As Hurricane Katrina dissipated on its way toward the northeastern United States on Tuesday, the threat only grew for New Orleans and other Gulf towns. Monday afternoon's seeming reprieve in New Orleans evaporated as two breached levees flooded the city.
New Orleans is a bowl-shaped depression set just below sea level, surrounded by water on all sides. Lake Pontchartrain to the North, the 7th Street canal on the east and the Mississippi River to the south and west had enclosed the city in a watery embrace, held off only by flood walls and levees reinforced to withstand a Category-3 hurricane, according to the U.S. Army Corps of Engineers. After the worst of Katrina had past, high water levels caused the flood walls holding back both the lake and the canal to breach.
Hurricane Katrina was a Category 4 when it touched down in New Orleans on Monday. The condition of the levees has been a concern over the past decade, says Peter Scholle, president of the Association of American State Geologists and New Mexico State Geologist. The body of literature published on the Mississippi basin and other discussions on the state of the levees have led to "a lot of public praying about New Orleans," he says, in the event of a large storm or flood. The whole system has been "a faith-based approach to coastal management."
One reason the levees were not fortified to withstand more than a Category-3 storm is because building levees is expensive: "The higher you build them, the more it costs," Scholle says, taking up more land in an expensive city; plus, they have to be built up river and down, otherwise water can flow in from the sides. On geologic timescales, the Mississippi would have switched to flow through the nearby Atchafalaya River long ago but for the controls placed on it by the U.S. Army Corps of Engineers. The levees that entrain the river stop it from flooding in normal conditions, also preventing it from depositing the huge amounts of sediment it carries from the Rocky Mountains and the Midwest, out beyond the continental shelf. That lost sediment could have been deposited in what is now New Orleans, and it would have made up for part of the massive subsidence and compaction the area is experiencing, "but it's gone," Scholle says.
"Several government policies have made this disaster possible, or at least exacerbated it," Scholle says, from subsidized federal coastal home insurance that encourages people to build and rebuild in hazardous areas, to the levee system.
News reports indicate that New Orleans, with a pumping system that can only remove half an inch of water an hour, may be inundated for weeks, and officials have begun a complete evacuation of the city, which has been in disarray. Once the breaches are mended, the U.S. Army Corps of Engineers estimates that it can pump out a foot of water a day, but that is the best-case scenario, according to a briefing document.
Thousands of people remain displaced, and bodies have been left floating in the flood waters as rescue workers focus on finding survivors. Yesterday, the mayor of New Orleans gave what the New York Times called the first official estimate on the scope of deaths caused by the hurricane, saying that thousands could have died in New Orleans alone. Heart-wrenching stories of loss and survival continue to be told, and reports from Mississippi and Alabama on Tuesday showed a much worse scene of devastation than initially thought.
More storms are expected, as hurricane season typically peaks in September.
NOLA.com, the online edition of the Times-Picayune
New York Times coverage and photo essay
"Hurricane Katrina hits hard," Geotimes Web Extra, Aug. 30, 2005
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