Portsmouth planning board grants Liberty Mutual wetland permit to repair detention pond with conditions

The Portsmouth Planning Board on the evening approved a wetland conditional-use permit allowing Liberty Mutual to repair a failed culvert and remove accumulated sediment at a detention pond at 0 Borthwick Avenue.

Proponent Damon Burt of Fraggle Rock Environmental told the board the project will restore the original stormwater function by replacing a buried or failed 24-inch culvert, dredging about 2 feet of sediment from the existing detention pond, and unburying and resetting two existing culverts. Burt said the application estimates roughly 9,500 square feet of temporary impacts during dredge work, 700 square feet of wetland buffer disturbance and about 225 square feet of direct prime-wetland impacts. "They were in favor of recommending approval for our project," Burt said of the Conservation Commission's April review.

Why it matters: the work is meant to restore a failing stormwater outlet and to reduce future ponding and maintenance problems. The board recorded commitments from the applicant for invasive-species management and replacement plantings to offset tree removal.

Board members asked about construction access, timing and nuisance trash. Burt said that excavators will likely need access into the pond in places to reach the culvert and that sediment and contaminated soils would be trucked off-site and buried to avoid spreading phragmites seed. He confirmed the project proposes removal of 15 trees (3 red maples, 2 white pines and 11 quaking aspen) and that Liberty Mutual has committed to plant 13 native trees along the parking lot; exact planting locations were not finalized.

The applicant also provided a snow-management plan intended to prevent snow-storage piles (and associated debris) from accumulating inside the 100-foot buffer. Board members urged scheduling the work during a dry period and requiring site controls; Burt said the contractor plans to do the excavation during dry weather and to berm the outlet and hold stormwater on-site if needed.

The public hearing produced no speakers. A board member moved to find the application met the requirements of section 10.10.17.50 of the ordinance and to adopt the staff findings; the board approved that motion. The board then granted the wetland conditional-use permit with the staff conditions referenced in the staff memo. An administrative correction accepted during the motion clarified that "the applicant shall provide a planting plan to the Planning Department" rather than to the Planning Board.

The permit was approved with the conditions outlined in staff materials; applicants and staff will coordinate final planting plans and invasive-species disposal logistics before work begins.