Digging Deep to Improve Water Quality
Workers sink a shaft at street level in a dense neighborhood at 1st and V Streets, N.W., in Washington (below) as part of a tunnel-building project below. Pieces of a giant tunnel-boring machine are lowered (above) and assembled underground. The machine bores space for a tunnel 23 feet wide to store a mix of stormwater runoff and sewage to improve water quality. Photographs, DC Water
IT'S THE BIGGEST PUBLIC-WORKS PROJECT IN WASHINGTON, D.C., since the construction of the Metro system. It is also the single most expensive project aimed at improving water quality in the Chesapeake Bay. But to many people, it's invisible and little known.
It's the $2.6-billion project in which DC Water, the city's water-and-sewer authority, is building 18 miles of tunnels under the nation's capital. They are designed to address a water-quality problem known as combined sewage overflows. A giant machine began digging the first section of tunnel starting in 2013, and the work will continue one section at a time through 2030.
These underground tunnels are a large part of DC Water's plan to correct a big above-ground problem: in about one-third of Washington, untreated sewage empties into the same pipe network that carries stormwater, and during big rains, the mix overflows directly into local waterways. The "combined sewer system" of pipes sends the mixed flow to the Blue Plains sewage treatment plant that DC Water operates in the city's southwest corner, along the Potomac River. The flaw in the system: even during a rainstorm as small as one-tenth of an inch, the flow can exceed the system's capacity. When that occurs, the pipes are designed to let the untreated overflow run directly into the Potomac and Anacostia Rivers and into Rock Creek. In an average year, nearly three billion gallons are released.
The water authority began this massive subterranean project after the Environmental Protection Agency and environmental organizations filed legal challenges against DC Water in the early 2000s for violating the Clean Water Act. In 2005 the water authority agreed to settle the case by building a network of three tunnels to fix the problem — one each for the Anacostia and Potomac Rivers and Rock Creek. For all but the largest rainstorms, tunnels will store the backed-up mix of sewage and stormwater until it can be pumped or flow by gravity to the Blue Plains plant for treatment.
The tunnels are being built in sections to spread out the cost and to address the Anacostia River's pollution problems first. Two-thirds of the city's total combined sewage overflow now enters the Anacostia. The first tunnel section, completed in 2015, stretches four-and-a-half miles north from the Blue Plains plant, crosses the Anacostia River, and ends near Nationals Stadium. Also in 2015, construction began on the next section, stretching southwest two-and-a-half miles from Robert F. Kennedy Stadium to link up with the first tunnel section. That work is to be finished by 2018.
The project's vast scale recalls the Chunnel rail tunnel built under the English Channel during the 1980s. The Washington tunnels are cylinders 23 feet in diameter — big enough to fit a Metro rail car — and located more than 100 feet deep. That depth will allow the tunnels to run below other utilities and Metro tunnels.
The machines that build the tunnels share a similarly grand scale. The one that built the first tunnel section weighed 1,300 tons and measured 442 feet, longer than a football field. To get this massive digger in place, workers built a vertical shaft, lowered the various parts to the bottom, and assembled the machine underground.
Washington's water-and-sewer authority is building 18 miles of tunnels to store a mix of stormwater and sewage and keep the city's "combined sewage system" (CSS) from overflowing into local waterways. The black line shows the CSS boundary. DC Water will build "green infrastructure" within a fraction of this area (shown in green) to help reduce stormwater flowing into the CSS pipe network. Graphic, adapted by Sandy Rodgers from a DC Water image
During 2015, DC Water used two of these boring machines at once to dig the tunnel sections then under construction. The water authority tried to give the mechanical behemoths a personal face by naming them: The machine that built the first section was called Lady Bird after the wife of President Lyndon Johnson who was famous for her campaign to beautify public spaces in America. Nannie, the machine that attached the next section, was named for Nannie Helen Burroughs, an educator and civil-rights activist from Washington.
While digging, the tunnel-boring machines creep along 24 hours a day, six days a week, at a speed of about 100 feet per day. Machines are boring the Anacostia tunnel through clay and silt, avoiding bedrock, but constructing the Potomac tunnel will require boring through rock. The machines chew through this material using a spinning face studded with scrapers made of tungsten carbide. As a machine moves forward, workers assemble pre-fabricated concrete rings in its wake to extend the tunnel. Each ring — six feet in width and weighing 40 tons — is lowered into the tunnel, placed on a rail car, and rolled out on tracks to the boring machine. There workers use a mechanical arm to lift the ring sections into place.
The Anacostia tunnel sections are scheduled to be completed by 2022; construction of the Potomac tunnel would begin that year and run through 2030. When all is done, the net result is expected to be a big reduction — a 96-percent drop — in the total volume of overflow from the combined sewer system. The remaining four percent represents the runoff from the biggest storms, which can exceed even the capacity of the tunnel storage system.
Other cities, including Chicago, Cleveland, and Milwaukee, have also built or are building tunnels to reduce overflows, but the Washington tunnel will achieve one of the country's largest reductions in terms of percentage for a combined sewage system.
The result will be a lot of underground, concrete gray infrastructure, but DC Water also plans a green infrastructure component visible above ground in parts of the city. The authority will build 500 acres of bump-out boxes and other measures to reduce the amount of stormwater flowing into the tunnel system.
In 2014, DC Water proposed installing the 500 acres of green infrastructure as part of a broader pitch to reduce the scale of the concrete tunnel network. The authority estimated that because the green acreage would lower stormwater volumes, the authority could reduce the size of the tunnel system by a corresponding amount and still meet the project's water-quality goals. The EPA approved this change in plan, allowing DC Water to scrap the part of its original plan that called for building a half-mile stretch of tunnel serving the city’s Rock Creek area. In addition, the width of the Potomac tunnel was narrowed.
The costs of building green infrastructure will offset the savings from trimming tunnels, so the new plan did not change the project's overall cost; DC Water's customers are footing the bill through rate hikes. But Ge Hawkins, the authority's general manager, sees another benefit of installing bioretention bump-out boxes beyond controlling stormwater: they give ratepayers an obvious return on their spending.
“Green infrastructure is at the level of the customer’s experience,” he says. “You can go see it. Your investment in us isn’t just something we tell you about.”