By james art In late July 2023, 3.07 inches of rain fell over Boston in one day. The city’s sewage systems were overwhelmed, causing sewage to leak into Boston Harbor, prompting a public health alert. The summer of 2023 will turn out to be Boston’s second wettest on record.
About two months later, New York City received 8.65 inches of rain – more than any September day since Hurricane Donna in 1960. Low-lying areas of the city were flooded and half of the subway lines were shut down as water flooded underground stations.
Cities along the East Coast are increasingly prone to flooding due to climate change, Grist reports. But changing weather patterns are only half the problem – the other is inadequate infrastructure. In particular, these recent floods were exacerbated by the combined sewer systems of Boston and New York, which carry both stormwater and sewage in the same pipes. When such a system reaches capacity during a heavy rainfall or storm, it activates, sending a mixture of stormwater and raw sewage into waterways (and sometimes into streets and homes).
Many other cities across the country also have combined sewer systems, but as two of America’s oldest and most densely populated major cities, Boston and New York face an uphill battle when it comes to the climate resilience of their sewer systems. And the cities chose two very different paths: Boston chose to separate the combined portion of its sewer system so that sewage no longer mixes with stormwater during floods, while New York is betting on a new, separate storm management infrastructure to to relieve the burden of its combined sewer when it rains.
The success of their respective solutions is not only a matter of reducing flood hazards for the city’s residents and surrounding ecosystems – it is also required by law. That’s because flood-related backups that send sewage into waterways, known as the system’s combined spillways, are a violation of the Clean Water Act. In response to the system’s combined overflows, the U.S. Environmental Protection Agency entered into consent decrees with the municipal governments of Boston and New York—legally binding agreements requiring the cities to prevent further overflows. John Sullivan, chief engineer for the Boston Water and Sewer Commission, estimates that 90 percent of the nation’s sewer systems are under consent decree.
The consent decree “outlines how many years you have to fix this problem,” Sullivan said, “and they give you plenty of time, but you have to take action to do the things that you haven’t done.”
Boston is in dire need of better flood management infrastructure right now. Sea-level rise is occurring disproportionately faster on America’s East Coast due to factors including wind patterns and the changing Gulf Stream. Meanwhile, climate change is also increasing the amount of moisture in the atmosphere, making heavy rainfall more likely. The double whammy of rising sea levels and increasingly heavy rainfall is exacerbating the impact of flooding during storms. In Boston, in particular, tidal flooding is increasing more than three times faster than the national average, according to the National Oceanic and Atmospheric Administration. Such floods can engulf culverts, the pipes that send excess sewage into waterways when the system is flooded, further reducing the rate at which water drains from cities.
Boston has actually been working on splitting its sewer system since before climate-related flooding became a major threat. In response to a 1987 court order, the Massachusetts Water Resources Authority undertook nine sewer separation projects in the Boston area between 2000 and 2015, along with dozens of other sewer improvements designed to prevent combined system overflows . Today, only about 10% of the Boston Water and Sewerage Commission’s 1,538 miles of sewer pipes are combined.
The commission is currently working on two additional sewer separation projects in South and East Boston. The city is prioritizing spill prevention in areas where the risk of human contact with contaminated water is considered highest, such as public beaches.
These projects are expensive. According to estimates by the Sewer Commission, sewer separation work in the city in 2021 will cost an average of $340,000 per acre. About 88 acres of work was done that year, at a cost of over $30 million. From 2024 to 2029, the city plans to divide the sewer into 230 acres in East Boston and 400 acres in South Boston. These planned works make up about 3% of Boston’s sewer system, which covers approximately 20,500 acres, including portions that have always been separate.
Another problem is finding enough space to add new pipes – a luxury in some areas of the city. Sullivan sent Grist a plan to split the sewers in South Boston. It contains a wave of lines of varying thickness and color, some solid and some dashed, stacked on top of each other. Each represents a different underground pipe.
“You can see how messy they can be trying to fit these pipes under the gas pipes, under the electric pipes, under the phone,” Sullivan said.
Sewer replacements—most of which are done deep underground—also pose safety concerns for workers, such as low oxygen levels, inhalation of fiberglass resin fumes, and the presence of foreign objects in the drain. Contractors monitoring the work install oxygen meters in tunnels that sound when oxygen levels drop dangerously low.
According to Sullivan, the risks of replacing the sewers are worth it given the increasing rainfall.
While he says 90 percent of storms in Boston don’t exceed an inch of rainfall and can be mitigated by existing flood infrastructure, sometimes they can dump up to six inches of rainwater on the city.
“You need infrastructure to get that five inches of water out,” Sullivan said. “And it’s not done by any green infrastructure, it’s the pipes.”
But New York is betting that green infrastructure will do the trick. About 60 percent of the Big Apple’s 7,400 miles of sewer lines are combined, and the city has estimated that a complete system upgrade would cost about $100 billion and take decades. The New York City Department of Environmental Protection’s alternative to separating significant portions of its combined sewers is the Cloudburst plan, developed in partnership with the city of Copenhagen, Denmark, beginning in 2017. The plan uses newly constructed gray infrastructure such as underground storage tanks and green infrastructure, such as rain gardens, to divert rainwater during heavy rains. A presentation of Cloudburst Infrastructure on the Department of Environmental Protection’s website illustrates how porous concrete in parking lanes can capture stormwater and direct it to underground tanks for temporary storage.
