During Storm, New Tunnel Kept 233 Million Gallons Of Sewage Out Of Anacostia River

Tuesday was a wet one, with 2.24 inches of rain falling in the District — the highest single-day rainfall total in the past year. It was also the biggest test of a giant new sewer tunnel running beneath the city, meant to prevent sewage overflows into the Anacostia River and reduce neighborhood flooding.

The Northeast Boundary Tunnel, or NEBT, went online in September, promising to reduce sewage overflows in the Anacostia River by 98%.

During Tuesday’s storm, the tunnel captured 267 million gallons of stormwater and sewage, preventing it from spilling into the river.

“Before this tunnel was built, all of that flow would have overflowed into the Anacostia River,” says John Lisle, a DC Water spokesperson.

The tunnel did fill to capacity at about 8pm on Tuesday. About 112 million gallons were discharged into the Anacostia once the tunnel was full.

Water and sewage captured by the tunnel is later treated at DC Water’s facilities, and then discharged into the Potomac River, sans harmful bacteria and other pollutants. As of Wednesday afternoon, the tunnel was still being pumped out.

The new tunnel is about 5 miles long and is part of a new 13-mile system that has a total capacity of 190 million gallons. Before the project, the District’s old sewers would dump overflow into the Anacostia 84 times a year. Now it’s expected to happen just twice a year.

While the NEBT was designed to help clean up the river, as required by under a consent decree with the U.S. Environmental Protection Agency, it also helps drain stormwater from low-lying neighborhoods that have a history of flooding including Bloomingdale and LeDroit Park. The tunnel reduces flood risk in the area along Rhode Island Ave., NE, that flooded District Dogs in August, killing 10 pets.

The tunnel was designed to handle up to a 15-year storm — a storm with a 7% chance of occurring in any given year. Previously, the city’s antiquated sewer system could only handle a 2-to-5-year storm without causing flooding — meaning as much as a 50% chance of flooding each year in low-lying neighborhoods.

While there was some localized flooding in the city on Tuesday due to clogged drains and other issues, there wasn’t major flash flooding like there has been in past storms. Lisle says it’s not clear that there would have been flooding without the tunnel, but he says the tunnel’s performance is encouraging.

“We’re not going to judge the performance of the system or the impact of the system on mitigating flooding on just one rainstorm. We’re going to look at it over a number of storms over a period of time,” Lisle says.