It’s not necessarily "just overtopping"

A quick note on what’s happening in New Orleans, which looks like it could be fairly harmless -- except for the way those floodwaters are overtopping the Industrial Canal levee near the Lower Ninth Ward. Here’s a picture from Tulane University geologist Stephen Nelson, who gave me a tour of Katrina flood damage in 2006:

See that trench along the floodwall? It’s important to understand something: that’s the "dry" side; the flooding was happening on the other side. But as it splashes over the top of the wall, it lands, with some force, on the "dry" side and starts eroding a trench, quite rapidly. And if the floodwall isn’t dug down deep enough, that erosion can combine with the pressure from the other side to tip part of the floodwall over. It’s quite clear that happened along some of the levee breaches in 2005 (there were several breaches, cause by  different failures in different locations). That’s why the footage of water splashing over the top of the Industrial Canal floodwall is causing a lot of worry right now.