Hurricane Katrina came ashore slightly east of New Orleans. Tremendous rainfall preceded its arrival, filling up Lake Pontchartrain and Lake Borgne even before its storm surge arrived. As a result, on the morning of 29 August 2005, many of the levees on the eastern side of New Orleans were already at the brink of being overtopped. When the storm surge came, ranging from 3 meters to 9 meters, it added force to flow, and weaknesses in the levee system were revealed, resulting in key breaches that compromised the entire system. About 80 percent of New Orleans was flooded. The population was immediately reduced to a quarter of its prestorm numbers, and even today, more than two years later, the city struggles to redefine and rebuild itself in the wake of Katrina.

Where did the engineering fail in this massive disaster? In the design of the New Orleans levee system? In the maintenance of the system? In transitional engineering phases during the continuing construction of the New Orleans levee system? Who is responsible for this tragedy? How will it be prevented in the future?

An examination of the levee system conducted by an independent team of investigators in post-Katrina New Orleans has clearly shown that the answers to these questions are not simple at all. In fact, most of the 350 miles of levees in New Orleans survived the storm intact, despite the fact that they were specified to contain nothing stronger than a Category 3 storm. Hurricane Katrina was classified as a Category 3 but gave us a drastically different storm from the typical Category 3. Herein lies the first failure. The Saffir-Simpson scale, originally created in the 1970s to communicate to the public the risk implied in impending storms, did not reflect what Hurricane Katrina delivered. A slow-moving storm, massive rainfall, storm surge beyond that expected from the winds—all contributed to a fundamental misunderstanding of what Hurricane Katrina was bringing to bear on the Gulf Coast. Although this misunderstanding was resolved in the scientific community shortly before the storm, a better assessment of what Katrina had in store was not communicated quickly enough to the general public to facilitate the evacuation process.

The tragedy of Katrina in New Orleans lay in the breaching—not in the overtopping—of the levees. Breaching came about as a result of levee construction, not levee design. Almost all of the levees that were breached during Katrina did so because they had been compromised prior to the storm. Either the levees had been modified after their original construction to accommodate multiple purposes or they were not constructed of materials that met original design specifications. In one case, the placement of a railroad track over the levee compromised the designed height of the levee, as water passed right through the porous gravel underneath the track. In other cases, these urban levees were filled with some combination of originally specified materials (typically claylike to accommodate the space-conserving steepness of most urban levees) and substandard materials such as local sandy soils or shell-filled soils. These substandard soils resulted in water seeping underneath the foundations of some levees, and massive breaches and explosive collapses of other levees along Lake Pontchartrain and the Mississippi River Gulf Outlet.

The ultimate failure of the levee system in New Orleans, however, was not in the engineering or in substandard implementation. A storm-protection system, by definition, demands redundancy, either through redundancy in strength (the safety factor) or in structure (multiple levees operating in tandem). The levee system in New Orleans had neither of these. In most cases, only one levee stood between an inhabited area and a waterway. The U.S. Army Corps of Engineers stated that the system itself was designed for Category 3 storms, and since, historically, multiple Category 3 storms reach the Gulf Coast every century, we can be assured that the levee system is likely to fail at least once, if not more, every century.

Why did the levee system have neither additional strength nor parallel protection structures considered mandatory for storm-protection systems? The answer to this question is both simple and enlightening: the original levee system of New Orleans was designed for flood protection, not for storm protection, leaving the city extremely vulnerable to a hurricane of Katrina's force.

Click here for more articles in the “Learning From Katrina” series. See the video “Flood or Hurricane Protection: the New Orleans Levee System and Hurricane Katrina.”