When the Levee breaks: An explainer


Water splashes over a levee in New Orleans, Louisiana, on Sept. 1, 2008, during Hurricane Gustav. At the time Gustav was a Category-Two hurricane.


Jim Watson

On Monday, Aug. 29, 2005, Hurricane Katrina struck New Orleans and floodwater engulfed over 80 percent of the city after levees failed. Investigations followed, and blame for the levee failure eventually fell on the US Army Corps of Engineers. Their construction was faulty. Their design shoddy. In the flooding, at least 1,800 people died.

Two years later, Professor Raymond B. Seed from Berkeley's Department of Civil and Environmental Engineering wrote a letter to Dr. William F. Marcuson, then President of the American Society of Civil Engineers.

Seed said Katrina had destroyed three "precious" things: New Orleans, public confidence in civil engineering, and the "integrity" of the profession.

Now, as Isaac lashes New Orleans, a new flood protection system — built again by the Army Corps of Engineers — is being tested for the first time. It cost $14.5 billion and extends over 133 miles. Is it working?

To understand what's happening now, we have to go back seven years, to the day.

What happened to the levees in 2005?

A levee is basically a floodwall. In 2005, Tom Zimmie, a professor of civil and environmental engineering at Rensselaer Polytechnic Institute, testified before the US Senate Committee on Environment and Public Works that the walls around New Orleans failed for a number of reasons. 

Among them: "Overtopping of the levees (the levees were too short), erosion, failure in foundation soils underlying the levees, and seepage through the soils under the levees causing piping failures." When the 17th Street levee, London Ave. levee, and about 50 others failed, New Orleans flooded.  

In 2005, levees were basically all New Orleans had to hold back floodwaters. As Ken Holder of the Army Corps of Engineers told MSNBC, back then New Orleans "had a [flood protection] system in name only."  But now the city is protected by more than sandbags and 8-foot embankments.

For example, New Orleans now has a pump system. According to the New York Times, pumps installed after Hurricane Katrina are "pushing 8,800 cubic feet of water out of the canal every second."

Here's how that works:

Is the levee system working now?

By many accounts the New Orleans' flood protection system is performing admirably. Army Corps spokeswoman Rachel Rodi told Seattle PI the system was "withstanding the assault."

Isolated cases of levee failure are being reported during Isaac. The Washington Post reported floodwater had overtopped "a 8 or 9 foot levee between the Braithwaite and White Ditch districts," south of New Orleans in Plaquemines Parish. Rescue operations are underway to help people trapped by the flooding.

Louisiana Gov. Bobby Jindal announced earlier today that the slow moving storm will likely remain over his state until Friday. He also said local authorities are considering an intentional Levee breach to release pressure — the video below explains why:

Praise for the new system:

"The system is performing as intended, as we expected," she said. "We don't see any issues with the hurricane system at this point."

Louisiana Sen. Mary Landrieu told CNN, Isaac is a “nasty, determined storm,” but the flood protection system is “absolutely paying off.”

Isaac was a Category 1 hurricane Wednesday morning, but was downgraded to a tropical storm later in the day. (For comparison, Katrina was a Category 5.) Still, New Orleans, the state of Louisiana, Mississippi and the Alabama border are not out of the weather yet.

[View the story "Isaac downgraded, but damage continues" on Storify]