Challenge

Faced with an Office of Dam Safety Order to repair the dam or take it down, the City asked GZA to identify the most cost-effective approach, provide design plans, obtain necessary permits, and act as the resident engineer during construction. The earthen dam with a stone masonry spillway is an approximately 35 feet tall and 125 feet. This High Hazard, former water supply dam had been in a Poor/Unsafe condition since the 1970’s.A dam breach would require tackling the issue of dealing with approximately 27,000 cubic yards of sediment impounded by the dam.

 


 

Solution

The City concurred with GZA’s recommendation for dam removal as the preferred alternative. Connecting high-quality trout habitat in the stream above and below the dam made the project a priority for MA Division of Ecological Restoration who eventually became a project partner. After years of meetings and deliberations with city residents and regulators, the City, with the help of GZA and MA DER, was able to obtain concurrence to permit the project as a staged dam removal with a controlled sediment release. The project approach was developed to “throttle” the release of about 10,000 to 11,000 CY of sediment to the downstream sediment starved reaches. A post-construction sediment survey completed in the early fall of 2018 revealed very good agreement with GZA’s design, specifically, the amount of sediment mobilized (about 10,800 CY) and the location of the newly formed stream within the former impoundment.

 


 

Benefit

Practitioners from multiple GZA core service areas comprehensively and successfully addressed the City’s engineering, permitting, and construction support needs for the project. GZA also was successful in securing an EOEEA grant to cover 75% of the construction costs. The City’s liability was reduced by removing the Poor condition, High Hazard dam which also resulted in the removal of the tallest dam removal in Massachusetts. Project costs were reduced by using the innovative sediment release approach in lieu of dredging/offsite disposal, saving the City nearly $500,000. The newly formed stream re-established the connectively between two high-quality cold-water fishery habitats.

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