4.5m tonnes of earth excavated for London's Crossrail project used to create Wallasea Island nature reserve

Everyone knows that when a dog digs a hole, it throws up a big pile of dirt behind itself.

Replace the dog with a 1,000 tonne drill and the pile of dirt with Europe's largest man-made nature reserve, and you have the principle behind the Wallasea Island project.

Dreamed up four years ago by the Royal Society for the Protection of Birds (RSPB), the scheme will see 4.5 million tonnes of earth - dug out from under London by the Crossrail tunnelling machines - transported to the Essex coastline to create a nature reserve twice the size of the City of London.

The first shipment of London clay arrived at Wallasea Island today. Over the next seven years, 670 hectares of arable farmland will be transformed into a thriving wetland habitat of mudflats, salt marshes and lagoons - the landscape that last existed here 400 years ago.

The excavated earth will create areas of high ground, before engineers breach the levees that currently protect the farmland, flooding the plain and restoring this human landscape to nature.

Rare wetland birds like the avocet, dunlin, redshank and lapwing are expected to flock to the reserve to breed, following centuries in which their intertidal habitat has been in retreat all along the British coast. The spoonbill and the Kentish plover, which no longer breed or very rarely breed in this country, could also return.

The project, which will be completed by 2019, has been held up by the Government as "the gold standard" of how to marry economic growth with environmental protection. Crossrail is Europe's largest construction project, with a total cost of £14.8 billion. It will link Maidenhead to the west with Shenfield to the east of the capital, cutting journey times for London's commuter belt.

But what to do with the six million tonnes of material displaced by eight huge tunnelling machines boring a 21km twin tunnel underneath London, was a dilemma for planners whose contract was granted on the condition of meeting strict environmental targets.

The answer, from the RPSB, was the culmination of their long fight for threatened wetland habitats. Four hundred years ago there were 30,000 hectares of intertidal salt marsh along the Essex coast, home to wide variety of wading birds as well as mammals such as otters and seals.

Over the centuries more and more of this type of landscape has been drained, or protected from rivers and the sea by levees - all to produce more arable or grazing land. Today there are just 2,500 hectares of salt marsh in Essex - a decline that has been mirrored all along the coast.

Climate change has also taken its toll. The south east of England is particularly susceptible to drought and during the long spell of dry weather that finally ended in spring, it was wading birds that suffered the most among Britain's wildlife. Another threat comes from the other direction. Rising sea levels caused by the Arctic sea ice melt, threaten the lowlands of Britain's east coast. The RSPB predicts that 1,000 more hectares of wetland could be lost in the next decade - either for a want of water, or a catastrophic flood of it.

What the Wallasea Project proposes, is to manage a middle way between the two scenarios, where a controlled landscaping will recreate the ideal habitat that once existed before man began meddling with the coastlines and the climate.

Dr Mike Clark, the chief executive of the RSPB said that the project demonstrated how environmental concerns could go hand-in-hand."This is about a vision for a world in which you can have a first class economy, but also a world rich in nature. It is a symbolic project showing how we can meet both the needs of people and of wildlife," he said.

The new environment minister, Owen Paterson, also backs the scheme, although admitting at today's launch that he is "no expert" on coastal erosion. Mr Paterson's appointment to Defra raised eyebrows. He has a record of opposing wind energy as a local MP and has been called a climate-sceptic.

Today, confronted with the rising waters at Wallasea, he said that climate change was happening and that there was a "man-made contribution" but declined to say whether or not he believed that human activity was the main cause, or whether Britain should be cutting back on carbon emissions.

"We shouldn't be frightened of big infrastructure, of doing things that really enhance the economy," he said of the project. "You can turn what is a problem into a huge environmental gain."

Eighty-five per cent of the material unearthed by Crossrail will go to Wallasea (the rest will go to other landscaping projects such as golf courses). The earth will be gathered from tunnel entrances at either end of the line and be transported by rail, lorry and ship to Wallasea. At the project's peak 10,000 tonnes of earth a day will arrive on ships at a custom-made pontoon. In total 2,000 shiploads will be transported. Crossrail will meet the £50m bill.

"Wallasea Island will show for the first time on a large scale, how it's possible to 'future-proof' low lying coasts against sea level rise caused by climate change," said Dr Clark. "This will deliver benefits to wildlife and provide a wonderful place for people to enjoy."

A new home for thousands of animals

The new nature reserve at Wallasea Island will be complete by 2019, by which time it is hoped that thousands of wetland birds will have made their home their, along with marine animals and an array of hardy coastal plants.

Kentish plover

The small wader, despite its name, is no longer believed to breed in Britain because of the loss of its wetland habitat. Conservationists hope that the reserve at Wallasea will provide a safe haven for its return.

Spoonbill

The RSPB estimates that there are at most four breeding pairs of spoonbill left in Britain. Tall white birds with long black bills whose distinctive shape gives them their name, their return to Wallasea would be a conservation coup.

Seals

Small grey and common seal colonies already exists at Wallasea but the increased numbers of fish likely to use the flooded site as a nursery could see their numbers increase significantly.

Avocet

The return of the unmistakable black and white avocet to British coasts in the 1940s was one of the most successful conservation projects in the RSPB’s history. Wallasea Island could provide more breeding space for the species which is still rare.