I was just poking around the Army Corps of Engineers website and I found the following (and apparently recent) FAQ on unwatering New Orleans in Word format. I’m reprinting it here so that it will be more accessible and in a web friendly format.
Q. How long will it take to get the water out of New Orleans?
A. We are unsure. A number of factors play into this. First, Lake Pontchartrain is at roughly 4.5 feet above sea level and falling. The city is at a lower elevation so water will continue to flow into it until it equalizes.
Once the breach on the 7th Street Canal is closed, Pump Station 6 can pump 10,000 cubic feet per second.
Once the breaches are closed and all of the pumps are running, the pumps can lower the water level 1/2 inch per hour or about a foot per day. We can get the water level to sea level in four and a half days. The 1/2 inch rate assumes the late is at normal levels. That would create pumping inefficiency, as could trash in drains and canals that feed into the pump stations.
That’s a “Best Case” scenario. We don’t know the conditions of all of the pumps. Fortunately most of the pump motors and controllers are at an elevation greater than 5 feet and we hope they weren’t submerged. There could be other unforeseen problems.
We assume the pumps have not been submerged since most pumps are at an elevation greater than five feet above sea level.
Pumps are operated and maintained by the local sewage and drainage districts.
Q. Why did the levees fail?
A. What failed were actually floodwalls, not levees. This was caused by overtopping which caused scouring, or an eating away of the earthen support, which then basically undermined the wall.
These walls and levees were designed to withstand a fast moving category 3 hurricane. Katrina was a strong 4 at landfall, and conditions exceeded the design.
Q. How many other areas do you need to get water out of?
A. There are at least five ringed levees (areas surrounded by levees) that need to be emptied. New Orleans and Jefferson; New Orleans to Venice (Hurricane Protection project — Port Sulfur to Venice, LA); Chalmette Loop (lower 9th ward of Orleans Parish and Urbanized part of St. Bernard Parrish ); and, Plaquemines Parish non-federal levees have also been overtopped.
Q. What will be done to unwater these areas?
A. The unwatering plan will be used in these areas as well. Crews and equipment will be mobilized to breach the levees at predetermined locations and allow for gravity drainage into Lake Bourgne or other surrounding water bodies.