Feds: Levee System Did Not Cause Isaac Flooding

November 12, 2012

Federal officials say in a report released Nov. 9 that improvements to the New Orleans-area levee system did not cause Hurricane Isaac’s storm-surge flooding of areas in Louisiana that were not inundated during Hurricanes Katrina and Gustav.

The Army Corps of Engineers said surge flooding during Isaac was the result of the storm’s landfall just west of the Mississippi River, rainfall amounts of 8 to 20 inches, a forward speed three to four times slower than Katrina and an extended period of easterly winds that pushed surge heights to record levels.

The Times-Picayune reports that residents and elected leaders in several area parishes, including St. John the Baptist, St. Tammany, Plaquemines and Tangipahoa, had pushed for the study after record flooding in some areas last August.

The study found the unusual combination of Isaac’s storm characteristics resulted in surge heights greater than those typically associated with a Category 1 hurricane. Isaac was a weak Category 1 storm on the National Hurricane Center’s Saffir-Simpson Scale, which is based only on maximum sustained wind speeds, not on surge heights. Isaac’s maximum sustained winds were 80 mph, or just 5 mph above hurricane-strength.

High-water-mark measurements collected after Isaac indicate its surge heights were below elevations of the pre-Katrina hurricane levee system in all but three areas: the Caernarvon-to- Highway 46 stretch of levee in St. Bernard Parish; the Inner Harbor Navigation Canal-Gulf Intracoastal Waterway area on the northwest edge of Lake Borgne that is now protected by a 25-foot-high barrier wall; and a segment of improved levee in St. Charles Parish west of the West Return Floodwall at the Jefferson Parish border.

The study used two computer model simulations of Hurricane Isaac’s storm surge to test whether it was affected by the levee system one with the completed levee improvements, the other without the improvements.

For St. John the Baptist and St. James parishes, which experienced significant flooding during Isaac, the surge heights reached record levels as a result of the lengthy period of time — 45 hours from mid-day on Aug. 28 through mid-day on Aug. 30 — that tropical storm force winds, generally from the east, blew water into Lake Pontchartrain.

The levee improvements actually resulted in about a one-inch decrease in surge levels in that area, according to the modeling, although at LaPlace, where the surge reached 8.4 feet, there was no change resulting from the levee improvements, according to the modeling.

In Madisonville, where the surge reached 9.4 feet, the new levees had no effect on its height.

Concerns also had been raised after Isaac that the huge West Control Complex, which is designed to block surge moving north from the Barataria Basin from affecting West Bank communities, might actually increase the amount of flooding in Crown Point and other locations near Lafitte.

At Braithwaite in Plaquemines Parish, where the surge reached 13.9 feet and overtopped a parish levee, flooding hundreds of homes and the Stolthaven chemical storage facility, the levee system increased the water height by 1 inch.

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