Project Profile: Treating Runoff, Teaching the Public
In Cambridge, MA, a 120-acre protected public reservation will soon have an unusual amenity alongside its “rails-to-trails” bikeway and pedestrian boardwalk: a constructed stormwater wetland and flood basin. Funded by the city of Cambridge and overseen by the Massachusetts Water Resource Association (MWRA), the new Cambridge Stormwater Wetland will limit the effects of the region’s combined sewer overflow (CSO) activations and provide 10.3 acre-feet of flood protection. The result of a holistic approach to confronting both unique engineering challenges and design opportunities, the project, when opened in 2012, will offer benefits beyond ecological sustainability, helping transform an underused urban wild into a passive interpretive landscape complete with educational amenities.
The project addresses increasing CSO activations in Cambridge and its surrounding communities. As development has strained the region’s century-old stormwater pipes, sewer overflows have increasingly polluted the Alewife Brook, a Massachusetts Department of Environmental Protection 303(d)-listed impaired stream contaminated with pathogens. In studying the issue, the city of Cambridge concluded that a central detention basin within Alewife’s low-lying 100-year floodplain could treat as much as 80% of the polluted runoff from the area’s sprawling network of stormwater channels. The city tapped three engineering and design firms–Kleinfelder/SEA Consultants of Cambridge, MWH of Boston, and The Bioengineering Group of Salem, MA–to locate an appropriate site and design the solution.
Evaluating the Alternatives
The team weighed two primary options for the basin. The first centered on a privately owned site just uphill from the Alewife Brook. Removed from public view and sizable enough to contain the treatment basin, the area shielded the project from many potential regulatory and community hurdles. But those advantages were costly. The property would need to be purchased via eminent domain, and its uphill location required the installation of a pump and large channel to transport water to and from the brook.
The second alternative was cheaper, but more complex: situating the treatment basin and constructed wetland directly along the brook, within the state-owned Alewife Reservation. While this option eliminated the need for costly pumping–gravity would feed the basin–its position in one of the largest urban wilds in the region and a public rails-to-trails bike path meant additional stakeholders and community involvement. Any solution would thus have to reconcile MWRA goals for flood control with the public and recreational aspirations contained in the reservation’s master plan.
Despite the challenge, the team chose this path. For starters, it saved the state upward of $15 million in infrastructure and acquisition costs of the uphill site. Just as significantly, it offered an opportunity to combine watershed engineering with community placemaking. As Duke Bitsko of Bioengineering Group notes, “We believe in combining beauty with brains. This project offered a rare opportunity: to naturally contain and treat runoff while creating a great public space.” Solving the CSO issue while simultaneously improving the ecological integrity of the reservation thus emerged as a goal from the outset.
Overcoming Site Challenges, Community Concerns
Once the team set to work, the inherent challenges of constructing a stormwater treatment facility in a place once known as the “Great Swamp” were compounded by permitting obstacles. The most difficult of these, a state wetland protection bylaw enforced by the Department of Environmental Protection, required all water be treated at least 25 feet from waterways (problematic for the riverfront project). A small but vocal group of citizens also expressed concern about building within an urban wild adjacent to a river. Jim Whitehead, Bioengineering Group’s project manager, explains: “A few folks thought that, however underused it was, a public reservation was simply the wrong place to put a detention basin.”
The team responded to the challenges holistically, winning public approval by incorporating park-like elements and amenities that would enhance the reservation, overcoming the DEP’s objections through foot-by-foot floodplain compensation, and using HydroCAD modeling to design a basin that could contain and treat urban stormwater runoff.
“We had to shoehorn the basin in, given the tough dimensions of the site,” explains Whitehead. “It was the best alternative, for sure, so we were committed to the solution, but it took eight months for us to get the model to work.”
The final design incorporates a sediment forebay large enough to accommodate maintenance truck access and voluminous enough to contain the sediment. A bending weir ultimately releases the treated water to a grass swale and secondary forebay area. U-shaped to maximize time the water spends being naturally filtered, the wetland leads to a final open-water area just before the outlet.
The benefits of this open-water design range from aesthetics to accessibility. Runoff moves slowly through the constructed wetland, a process allowing sediments to settle and gases to escape while helping rejuvenate the reserve ecosystem. “Meeting the desedimentation and flood control requirements was the priority and toughest aspect of the design,” notes Whitehead. “But designing with public visibility and use in mind helped us find areas where we could combine aesthetic form and function.” The boardwalk over the flood basin, for instance, allows both public access and easier maintenance, as does the bikeway on the south side of the basin, which is usable by a vacuum truck to reach the forebay and constructed wetland. Other park amenities, including an amphitheater to be used by Tufts University science and engineering classes, and a diverse combination of native upland and wetland plantings round out the benefits by engaging the public in the natural processes at work in the basin.
Transcendent Results
If successful, the Cambridge Stormwater Wetland will have transformative effects for the watershed–at a major cost savings over the alternatives. Beyond meeting the environmental threshold of an 80% reduction in stormwater pollutants and sediment from incoming streams, the project will restore a long-neglected urban wild into a diverse biohabitat. Meanwhile, welcoming natural amenities will engage the public and showcase a type of engineering typically hidden from public view. In fact, The city of Cambridge has required that a portion of the wetland plantings be installed by volunteer stakeholders and students.
The project also anticipates a future of ever-stricter water regulations. While current requirements focus on sediment, the team designed the forebay solution to accommodate a “trickle filter” to filter pollutants via naturally occurring bacteria. “It took years to get the model to work, years to create workable solution, so we wanted to make sure it was built to last,” says Whitehead. “The state is thrilled to save money over the alternatives, but we’re most proud of the design. We think this will rejuvenate an underutilized preserve, improve its ecosystem, and address a major CSO problem. It proves that high-performing environmental projects can work within a beautiful landscape.”