U.S. reassesses safe water levels in New Orleans' outfall canals

Published 18 May 2009

New Orleans has three outfall canals, the role of which is similar to that of a storm drain under a city street; since Katrina, there have been disagreements among engineers as to how much water would it be safe for each of the three canals to handle during a storm

New analyses are under way to determine how much water the worrisome floodwalls on New Orleans”s three outfall canals can safely hold now that new, more conservative design standards gradually adopted since Hurricane Katrina are finally in place. If current safe water levels still prove safe enough to meet the stricter criteria, there will be no need further to restrict the surge allowed into the 17th Street, London Avenue and Orleans Avenue canals before floodgates must be closed. If any of the current levels would put more water into the canals than the new standards permit, however, it would be necessary to either reduce the safe water levels or, if possible, strengthen trouble spots so that existing levels could be maintained, Army Corps of Engineers officials said.

The Times-Picayune’s Sheila Grissett writes that Corps supervisory geotechnical engineer John Grieshaber said he is optimistic that safe water levels won’t have to be reduced — even if it requires driving sheet piling, building coffer dams or taking other affordable steps to bolster any weak sections that would otherwise force reductions. “We’re confident with the safe water levels we have today because we’ve been using conservative assumptions to set them,” Grieshaber said.

If it turns out that any one of them is no longer conservative enough for the new criteria, Grieshaber said, it may only take relatively simple remedial projects to maintain the current levels.

The analyses and final reports are being done by Black & Veatch, an international engineering firm overseeing the canals for the corps.

As part of that, consultants will assess the constructibility and cost of various remedial actions that could help maintain, or even potentially increase, safe water levels.

The London Avenue Canal’s current safe water level of five feet is particularly crucial. If lowered by even one foot, experience has shown, the reduction would reduce by 30 percent the amount of storm water that New Orleans Sewerage & Water Board pumps can move out of surrounding neighborhoods. That happened three times before the corps raised London Avenue Canal to five feet in late 2007 after conducting a $4 million load test that corps officials said proved the higher level was safe.

It is not an opinion shared by critics pushing the corps to redesign and replace the existing floodwalls instead of making piecemeal repairs or relying on floodgates and complex pumping strategies to maintain safe water levels.

Southeast Flood Protection Authority-East member Stradford Goins, an engineer, opposed the corps’ decision to raise the London Avenue Canal’s safe water elevation to 5 feet. “I don’t trust those floodwalls,” he argued. “In my mind, all the canal floodwalls would have to be rebuilt with pilings substantially deeper than those that are out there now.” Engineer and authority member Tom Jackson said he felt he had no choice but to approve it: “Not raising it would have meant flooding the city during the next big rain.”

The corps’ Grieshaber has far more confidence in the ability of current safe water levels and floodwalls to prevent future flooding. “But it’s too soon to say what (the new analyses) will mean for the safe water elevations,” he said. “We have to see what the analyses show.”

The results of analyses on the London and Orleans Avenue canals — considered the sturdiest of the three with a water level of 8 feet — are due by the 1 June start of hurricane season. Grieshaber predicts that would give time before the season’s traditional August-September peak to complete any residual work that is needed.

It is a different timeline for the 17th Street Canal, which must be resurveyed before the safe water level can be recalculated. The new survey, to include canal depths and widths at multiple locations, is required because some engineers have lost confidence in parts of the previously collected data, Grieshaber said.

The need to resurvey will delay the decision until it is too late to make changes, if any are needed, before the peak of hurricane season.

Still, Grieshaber told Grissett he is comfortable that the new analysis would not force a reduction in the current level. If remedial work is required, he said, residents should remember the canal held six feet of water when floodgates were closed against storm surge twice last season. “Six feet is a conservative number,” he said. “When we looked at the different canals, we always erred on the side of caution.”