Consultants tell Belfast council breakwater extension would cause only small, slow sedimentation near its end

WSP consultants presented the final harbor sedimentation study to the Belfast City Council on Aug. 20, concluding that raising and extending the town’s breakwater would not dramatically change local hydraulics but would likely create a slightly larger depositional area immediately downstream of the structure over months or years.

Todd Hopkins, an environmental scientist and coastal geologist with WSP, told the council the team ran hydraulic (HEC‑RAS), wave and bed‑level models using Army Corps hydrographic surveys from 2011, 2018 and 2022. He said the models examined existing and proposed breakwater configurations under a range of flows, tides and storm conditions and found that most high‑velocity flows remain in the channel while low‑velocity, depositional areas are pushed slightly eastward under the proposed design.

"We found that over the long term from 2011 to 2022, 98% of the surveyed area changed less than 0.05 feet per year," Hopkins said, adding that short‑term variability of up to about 0.5 feet per year appears in limited spots. He said the proposed extension would likely produce inches of accretion at the new breakwater tip over months or years rather than immediate, large shoaling.

Jillian Whiting, who ran the model results, said the sediment‑plume work showed most material remains in the channel and is carried downstream rather than re‑entering the area immediately upstream of the breakwater. "We don't see it coming back on the downstream side due to the extension," she said, characterizing the change as modest and localized.

Council members and harbor stakeholders pressed for quantification. Councilor Paul (last name in packet) asked for an estimate of sediment volume and potential impacts to downstream uses; consultants said precise volumes would require additional local data and targeted simulations. "With the available data we had, we predict a slight increase in depositional zone right south of the proposed extension," a WSP presenter said. "We’re not talking about feet of accumulation, but more like inches of accumulation over time."

Former Harbor Committee member Jerry Brown praised the study as a strong baseline but stressed the need for local sensors and repeated surveys. "This study is establishing a baseline," Brown said. "Down the road we'll have better local data — tide sensors and current sensors — so we can repeat this when we think about bigger breakwaters."

City staff and WSP said the proposed project should be paired with monitoring: repeat hydrographic surveys, periodic model updates and local current or tide sensors to detect any unanticipated change. The consultants also noted the city has a climate and resilience action committee and that additional funding recently obtained could support broader resilience planning.

Next steps discussed by the council included keeping the study on file as a baseline, soliciting competitive bids if the council moves forward with design, and continuing community outreach and monitoring. No formal vote on a construction project was taken the night of the presentation.

The presentation and council discussion took place during the Aug. 20 meeting in Belfast City Hall. The consultants repeatedly emphasized that increased sedimentation resulting from the breakwater expansion would be limited in extent and accrue slowly, recommending monitoring and adaptive management rather than immediate major dredging.