The $14 billion system of levees and floodwalls built around New Orleans after Hurricane Katrina is already sinking just 11 months after it was completed, experts have said.

The Army Corps of Engineers said in an impact study notice earlier this month that the flood control system could loose its intended ability protect against a 100-year flood as soon as 2023.

The agency said it it was concerned by 'weak soils, general subsidence, and the global incidence of sea level rise that will cause levees to require future lifts to sustain performance.'

The Corps of Engineers said it needs to study whether it would be worthwhile to undertake a costly project to lift hundreds of miles of floodworks that protect New Orleans.

A map shows the hundreds of miles of levees that protect New Orleans in red. The Army Corps of Engineers says the levees are already sinking

The Lower Ninth Ward lies next to the repaired Industrial Canal levee wall in 2010 in New Orleans, Louisiana. A new report suggests the levees are sinking faster than expected

'These systems that maybe were protecting us before are no longer going to be able to protect us without adjustments,' Emily Vuxton, policy director of the Coalition to Restore Coastal Louisiana, an environmental group, told Scientific American.

She said repair costs could be 'hundreds of millions' of dollars, with 75 per cent paid by federal taxpayers.

'I think this work is necessary. We have to protect the population of New Orleans,' Vuxton said.

Corps officials won't have an estimate of exactly what work is needed or how much it will cost until their reports are completed.

The existing levee system was constructed at taxpayer expense in the years after some 1,800 died in Hurricane Katrina in 2005.

‹ Slide me › The Lower Ninth Ward in front of the Industrial Canal is seen after the devastation of Hurricane Katrina in 2006 (left) and following the recovery in 2015 (right)

An elevation map shows the areas of New Orleans above sea level in red, yellow and green, and those below sea level in blue

It is designed to protect against a flood level that has a 1 per cent chance of occurring on any given year, a so-called 100-year flood.

That level of protection is required in order for property behind the levees to participate in the National Flood Insurance Program.

In 2023, the levees must be recertified as still providing that level of protection, or the homes behind them run the risk of being declared ineligible for coverage.

In its April second notice in the Federal Register, the Army Corps of Engineers says that 'absent future levee lifts to offset consolidation, settlement, subsidence, and sea level rise, risk to life and property in the Greater New Orleans area will progressively increase.'