About 25,000 people and 11,000 structures could be in harm's way when the gates on the Morganza spillway are unlocked for the first time in 38 years.
Opening the spillway will release a torrent that could submerge about 3,000 square miles under as much as 25 feet of water but take the pressure off the downstream levees protecting New Orleans, Baton Rouge and the numerous oil refineries and chemical plants along the lower reaches of the Mississippi.
Engineers feared that weeks of pressure on the levees could cause them to fail, swamping New Orleans under as much as 20 feet of water in a disaster that would have been much worse than Hurricane Katrina in 2005.
