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09/23/2005: About Those NOLA Levee Design Specs...
CNN is reporting breaking news (9:30 am CST) that there is already a levee breach in the 9th ward in NOLA. They say the structure of the levees are still intact but there is rain that is overtopping the walls and about 2 ft of water in the ward.
CNN reports "This is the first test of these levees" which were strengthened since Hurricane Katrina - and they are failing again in the first rainfall to hit the area.
Reminds me about this aricle I hadn’t yet posted from the NY Times: Design Shortcomings Seen in New Orleans Flood Walls.
Click on the "more" button to read further.
“Along the Mississippi River and Lake Pontchartrain, great earthen levees were ample to hold off much of the surging water propelled by Hurricane Katrina.
But concrete flood walls installed over the last several decades along the drainage and barge canals cutting into New Orleans were built in a way that by Army Corps of Engineers standards left them potentially unstable in a flood, according to government documents and interviews. The walls collapsed in several places during the storm.
A corps engineering manual cautions that such flood walls "rarely exceed" seven feet because they can lose stability as waters rise. But some of the New Orleans canal walls rose as high as 11 feet above dirt berms in which they were anchored.
As a result of federal budget constraints, the walls were never tested for their ability to withstand the cascades of lake water that rushed up to, or over, their tops as storm waves pulsed through the canals on Aug. 29, corps and local officials say.
Hurricane Katrina was the first serious test of the flood walls, said Stevan Spencer, chief engineer for the Orleans Levee District, and it "just overwhelmed the system."
Since the storm, corps officials have said that there is a simple explanation for the devastation: Hurricane Katrina was a Category 4 storm and Congress authorized a flood control system to handle only a Category 3 storm. "Anything above that, all bets are off," said Al Naomi, a senior project manager in the corps's New Orleans district.
But federal meteorologists say that New Orleans did not get the full brunt of the storm, because its strongest winds passed dozens of miles east of the city. While a formal analysis of the storm's strength and surges will take months, the National Hurricane Center said the sustained winds over Lake Pontchartrain reached only 95 miles per hour, while Category 3 storms are defined by sustained winds of 111 to 130 m.p.h.
This raises a series of questions about how the walls that failed were designed and constructed, as well as whether the soil in some spots was too weak to hold them. Investigations by federal engineers and outside experts are just now beginning.
The 2000 edition of the Army Corps of Engineers manual "Design and Construction of Levees" says that the height of flood walls built on levees is an important factor in their ability to withstand a flood. For that reason, the manual says walls like those used in New Orleans "rarely exceed" seven feet. But on two of the three canals where breaks occurred - the 17th Street and London Avenue canals - the concrete sections rise 11 feet above the dirt berms.
Corps veterans said it was not surprising that federal engineers did not issue more vocal warnings.
"I don't think it was culturally in the system for the corps to say 'this is crazy,' " said William F. Marcuson III, the former director of the Waterways Experiment Station for the corps in Vicksburg, Miss., and president-elect of the American Society of Civil Engineers.
"The corps works for Congress," Mr. Marcuson said, "and when the boss says design for a Category 3 storm, culturally the corps is not going to go back and say this is wrong."
Karen on 09.23.05 @ 09:35 AM CST
