defences in the dynamic coastal environment. In the UK, much of the coastline is internationally designated for its conservation value. However, as coastlines naturally evolve and as sea levels rise through climate change impacts, coastal habitat is being lost where there is a sea defence in place – a process known as “coastal squeeze”. This creates a driver for managed realignment sites to provide replacement coastal habitat to compensate for losses at locations where the existing defence line must be maintained (or even advanced seawards), such as at ports, urban areas and other high value sites. Managed realignment at low priority coastal frontages can also alleviate the pressure of coastal forces on adjacent/nearby sites of higher value, which reduces the costs associated with maintaining essential coastal defences.

Typically, managed realignment involves breaching or removing the existing coastal defence – this can range from simply halting current management practices and allowing failure and breach of the defence line, to active removal of a defence in whole or in part. New defences are often constructed behind the original line to continue to protect key assets, and because they are usually less exposed to waves, and in estuaries the water level is also reduced, they can be lower in height and are not as expensive. In some cases it is possible to make use of existing high ground as the new line of defence; the land between the new and old defence is then opened up to the sea, with the resulting habitat created depending on the level of the ground relative to the tidal sea. The creation of coastal habitat such as saltmarsh also helps to absorb wave energy as it approaches the new line of defence. The result is an effective, sustainable solution to flood and erosion risk at the coast.

The need for the scheme

Around 90,000ha of land surrounding the Humber Estuary is at risk from flooding. The estuary has a highly dynamic tidal range, and much of the land sits below the high tide level. Major industries, power stations, farmland, the country’s biggest port complex and the homes of 400,000 people are all based on the floodplain.

The effects of climate change have led to increased flood risk in the UK, through both sea level rise and higher frequency of severe storms. Frontline flood defences have existed at the estuary for many decades, including an embankment wall built in the 1950s, but their standard of protection has significantly reduced.

The estuary also holds environmental importance and is designated under UK Habitats Regulations. Environment Agency studies have shown that valuable inter-tidal habitat around the estuary is at risk of loss from sea level rise caused by coastal squeeze.

The managed realignment scheme at Alkborough Flats is a key part of the Environmental Agency’s long term strategy to reduce flood risk at the Humber Estuary. It also delivers important environmental benefits by creating and preserving wildlife habitat.

Community Engagement

Most managed realignment schemes will seek to engage with local communities and stakeholders because of the level of impact on the surrounding region.

From the outset, the development team engaged with a wide group of stakeholder parties including landowners, farmers, ramblers, parish councils and local residents. Working groups were set up to consider specialist areas such as conservation and farmland management.

The contractor, Volker Stevin, placed particular emphasis on maintaining a good relationship with the local community throughout the entire duration of the project. Residents, farmers and parish councils were kept informed at all stages of the project. A traffic management plan ensured that local roads were not overloaded with heavy vehicles and the construction programme itself was adapted to align with farming cycles.

Delivery

The Alkborough Flats scheme was developed via a partnership approach involving the Environment Agency, Natural England, Associated British Ports and Lincolnshire County Council. Designed by Halcrow and constructed by Volker Stevin, the project cost £10.2million to deliver. Upon completion in 2006, it was the largest managed realignment scheme in Europe.

The existing flood embankment wall was retained, both to prevent re-meandering of the river and to protect navigation in the estuary. A 20metre-wide breach was created, through which water can flow in and out to inundate the site, according to the tidal cycle.

The remaining 1500m of the wall was lowered to act as a weir and prevent overtopping in extreme flood events. A new section of floodbank was also built on the landwards side, to protect other assets including a sewage treatment works.

Completion & Benefits

The breach in the flood bank was made on 6 September 2006, with the first tidal waters entering the inundation site two days later. Around 170ha of the site is permanently exposed to flooding, with the remaining area acting as extra storage capacity during extreme surges.

The scheme has increased flood protection over an area stretching along both the River Ouse (from the Humber Bridge to Goole) and the River Trent (as far as Keadby Bridge). For a 1-in-200 chance flood event, the scheme reduces extreme water levels in the upper estuary by over 150mm. Overall, the scheme is estimated to have delivered a financial benefit of £400,000 per year.

New inter-tidal habitat has developed at the inundation zone, including mudflats, saltmarsh and reedbeds. These areas provide food and habitat for a wide range of bird and fish species, allowing the estuary to meet the requirements of habitat directives. The estuary is one of the most important areas in Europe for birds, especially during winter. The new habitat will also help replace mudflats and saltmarsh lost elsewhere in the estuary due to rising sea levels.

The scheme has also delivered recreational benefits – a new network of footpaths, birdwatching opportunities and public information boards.

Further Information

Read our case studies on other UK managed realignment schemes:

Read our paper The Role of Coastal Engineers in delivering No Net Loss through Biodiversity Offsetting for further information on offsetting policies and their application to civil engineering in coastal and estuarine areas.

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