New Orleans Floods Despite Having a Properly Working Drainage System
By Jeff Adelson
The flooding in New Orleans on Saturday happened even though the drainage system was working as it was supposed to and didn't have any unexpected failures, Sewerage & Water Board officials said Monday.
They also said there are no simple upgrades that can prevent such an event from occurring again in a heavy rain, given the complicated and interconnected nature of the pipes, pumps and canals designed to move water out of the city.
While many in New Orleans, including some city officials, are looking for points where the system broke down or casting blame with talk about idle pumps, the S&WB officials once again said Monday that the problem was a storm that dumped more water on the city faster than the system is designed to handle.
The widespread flooding has raised many questions about whether something went wrong. But many of those questions, officials said, are based on an incomplete understanding of how the system works.
Outfall canal pumps at the lake, which are part of the hurricane protection system, remained idle because they would at best be useless and at worst hinder the flow of water in an event like Saturday's, they said. The cracked infrastructure below the city doesn't hinder drainage and may, in fact, aid it in some cases. And upgrades, including the massive Southeast Louisiana Urban Flood Control Project in Uptown, have only relatively modest impacts.
"It's tough to say one project is going to make all the difference in the world," S&WB General Superintendent Joe Becker said.
Probably every New Orleanian has heard what's become a mantra for the S&WB during both floods over the past month: The drainage system is designed to handle 1 inch of rain in the first hour and a half-inch every hour after that.
That standard is built into nearly all parts of the system, from the pipes to the pumps.
Short of replacing the system as a whole -- to the tune of tens of billions of dollars and an amount of construction that would dwarf what the city has ever contemplated -- any upgrades will buy only a few extra minutes or a few fractions of an inch in storms such as Saturday's, which dropped nearly 10 inches of water in Mid-City in a few hours and more than half that in other neighborhoods.
There are no indications that any of the city's 24 pumping stations, housing 121 individual pumps, weren't turned on when they were supposed to be or weren't able to operate continuously, Becker said.
Seven individual pumps were offline throughout the storm because of routine maintenance, but Becker said that had a negligible impact on the overall system and some of those pumps were in areas that didn't flood, such as the West Bank and New Orleans East.
All water that falls in New Orleans must either be pumped out or absorbed into the ground.
The pumps are part of a system that begins with catch basins, which collect water off the streets and direct it into increasingly larger pipes and culverts. Those pipes buy the city a little time at the beginning of a storm, holding a half-inch of water between the catch basins and the pumps, Becker said.
That storage capacity explains why the system can handle twice as much water in the first hour of rain, when the water flows into empty pipes, as afterward. But after that capacity is filled, water will begin pooling in the streets.
The water on the surface will flow to lower areas, as happened Saturday in Mid-City and other areas. That flow can cause water levels to rise in some places even after the rain has stopped, as more water comes in from higher parts of the city, Becker said.
Increasing the system's storage capacity is not cheap. Once complete, the half-billion-dollar SELA work Uptown -- which includes installing bus-sized culverts under major avenues -- will add only enough capacity in that area to hold another half inch of rain, so water would begin pooling on the street after more than 1.5 inches falls in the first hour, Becker said.
That said, the S&WB is trying to make minor improvements where it can. Undersized pipes are being replaced by larger ones as road reconstructions are underway, Becker said. But those would not have prevented the flooding on Saturday, he added.
Interestingly, the cracked pipes that cause problems for the city's water and sewer systems are not a problem when it comes to the drainage lines and actually may be a benefit, Becker said. Some lines used by the city are in fact designed to have holes, something that helps water in the ground make its way to the pipes to be pumped out, he said.
Gravity -- typically more efficient than pumping -- carries the water through the pipes to pumps with large storage areas below low points of the city. From there, the water is pumped up between 10 and 15 feet to a second storage basin, this one feeding larger pumps such as those that feed into the outfall canals, which in turn allow the water to flow out and into the lake.
Those pumps do not actually push water through the canals. Once again, the S&WB relies on gravity for that. Instead, they're there just to get the water to the height of the canals' starting points.
Questions about the outfall canals have been raised by some New Orleanians, including members of the City Council, who suggested the flooding could have been avoided had pumps at the lake end of the canals been turned on.
But those pumps are specifically designed for tropical storms and hurricanes and would not have helped on Saturday, officials insist.
The pumps, both interim ones in place in recent years and the permanent ones being completed by the U.S. Army Corps of Engineers, are designed to pump water over gates at the ends of the canals that can be closed to prevent storm surge from pushing into the city from Lake Pontchartrain.
But when those gates are open, as they were Saturday, gravity moves water out of them more quickly than the pumps would, Becker said.
On Saturday, the lake wasn't high and water levels in the canals weren't anywhere close to the point at which the pumps would have to be turned on. The canals can now handle 8 feet of water. The greatest depth recorded was in the London Avenue Canal, which reached about 3.7 feet, corps spokesman Ricky Boyett said.
There have been only two times any of the pumps had to be turned on over the last decade, outside of a tropical storm or hurricane. In both those cases, the London Avenue Canal pumps had to be used because water in the canal was nearing 5 feet deep, its limit at the time, Boyett said.
Turning on the pumps Saturday could have been counterproductive, he said.
"You'd be adding another step in the process of the water getting out the city," Boyett said.
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