Even before Hurricane Katrina struck, experts warned that the network of earthen, steel and concrete barriers that protected New Orleans was inadequate - and they proved tragically correct.
On Tuesday, a day after the storm barreled past, sections of two levees broke, spilling water into the streets and inundating an estimated 80 percent of the city, much of which lies below sea level.
In all, the New Orleans district contains 350 miles of hurricane levees, mostly along Lake Pontchartrain to the north and the Mississippi River on the south - part of an even larger network constructed over many decades along the Mississippi by the Army Corps of Engineers.
The earliest levees were erected soon after the city's founding in 1718; the system has been expanded and strengthened ever since.
"This levee system is to levees around the world the way that the Great Wall of China is to walls around the world," Tulane University environmental expert Oliver Houck told American RadioWorks in a 2002 documentary about New Orleans' vulnerability to hurricanes.
Repeatedly, almost miraculously, the city escaped major damage in the past century as hurricane after hurricane wreaked havoc elsewhere along the Gulf Coast. But as a consequence of Hurricane Betsy in 1965 - the last major hurricane to strike close to New Orleans - the levees encircling the city and outlying parishes were raised to heights of up to 23 feet.
Nonetheless, experts repeatedly cautioned that the protective system was unlikely to prevail if a Category 4 or Category 5 hurricane like Katrina hit the city.
Marine scientist Ivor van Heerden of Louisiana State University, who has developed flooding models for New Orleans, was among those issuing dire predictions as Katrina approached, warnings that turned out to be grimly accurate.
"We're talking about an incredible environmental disaster," van Heerden said before the storm arrived.
He predicted that floodwaters would overcome the levee system, fill the low-lying areas of the city and then remain trapped there well after the storm passed - creating a giant, stagnant pool contaminated with debris, sewage and other hazardous materials.
Van Heerden and other experts put some of the blame on the Mississippi River levees themselves, because they channel silt directly into the Gulf of Mexico that otherwise would stabilize land along the riverside and slow the sinking of the coastline.
Despite warnings from experts, the levees may also have contributed to an unwarranted sense of security among residents. Only 34 percent of respondents in a University of New Orleans survey earlier this year said they would definitely leave home if evacuation orders were issued ahead of an approaching Category 3 hurricane.
When Betsy struck in 1965, water flowed over the top of the levees and flooded the pumps intended to keep the city dry. The same phenomenon has happened now, except that authorities say it will take far longer to get any pumping operations started.
The Army Corps of Engineers said Wednesday it planned to use heavy-duty Chinook helicopters to drop 15,000-pound sandbags into the 500-foot gap in the failed 17th Street Canal levee. Walter Baumy, chief of the engineering division of the New Orleans district of the corps, said 200 bags had been filled with sand and gravel, and that more than 1,000 were being prepared.
Officials said they were also looking at the possibility of finding a barge to plug the 500-foot gap.
Once the breaches are plugged, engineers can start pumping water out of the city, if failed pumps can be repaired.
At the same time, Army Corps engineers said Wednesday they are preparing to intentionally breach some levees near the bottom of the New Orleans bowl to let trapped water out.
Then the task will be to rebuild levees, probably to a higher level. But taller levees must also be wider levees: Typically, they add two feet in width for every foot in height.
Another possibility would be to add concrete walls anchored atop levees to increase their height.
"My recommendation would be to build the levees of something other than earthen materials that would not be so subject to erosion, like concrete," said Richard Weggel, a civil engineer at Drexel University.
"The real thing they need to do is to isolate the pumps from damage so they can remain operational," Weggel said.
All of the pumps in New Orleans failed during Hurricane Katrina and none were working Wednesday, the corps said. Several in neighboring Jefferson Parish, along the lakefront, are operational.
Information from Knight Ridder news service was used in this report.
