On June 1, just at the start of this year’s hurricane season, the team investigating the New Orleans levee failures released its final report. Known as the Interagency Performance Evaluation Task Force (IPET), the team consists of several US Army Corps of Engineers representatives plus scientists from the National Oceanic and Atmospheric Administration, FEMA, the US Geological Survey, and many other federal and local flood control agencies; about two dozen private companies; and 25 universities.

The report is intended to finally answer the question “What happened?” and, just as importantly, “How can we stop it from happening again?” As many in the ESC industry suspected as they watched the flooding during Hurricane Katrina, much of the failure was caused by the erodibility of the levees, which were susceptible to scour by the waves that overtopped them, and of the ground at the base of the floodwalls.

Although headlines on the day of the report’s release emphasized the corps’ failings—“Corps of Engineers Contrite for Katrina Flooding,” for example—and though the corps did accept and acknowledge blame for some less-than-adequate construction and materials, the situation is actually more complex. The report looks at the entire hurricane protection system surrounding the city, including the levees and floodwalls. Part of the system’s failure was due to errors in calculation and faulty analysis of soil conditions, and part to the fact that the system as a whole simply wasn’t designed and built to handle a storm like Katrina.

The report concludes that of the flooding that occurred throughout 80% of the New Orleans metropolitan area, two-thirds can be attributed to the breaches, while the rest would have occurred even if the system had not been breached, simply because of waves overtopping the levees and because of the sheer amount of rainfall that landed inside the bowl-shaped, drainageless city. Although Katrina had slowed and ceased to be a Category 5 storm before it hit land, the report notes that it brought with it record waves and storm surges, ranging from 10 to 20 feet, “larger than any previous storm to strike the area, or the North American continent.”

“There was no evidence of government or contractor negligence or malfeasance,” the report states, but two errors are singled out. First, the report says, “Sections of the system were built below specified design elevations due to the use of an inaccurate relationship between the geodetic datum and mean sea level.” The difference in elevation was as great as 2 feet in some areas—in other words, the walls weren’t as high as they should have been even to protect against a Category 3 storm. Second, too few soil samples were taken to provide an accurate picture of subsurface conditions: “Foundation soil strengths were derived from relatively widely spaced borings and at times using average values that may not capture the high variability inherent in this type of geology.” This, the report says, led engineers to overestimate the subsurface soil strength in some places, particularly at the toe of the 17th Street Canal levee, which experienced a blocks-long breach. Areas of the system with more erosion-resistant materials and with more conservative design performed well, despite being hit with a storm that exceeded their design parameters.

“The value of added erosion resistance was clear,” the report states, and the corps says it’s taking the report’s findings into account as it continues working on the system. Although everyone working within the industry has probably witnessed similar situations at one time or another—though most likely on a smaller scale and with far less drastic consequences than in this case—the team’s conclusions reemphasize the importance of doing the groundwork, literally, on any project.

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EC - July/August 2006