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Consultants present green‑lidar hydraulic model and early flood projections for Stevens Branch
Summary
SLR Consulting briefed the council on green‑lidar data, a 2D HEC‑RAS hydraulic model and hydrology results; early modeling shows peak flows in the system reaching roughly 10,000 cfs (upper Stevens branch), 14,000 cfs (confluence) and 18,000 cfs (downstream main stem) for a 100‑year scenario, and staff said the model will be validated against recent flood marks.
SLR Consulting gave a detailed progress report to the Barre City Council on the firm’s hydraulic, hydrologic and storm‑sewer analyses for the Stevens Branch watershed and associated urban drainage.
Alex Marhushy of SLR (S18) described how 2025 green‑lidar data collected by WhiteHat Solutions allows the team to penetrate water surfaces and more accurately map streambeds, channel banks and infrastructure. "Green lidar is able to penetrate the water and capture stream beds," Marhushy said, explaining that the higher‑resolution point cloud improves cross‑section views used in the hydraulic model.
Leah Cromer (S5), SLR’s hydrologic modeler, summarized preliminary hydrology outputs. She said the project’s modeled drainage area is about 96 square miles and that preliminary peak flows for a 100‑year event are roughly 10,000 cfs for the Stevens Branch subwatershed above the Bridge Street location, about 14,000 cfs at the confluence with Veil Branch, and about 18,000 cfs at a downstream main‑stem location. "These are preliminary; hydrology is conservative and will be validated against observed high‑water marks," Cromer said.
SLR also described a focused drainage‑system survey for the Berlin Street network and an invert/rim survey of storm pipes and catch basins to build an existing‑conditions drainage model. Brian Cody (S11) said that field survey work will allow the team to identify undersized pipes and locations where backflow from full channels causes street flooding. Councilors and residents asked whether the model can be run for short, intense storms (for example, five inches in four hours) and whether model outputs could be used in near‑term operational alerts; SLR replied that the hydraulic/hydrology models can be run against different intensity scenarios and that the team plans validation against recent flood events.
Staff and councilors discussed next steps: finishing surveys, validating the model against recent flood data, using the model to test alternatives such as bridge or channel changes, and preparing public figures and outreach materials. SLR said it will provide more results at the next council briefing; the council asked staff to ensure the survey and model outputs are available to support near‑term maintenance and contracting decisions for storm drains in at‑risk neighborhoods.
