After Hurricane Katrina flooded low-lying parts of New Orleans in 2005, the Army Corps of Engineers spent billions of dollars repairing and improving the ring of levees that encircles the city. The Corps considers the project a success — and so does the National Flood Insurance Program, which doesn't require insurance or mandate homes to be raised within the city's protective ring.

But the New Orleans Times-Picayune reports this month that the updated levee system isn't quite all it's cracked up to be (see: "New Orleans area's upgraded levees not enough for next 'Katrina,' engineers say," by Mark Schleifstein). That's because the design of the levee wasn't driven by the historic experience of Hurricane Katrina, but by another, lesser goal.

"The problem, in part, is the result of a devil's bargain' hammered out between the Army Corps of Engineers and the National Flood Insurance Program in Katrina's wake: Allow residents and businesses within the levee system to remain eligible for federal flood insurance while the corps redesigned and built the system to protect from the insurance program's so-called 100-year flood event," the paper reported. "That event is storm surges caused by a hurricane with a 1 percent chance of occurring in any year, the so-called 100-year storm. But the surge created by Katrina in St. Bernard Parish was that of a 200-year storm, overtopping levees in that area. The levees along the south shore of Lake Pontchartrain saw surge of a 150-year storm, scientists say."

Even worse, the paper reports, with the passage of time, the repaired levees will become less capable. That's because the land they're built on is itself sinking. "Bob Jacobsen, a Baton Rouge-based engineer hired by the east bank levee authority, conducted a surge modeling study that indicated some earthen levee locations could be overtopped today by waves atop storm surges from a 100-year storm," the Times-Picayune reported. "The model also showed that significant parts of the east bank system will have subsided below the 100-year design level long before the levees' 50-year life expires in 2057. Earlier this year, the corps agreed with part of his findings and recommended that its levee designs be reanalyzed by 2018."

Historian John M. Barry is something of an expert on floods and New Orleans: Barry's 1997 book Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America was a best-seller, and Barry served on the the Southeast Louisiana Flood Protection Authority - East, the levee board responsible for overseeing protection for localities situated between the river's east bank and Lake Ponchartrain. In recent opinion writings, Barry has cast doubt on the value of the 100-year storm as a benchmark for levee construction.

Writing in the Times-Picayune (see: "New Orleans needs you to help hold back the sea: John Barry"), Barry argued, "One hundred year protection, an Orwellian phrase, is woefully inadequate. In an average person's lifetime there is a better than 50 percent chance he or she will experience at least one greater-than-100-year flood. The 100-year standard was never supposed to define the level of flood protection; both FEMA and Corps of Engineers experts agreed that cities should have 500-year protection. The 100-year standard was supposed to be used only for insurance purposes, when the National Flood Insurance Program began in 1973."

Writing in the New York Times (see: "Is New Orleans Safe?" by John M. Barry), Barry expanded on the point: "Indeed, before 1973, the Army Corps of Engineers built flood-protection systems to guard against the worst likely flood. That was the standard used in the 1930s when, following a 1927 Mississippi River flood more disastrous than Katrina, the corps built levees and floodways along the lower Mississippi River. That was fortunate, since floods also smashed the 100-year standard along the lower Mississippi in 1937, 1973 and 2011 but did little damage because of the levees and floodways. By contrast, areas of the country with levees built to the 100-year standard have been repeatedly devastated."

Barry recommends a 500-year flood standard for civil works. And in New Orleans, he says, that goal is achievable only by working to restore and rebuild the eroding land outside the levees, between the city and the Gulf. The cost of that effort could be astronomical, however. So Barry wants to turn to the region's oil and gas industry — which Barry, with considerable backing from scientists, blames for much of the erosion. "Oil, gas and pipeline companies have dredged an estimated 10,000 miles of canals through the coast; ensuing salt water intrusion killed plants, without whose roots land dissolved," Barry wrote in the Times. "Companies also sucked so much material from below ground that the surface sank. No one seriously disputes industry’s role in land loss."

The Southeast Louisiana Flood Protection Authority - East has sued the oil and gas industry seeking money to fund coastal restoration, which they say would help the rebuilt levees withstand another Katrina (or worse). Louisiana's legislature, however, has passed a law that prohibits the Authority from pursuing the legal case. The levee authority is challenging that law in court. So for now, that issue is bogged down — as New Orleans, and its levees, continue to sink.

For more about the loss of land outside the levees, see: "Gulf Eats Away at Coast Outside Levee-Protected New Orleans," by Cain Burdeau.