Lowering New Orleans floodwalls could improve canal safety, engineers say

on November 22, 2010 at 6:00 AM, updated November 22, 2010 at 9:40 AM

As if there's not been donnybrook enough around the failed New Orleans outfall canal floodwalls, regional levee commissioners are floating a counterintuitive idea: After the Army Corps of Engineers makes the walls safe enough to hold eight feet of water next year, let's consider reducing their height to make them even safer.

17th.jpgLooking east into New Orleans, the 17th Street Canal floodwall could be lowered once remediation work on the canal is complete.

The prospect of lowering any flood control structure might stun central New Orleans and East Jefferson residents whose homes and businesses flooded during Hurricane Katrina when floodwalls failed catastrophically and let Lake Pontchartrain pour in through the breaks.

But engineers on the Southeast Louisiana Flood Protection Authority-East insist that professionals are obligated to ask and answer the question: Once the 12-foot walls are repaired to hold 8 feet of water, would removing the top 4 feet actually reduce the potential for a future breach?

The supposition is that if the walls were cut down to leave only their "safe" sections standing, water that exceeded 8 feet would simply cascade over like an overfilled bathtub. That's a far less damaging scenario than having high water climb the unsafe sections, rupture the wall and send water exploding into the nearest neighborhood, said levee authority commissioner Tom Jackson.

"I know that this concept is a political hot potato. It's not going to be politically popular," said Jackson, chairman of the authority's engineering committee and a former president of the American Society of Civil Engineers.

Jackson Tom.jpg'We're not politicians. We are engineers,' says Tom Jackson, chairman of the levee authority's engineering committee.

"But we're not politicians. We're not elected. We are engineers ... and I think these are the types of discussions that we should initiate."

On the recommendation of Jackson's committee, the full authority agreed Thursday to initiate a four-way discussion of the concept with the New Orleans Sewerage & Water Board, the state Coastal Protection and Restoration Authority and the corps.

"So as not to start a riot out there, this is nothing more than an investigation of the benefits that we might derive from such an action," authority president Tim Doody said.

Two canals at issue

The study will focus on London Avenue and 17th Street canal floodwalls, which as a result of breaching during Katrina now have "safe water" levels of only 5 and 6 feet, respectively. The Orleans Avenue Canal would be excluded from the study because an existing low spot in its eastern wall already lets high water flow out instead of build up, which some think helped keep its floodwalls from failing in Katrina.

The corps will soon start millions of dollars of remediation work to reset the safe-water levels to eight feet. Once that's done, levee authority commissioner Steve Estopinal likens removing floodwall sections above that elevation to equipping boilers with safety valves.

"It relieves the pressure: There wouldn't be blow-out," said Estopinal, also an engineer.

A corps official expressed skepticism at the prospect of reducing wall height, but the authority's decision to study the concept was applauded by S&WB Executive Director Marcia St. Martin. Her agency operates the primary municipal pump stations in all three canals, and a few years down the road, the S&WB likely will also be operating new pump stations that the corps plans to build near the mouth of each canal.

london-avenue-canal.JPGView full sizeThe study will focus on the 17th Street and London Avenue canal floodwalls. As a result of breaching during Katrina, The London Avenue Canal, above, now has a 'safe water' levels of only 5 feet.

Designed for use only during tropical storms and hurricanes, the new stations must operate in tandem with S&WB pumps to move stormwater out of the surrounding drainage basins, through the outfall canals and into the lake without exceeding the safe water levels in each canal.

"The citizens of this region paid for a portion of these floodwalls to levels of (12) feet, and now the corps is patching them to have an elevation of eight feet," St. Martin said. "I think having a delta between the safe water elevation and top of the wall gives people a false sense of confidence.

"Perhaps they should be brought down to grade; I don't know," she said. "But I think it's a good thing that the levee authority responsible for the walls is willing to initiate a study group to investigate.

"A decision needs to be driven by science ... and we would participate with our partners in it."

Corps is skeptical

It is during the tandem operation of pumps on each canal that Jackson and others say an operational foul-up mostly likely could occur, causing water to build up in the canals and potentially exceed eight feet. Jackson said he doesn't except such a failure, but he said Katrina taught to anticipate the unexpected.

In a worst-case scenario, water climbing above eight feet would so stress the 12-foot floodwall and trigger other changes that it could suddenly breach and pour water into the adjacent neighborhood. Were the walls only eight feet high, Jackson said, breaching is less likely and any high water would flow more gently -- and at lower rate -- over the walls.

Jackson said the floodgates built in each canal after Katrina virtually eliminate the possibility that the lake could ever again surge unrestricted into the canals and through another floodwall breach.

"With gates closed, a breach wouldn't be catastrophic to the city as the breaches were in Katrina," he said.

But even with the gates closed, a deep break could easily spill enough water to damage residences taking that took the brunt of a concentrated flow.

"A breach, depending on how it occurred, it could partially drain or empty a canal. If a breach occurs, there would be a sudden failure and the pumping system would be closed down until the breach could be repaired, which means that even when the water drops low enough for the S&WB pumps to work again, they would have to remain shut down until the breach was repaired," Jackson said.

John Grieshaber, a geotechnical engineer and executive officer of the corps' Hurricane Protection Office, said his agency sees no imperative to reduce the height of floodwalls and isn't recommending it.

"The corps would have no objection to them doing it, but we're not going to do it," said Grieshaber, whose division is planning and executing the remediation work and overseeing design and construction of the new pump stations.

"I don't see any advantage to doing it ... and we see no way now, under existing conditions ... or once the new stations are in, that water in the canals can even get to eight feet," he said. "It's our understanding that the S&WB cannot even pump that much water into the canals, because at elevation eight, water would be coming into the pump stations and they wouldn't even be able to pump."

Still, Grieshaber said corps representatives would certainly look at the issue again in any discussion its "local partners" want to initiate.

"We can participate in a conversation and help flesh out everybody's concerns and learn together."

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Sheila Grissett can be reached at or 504.717.7700.