Sewerage & Water Board officials have now examined most of the major underground canals in and around downtown New Orleans, hoping the investigation would offer an explanation for recent flooding after heavy rains in the area.

It hasn't.

"We haven’t had anything that jumps out at us as saying, 'This is the reason we’re seeing flooding,' " S&WB General Superintendent Bob Turner told a City Council committee on Wednesday.

Crews are now delving deeper for answers by looking in the smaller pipes that crisscross the area, Turner said, trying to determine whether they're blocked or possibly just too small to carry the huge amounts of water that have fallen on the Central Business District in recent storms.

The search for obstructions was sparked by the Aug. 26 storm that flooded much of the CBD, inundating some buildings and streets in the area for the fourth time this year.

After noticing water pooling without draining in the area during the storm, S&WB Executive Director Ghassan Korban suggested the culprit could be some problem in the drainage lines. That prompted an inspection of the four major underground canals that drain the area.

But workers have now gone through all those culverts save the one that runs under Canal Street and have not found any major problems, Korban said.

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"We've kind of ruled those out, almost," Korban said at the meeting.

The decision to look at the downtown canals came after the S&WB pulled a car and other debris out of an underground canal near the Lafitte Greenway earlier this month. There were 22,000 tons of debris causing a major blockage in the 2.5-mile canal, and it will cost about $10 million to clear it, Korban said.

About 950 tons have been removed so far, he said.

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But while the Lafitte Canal includes an open-air section that could allow large debris to enter the underground segments, the downtown canals are completely covered. That means only relatively small debris would be able to get in through catch basins, Korban said.

With the canals now largely crossed off the list of potential causes, officials still can't explain why the CBD has been seeing more frequent flooding. The lack of answers has residents and elected officials on edge.

"What’s frustrating for people in the city is that whether you’re in your neighborhood or you're downtown or you're by the Zulu Club (on North Broad Street) and you stand there and you don’t see the water moving," Councilwoman Helena Moreno said. "Particularly what we saw that day was that it's just not going anywhere."

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The Department of Public Works next will look into the smaller pipes that lead from catch basins to the canals. There's some indication that those pipes could be a bottleneck.

Public Works Director Keith LaGrange said Wednesday that crews have been looking at pipes on St. Charles Avenue. The pipes are only 10 inches in diameter, which may be inadequate for the area's needs, he said.

Replacing smaller pipes with larger ones citywide would be extremely expensive. LaGrange said a study after Hurricane Katrina found it would cost $3 billion to upgrade the city's pipes to be able to handle a rainstorm that has a 10 percent chance of hitting the city each year.

Public Works is responsible for the smaller pipes in the drainage system, while the S&WB is in charge of the larger culverts and canals.

The inspections by Public Works have so far focused on areas that were particularly hard hit during this year's Mother's Day flood, LaGrange said.

Like much of the rest of the system, those pipes have largely been neglected when it comes to inspection, maintenance and repairs over the years.

LaGrange said less than 4% of the 8 million linear feet of pipe running under the city has been inspected this year.

While the city will never be able to inspect every pipe every year, LaGrange said the goal is to increase inspections so that each pipe is looked at once every three years.