Massive Tunnel Project Diverts Washington, D.C., Stormwater Better Than Expected

Two completed segments of the tunnel have diverted more than 2 billion gallons of stormwater to a treatment plant and have prevented more than 100 tons of trash from entering area rivers

Massive Tunnel Project Diverts Washington, D.C., Stormwater Better Than Expected

A segment of a tunnel-boring machine is lowered into a tunnel as part of DC Water's Anacostia River tunnel project. (Photo Courtesy of DC Water)

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A large river flows through Washington, D.C., but if you’re imagining a crystal-clear, picturesque river flowing through the nation’s capital, think again.

For decades, the Anacostia River in Washington has been unusable. No swimming, no fishing — completely unsafe to use for pretty much anything that requires direct contact with the water.

In 2017, the river was given its first “passing grade” in 10 years of analysis by the Anacostia Watershed Society. A history of pollution goes back much, much further.

“We have an old, antiquated combined sewer system, where about a third of this district has these combined sewers,” says Carlton Ray, director of the Clean Rivers Project for DC Water. “In wet weather, those sewer lines fill up, and 75 to 80 times a year, we actually have raw sewage, or combined sewage that overflows in the Anacostia River, Potomac River, and to a lesser degree, Rock Creek.”

The largest infrastructure project in DC Water’s history is underway to reverse the decades of damage done by combined sewer overflows.

When the project is finished, 13 miles of large-diameter tunnels will reduce CSOs to the Anacostia by 98 percent. A number of green infrastructure projects throughout the city will aid in a 96 percent reduction for the three bodies of water included in the city’s consent decree.

“The goal was to develop this plan to ultimately make our rivers, those three streams, fishable and swimmable. That’s the goal,” Ray says. “Now, there are other sources of pollution — stormwater or urban runoff — but this is like the cat’s meow, it’s the big source of pollution that’s going into the rivers. And if we can get this out of the equation, then it’ll go a long way toward ultimately making the rivers fishable and swimmable.”

Better than planned

DC Water began developing the CSO control plan in 1999 and gained approval under consent decree requirements in 2002. Initially a 25-year plan, it was modified along with the consent decree to include green infrastructure as it has become more viable.

Now, they plan to finish the project by 2030.

“When we started this program back in the ’90s, we had about 3 billion gallons of sewage get away from us, 75 to 80 times a year. So that’s a total amount, on an average year,” Ray says. “Obviously with a lot of sewage going to the river, that’s causing water-quality problems, and it’s actually a violation of the Clean Water Act. So like many other communities, we developed a plan.”

In addition to the extensive large-diameter tunnels, DC Water will need to construct a series of shafts to drop the flow down over 100 feet, below the metro grade, into the diversion tunnels. A patent-pending process uses a spherical flow and deaeration run to avoid geysering effects and capacity reduction.

Two segments of the overall tunnel system are online, and in less than five months, they have diverted more than 2 billion gallons — which was originally the one-year goal. The city is already at 70 percent CSO reduction with only part of the system online. It has also prevented over 100 tons of trash from entering the bodies of water in that time.

“Basically the lower half of this tunnel system is already operational,” says Pamela Mooring, external communications manager with DC Water. “The numbers have been much better than anticipated. We’re well ahead of our projections. 

Awarding a half-billion contract

Choosing a company to take on the largest contract in the department’s history was not a decision made lightly by DC Water. They follow a rigorous bid procurement process, meeting with all the finalists and ranking bids.

“We spend a lot of time going through our shortlisting process, and selecting the three or four best-qualified design-build teams,” “Then we work with each of those design-build teams to understand their approach, so they meet all our mandatory requirements.”

They rank bids by assessing the technical design independent of the bid price. After scoring them on a 40-point scale, they do the same for the price sheet, scoring out of 60. The highest combined score wins.

That process took about a year before a design-build team was chosen in September 2017.

From developing the project guidelines to choosing the bid was about two to three years. The tunnel-boring machine that will be used on this segment is already in the ground, and mining should begin in mid-September, with a project completion target in 2023.

After that, this northeast segment will connect to the 7 miles of tunnel already in operation, the full 13 miles conveying by gravity to the Blue Plains Wastewater Treatment Plant, where a three-stage process treats over 500 mgd.

Tunnel-boring machines

With an outside diameter of just under 26 feet, this tunnel project requires a massive piece of equipment to dig. The contractor chose an earth pressure balance tunnel-boring machine from German company Herrenknecht.

The machine is nearly the length of a football field, with a conveyance system that extends even farther — the length of the entire tunnel, in some cases.

“We have a conveyor system to pull material from the tunnel-boring machine all the way back, so for example, the Blue Plains Tunnel, that was 24,000 lineal feet,” Ray says. “By the time we got to the end, that conveyor belt was 24,000 feet times two, because it’s up on top as well as on bottom, so it’s 48,000 feet of conveyor belt.”

Steel-fiber concrete was used to construct the tunnel; the precast segments put into place as the machine progresses. It will average 40 feet per day along the 27,000 lineal feet of this project. For more about the machine, see the video below. 

Long in the making

Like many communities throughout the country, CSOs have been an issue for DC Water as far back as anyone can remember. In addition to eliminating CSOs, this program has remediated chronic flood zones in the city.

It’s a major step toward clean rivers and consent decree compliance, and already there has been a noticeable difference. “We’re preventing even more CSOs than we expected, with just this portion of the tunnel, so we are looking forward to putting the rest of the tunnel in service in 2023, and seeing some really good numbers,” Mooring says.


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