The Corps Strikes Again

The Washington Post reports a new study by the Corps of flood risks in New Orleans:

After nearly two years of levee repairs, the chances are 1 in 500 that nearly all of the city will be flooded again this year with more than six feet of water, according to flood risk maps issued today by the Army Corps of Engineers.

There is a 1 in 100 annual chance that roughly one-third of the city will be flooded with as much as six feet of water. For dozens of city blocks, the chance of significant flooding is twice as high.

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The study of flood risks was conducted by the team of academic and private-sector engineers that issued an eight-volume report on the levee system's failures after Hurricane Katrina. It was led by Ed Link, a senior research engineer at the University of Maryland.

"There is a gnawing sense out there -- 'Can we trust the Corps?' " Powell said. "This team was independent."

The research team based its estimates of the flood risk on the odds of hurricane strikes and an assessment of the region's flood defenses, which consist of 350 miles of levees and flood walls, as well as pumping stations and flood gates.

One of the key findings of the group was a departure from the widely held idea that the flooding a hurricane can cause is almost entirely dependent on its intensity. Instead, the group found, the surge depends significantly on the hurricane's track and its radius.

The study showed that the levee work completed has significantly reduced the flood risks in some neighborhoods, particularly in Lakeview, the site of one of the catastrophic levee breaches caused by Hurricane Katrina. Elsewhere, only slight reductions in risk have been achieved.

Unfortunately, this study probably significantly underestimates the risk. The study assumes that the components of the flood control system will actually function as designed, which seems unlikely given the performance of the system two years ago. Also, the study may assume that system failures are independent, whereas failures of one component may be correlated with failures of others. Finally, the study may not account for the increased risks posed by climate change.

The study is particularly bad news for residents of the Ninth Ward and Gentilly. Even with current improvements (and even assuming the levees function as designed), these areas would be flooded about as badly as they were in 2005.