The EA said flood defences at Polperro in Cornwall were "overtopped" by "unprecedented" levels of floodwater.

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Local reports later said the levee was overtopped, not breached, but the Corps of Engineers reported it Tuesday afternoon as having been breached.

CNN: Mayor blasts failure to patch levee breaches

At first the Army Corps of Engineers said they thought the storm surge had overtopped the concrete flood walls, washing them out from behind.

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For places like Plaquemines Parish, the wall of water overtopped levees, flooded the town of Braithwaite with 12 feet of water, and killed two people.

FORBES: New Orleans Getting Back To Business after Hurricane Isaac

And so that obviously contributes to them being overtopped.

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The worst hit area is Plaquemines Parish, which juts out into the Gulf of Mexico, and where the storm surge overtopped an 8-foot-high, 18-mile levee, flooding an estimated 800 homes and forcing residents onto their roofs.

FORBES: Levee Breach Floods 800 Homes In Louisiana, And Other Notes From Isaac's Landfall