A scheme to combat flooding by surrendering land to the sea will be completed on Monday on the south coast.
The £28m "managed realignment" at Medmerry in West Sussex has seen the building of 7km (four miles) of new sea walls up to 2km inland.
By letting the waters in, the Environment Agency says the risk of flooding for hundreds of homes will be reduced.
The surrendered land will become a wetland habitat for many species.
The sea has long been a threatening presence on the flat land of the Manhood Peninsula that juts into the English channel on the coast between Portsmouth and Worthing.
Caravan parks in Selsey and Bracklesham Bay have been flooded a number of times in recent years, as the shingle defences have yielded to the surging seas.
Since 2011, the Environment Agency have been working on a plan that they believe curbs that threat.
The project is billed as the country's largest ever coastal flood realignment scheme, but it has required the destruction of the existing sea wall at Medmerry and giving back to the sea some of the land nearest to the coast.
"We have made a hole in the sea wall," the Environment Agency's flood and coastal risk manager, Andrew Gilham, told BBC News on a visit to the site.
"But before doing that we've actually built 7km of new defences further inland from the coast. What we've been able to create here is an area that can absorb the energy of the waves and reduce the flood risk to people."
Although the new walls are much closer to local communities, the Agency says homes are much better protected as a result, with the development able to withstand a once in a thousand year flood.
The change is partly being forced on the Agency because of EU legislation that requires compensation for the loss of wildlife habitat through development. The Medmerry scheme makes up for the loss of similar conservation areas in and around the Solent.
"It is not politically driven," said Mr Gilham.
"But the benefit of creating the habitat here is not only to reduce flood risk in this area but also to reduce flood risk to communities in Portsmouth and Southampton.
"If we weren't able create habitat here we wouldn't be be able to do those defences."
The scale of the £28m development is enormous. Between the new earthen walls and the sea stand 183 hectares of land that the Agency says will rapidly become a saltwater marsh, with potentially huge benefits for wildlife in the area. It will be managed by the RSPB.
"Even in construction, we're seeing lots of migratory birds using this area," said Andrew Gilham.
"It is already starting to be used by the wildlife. It's a massive nature reserve and a massive opportunity on the south coast."
Local people are hoping that letting the sea in will not only protect their homes but boost the local economy as well.
Alan Chamberlain is the estate manager of the newly renamed Medmerry Park holiday village. His business, located next door to the new habitat, is actually below sea level.
"In the past we have been very close to flooding, and now we've got a one-in-one-thousand-year protection, one of the best in the country actually," he told BBC News.
"For us it's a win win, it will encourage visitors to the site, birdwatchers and those interested in nature."
The new development is facing its first big test on the day it opens as it will see one of the highest tides of the year. But the Environment Agency are fully confident it will pass with flying colours.
Medmerry is one of a number of managed realignment projects that the Environment Agency are involved in. Andrew Gilham says it's part of a new approach.
"Rather than fighting it, we are working with nature," he explained.
"It's an important change in approach, you can only keep building bigger and bigger defences for so long. We have to ask if we can make better use of public money.
"Certainly the habitat we are creating here is important to the broader ecosystem and the broader economy of the country by encouraging people to visit remote areas."
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