Coastal, River, & Wetland Restoration
- Neponset River Dam Removal
Feasibility Study
the challenge The objective of the proposed project was to assess options for restoring fish passage, aquatic habitat and riparian habitat upstream of the Baker and T&H Dams on the Neponset River, located in Milton and Boston, Massachusetts. Although in the Algonquian Indian language, Neponset means the "Harvest River," referring to the exceptionally rich stock of shellfish and finfish in the river's estuarine and upstream reaches, the Neponset River has been dammed for waterpower since 1634. These dams continued to block passage for anadromous fish to spawn upstream even though the dams had outlived their usefulness, and contaminated sediments lie behind the dams and throughout the river channel. Located in one of the most densely settled watersheds in New England, the river is abutted by active industry, socio-economically diverse neighborhoods, recreation users, and developers/occupants of historic mill conversion condos, creating a vocal and frequently opposing set of special interests.
the interdisciplinary approach The Bioengineering Group conducted a full geomorphic analysis of 8.5 miles of river from the head-of-tide to a large wetland complex outside the urban limits. We compiled data from our survey and calibrated it using existing FEMA data to form the basis of our HEC-RAS model. A hydraulic analysis was then conducted for three alternatives at six different flow regimes including: the existing conditions with dams in place, complete removal of the dams, and a partial breach of the dams. We conducted a bathymetric survey and sediment sampling/ analysis. When test results showed that the impounded sediments contained PCBs and heavy metals, we modified the design to provide for consolidation of contaminated river sediments on a smaller footprint with containment using a stone toe and vegetated soil cap, minimizing cost while being protective of downstream environments. The design prescribed in-stream structures and bank stabilization measures for establishing a suitable stable channel upon dam removal.
Extensive coordination with state and federal agencies and a stakeholder engagement process including interactive public meetings helped build support for the first dam removal in a major urban area in New England. Complex issues included cultural and historic values of the dam site and surrounding mill complex, and competing desires about level of access for recreation along the river corridor.
the results The recommended alternative was the removal of a substantial portion of the existing structures at the Baker and T&H Dams to create a free flowing river, while leaving mill foundations intact. The alignment and profile were designed to provide a geomorphically stable channel that improves fish passage, aquatic substrate, bank stability, and riparian wetland quality, opening 22 miles of high quality river habitat to shad, alewife, herring, and eels. The project also instigated a very constructive dialog between state and federal agencies on the subject of management of contaminated river sediments, representing the first systematic assessment of state standards for clean-up in physically dynamic settings such as rivers and floodplains. The project laid the groundwork for future pro-active restoration projects to proceed more smoothly based on updated policies and procedures.