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    New Orleans Levees About to Get Near-Failing Grade

    By By Heather Buchman, Meteorologist
    September 05, 2011, 12:37:24 AM EDT

    In the face of the threat of Tropical Storm Lee in the Gulf of Mexico, the New Orleans area dikes are about to get a near-failing grade from the new Army Corps of Engineers rating system.

    "Preliminary rankings... show that the corps believes there's still a significant risk of flooding from major hurricanes or river floods that are greater than the design heights of Mississippi River levees and hurricane levees on both the east and west banks," Mark Schleifstein with the Times-Picayune reported Monday.

    Schleifstein stressed, however, that the near-failing grades applied to storms larger than what the levees are built to withstand.

    "The hurricane and river levees are designed to protect from surge created by a so-called 100-year hurricane, or a storm with a 1 percent chance of occurring," Schleifstein explained. "The ratings show that 500-year events, with a 0.2 percent chance of occurring in any year, will overtop the levees and cause significant flooding."

    "The chances of failures for flood events involving water levels below the authorized 100-year heights were adequate," he went on to say.

    So while it would take an extremely rare hurricane, such as Katrina, to cause the levee systems in New Orleans to fail, there are still major concerns about the flood potential from the system currently developing in the Gulf.


    Flood Threat to New Orleans "From 10 to 20 inches of rain may fall on part of the north-central Gulf Coast beginning late this week and continuing into next week, and could in itself result in disastrous flooding," warned AccuWeather.com Expert Senior Meteorologist Alex Sosnowski.

    Schleifstein explained that New Orleans' internal drainage system can only handle 10-year rain events. "The pump system handles an inch of rain per hour for the first few hours. After that, all bets are off," he said.

    "There is potential for some places to get rainfall rates of an inch per hour or more for a prolonged period," Sosnowski stated.

    If the pumps are overwhelmed, that could have an effect on the levees.

    "If we would get 20 inches over the course of three or four days, we will have street flooding, but the chances of actual structures getting flooded is not that great," said Robert Turner, regional director of The Southeast Louisiana Flood Protection Authority - East.

    "If we would get 20 inches of rain in a day, that would be a different story," Turner added. "But we would need to have prolonged periods of extremely heavy rain to have flooding of structures."

    He said the main concern for the levee system would be with the outflow canal levees. "It would be a 'synchronization dance' between pump stations and temporary stations on Lake Pontchartrain," Turner explained. "It is conceivable that water could rise too high in the canals."

    He added that any problems would be extremely remote, however.

    Sosnowski pointed out that there are other issues to consider with a long-duration, meandering tropical system, including potential for long-duration storm surge.

    "It's not just the rainfall, but perhaps days of pressure on levees, as storm surge water could be driven into Lake Pontchartrain if a tropical storm or hurricane hangs out over the north-central Gulf of Mexico," AccuWeather.com Meteorologist Mark Mancuso said.

    For more information on the Army Corps of Engineers new rating system and the preliminary ranking for New Orleans levees, see Mark Schleifstein's full report.

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