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Letting it flow

England’s first managed realignment on the open coast has been completed, just ahead of the biggest storm so far this autumn. Jackie Whitelaw reports.

When the winds whipped up by the St Jude storm hit southern England at the end of last month, there was one stretch of the English south coast that for the first time was not a flooding concern for the Environment Agency.

The area of Medmerry near Selsey in West Sussex has increasingly been flooded in high seas and bad weather. But a few weeks ahead of the storm, in September, the Agency had opened up an 110m wide breach in the sea wall as one of the final steps in the creation of its latest managed realignment coastal flood management scheme.

And instead of fighting to keep the sea back with bulldozers and shingle - 250,000t of it every year on average - the Agency could welcome the water to a defined area inland, and that way stop it threatening 350 properties, the coast road to 5,000 homes on Selsey Bill and the local sewage treatment works.

Shingle bank

“Had we not completed the scheme and been relying on the shingle, the defences would have been breached in the St Jude storm,” says Environment Agency flood and coastal risk manager for the Solent and South Downs area Andrew Gilham. “Sustaining the shingle bank had become difficult and with sea level rise and increasing storm events it was becoming impossible.

“There was a lot to worry about that stormy night, but for the first time in my 15 years here I was confident that the Medmerry area would be fine,” he adds.

Managed realignment is now a proven response to rises in sea level and increasingly stormy seas - problems that are predicted to increase in the next 100 years.

Surrendering land to the sea and realigning defences inland protects people and vital infrastructure but also creates new habitat to replace that lost to rising water levels. But Medmerry is a new departure for the Environment Agency. It is its first open coast realignment scheme.

“Previously defences have been taken down and areas allowed to flood on estuaries where the wave climate has been less severe,” says Gilham.

“Had we not completed the scheme and been relying on the shingle, the defences would have been breached in the St Jude storm”

Andrew Gilham, Environment Agency

Medmerry is right on the South Coast and is fully exposed to the South West storms that drive up the English Channel from the Bay of Biscay, so it is quite unique.”

The site is also important because it provides 183ha of new inter tidal habitat. Coastal squeeze is a big issue for the Solent coastline. The area is very urbanised. Existing hard defences and new ones needed for cities like Portsmouth and Southampton as they grow are exacerbating the problem.

“As sea levels rise the low tide mudflats can’t migrate inland because the hard defences prevent it,” says Gilham “and so the habitat is lost.

Medmerry is replacing lost habitat, and the site is so big it provides for the Solent’s needs for the next 20 years.”

New flood defence

The Medmerry scheme has involved the construction of a 7km long, 3m to 4m high earth bund to act as new flood defence formed of 450,000m3 of spoil from the site. Bund toes are reinforced with 60,000t of rock brought to site by sea.

These are designed to take account of the more dynamic and aggressive nature of the open coast and tie the new defences into the existing coastal defences. And four freshwater outfalls have been constructed to drain existing rivers and streams out to sea.

The total cost of the job is £28M including purchase of the land, construction of the new defences and habitat creation. The job was officially opened at the start of this month.

Construction had begun in 2011, and before the sea could be allowed in, the Agency had to relocate wildlife such as voles and reptiles to new areas of meanders, grassland and reed bed on the landside of the new defences.

The biggest challenge for the project was wet weather and particularly the soaking summer of 2012 which delayed earthmoving and added six months to the programme.

One of the great successes was the relationship with the local community that was concerned at a plan to move the sea defences closer to their properties.

“It will be very interesting to discover how nature develops the site over the next few years”

Andrew Gilham, Environment Agency

“The Medmerry Stakeholder Advisory Group has been a good sounding board for us,” says Gilham, “and it has been instrumental in helping shape the final design of the site.”

When the scheme opened at the start of this month it included 10km of footpaths, cycle paths and bridleways, a car park and viewing point, creating a valuable new amenity. Medmerry already attracts tourists, but the investment is helping expand the local economy.

The West Sands Holiday Park on the eastern edge of the site is the second largest of its kind in Europe, and thanks to the creation of the new habitat, is attracting new visitors and extending its season by four months.

On the western side of the breach the Medmerry Park Holiday Village is seeing similar benefit.

The Royal Society for the Protection of Birds is managing the site on behalf of the Agency, and both bodies are eager to see how it develops.

“We are very much at the beginning of the process, and it will be very interesting to discover how nature develops the site over the next few years, how the natural meanders form and so on.

“Birds are already using the site as a resting spot, and as the invertebrate population recovers after the earthmoving we expect the bird population will thrive.”

He is very confident about the integrity of the new flood defences.

“They are designed to resist a one in 1,000 year event,” he says. “But we are all aware that as sea levels rise, 100 years from now that event horizon will change but it will still be one in 200 years.”
Medmerry then, is an investment with a long future.

Precast aids outflow construction

Brought in at an early stage in the design process for the Medmerry scheme, Kijlstra developed the design of the outflows so that the headwalls are supported on insitu concrete bases. The structures are all similar, though not identical.

Contractor Mackley assembled the 3.5m high headwalls on site, simply lifting the units into place on the prepared bases. The modular design concept allows various types of flow control devices - such as flap valves, penstocks, weirs and hydrobrakes - to be fitted at the factory, speeding up site installation and ensuring a quality finish. Mackley project manager Terry Gretton said that precast had clear benefits over traditional in-situ methods: “This was much quicker and less likely to encounter snags on site,” he said.

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