Consultants: Proposed Francis Street dune will cut overwash, need small landside wall

Woods Hole Group coastal modelers reported that the proposed Francis Street Beach dune designs — the “dune swoop” and the “beach overlook” — produce similar erosion outcomes and offer near- and mid-term flood risk reduction while requiring only modest maintenance.

The firm’s summary explained the modeling looked at both sediment movement (how the beach profile changes during storms) and flood redirection/velocity concentration (how the project affects where water flows during storm events). Kirk of Woods Hole Group said the team ran simulations using observed tide-gauge data and regional projections and evaluated storms from present-day 100-year events to future-event scenarios. “We don’t think yet we need to have any kind of internal dune core for this,” Kirk said, “however, we do think you’re gonna probably want some kind of landscape wall to limit sediment overwash.”

Why it matters: The modeling found the proposed dune raises the representative cross-shore profile (crest elevations discussed around elevation 8) and shifts how waves and sediment behave during storms. In the smaller and moderate storms modeled, the proposed profile moves less sand onto Washington Street than existing conditions; in a very large event the existing profile showed as much as about a foot of sand pushed landward onto Washington Street, while the proposed profile reduced that overwash.

Key technical findings: Models that combine water levels and waves showed a peak significant wave height of roughly 3 feet during the simulated large storm passage. Sediment-transport runs showed typical offshore movement of sand under storm wave action and occasional overwash into landward areas; the dune reduces but does not entirely prevent overtopping during the largest events. Velocity and flood-depth modeling compared existing and proposed conditions and found localized reductions in flood depth and velocity behind the dune; flows that cannot overtop the dune tend to move around the project and enter the low basin behind it from northern approaches.

Regulatory safety: The team calculated peak velocities at three monitoring points and plotted changes against industry damage curves. The increases in velocity that appear on Washington Street under some scenarios remain “well under all the curves,” the consultant said, indicating the project is unlikely to induce scour or building damage at modeled storm levels. The analysis also flagged modest velocity increases at one northwestern connection point and recommended prioritizing adjacent connections to avoid localized impacts.

Maintenance and design implications: Consultants recommended no structural core or heavy engineered armoring inside the dune but advised a short, low landside retaining/landscape wall—on the order of about 1 foot high—to capture overwash sediment and limit maintenance burdens on Washington Street. They also recommended coordinating the dune tie-ins with adjacent properties (the consultant cited the neighbor to the north and a nearby cottage as relevant connections) so the project’s protective function is continuous alongshore.

Context and next steps: Presenters noted the two layout options (dune swoop vs. beach overlook) produced similar coastal-performance results, so selection can be guided by public-access and design preferences. The team said further work by the PAR (design) team will present cross-section concepts and material/visual treatments; they suggested early consultation with historic-preservation stakeholders to refine aesthetics and materials. The group will review a preferred alternative at a subsequent meeting.

Ending: Project staff will bring the technical memoranda and recommended tie-in strategies to future meetings and to the PAR design review next week; consultants said the dune approach is likely to reduce routine overwash while keeping required maintenance modest, provided the landside wall and adjacent connections are implemented.