In 2011, the Philadelphia Municipal Water Works attracted national attention. Green cities, clean wateris a 25-year program to manage increasing stormwater volumes by using primarily “green infrastructure” such as rainwater gardens and porous pavement to allow stormwater to soak into the ground rather than run off and pollute rivers and streams.
The Philadelphia Water Department, with its green infrastructure plan to drain about 9,500 acres of land in the city, is considered by many to be on the cutting edge of stormwater management, which has become an increasingly urgent issue for city government as climate change impacts now result in larger and more frequent storms, especially in the Northeast United States.
The program prompted Philadelphia and other U.S. cities to install nature-based solutions to absorb and filter stormwater in addition to traditional “grey infrastructure” like pipes, tunnels, and pumping stations. But while other cities like Milwaukee and Boston viewed green measures as a complement to grey infrastructure, Philadelphia put rainwater gardens and bioswales (vegetated ditches that collect rainwater) at the center of its strategy. But critics say these innovations are now proving insufficient to deal with the rise in extreme precipitation. In fact, the amount of overflow from pipes, where stormwater and raw sewage mix, has actually increased since the Green City program began.
Implementing grey infrastructure can take decades and cost billions of dollars, but many cities don’t have much of a choice.
According to a new report titled “Setting the Facts,” by the River Advocacy Team, a group of water experts and environmentalists including the former heads of the Interstate Delaware River Basin Commission, which manages wastewater in southern New Jersey counties, and the Camden County Public Works Department, an average of 14 billion gallons of polluted storm water has overflowed Philadelphia’s 164 sewer outfalls each year since “Green City, Clean Waters” began.
“Due to climate change, the City’s wastewater pollution reduction goal – a reduction of approximately 8 billion gallons of combined sewer overflow per year from the 2006 baseline of 13 billion gallons – no longer relates to the reality of the climate-driven precipitation Philadelphia experiences,” the report states.
According to the Environmental Protection Agency, roughly 700 municipalities in the U.S. rely on combined sewer systems, most of them in the Northeast and around the Great Lakes. As these municipalities grapple with the latest climate projections that show an increase in heavy rainfall, many are focusing on gray infrastructure projects, like concrete retention tanks, tunnels, and pipes that can divert and hold the combined flow until the rain stops and treatment plants can be restored. Such projects can take decades to implement and cost billions of dollars (green infrastructure is cheaper and quicker to build), but many municipalities are finding they don’t have much of a choice.
A tree planter designed to capture stormwater on Osage Avenue in Philadelphia.
Philadelphia Water Department
In Milwaukee, the Metropolitan Sewerage Authority, which serves 1.1 million people, relies on both green and gray infrastructure. But Executive Director Kevin Schafer says it’s the authority’s retention tunnels and tanks that have done the most to reduce the amount of water that spills out of the combined sewer system into Lake Michigan and local rivers. The authority’s gray infrastructure can currently handle 2.5 to 3 inches of rain at a time, while green infrastructure can only absorb about half an inch, he estimates.
Still, he calls green infrastructure “the best icing on the cake,” because managing water where it falls can keep it free of bacteria, heavy metals, trash and other contaminants and reduce the risk of water backflow in basements. “You need both,” he said.
Despite limited funding and aging infrastructure, the Milwaukee Sewerage Department has a goal of eliminating combined sewer overflows (CSOs) by 2035. “When you think about the climate change we’re facing, it’s like we’re running uphill, and it’s going to get steeper and steeper,” Schafer says.
Boston is also focusing on grey infrastructure to improve stormwater management. The city operates under a long-range management plan, a federal mandate to reduce CSO volumes until waterways comply with the Clean Water Act, which sets a goal of making all rivers and streams “fishable and swimmable.” To achieve these goals, the city has gradually separated stormwater and sewer pipes, so that stormwater flows into streams and rivers and only sewage remains in pipes that lead to treatment plants. Before stormwater overflows into waterways, the city also directs it to a “partial treatment plant,” which screens out trash and other solids and disinfects the water with high levels of chlorine before it is discharged down the drain.
The new tunnel in Alexandria is designed to reduce the number of floods from 70 per year between 2000 and 2016 to four.
But the management plan’s current goals may be unattainable, said Max Rome, stormwater program manager for the nonprofit Charles River Watershed Association. For example, in 2023, Boston saw a slightly above-normal rainfall of about 55 inches, but about 70 million gallons of sewage and stormwater flowed into the Charles River, about five times the amount allowed in the long-term management plan.
