Study finds rapid pre-Katrina sinking in New Orleans
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(CNN) -- Parts of New Orleans sank rapidly in the three years leading up to Hurricane Katrina, which might have made the already low-lying city even more vulnerable, a new study found.
That may explain why some levees failed during Katrina and raises serious concerns about the future of the city, according to researchers.
The study, released Wednesday by the journal Nature, found that some areas subsided 28.6 millimeters (about 1 1/8 inch) per year between 2002 and 2005. The average decrease was about 5.6 millimeters (almost one-quarter inch).
The area around the Mississippi River-Gulf Outlet shipping canal in St. Bernard and Orleans Parishes sunk 20 millimeters (more than three-quarters of an inch) per year during that period, according to the study.
The canal's levee was one of the ones breached by Katrina's floodwaters.
"What we found is that some of the levee failure in New Orleans were places where subsidence was highest," University of Miami professor Tim Dixon said in a news release from the school. "These levees were built over 40 years ago, and in some cases, the ground had subsided a minimum of 3 feet which probably put them lower than their design level."
Scientists made the measurements by studying more than 100,000 images taken by a Canadian satellite monitoring the wetlands around New Orleans, study co-author Shimon Wdowinski said. Scientists did not know about the images until after Katrina.
"Not only is New Orleans subsiding, but the whole Mississippi delta has big problems with disappearing wetlands," Wdowinski said. "Eighty acres of wetlands a year disappear because of subsidence and sea-level rise."
He said that some places, including the Lakeview and Kenner areas, would continue to sink about an inch per year over the next 10 years but that the average would be a fraction of that.
"We need to think long term, think of what will happen in the city in 50 or 100 years," Wdowinski said. "Some areas will continue to subside, the sea level will continue to rise. Places like the Lower Ninth Ward will be 10 feet below sea level."
He said the findings raise serious concerns as officials work to rebuild the city.
"I don't think anybody wants to live in a place like that. It's just not a good idea."
An independent study into New Orleans' flood control system found serious design and construction flaws. (Full story)
The report was highly critical of the Corps of Engineers, which is in charge of the levees.
U.S. Army Corps of Engineers civil engineer Allen Coates said on Friday that the subsidence findings were surprising but would not delay plans to improve the levee system.
"I'm certain that it will have some impact. The only thing we can do at this point in time, until we survey the data, is to keep building," he said, adding that scientists and engineers would have to investigate further.
The Corps has been working to rebuild the system before the start of hurricane season, but said some parts will not be ready until later in the summer.
CNN's Marsha Walton contributed to this report.
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