After hurricane Katrina devastated New Orleans in 2005, many asked: “Can’t we just build a giant wall around the town and protect it?” The answer is yes, and no.
Let’s compare it with a North Georgia town that had a recent flood event, but in a less complicated physical setting than New Orleans. On May 30, 2018 the Chattahoochee overflowed its banks in Helen after torrential rains, causing damage to businesses, roadways and homes. A wall around the town wouldn’t have helped because the water came from two directions: the upstream part of the river, and the groundwater below Helen. Only a dam to create a lake just north of the main city area would have slowed drainage from the river. But it would have put other places underwater, such as the Alpine Valley Shopping Center, River View Lodge, and the Chattahoochee Cheese Market. The first lesson is, then, that we can’t easily get rid of floodwaters without dumping the trouble on someone else. The same is true if a giant spillway were to divert water before it reaches Helen, diverting it to Cleveland instead.
Like Helen, New Orleans also has a river flowing through it. On August 29, prior to Ida’s landfall, the Mississippi was at about 10 feet. That’s 7 feet below flood stage, which could easily be reached during a hurricane. Increased flow from upstream due to rain is a concern. But more worrying is the water backing up downstream because of the storm pushing ocean water into the river mouth. In the comparison to Helen, we now have a third direction from which the push comes: Downstream.
Zoom out in Google Earth, and two additional large water bodies enter the picture: Lake Pontchartrain and Lake Borgne. Heading along Pontchartrain’s shore on Lakeshore Drive, there’s an 18-foot tall levee, an artificial ridge to protect the homes behind it. But those residences on streets like Amethyst, Topaz and Crystal, are barely 2 feet above sea level. If the Mississippi gets in trouble, Bonnet Carré Spillway can release the excess into Pontchartrain. But this will simply move the floods to the other side of New Orleans, and into these neighborhoods.
With major lakes, America’s largest river, and a storm-prone ocean in its neighborhood, New Orleans probably has the most complex water environment of all. Protecting homes and businesses is much more complicated than merely building a wall.
Rudi Kiefer, Ph.D., is a professor emeritus of physical science at Brenau University His column appears Sundays and at gainesvilletimes.com.