20 years after Katrina, New Orleans remembers

“They also ignored some of their own geotechnical science when designing the levees,” he continued. “Some were built in sand with very shallow footings, so the water just went underneath and blew out the levee. Some were built on piles of earth, again with very shallow footings, and they just fell over. The 17th Street Canal, the whole levee structure actually slid 200 feet.”

There had also been significant alterations to the local landscape since Hurricane Betsy. In the past, the wetlands, especially the cypress tree swamps, provided some protection from storm surges. In 1992, for example, the Category 5 Hurricane Andrew made landfall on Atchafalaya Delta, where healthy wetlands reduced its energy by 50 percent between the coast and Morgan City, per Van Heerden. But other wetlands in the region changed drastically with the dredging of a canal called the Mississippi Gulf Outlet, running from Baton Rouge to the Gulf of Mexico.

“It was an open conduit for surge to get into New Orleans,” said Van Heerden. “The saltwater got into the wetlands and destroyed it, especially the cypress trees. This canal had opened up, in some places, to five times its width, allowing waves to build on the surface. The earthen levees weren’t armored in any way, so they just collapsed. They blew apart. That’s why parts of St. Bernard saw a wave of water ten feet high.”

Just trying to survive

Add in drastic cuts to FEMA under then-President George W. Bush—who inherited “a very functional, very well-organized” version of the department from his predecessor, Bill Clinton, per Van Heerden—and the stage was set for just such a disaster like Katrina’s harrowing aftermath. It didn’t help that New Orleans Mayor Ray Nagin delayed issuing a mandatory evacuation order until some 24 hours before the storm hit, making it much more difficult for residents to follow those orders in a timely fashion.