Large engineered wetland combines form and CSO function

April 2014 » Project + Technology Portfolio » Water
Massachusetts completes stormwater project as part of a long-term plan to reduce combined sewer overflows by 84 percent, or 43.6 million gallons annually.
John Struzziery, P.E.

The recently completed $26.4 million Alewife Reservation stormwater wetland project is a model for combined sewer overflow (CSO) compliance, ecological value, and community spirit. The 3.4-acre project, part of the federally mandated Boston Harbor Cleanup, is the largest engineered wetland in the Eastern United States. With the goal to combat CSOs, the project is making sewer separation possible in West Cambridge, Mass., by retaining separated stormwater in a constructed wetland before being discharged to the Little River and ultimately Alewife Brook, Mystic River, and Boston Harbor.

While the primary objective of the project is to store, reduce, and release collected stormwater, the wetland provides recreational amenities, enhanced habitat for native plants and animals, and educational opportunities. It’s a powerful example of how innovative science, engineering, and ecology can concurrently resolve infrastructure challenges. As Richard C. Rossi, the Cambridge city manager said, “This newly constructed wetland not only improves water quality in the Little River and Alewife Brook, but also provides a new and unique recreational and educational open space for the community to enjoy.”

Muddy background

The Alewife Stormwater Wetland project is part of the Massachusetts Water Resources Authority’s (MWRA) long-term CSO control plan (LTCP) for the Boston Harbor Cleanup, and is a joint effort between the City of Cambridge Department of Public Works (DPW) and the Massachusetts Department of Conservation and Recreation (DCR).

As part of the cleanup, CSO events in the Alewife Brook had to be significantly reduced. The plan called for construction of a 3.4-acre wetland to store and treat stormwater while also meeting key components of the DCR Master Plan for the Alewife Reservation. The reservation is an area of historical significance: The 120-acre park located in Cambridge, Arlington, and Somerville, Mass., was established in 1893 as part of Boston’s Metropolitan Park District. It was originally landscaped by the Olmsted Brothers as a planned “urban wild” reminiscent of Central Park in New York City, and was among the first public parks in the United States.   

With the court-mandated schedule ticking away, MWRA and DPW contracted Kleinfelder to manage the sewer separation design and stormwater management program, along with MWH to conduct hydraulic modeling and perform civil design, and the Bioengineering Group to design the wetland, habitat improvements, and park amenities.

DCR collaborated with DPW and the project team to design features that would provide significant public benefit and ecological value, meeting the 2003 Alewife Reservation Master Plan goals of improving water quality and hydrology; protecting and enhancing wildlife habitat; improving recreational, educational, and other cultural opportunities; and providing for maintenance that minimizes costs and maximizes efficiencies.

The wetland also would minimize flood impacts to the City of Cambridge, protect Fresh Pond Reservoir from floods, and reduce pollutant discharge to the Little River — a tributary to the Alewife Brook, Mystic River, and Boston Harbor.

The idea to develop the engineered wetlands received a great deal of support from the U.S. Environmental Protection Agency (EPA), the Massachusetts Department of Environmental Protection (DEP), and the U.S. Department of Justice — all of whom are parties to the Boston Harbor Cleanup court case.

Stormwater treatment

The Alewife wetlands plan included several interdependent, complementary elements that included treating stormwater, managing flooding, and meeting pre- and post-peak discharge requirements while simultaneously managing to restore an ecological wasteland. Every element integrated, influenced, and complemented the other.

When stormwater flows out of the separated sewers, the wastewater’s velocity will be reduced through baffles that prevent re-suspension of settled solids. From there, the water flows through 3,000 feet of new 8-foot by 4-foot box culvert installed to collect the separated stormwater in the upstream areas. As part of the hydraulics for the system, a bending weir is used to divert stormwater to the wetland basin. In the event the predetermined volume of stormwater is reached, flow will be diverted across the bending weir for discharge directly to the river. 

To construct the wetland, existing utilities that crossed the area needed to be relocated. An innovative trenchless technology approach of using pilot tube auger boring was used to relocate a 1,300 KV electric cable, 8-inch-diameter gas main, and three 6-inch-diameter telecommunication systems from one side of the project, across the wetland area and river, and onto the other side where each utility was reconnected to existing infrastructure. The high-voltage electric cable was included as part of a 48-inch-diameter pilot tube auger bore, which at that time was the largest and longest (410 feet) of its kind.

The constructed wetland was designed to be a 3.5-acre stormwater wetland that holds up to 10.3 acre-feet to reduce peak flows to the Little River during major storm events. The more than 120,000 wetland and 4,000 upland plants located in the reserve act as a habitat for wildlife while cleansing the stormwater through sedimentation and floatables control absorption, physical filtration by plants, and microbial breakdown and plant uptake. 

The native plantings created diverse ecological communities that include deep, emergent, and high marsh, broadleaf floodplain, open water, and riparian woodland habitats to provide food and cover. Several islands provide breeding grounds, and an oxbow-like channel connected to the Little River provides improved spawning habitat for migratory fish such as Alewife and Blueback Herring.

A greener world

Since the park opened in October 2013, the community and ecological response has been outstanding. “A parcel of what was considered wasteland has been transformed into an environmental miracle as well as what (I am confident) will come to be regarded as one of the Boston area’s most beautiful urban parks,” said Judge J.D. Stearns, the federal judge overseeing the Boston Harbor Cleanup.

Social benefits include passive recreational amenities such as interconnected trails for recreational walking and running, access for bird watching, nature walks, and scenic overlooks. A multi-use pathway immediately adjacent to the wetlands provides connection to the Minuteman Bikeway and a public-transit station.

Outdoor educational opportunities include informational signage, a stone amphitheater, interpretive signage and engraved boulders, and a trail/boardwalk system providing a close-up view of a functioning wetland as well as information about the management and impact of urban stormwater runoff.

Ecologically, the wetland now enables the sewer separation and infrastructure renewal for 420 acres of the upstream catchment area, scheduled to be complete in December 2015, at which point CSOs to the river will be reduced by 84 percent, or 43.6 million gallons annually.

John Struzziery, P.E., a senior program manager in Kleinfelder’s Cambridge, Mass., office, has more than 31 years of experience with the firm focusing on wastewater, water, stormwater, and solid waste projects. He can be reached at jstruzziery@kleinfelder.com


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