Les Jones

News > USGS: Overtopping Caused Most New Orleans Levee Failures

From a December 2005/January 2006 USGS report linked in the comments on Nashville is Talking. According to the report, the most severe breaches were caused by soil failure under the levees, but the majority of breaches were caused by overtopping waters scouring away surface soil on the dry side of the levees.

The storm surges produced by Hurricane Katrina on August 29, 2005, breached the levees protecting New Orleans in numerous places, flooding approximately 75 percent of the metropolitan area. Most of the levee failures were caused by overtopping, as the storm surge rose over the top of a levee and scoured out the base of the landward embankment or floodwall. Three major and costly breaches appear to have been caused by failure of the soils underlying the levees or failure of the earthen levee embankments themselves; in several places, levee foundations failed when water levels were below the tops of the levees. Transitions between levees of differing heights or materials proved to be weak points in the flood-protection system; a significant number of levee washouts occurred, for example, where the weaker of two adjacent materials was at a lower elevation.

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Farther west, in the New Orleans East Bank Canal District, three levee failures occurred along the banks of the 17th Street and London Avenue Canals. Kayen and Collins observed evidence indicating that the failures occurred when water levels were below the tops of the concrete floodwalls lining the canals. These three levee failures were likely caused by failure of the foundation soils underlying the levees. Signs of an incipient failure were observed at a fourth distressed levee/floodwall segment on the London Avenue Canal, where lateral displacements, sinkholes, and sand boils all indicate that water was flowing through a weakening embankment.