Q&A: Draining New Orleans — Sept. 9 Update

Water flows through breach i i

Water flows back into the Industrial Canal in New Orleans through a floodwall breach caused by Hurricane Katrina. The inundated neighborhood in foreground is known as the Lower 9th Ward. U.S. Army Corps of Engineers hide caption

itoggle caption U.S. Army Corps of Engineers
Water flows through breach

Water flows back into the Industrial Canal in New Orleans through a floodwall breach caused by Hurricane Katrina. The inundated neighborhood in foreground is known as the Lower 9th Ward.

U.S. Army Corps of Engineers
Map of flooding in New Orleans i i

A map from the U.S. Army Corps of Engineers sets out areas of flooding in New Orleans following Katrina, Sept. 2. U.S. Army Corps of Engineers hide caption

itoggle caption U.S. Army Corps of Engineers
Map of flooding in New Orleans

A map from the U.S. Army Corps of Engineers sets out areas of flooding in New Orleans following Katrina, Sept. 2.

U.S. Army Corps of Engineers
Breaches in canal walls i i

In some parts of New Orleans areas, the major sources of water appear to have come from breaches in canal walls that occurred Monday night or on Tuesday, Aug. 30. (Enlargement shows more detail.) U.S. Army Corps of Engineers hide caption

itoggle caption U.S. Army Corps of Engineers
Breaches in canal walls

In some parts of New Orleans areas, the major sources of water appear to have come from breaches in canal walls that occurred Monday night or on Tuesday, Aug. 30. (Enlargement shows more detail.)

U.S. Army Corps of Engineers

The Army Corps of Engineers estimates that more than half of New Orleans is still flooded, although they are making slow but steady progress in draining water from the city. NPR Science Correspondents David Kestenbaum and David Malakoff, and Reporter Nell Boyce provide an update on efforts to drain the floodwaters.

Q: What's the latest news?

The Corps now says it may take 40 days or less to fully remove water from the city. Earlier this week, a computer simulation that engineers are using to guide water removal efforts suggested it could take up to twice that — 80 days. The quicker pace is due to success in getting more pumps working, in closing levee breaches caused by the hurricane, and in intentionally opening other breaches for water to flow out. Water is reportedly dropping by up to half a foot a day in some areas, and those areas could be dry within two weeks.

Q: What is the strategy for getting the water out?

Pumping is the major method now. The U. S. Army Corps of Engineers says that 37 of 174 pumps around the city are now working. Not all of these pumps are in flooded areas, however. In addition, they have brought portable pumps to some neighborhoods. Engineers are still working to get more pumps started. Teams are also working to clear debris, reportedly including corpses, blocking some pumps and storm drains. Equipment is pouring in from around the world.

Q: How did the water get into the city?

Some parts of the city were flooded by the initial hurricane. It now appears one wall of water overtopped a floodwall protecting the city's Ninth Ward during the peak of the hurricane on Monday, Aug. 29, causing water levels to rise 8 feet or more in less than an hour in places.

In other areas, the major sources of water appear to have come from breaches in canal walls that occurred Monday night or on Tuesday, Aug. 30. One was the 17th Street Canal, which is used for drainage and runs through New Orleans to Lake Pontchartrain on the city's northern edge. A second breach was in a wall in the London Avenue Canal. There were also breaches in the wall of the Industrial Canal (also called the Inner Harbor Navigational Canal), on the east side of town.

Q: What is the difference between a floodwall and a levee, anyway?

The canals walls that broke are technically called floodwalls. They are made of concrete and steel, are 6-to-10 feet tall, and about a foot wide at the top and 2 feet wide at the bottom. They stand on top of an earthen base.

A levee is a broad mound, 50 feet or more wide at the base, that rises slowly to a broad crest at the top. You could easily walk or drive up the side of one. These are far more stable than floodwalls. Water can spill over the top and erode some of the levee, but it will still function. When a floodwall fails, it fails catastrophically.

Q: Is water still flowing into the city?

No, although engineers are carefully managing several of the holes they intentionally put in levees to drain water. The Mississippi River is tidal near New Orleans, so when the tide rises, they add extra fill to the holes to make sure no new water flows in to the city. When the tide falls, they open the breaches again so that water can flow out.

Q: Why did the canal walls break?

The U. S. Army Corps of Engineers says the system simply got more water than it was designed to withstand. Standing on the street, you look up to the 17th Street Canal. It has a small earthen base, with a tall concrete and steel floodwall on top that holds back the water. The Army Corps says the breach apparently occurred when water levels in the lake rose after the storm, causing water in the canals to flow over the top of the floodwall. The water likely eroded the foundation under the wall.

Q. What is the difference between "dewatering" and "unwatering"?

Some engineers use the terms interchangeably. But "dewatering" technically refers to removing water from under the ground. "Unwatering" means removing standing water, as they are doing in New Orleans.

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