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Engineers took the first steps Thursday to closing the 700-foot levee breach that flooded New Orleans in the aftermath of Hurricane Katrina, and Louisiana state officials said water removal from the stricken city could begin by Saturday.

Early in the evening, a local contractor began driving steel slabs into the breach in the 17th Street Canal. Once the job is finished, engineers will begin entering pumping stations in the flooded areas of the city and activating pumps to carry water away.

The U.S. Army Corps of Engineers said the speed of process, predicted by some to take a month, will depend on several factors, including the number of pumps in working order and how much floodwater will escape back the way it came as Lake Pontchartrain continues to subside to pre-hurricane levels.

Rain and Katrina’s storm surge caused the lake, which borders New Orleans on the north, to rise more than 4 feet above its normal level in the 36 hours after the hurricane plowed into the city early Monday.

The swollen lake backed into the 17th Street Canal and surged over a section of its earthen levee on the east side, a catastrophic phenomenon known as “overtopping.”

“Erosion occurs rapidly,” said civil engineer Rick Stephenson of the University of Missouri at Rolla. “You have a concrete retaining wall on top in some areas, and it just collapses the levee as the earth foundation is eroded.”

Water surged through the breach and through a second, much smaller break in another levee at the London Avenue Canal to the east. By Tuesday evening, 80 percent of the below-sea-level city was submerged, with some sections flooded to a depth of 20 feet.

Since then, however, “the water in the lake has been dropping a tenth of a foot per hour,” said Bob Anderson, a spokesman for the Army Corps of Engineers in Memphis, which is overseeing the water removal. “It’s getting close to its natural level.”

At first, the corps contemplated using helicopters to drop large sandbags or concrete blocks to fill the 17th Street breach, but the corps’ chief, Lt. Gen. Carl Strock, said Thursday that federal relief officials decided to dedicate all the available aircraft to rescuing survivors.

A helicopter drop probably would not have helped anyway. “You’ve got a great big spillway,” said William Marcuson, director emeritus of the corps’ Geotechnical and Structures Laboratory in Vicksburg, Miss. “At some point the lake will subside, but until it stabilizes, you might as well leave the breaches there.”

Equilibrium came early Wednesday. New Orleans’ harried residents noticed that the floodwaters had stopped rising, and as the day wore on, the level began to fall. This, officials said, was caused by two factors.

First, Anderson said, many of the city’s pumps, particularly in higher-ground areas, never stopped operating and were continuously flushing water into the Mississippi River south of the lake and into undamaged canals.

Marcuson explained that New Orleans has a compartmentalized flood control system in which an internal system of floodwalls and levees divides the city into a series of cellarlike sections, which are drained during storms by dedicated pumps. “Think of it like a set of basements with sump pumps in each one,” he said.

The second reason for the receding waters was that the lake had subsided so much that the water inside the city was able to flow back the way it had come. The 17th Street and London Avenue breaches became drains. By Thursday afternoon, 53 percent of the city was dry.

Anderson said the Corps planned to leave the 250-foot-wide London Avenue break alone so water would continue to flow out, and “notches” are being cut in the levee protecting eastern New Orleans — makeshift gutters so water will spill out.

But if the city relied only on gravity, months would elapse and there would still be two feet of water on the ground. Engineers needed to get pumps working in the areas of heavy flooding, and to do that they needed to close the 17th Street breach.

For three days, Corps officials had lamented the difficulty of gaining access to the canal, but Thursday a local contractor, Boh Bros. Construction Co., apparently drove to the mouth of the canal and started placing a set of steel sheet pilings to isolate the canal from the lake. This job was finished Thursday afternoon.

Only then, Louisiana Secretary of Transportation and Development Johnny Bradberry explained, would the canal waters subside enough that Boh Bros. could get to the breach itself and start driving piles there to fill it. Anderson said this began Thursday evening.

Bradberry said the breach would be plugged by Saturday.

DIAGRAM: Plans to drain New Orleans. Now that water inside New Orleans has reached the same level as Lake Pontchartrain, the Army Corps of Engineers will attempt to block the 17th Street Canal to prevent any more water from flowing into the city. Sources: U.S. Army Corps of Engineers; U.S. Enviromental Protection Agency; KRT; MWI Corp. Staff graphic/Len De Groot, Karsten Ivey