Merrimack Commons plan to add emergency overflow pipe prompts neighbor flood concerns; commission asks Natural Heritage check

Doug Lees, representing Merrimack Commons Rentals Associates, said the project's infiltration pond currently has no outlet and the applicant proposes a buried 12-inch HDPE emergency overflow tied to an existing stone box culvert under the railroad to prevent pond overflows from backing up on-site. "The pond is designed to fill up to a 105.7. We put the pipe coming out at a 106," Don Lees said, describing the proposed invert elevations and the temporary nature of the construction disturbance.

Several commissioners and residents recalled historical flooding in the area, including a heavy event that inundated condominium first floors. Residents asked whether routing an overflow into the culvert could cause backwatering and neighborhood flooding in rare high-water events. A neighbor said profile graphics for the proposed berm and basin appeared to understate adjacent slopes and that foundation setbacks for nearby garage foundations might be insufficient under stormwater regulations; he urged further verification of slope and setback compliance.

The applicant said Natural Heritage (NHESP) had issued a "no adverse effect" letter on the project footprint, but commissioners voted to refer the matter back to Natural Heritage/Division of Fisheries & Wildlife to confirm the timing and potential seasonal restrictions (for example, nest season) and to ensure temporary work windows do not conflict with habitat protections. The hearing was continued to Aug. 11 to permit that follow-up and additional review of flood-history and elevation data.