Commission recommends Liberty Mutual pond repair with tree replacement, boundary markers and recorded maintenance plan

The Portsmouth Conservation Commission on April recommended approval of Liberty Mutual’s restoration plan for a failing stormwater detention pond at 0 Borthwick Avenue, but attached conditions to limit future impacts and ensure long-term maintenance.

Damon Bert of Braggle Rock Environmental, representing Liberty Mutual, told the commission the project will replace a failed 24-inch outlet culvert, clean and regrade the pond bottom and remove approximately 9,500 square feet of material within the 100-foot wetland buffer. He said the team has identified phragmites in the pond and proposed that contaminated sediment and vegetative material be removed and disposed off-site to prevent spread. “We’re representing Liberty Mutual … the plan is basically to clean up the stormwater area and replace the culvert,” Bert said.

Commissioners pressed the applicant on invasive-species controls, tree removal and long-term obligations. Commissioners noted roughly 11 trees proposed for removal (red maples, white pines and quaking aspen) and asked whether the property provides space to replant. The applicant said it would raise the question with Liberty Mutual and, where planting on the parcel is not feasible, explore shrubs or off-site plantings. The commission also sought a clearer monitoring schedule; Bert said the team will perform annual inspections with the option to increase frequency if conditions warrant.

The commission’s recommendation includes four written stipulations: (1) the applicant must prepare and submit a planting plan that replaces removed trees “in kind or with appropriate species” where feasible; (2) wetland boundary markers shall be installed consistent with city guidance (markers may be placed on existing guardrail or stakes as appropriate); (3) the maintenance plan for the pond and its invasive-species protocol must be recorded in the chain of title at the county registry of deeds; and (4) the applicant should consider a monitoring cadence more frequent than annual to ensure the culvert remains clear and the pond does not re-establish invasive stands.

The commission discussed whether markers should be placed every 25 or 50 feet and suggested guardrail-mounted markers as a practical approach for this site. After commissioners confirmed staff would receive a scanned, stamped wetland delineation page for the file, the body voted in favor of the recommendation.

The commission’s action is a recommendation to the city planning authority; it does not itself execute permits. The applicant indicated they plan to perform the work in summer months when the pond is dry and will include sediment controls and the invasive-species plan in contractor bidding documents.