...and breaches resulted, destroying 1000's of homes. Overtopping did cause some of the breaches. Other causes listed below. Overtopping is a cause, a breach is an effect.
And if over a dozen breaches were observed, that seems like a lot, if a so called "levee breach" was supposed to be a real rare event, and not anticipated.
Here's the report:
Hurricane Katrina: Why Did the Levees Fail?
Testimony of Peter Nicholson, Ph.D., P.E. Associate Professor of Civil and Environmental Engineering and Graduate Program Chair University of Hawaii
On behalf of the AMERICAN SOCIETY OF CIVIL ENGINEERS
Before the Committee on Homeland Security and Governmental Affairs U.S. Senate November 2, 2005
"What we found in the field was very different than what we had expected, given what we had seen in the media reports. Rather than a few breaches through the floodwalls in the city caused largely by overtopping,
we found literally dozens of breaches throughout the many miles of levee system. A number of different failure mechanisms were observed, including
scour erosion caused by overtopping, seepage, soil failure, and piping. As geotechnical engineers, we were particularly interested to find that many of the levee problems involved significant soil-related issues."
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"Outside New Orleans - It is important that the impact of the levee breaches outside of the city of New Orleans not be overlooked.
Many sections of the system were severely tested by overtopping from a direct onslaught of the storm surge. Many portions of these levees were breached and/or severely distressed, causing severe flooding and, in many cases, complete destruction of thousands of neighborhood homes.Some of the levee sections were nearly obliterated and were observed to have been constructed of highly erodable materials"
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http://www.ewrinstitute.org/files/pdf/katrinalevees.pdf