![](http://soundwaves.usgs.gov/2006/01/OvertopCartoon.jpg)
Above: Diagram showing
how overtopping caused levee breaches. The levee in the diagram is composed of concrete surrounding a corrugated steel "sheetpile," so called because it serves as a piling when driven into the underlying earthen embankment. Water overtopping the floodwall eroded the soil embankment on the landward side, undermining the wall and causing it to fail. In other areas, water seeped below the sheetpile and weakened the embankment, causing failure even at water levels below the height of the floodwall. Modified from a Wall Street Journal graphic based on the 17 MB PDF NSF-ASCE report available at URL
http://hsgac.senate.gov/_files/Katrina/Preliminary_Report.pdf.http://soundwaves.usgs.gov/2006/01/