POORLY DESIGNED and constructed levees aggravated flooding in New Orleans after Hurricane Katrina, senior United States engineers said this week.
Levees failed in some places without being overtopped or exposed to a storm surge greater than that for which they were designed.
Shallow sheet piles, bad ground conditions and inconsistent use of construction materials all contributed to the failure of the city's fl ood protection system, the engineers said.
These conclusions were published last week in a report by the American Society for Civil Engineers (ASCE) and University of California Berkeley.
'In Lake Pontchartrain the levels of the actual winds were much less than a Category 4 storm and may have been as low as Category 2 or 1, ' said ASCE team leader Peter Nicholson.
The report says levee breaks in this area of the city were caused by 'massive underseepage' passing beneath 'relatively short sheet piles', and through foundation soils, which were a mix of sand, marsh and peat.
This removed support for the flood walls, which then gave way to the force of the storm surge.
Where flood walls were overtopped the report found that the levees' resistance was undermined by transitions between concrete, steel and earth levees, sometimes within 15m to 30m.
Weak points in the levees were often caused at transition points between concrete, sheet piled and earth constructions.
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