Engineer Manual No. 1110-2-2502 is not a publication that would normally excite interest from the general population. But in the weeks and months ahead, as New Orleans struggles to rebuild from the floods of Hurricane Katrina, Section 4 of that Army Corps of Engineers book could be scrutinized intensely by city and state officials.
The manual sets the performance standards for, among other things, inland floodwalls: the kind the corps built along the city's drainage canals and that failed spectacularly during Katrina, flooding much of the city and leading to many of the 1,100 deaths thus far confirmed from the storm.
...Forensic engineers investigating the levee failures say the layman's translation of that section amounts to a "gotcha" clause for those who believe the walls failed through faulty design and not because they were overwhelmed by a storm that exceeded design limits.
"It says what every engineer knows: If you build walls to 14 feet, regardless of the design specifications for the expected storm -- 12 feet or 10 feet or 13 feet -- those walls must hold water to their tops," said J. David Rogers, a forensic engineer on the National Science Foundation team investigating the failures.
..."So, yeah, this was a human failure, not a natural disaster."
Since overtopping has been ruled out as the cause of failure along some canal walls, experts in and out of the corps have debated whether the water inside the failed floodwalls was higher than the 12.5 feet maximum listed as the design capacity. But Rogers and other engineers said Section 4b makes that discussion moot. Because the walls were built to 14 feet, to account for wave splash, any collapse below that level means the design failed.
"Their own manual makes it pretty clear this was a failure, " Rogers said, "although they might try to argue it was something else."
...But the debate continues, because the stakes are high. If the walls yielded to forces lower than their design specifications, the city will be on firmer moral and legal footing in asking Congress to pay for the all the property damaged and destroyed when the waters of Lake Pontchartrain poured into the city. If not, the government can say the cost should be borne by flood insurance and homeowners.
Much more at the link.