Consultants present hydraulic modeling, propose three HMGP grant projects and expanded watershed study

Consultants presented updated hydraulic modeling and initial benefit estimates for three river projects the council may submit for federal Hazard Mitigation Grant Program (HMGP) funding, and recommended an expanded hydrologic and hydraulic study to link separate FEMA models for better watershed-level decision making. Alex Marcucci, water-resources scientist with SCLR Consulting, and Roy (consultant) described modeling done with HEC‑RAS using the FEMA flood-insurance study geometry and newer field reconnaissance.

The consultants said they compared alternatives to a locally realistic “blocked bridge” baseline that matches repeated debris clogging observed at the Berlin Street bridge. For the Stevens Branch corridor, the modeling suggests removing the Berlin Street bridge could reduce small-to-medium flood depths by about 2 feet in the immediate vicinity and a little upstream; creating a lowered floodplain at Scampini Square (about an acre lowered roughly 3 feet) produced smaller, localized reductions (roughly 0.1 foot across modeled flows). A combined Berlin‑removal plus Scampini floodplain scenario added modest additional benefit compared with bridge removal alone.

At Willie Street the team modeled a feasible replacement that would widen the opening by about 10 feet and raise the deck about 3 feet. That replacement, modeled with the Berlin structure assumed clogged, produced reductions in small‑to‑medium flood depths of roughly 1.5 to 2 feet upstream, with diminishing benefits for larger floods. The consultants also tested a scenario in which upstream debris (from a removed Berlin bridge or trestle changes) is transported downstream and blocks Willie Street; that scenario produced modeled increases in flood depths up to about 4 feet upstream, illustrating a risk that changing one choke point without addressing downstream constraints could shift or amplify impacts.

On Gunners Brook (Harrington Avenue area), modelers updated the FEMA geometry to include the restoration work done after prior buyouts and tested the effect of the site’s debris catcher becoming fully clogged. They found blockage could raise nearby flood depths by nearly 2 feet and send water onto Brook Street; reconfiguring the catcher into several smaller, partly offset elements and expanding the restored floodplain across newly acquired buyout parcels produced modeled depth reductions on the order of about 1 to 2 feet in the Harrington area and reduced overbanking onto neighborhood streets in small-to-medium events. The consultants emphasized the importance of making space for managed debris removal — including safe equipment staging pads on restored floodplain parcels — to prevent catchers from becoming full dams in floods.

Project-level next steps described to council included preparing concept designs and cost estimates to support three HMGP applications next month (Berlin Street area, Willie Street area including the 572 North Main buyout parcel, and Gunners Brook/Harrington Avenue). The consultants also proposed an expanded hydrologic and hydraulic study (green lidar bathymetry plus updated rainfall-runoff hydrology) to replace and link the two FEMA flood models that currently cover different reaches; that work would let staff and the council test scenarios at watershed scale (headwaters storage, land‑use changes, and combined alternatives) rather than relying on separate local models. The consultants said the green-lidar/topographic flights and follow-up field survey would be scheduled as soon as runoff conditions permit, likely late April–early May.

Councilors pressed for clarifications during a 90‑minute discussion. Topics included whether bridge removal upstream could worsen downstream conditions (consultants warned this is possible without accompanying downstream widening or debris-management sites), how recent floods map to the small/medium/large events used in analysis (July 2023 characterized as medium–large; 2024 generally small–medium on Stevens Branch, larger on Gunners Brook), and whether targeted sediment management in confined reaches could help (consultants said targeted, site‑specific sediment work is sometimes useful but broad, continuous channel dredging is not a recommended system‑scale solution). Multiple council members asked staff to return with grant‑application timelines and budget implications and to coordinate with neighboring towns and state watershed planning. No formal funding decision or vote was recorded on the modeling or any individual project: the consultants and staff framed the session as an update and direction-setting step before grant applications and a larger linked study.

Ending: Councilors and staff largely praised the technical presentation and pressed for follow-up: (1) prepare concept designs and cost estimates for the top three projects for HMGP submittal; (2) proceed with the expanded hydrologic/hydraulic study (green lidar, updated flows) so alternatives can be tested at watershed scale; and (3) develop debris‑management protocols and safe‑access designs tied to buyout parcels so catchers and bridge modifications will not worsen downstream risk. City staff said they would return with grant schedule, estimated local match requirements, and a proposed scope and schedule for the expanded study.