Northborough Reservoir Dam – Significant Hazard Dam Removal

Northborough Reservoir Dam is a 780-ft. long, 25-ft. tall SIGNIFICANT hazard potential embankment dam with an uncontrolled broad crested weir spillway owned by the Town of Northborough.

The dam was constructed in the 1880’s to create a reservoir for drinking water supply. The drinking water supply operations were discontinued in the 1950’s. The dam is currently in poor condition. The Town of Northborough is seeking to breach the dam to reduce the hazard potential associated with the dam and provide ecological restoration of the impoundment and stream.

The current design for the dam breach entails removal of the existing spillway. The newly created channel between the lowered pond level and the Reservoir Street culvert, will be designed to simulate a natural stream channel reach within Rawson Hill Brook located just upstream of the impoundment. This reference reach stream segment was assessed for the longitudinal profile of the channel thalweg, wetted channel width, bankfull channel width and depth, stream bed substrate and channel forming (grade control) features. The channel includes a two-tier cross section reminiscent of the reference reach and adjacent floodplain with boulders provided as grade control habitat elements along with bank stabilization and additional bioengineering measures along the stream bank for additional stabilization appropriate for the predicted stream velocities.

Weston & Sampson developed a hydrologic and hydraulic (H&H) model calibrated using data from river gauges within the drainage basin to better reflect the fluvial geomorphology of the proposed conditions to ensure a stable post-construction site. Unit hydrograph rainfall-runoff modeling, which is typically used for dam safety projects, would not provide a good prediction of the streamflow regime experienced by the brook post-removal. The model output demonstrated that breaching the dam at the existing spillway would meet hydraulic requirements and result in a structure which the DCR ODS would consider non-jurisdictional. This model further demonstrated that the discharge stream could be conveyed through the existing culvert below Reservoir Street which resulted in a significant reduction in the anticipated construction cost.

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