Shoreline Resiliency - The Merrimack Bank Stabilization Project at Wellman Avenue in Chelmsford

July 1st, 2021

As climate change continues to express itself in various ways, protecting waterfront property and infrastructure is an urgent matter. The longstanding traditional protection methods, such as seawalls and stone revetments, are effective, but in specific cases, they can have adverse effects on natural systems. Therefore, alternatives need to be sought when hardened shorelines are not the preferred solution. The Epsilon team has a depth of experience working to plan and permit resilient shorelines in tidal and non-tidal waters. A few examples include the MBTA Charlestown Bus Garage Resiliency Project on the Mystic River in Boston and Somerville, MA; the Senator Joseph Finnegan Park at Port Norfolk on the Neponset River in Boston, MA; the Sconset Bluff Stabilization Project on Nantucket, MA; and the Merrimack Riverbank Stabilization Project at Wellman Avenue on the Merrimack River in Chelmsford, MA.

The nearly 4,000-foot long Merrimack Riverbank Stabilization Project at Wellman Avenue demonstrates how combining natural (“soft”) solutions with traditional (“hard”) civil construction can protect important infrastructure while maintaining a natural system. The success of the project demonstrates that “hybrid” approaches should be examined and selected when site conditions allow.

Coastal Resiliency
Selection of the hybrid bioengineered bank stabilization alternative is quite notable. While hard solutions such as walls, riprap and revetments can effectively protect property and infrastructure along inland rivers and coastlines, there can be significant drawbacks: habitat degradation; destabilization of nearby, unfortified natural banks; and high initial construction and longer-term maintenance costs. This hybrid approach effectively defends against erosion; blends critical property and infrastructure protections with the silent needs of the natural world; protects the fragile interface between land and river habitat, and can be argued to have lower overall cost, both at the front-end and long- term back-end maintenance.

The project site supports a 700-unit waterfront condominium development built in the mid-1980’s. Since its construction, the riverbank had eroded, with some portions exhibiting nearly vertical slopes up to 16-feet high. The gravity sewer serving the development is parallel to the bank and portions of the sewer line were only 10-feet from the top of the bank when planning and design began. During previous intense single-storm and flooding events, 6- to 10-feet of the bank had eroded; thus, the sewer was at risk for exposure and damage. The bank needed to be stabilized to avoid environmental harm from a raw sewage discharge to the Merrimack River, should the sewer become exposed and rupture. Several bank stabilization options were evaluated, and from those options, the hybrid solution (i.e., a stone sill with a vegetated slope above it) was selected as the option that would protect the sewer for the long-term and concomitantly provide riparian habitat.

Above the stone sill, coir rolls were installed to form the soil slope and provide a more robust slope while planted vegetation becomes established and binds the soil. Existing mature and stable trees were retained to protect riparian habitat features and retain tree root matrixes that provided soil stabilization. To protect fisheries habitat, the stone sill was constructed using large stones to leave interstitial spaces, refugia, along the toe of the bank. On the re-constructed slope approximately 3,500 shrubs and 850 trees were planted above the sill, and the slope was over-seeded with grasses and forbs to stabilize the soils. Over time, vegetation is expected to develop and provide travel corridor and feeding habitat for terrestrial wildlife, and shade from overhanging branches will improve the riverine habitat.

Epsilon led the planning and environmental permitting efforts to bring this project to fruition. Epsilon staff worked with the Town and the condominium association to secure a FEMA Hazard Mitigation Grant to fund this project. Epsilon then worked to secure local, state, and federal permits, which included among others: MEPA review, Chapter 91 License, NHESP review, and a U.S. Army Corps of Engineers Individual Permit. Team Members GEI Consultants Inc. led the design, PAL provided cultural resources assessment and permitting, and SumCo Eco Contracting constructed the project.

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