city government flood protection and drainage

SELA projects didn’t increase flooding on July 10, report concludes

Rainfall amounts on July 10, 2019. The highest levels are in red and the lowest in blue. (Comprehensive S&WB-City of New Orleans Stormwater Management Model)

The intensity of the July 10 rainfall — with as much as 9 inches in three hours — overwhelmed the city’s pumping and drainage system. Streets were inundated, and many became impassable; cars were submerged; homes and businesses flooded; and the tally of damages went far beyond an annoying commute.

Among the rumors circulating after the deluge was the claim that the massive Southeast Louisiana Urban Flood Control Program, or SELA, construction project, designed to reduce the flood risk in certain low-lying areas, hindered the drainage capacity in other areas of the city, including Mid-City.

The S&WB and the U.S. Army Corps of Engineers are in the final stages of the east bank portion of the federally funded $1.5 billion SELA construction of underground drainage canals. The completed projects are primarily Uptown — in Hollygrove and along Jefferson, Napoleon and Louisiana avenues.

To investigate the SELA drainage canals’ impact on the July flooding, S&WB and the Mayor’s Office hired Ardurra, an engineering consulting firm, to digitally model and evaluate the July 10 flooding. In a report released Friday, Ardurra concluded that SELA improved drainage in the targeted areas and did not adversely impact other areas.

“Claims that SELA has caused flooding in certain areas of the City are unfounded,” the researchers state. “Some residents are saying that they never experienced flooding like this prior to the completion of SELA. The fact is that the City has not experienced a rainfall event equivalent to the recent events since 1995. In 1995, flooding in the SELA improved areas of Uptown was significantly more severe than in recent rain events.”

Researchers state that the SELA underground canals installed Uptown do not drain Mid-City or downtown, where separate canal networks carry water to different pump stations. The areas severely inundated on July 10 were in Mid-City, the CBD, the Warehouse District, Broadmoor, the Lower 9th Ward and Gentilly.

The city has 24 pumping stations that, when operating, have a combined capacity of nearly 400,000 gallons per second, according to the S&WB. Pumping this volume of water allows the drainage system to handle about 4.7 inches of rain in three hours.

Ten of the 16 rain gauges on the east bank of New Orleans measured more than 4 inches of rainfall in three hours on the morning of July 10. It fell over a relatively short period of time, so the intensity was much higher than the storms on April 4 and May 12, the report states.

Ardurra processed radar data to model the flooding. “Since the rainfall intensity and duration can
vary significantly within a short distance and there are only a few reliable rain gauges in the basins, it would be almost impossible to get this level of accuracy without radar data,” the report states. Average rainfall amounts ranged from 2 inches to more than 8 inches.

The analysis concluded that “the amount of rainfall on July 10 exceeded the expectations for monthly rainfall based on historical records.” In terms of the runoff produced, it went beyond a typical 100-year storm, or a storm that has a 1% chance of occurring.

The researchers point to climate change as a culprit in the July 10 flooding: “A recent study by a team of researchers at LSU (Brown et al., 2019) revealed that due to the change in global climate in the last few decades the hourly rainfall intensity significantly increased in the Southeastern area of the United States. The average duration of the rainfall has shortened as the global climate has gotten warmer.”

The report recommends improved preventive maintenance of both the drainage system, consisting of the drainage pipes and catch basins maintained by the city, and the pumping system, the S&WB canals and pump stations. This includes frequent, regular inspection of drainage canals; removal of debris from the open and closed culverts and from the canal banks; and reshaping of earthen canals to better handle water flow.

The report also encouraged green infrastructure projects, such as the Gentilly Resilience District and the Hagan-Lafitte project in Mid-City.

In a press release promoting the study, the Mayor’s Office states: “The SELA analysis is the latest step taken by the Cantrell Administration and SWBNO to methodically investigate the limits of New Orleans’ drainage system. Inspecting underground canals, cleaning clogged catch basins and identifying trouble spots for street flooding are all building blocks toward a master plan to re-envision stormwater management in the City.”

The full study can be found here.

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