By Alex Kirby BBC News Online environment correspondent

It allows the sea to flood low-lying land, creating saltmarsh which forms a barrier against the force of the tides.

The marsh attracts birds and other creatures, besides protecting the coast more cheaply than artificial defences.

Record numbers of avocets have already nested on the marsh, the first to breed there for a century.

The saltmarsh being created here will only replace what's being lost to rising sea levels around the UK coast this year

Grahame Madge, RSPB

The scheme, the largest of its kind in the UK, covers a eight km (five mile) stretch of the coast of Lincolnshire, near the town of Boston.

Three 50 metre breaches have been cut in the outer sea bank, letting salt water from the Wash encroach on 78 hectares (193 acres) of farmland.

Double benefit

The saltmarsh that has begun to form will absorb the waves' energy and improve the protection given by a newly-strengthened embankment further inland.

North Sea Camp prisoners built the defences

This form of flood protection, working with natural forces instead of trying to control them, is now known as managed realignment (it was formerly called managed retreat).

Chris Allwork, the project manager, said: "We've been able to construct a more reliable sea bank further back.

"We're creating saltmarsh which is being lost at a frightening rate, and we're helping to maintain the Wash's importance for global biodiversity by providing a habitat for birds such as the redshank, and a range of plants and insects."

The scheme involves the Department for Environment, Food and Rural Affairs (Defra), the Environment Agency, English Nature, the Royal Society for the Protection of Birds (RSPB), and North Sea Camp prison.

Prisoners' monument

The land being flooded was bought by the RSPB from the prison to form part of a new reserve.

Birds are returning to the coast

Most of the earth banks in the area, including the one that has been deliberately breached, were built by prisoners from North Sea Camp, and were known as borstal banks.

Disgraced peer Jeffrey Archer was until recently serving his sentence for perjury at the prison, but was not involved in the flood protection scheme.

Grahame Madge of the RSPB told BBC News Online: "We welcome this - it's fantastic for conservation.

Bargain breaches

"But you have to realise that the saltmarsh being created here will only replace what's being lost to rising sea levels around the UK coast this year.

"We hope the government will identify other areas where we can introduce managed realignment.

"The economics are startling. The cost of building a sea wall is 10 times less if you have a saltmarsh in front of it than if it has to face straight on to the incoming tide."

The Environment Agency says saltmarsh is now one of the UK's rarest habitats, with only 44,500ha (110,000 acres) left across the country.