New Orleans, La. -- Southeastern Louisiana's new levees and flood walls were not responsible for flooding areas outside the system during Hurricane Isaac, a U.S. Army Corps of Engineers study concludes.
"There was one inch difference when we ran Hurricane Isaac through the [computer] models," said Col. Ed Fleming, commander of the Corps New Orleans District.
The modeling found that many areas along Lake Pontchartrain took no additional water as a result of the levees. One of the more surprising, perhaps controversial findings was the effect on LaPlace. The models concluded that an area of St. John Parish actually took one inch less water because of the Corps risk reduction system.
Engineers reason high water that normally would have flowed through the Mississippi River Gulf Outlet, the Industrial Canal and into the lake was cut off. The new system closed the MRGO and sealed off the lake with flood gates.
"These models are consistent with the models that we ran prior to construction," Fleming said.
Following Katrina, the Corps constructed 150 miles of flood walls, gates, levees and pumping stations, a massive $14.6 billion effort to protect against a 100-year storm.
In Isaac's wake, Louisiana congressional leaders demanded the Corps take a second look at how the new projects affected areas outside the system.
Many residents who lived through Betsy and even Katrina had trouble believing the higher levees left them at no more risk.
"We need protection just like New Orleans," St. John Parish resident Mona Jacob said in the aftermath of Isaac. "St. James needs protection, all those surrounding parishes need protection."
"Every storm is different," Fleming said, noting that Isaac was "about three or four times slower [than Katrina] as it moved through."
The nearly-300-page report finds other factors were at play, including the size, path and duration of Isaac.
Fleming noted metro New Orleans endured tropical storm force winds piling water in from the east for 45 hours.
"As those winds are blowing in a counterclockwise direction, what you're doing is taking that surge and bringing it up across from the Gulf of Mexico," Fleming said.
The report finds the West Bank experienced the biggest difference in high-water levels. Nine more inches of water pressed against the giant "West Closure" pumping station and flood gates on the Intracoastal Waterway, according to the modeling. The surge in Crown Point rose about four inches to 4.4 feet.
The watchdog group Levees.org points to the disappearance of natural defenses as a factor in the unprecedented water heights during Isaac.
"They might have heard about the wetlands being lost," said Levees.org founder Sandy Rosenthal. "But they couldn't have known just how deadly, just how much it matters."