Corps releases evaluation of Isaac's storm surge

The improved levee protection around the greater New Orleans area had little effect on the extensive flooding in areas like LaPlace or Braithwaite during Hurricane Isaac, according to an evaluation released by the U.S. Army Corps of Engineers on Friday.

“The impacts of Hurricane Isaac show that these impacts would have been similar with or without that system in place,” said Col. Ed Fleming, New Orleans corps district commander.

Instead, it was the characteristics of a slow-moving, rain intensive storm that tracked in such a way that the storm spent a very long time pushing water to the east into Lake Pontchartrain and eastern Plaquemines Parish, Fleming said.

The report is in response to concerns voiced after Isaac that the improved levee protection in the greater New Orleans area pushed storm surge into surrounding areas. Fleming explained that corps staff used water marks measured in flooded areas after the storm and ran computer models that compared impacts with the New Orleans protection system in place and impacts without the protection system.

Overall, the computer models showed about a one-inch difference between the two scenarios, Fleming said. In one community north of Lafitte, of the 4.4 feet measured as a high water mark, about four inches of that could be attributed to the improvements of the New Orleans levee system, Fleming said. That was the highest level measured in an inhabited area.

The high water marks are where the water settled long enough to make a mark and don’t take into account any waves that may have been on top of that water level, Fleming said.

In addition, the computer model calculations match the results the corps found when computer models were run before construction in the New Orleans area, Fleming said.

St. John Parish President Natalie Robottom said she doesn’t believe the New Orleans protection system had no impact on the flooding she saw in her parish in places like LaPlace.

Robottom said she and other officials got a short briefing from the corps about the study results, but she hasn’t gotten a chance yet to review the 300-page document the corps released Friday.

“What I do know is St. John Parish has never flooded,” she said.

“What is clear is that the areas protected by the federal system were safe and areas outside were not.”

Robottom said there has been a protection plan for the area on the shelf for decades that needs to be implemented and added that Fleming and state officials have agreed to work on the issue.

“It’s been in the works an extremely long time,” she said of the West Shore Levee Project. “People want to know when we will have levee protection.”

Robottom said although she doesn’t agree with the corps study that found no additional impact to LaPlace because of the New Orleans levee system, she doesn’t want another study done.

“Our study was houses flooded. That’s the last study I want to see done,” Robottom said. “I want a levee.”

Some areas, particularly on the north and western sides of Lake Pontchartrain, actually saw a decrease of an inch or two of storm surge due to the greater New Orleans protection system, according to the computer models.

Fleming said the closure of channels at the Gulf Intracoastal Waterway and at Seabrook in New Orleans meant that water could no longer flow through the city into Lake Pontchartrain.

Fleming cautioned that the computer models showing impacts to surrounding areas were run only for the track and size of Hurricane Isaac.

“Another storm on another track would show different results,” he said. “All hurricanes are different. Each hurricane has its own characteristics and Hurricane Isaac is no different.”

People have different “baselines” for what damage a hurricane will do in their area whether that baseline is Hurricane Betsy or Hurricane Katrina, Fleming said. However, hurricanes can’t be compared that way with each other because the track, the forward speed, the size, the rainfall and many other factors play into the amount of damage areas might sustain, he said.

“Just because the National Weather Service comes out and says it’s a Category 1 (storm), that’s good information,” Fleming said. “But around here, that’s not enough.”

People need to make informed decisions by looking at the entire storm package including the size and forward speed of the storm, he said.

The slow-moving Isaac had a lot of time to push water into Lake Pontchartrain where it stacked up on the north and west side of the lake.

At the same time, rainfall north of the lake was making its way down the rivers, but because of the lake’s height, there was nowhere for that water to go, Fleming said.

People who didn’t flood before, but did during Isaac, want to know why and Fleming said he understands that, but the computer models show that New Orleans’ protection system had very little impact.

The report was done by the corps with help from the National Oceanic and Atmospheric Administration and the National Weather Service. The Water Institute of the Gulf and the Southeast Louisiana Flood Protection Authority-East also reviewed the work. An independent external review is also being done by the Louisiana Water Resources Council but those results may not be ready until spring.

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The entire study is available at http://www.mvn.usace.army.mil/