The $14 billion network of levees and floodwalls that was built to protect greater New Orleans after Hurricane Katrina was a seemingly invincible bulwark against flooding.

But now, 11 months after the Army Corps of Engineers completed one of the largest public works projects in world history, the agency says the system will stop providing adequate protection in as little as four years because of rising sea levels and shrinking levees.

The growing vulnerability of the New Orleans area is forcing the Army Corps to begin assessing repair work, including raising hundreds of miles of levees and floodwalls that form a meandering earth and concrete fortress around the city and its adjacent suburbs.

“These systems that maybe were protecting us before are no longer going to be able to protect us without adjustments,” said Emily Vuxton, policy director of the Coalition to Restore Coastal Louisiana, an environmental group. She said repair costs could be “hundreds of millions” of dollars, with 75% paid by federal taxpayers.

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

The protection system was built over a decade and finished last May when the Army Corps completed a final component that involves pumps.

The agency’s projection that the system will “no longer provide [required] risk reduction as early as 2023” illustrates the rapidly changing conditions being experienced both globally as sea levels rise faster than expected and locally as erosion wipes out protective barrier islands and marshlands in southeastern Louisiana.

Of primary concern are the earthen levees that form the backbone of the 350-mile maze of protection that includes concrete floodwalls, pump stations and gated structures, Army Corps spokesman Matthew Roe said.

The levees are losing height as they start to settle—a natural phenomenon that is exacerbated by the soft soils in southern Louisiana. Some floodwalls are built into the levees.

But “the global incidence of sea level rise” also is contributing to the inadequacy of the levees, the Army Corps said in the April 2 Federal Register notice announcing that it is studying system improvements.

“It’s happening a little bit faster than our projections in 2007,” Roe said.

Numerous studies in recent years have warned of New Orleans’ unusual vulnerability to sea-level rise. The National Academy of Sciences projected in 2016 that New Orleans could be one of the hardest-hit cities in the world along with Manila, Philippines; Jakarta, Indonesia; and Bangkok.

“The public may feel complacent. The [protective] system was built, and they think it’s done. They look at that levee as a static monolith,” said John Lopez, director of the coastal and community program for the Lake Pontchartrain Basin Foundation, an environmental group. “But the crisis never really was over. We improved the system, but we have always been under threat.”

Sea-level rise raises questions about whether the protective system—known officially as the Greater New Orleans Hurricane and Storm Damage Risk Reduction System—should be built to a higher standard.

When Congress approved funding after Katrina, it required the system to protect against a so-called 100-year flood, which has a 1% likelihood of occurring in any year.

“We should be looking at higher than a 100-year standard, but not through levees alone,” Lopez said, calling for Congress to pay for natural barriers that build up coastal buffers. “We need a higher standard, but it should never be a single-type solution because we’ve seen that doesn’t work.”

As the Army Corps studies reinforcing the system, it will model the effect of roughly 150 storms ranging from a small tropical depression to a mammoth 500-year storm that has only a 0.2% chance of occurring each year, said Roe, the agency spokesman. A first draft of a report is scheduled to be done by December, after which the Army Corps will accept public comments.

Reinforcing the levees—a process known misleadingly as “lifting"—involves scraping off the top layer of grass and a fabric mattress and piling on additional earth before restoring the surface layer. It is unclear how much earth will need to be added to the levees, which stand as high as 35 feet.

But the Army Corps said in its Federal Register notice 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.”

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.