New Orleans' levees are such a common feature of the landscape that they become nearly invisible: flat-topped, grass-covered pyramids adorned with bike paths and parks, in residential neighborhoods and industrial districts.

Only when they break, as they did this week, do the levees reveal themselves — and expose the city's vulnerability.

The levees are earthen embankments that stand between the city and Lake Pontchartrain on one side and the Mississippi River on the other. When topped by floodwaters, they tend to erode quickly and then give way, allowing water to rush into the city.

The levees then become part of the problem, trapping water in the city.

Plugging a breached levee can be difficult, as the Army Corps of Engineers found yesterday. Inrushing water can sweep away giant sandbags faster than workers can fling them into the hole, until water levels are equalized on both sides of the levee.

"Whatever you put in there washes away unless it's big enough or you overwhelm it with a whole lot of material in a short amount of time, and that's very difficult to do," said Richard Weggel, a civil engineer at Drexel University in Philadelphia who is an expert in coastal engineering issues.

The Army Corps of Engineers said yesterday 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 more than 1,000 were being prepared.

Officials said they also were looking at the possibility of finding a barge to plug the hole.

Once the breaches are plugged, engineers can start pumping water out of the city, if failed pumps can be made to work. But at an inch of water a day, it would take about four months to pump out the water that covers much of New Orleans.

At the same time, Army Corps engineers said yesterday they are preparing to intentionally breach some levees near the bottom of the New Orleans bowl to let trapped water out.

The task then will be to rebuild levees, probably to a higher level. Some already are 18 feet tall. But taller levees also must be wider levees: They typically add 2 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," Weggel said.

"The real thing they need to do is to isolate the pumps from damage so they can remain operational," he said.