Consultants: enlarging Harrison Street culvert and dredging would improve low‑tide drainage but increase future flood extent

Mary Scholl, a scientist with Woods Hole Group, said the team’s hydrodynamic models show that enlarging the Harrison Street culvert and conducting upstream/downstream channel dredging together provide the greatest improvement in low‑tide drainage and tidal flushing for the Upper Blue Fish River. “Enlarging the culvert and dredging as two different things that work together do provide a greater drainage benefit,” Scholl said.

The project area examined is the marsh and channel south of Harrison Street, including the Fairway Lane side channel and a maintained freshwater pond that forms the western boundary near the golf course. The consultants modeled present conditions and future sea‑level scenarios and tested alternatives that change culvert span (barrel count or single larger opening), invert elevation and top elevation, and dredging depths.

Scholl said the team evaluated alternatives against three criteria: how well the design drains the channel at low tide, how it affects future flood extent and intensity, and how it improves tidal flushing (which can improve water quality). The team’s preferred “D” alternative—which lowers the invert, raises the overt and includes dredging—produced the best drainage and flushing relative to existing conditions, but it also increased the projected flood footprint in adjacent areas, particularly toward the golf course.

Consultants and town stakeholders described the tradeoff this way: better low‑tide drainage and flushing can reduce residence time and help water quality and native marsh species, but a larger opening can propagate higher water levels during storm and higher‑sea scenarios and thus expand where flooding will occur. Scholl cautioned that the study did not model detailed mitigations for the expanded flood footprint and said additional design work would be needed to address that risk.

Several participants urged an adaptive, phased approach. A Coastal Zone Management representative and other consultants framed phragmites control as a stepwise strategy: use culvert changes to allow more saline water into the marsh and monitor plant response for a few growing seasons before adding targeted mowing or removal. “The single best thing you can do against phragmites in a coastal situation is to increase the amount of salinity getting out there,” one participant said, describing monitoring that shows salinity above 22 parts per thousand on flood tides in the area.

Speakers also flagged the condition of the Harrison Street culvert/stone structure and recommended a near‑term structural assessment to determine whether replacement or repair is required before larger design work proceeds. Stakeholders noted practical constraints—roadway driveway tie‑ins, construction logistics, permitting feasibility and project cost—and stressed aligning recommended actions with available grant programs.

The consultants said the final report will present short‑, medium‑ and long‑term actions, prioritize them by ecological benefit, climate resilience and construction feasibility, and identify likely funding avenues. Jason Bergner of the state coastal office noted several programs that commonly support these elements, including CZM habitat and water‑quality grants, state resiliency programs and Division of Ecological Restoration funding. The project team expects to deliver the final plan to the town by the end of the grant period.