The Bizarre Flaw in the New Orleans Levees — Practical Engineering

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[Note that this article is a transcript of the video embedded above.]<br>What happened in New Orleans in August 2005 wasn’t a natural disaster. Don’t get me wrong; Hurricane Katrina was a big storm. By many measures it was one of the strongest hurricanes to ever crash into a United States coastline. It’s in the top 10 for largest diameter, lowest central pressure, highest integrated kinetic energy and more. Storm surge along parts of the Gulf Coast crested at nearly 28 feet or 8.5 meters, the highest in US history.<br>There really are no words to describe the devastation Katrina caused, particularly in New Orleans and the surrounding areas. Roughly four-fifths of the city was flooded. In some areas the water rose above the rooflines of houses. The storm shredded the fabric of the city, and to this day, more than 20 years later, the population of New Orleans has not recovered to its pre-Katrina level.<br>If this were just a story about a major storm overwhelming the flood defenses, it wouldn’t undercut the tragedy, but it would be understandable. I think we generally recognize that these kinds of systems don’t offer absolute protection. The levees and floodwalls around New Orleans were a sprawling collection of structures built over many decades. It wouldn’t be out of question for a storm of the century to simply exceed what those protections were designed to handle. But that’s not what happened.<br>There were about 50 individual locations where levees or floodwalls were breached during Hurricane Katrina. Many of those were situations where the structure was overtopped by storm surge. But out of those fifty, only three of those breaches accounted for the majority of the flows that submerged the heart of New Orleans and led to nearly half of the disaster’s total fatalities and economic damage. All three of those breaches happened at surge levels below what the floodwalls were designed to manage. And I built a model in the garage to show you why.<br>This video is based on a chapter from my new Book, Disasters By Design: How Engineering Failures Shaped the Modern World, which you can pre-order right now at the links below. The book is all about engineering disasters through history and what we learned from them, and I really hope you’ll consider picking up a copy. I’ll tell you more about it at the end, but first, I want to show you this model. I’m Grady, and this is Practical Engineering.<br>The land we call New Orleans has a history stretching way beyond its 1718 founding or the birth of the United States. Originally inhabited by tribes like the Chitimacha, this area has always been a complex intersection of human settlement and flooding. It is strategically located on the Gulf Coast at the mouth of the Mississippi, the largest river in North America. But that prime location also comes with challenges, namely that the city’s exposure to tropical storms is extreme, and, being a river delta, much of its development happened on top of historically soft, marshy, and low-lying terrain.<br>In 1965 Hurricane Betsy rocked New Orleans, flooding more than 150,000 houses. At the time, it was one of the worst hurricanes in American history, earning the nickname “Billion Dollar Betsy,” because it was the first storm with damages in excess of a billion dollars. In response, the federal government, through the US Army Corps of Engineers, made a massive investment into the existing hurricane protection system around New Orleans. But they ran into an issue.<br>A levee is an earthen embankment meant to hold floodwaters back - kind of like a dam but parallel to coastline or a river instead of across it. We know that soil is naturally unstable on steep slopes. So levees have this trapezoidal shape in cross-section with gentle, stable slopes on the waterside and landside. Many of the levees in New Orleans were built decades before Betsy, and over that time, development encroached right up the toes of the structures. You can see the problem. If you want to raise the embankment to provide more protection, you need more space.<br>The solution they came up with is the “I-Wall” (not related to the eye wall of a hurricane). Steel sheet piles were driven into the top of the levee, creating a vertical floodwall on top. They got more height and more protection without the need to condemn property for a wider footprint, which was a major win.<br>All this flood protection infrastructure turned New Orleans into essentially a bowl. The levees keep the floodwaters out, but they also trap rainwater (or any other water) in. So in addition to the levees and floodwalls, the city is dotted with pump stations that move drainage within the city out of the bowl. The heart of this system is the trio of primary outfall canals that run south to north, carrying rainwater from New Orleans’ interior basins out to Lake Pontchartrain: the 17th...