Consultant asks Portsmouth council to adopt flood resilience strategy after pilot alerts predicted Oldtown flooding

Arcadus, the city's technical partner on resilience planning, asked Portsmouth City Council on April 13 to adopt a new flood resilience strategy that blends infrastructure projects and policy changes aimed at reducing coastal and rainfall-driven flood risk.

John Molaw of Arcadus said the strategy translates modeling and stakeholder input into an actionable roadmap. "This strategy takes aim at addressing this challenge for the city," Molaw said, noting the plan draws on the state's coastal resilience data sets and a citywide storm-water model to project flood exposure through 2100. Molaw told the council more than 10,000 stakeholders were engaged through open houses, surveys and events as the plan was developed.

Arcadus described a three-pronged approach — protect, adapt and move away from water — and recommended a hybrid program: approximately 30 infrastructure projects (storm-water conveyance, storage, pump stations, tide gates and coastal structures) and about 10 nonstructural actions (expanded building protections, nature-based solutions, and an interdepartmental resilience working group). "We recommend adoption of this flood resilience strategy as the city's official framework for reducing flood risk and guiding projects and policy," Molaw said.

Presenters highlighted a near-term pilot: a street-by-street storm-water platform tied to the city's model that can issue automated alerts to departments. Alex Carlson described how the platform flagged at-risk corridors in the days before a localized August event (referred to as Hurricane Aaron) that produced high tides and heavy rainfall aligned at rush hour. "We started picking up pilot notifications and could hone in on where flooding might occur," Carlson said, showing images of inundated Oldtown streets taken near 5:00 p.m. during the event.

Staff and Arcadus officials said the plan was funded largely through the state's Community Flood Preparedness Fund (CFPF): the 2021 grant supporting the plan provided approximately 90% of the plan's funding, and adopting the strategy would help preserve CFPF project eligibility. Arcadus also recommended an active public-awareness campaign and the formal creation of an interdepartmental resilience working group to push the roadmap toward design and construction.

Council members pressed for technical detail during a multi-member Q&A, asking how watersheds were delineated, where mapped detention ponds are located, and how current zoning requirements (three-foot freeboard in coastal A and V zones for the 100-year floodplain) might be expanded. Arcadus clarified the model used topography and aggregated smaller watersheds into five study watersheds, estimated that expanding protections to a 500-year future plane could extend protections to roughly 11,000 buildings, and said most recommended projects have only conceptual designs; none of the 30 recommended infrastructure projects have completed full design work.

Council members also asked about the intersect with other resiliency efforts (an OLDCC/OLDC program that previously identified around 50 projects). Arcadus said there is overlap and coordination is needed but that the plan was developed to avoid duplicating work and to inform future detailed design and priorities.

On funding and next steps, staff confirmed a formal request to the U.S. Army Corps of Engineers was submitted through Congressman Scott's office to initiate a Corps study, a step required to be eligible for some federal funding. Arcadus and staff emphasized large storm-water projects, such as a recommended Oldtown pump station, can be costly (presenters cited a $20 million order-of-magnitude figure for similar pump stations) and are typically pursued with federal and state partners.

The presentation closed with staff reiterating that adoption would not itself appropriate construction dollars but would create the strategy and project priorities needed to seek grants and advance design work. The council took no immediate recorded vote on adoption during the public works session; Arcadus said it stood ready to provide additional data and to support next steps if council decides to adopt the strategy at a future meeting.