Last year’s flooding reflects the increasing intensity of heavy rains that overwhelm wastewater treatment plants, Rome said. “This is a perfect example of how climate change is changing the way we rain. It’s not that we’re getting more rain, but it’s more rain over a shorter period of time. [number] Of the storm.”
Citing unpublished data from the Massachusetts Department of Water Resources, Rome said the city of Boston, working with the EPA, is revising its long-term management plan for the lower Charles River basin to set an annual overflow limit of about 38 million gallons, based on rainfall projections for 2050. That’s higher than the basin’s current limit of 13 million gallons, but lower than actual overflows in past years, suggesting that even the revised target will be difficult to meet.
Increased precipitation has also affected Western cities, including Portland, Oregon, which has significantly reduced pollution in its downtown and Columbia Slough sections of the Willamette River by building three stormwater interceptor tunnels, a project that took 20 years and cost $1.4 billion to complete in 2011 and nearly eliminated CSOs.
Utility companies would pay for such upgrades through increased water rates and federal grants, and the Biden administration’s bipartisan infrastructure legislation passed in 2021 currently provides funding for stormwater management measures.
In Alexandria, Virginia, the city’s sewer authority expanded the diameter of a new sewer tunnel from 10 feet to 12 feet based on federal projections that annual rainfall will increase from an average of 41 inches from 2000 to 2016 to an average of nearly 70 inches per year by 2100.
The 2.2-mile tunnel, which will cost $615 million to complete and won’t be operational until early 2026, will reduce sewage and stormwater overflows into the Potomac River from 140 million gallons a year to 17 million gallons a year between 2000 and 2016, said Justin Karl, CEO of AlexRenew, the city’s wastewater treatment authority. Alexandria’s new tunnel is designed to reduce the number of overflows to four a year, down from 70 a year between 2000 and 2016.
The project also includes green infrastructure such as bioretention basins and tree wells that filter and retain stormwater runoff, but the department decided to limit such measures because most of the CSOs occur in the city’s oldest neighborhoods, which have little open space and clay soils that infiltrate slowly when wet.
There was just one location in Washington state that was well suited to a nature-based solution because of low overflow and sparse development.
A much larger stormwater treatment project is being planned in Washington, D.C., the nation’s capital. DC Water, an independent municipality, is building an 18-mile network of tunnels that will store 249 million gallons of stormwater and sewage until it can be safely pumped to a treatment plant. When completed in 10 years, the system will reduce combined sewer overflows by 96 percent, says Moosa Warne, DC Water’s vice president of Clean Rivers programs. CSO discharges into the three city waterways are expected to fall to an average of 138 million gallons per year, down from 3.2 billion gallons in 1996, and the number of overflows is expected to fall from 82 to four over the same period.
Green infrastructure is only a small part of the project, costing $98 million of the total $3.29 billion budget, Warne said. The department conducted green infrastructure pilot projects in two locations and concluded that only one site, Rock Creek Park, which has low overflow and is less densely developed, was suitable for nature-based solutions.
Recognizing the climate change challenges facing U.S. cities, the Philadelphia Water Department (PWD) has issued a 182-page report in 2022. Report It projects a 9.5% increase in average annual precipitation by 2050 compared with the 1997-2017 baseline, and said stormwater management projects, whether grey or green, would need to be upgraded to reflect that scenario.
In April 2021, workers upgraded a combined sewer system in Northeast Washington, DC, to accommodate more storm water and sewage.
Drew Ungerer/Getty Images
“PWD knows there’s a problem. What they’re resisting is any kind of application of the Green City plan until it’s completed in 2036,” said Nick Pagon, an author of the “Unraveling the Facts” report and founder of Philadelphia Waterborne, a program that teaches teens how to build boats on the Delaware River. “They don’t want anything that would suggest a resumption of long-term management planning.”
The Philadelphia Water Department says it designs storm-control measures based on climate-driven rainfall patterns, but it needs at least 30 years of data before it can draw any conclusions about those patterns. The Green Cities program began in 2006.
“Natural variability in precipitation patterns has been a known challenge for centuries, but now, climate change has increased precipitation across a broad range of levels to uncertain degrees,” the agency said in a statement. “It would be unwise to use only observed data from the past few years to revise planning goals and assumptions.”
But John Rampler, an attorney for the nonprofit group Environment America, said cities across the country must accept the need to make significant investments to control increasing stormwater flows.
“Stopping billions of gallons of raw sewage from entering our rivers will require determined, massive investments in traditional wastewater infrastructure and repairs,” he said. “Increased storm events linked to climate change make this challenge even more difficult and underscore the need for further investments in wastewater infrastructure.”
