I'm fairly sure that's the case with most levees in the area. Along the MS, except for the downtown area. The one levee they are showing where the big breech is is actually concrete. But, you're right, most are earthen.
Down in Westwego (southwest NO) a couple of years ago, the shipyard workers told me the nutria (think of a rat crossed with a rabbit) were digging burrows into the earther levees, weakening them. This may have contributed.