This will return what is currently farmland back to its natural state as a coastal wilderness of mudflats, saltmarsh, saline lagoons and brackish marsh to support wildlife and act as a natural buffer against flooding.
Faber Maunsell has already designed a smaller wetlands scheme at Wallasea for the Department for Environment Food & Rural Affairs (DEFRA). It won this year's ICE East of England award for Overall Oustanding Achievement.
This scheme created 110ha of wetlands by breaching the sea wall following construction a new flood defence inland made from local clay.
But Faber Maunsell rivers and coastal associate director Phillip McLoughlin said his firm would be unable to repeat this method on the larger RSPB scheme.
"If we simply flooded the 736ha and took that much water out of the tidal prism, there would be no water left in the channels and estuary," said McLoughlin. "It could radically alter the way the estuary silts and erodes, with detrimental impacts for both wildlife and public access."
Instead, McLoughlin said the project would use flood control structures and earthworks to compartmentalise the island and manage the risk of over-flooding.
So far the RSPB has commissioned Faber Maunsell's design work and an environmental impact assessment by Associated British Ports Marine Environmental Research. It is now raising funds for construction, with contractors likely to be appointed by the end of 2008 and work on site due to start in 2009.