Commission denies detention‑waiver request for Bluebird edition site in floodplain

The Cabot Planning Commission voted down a request to waive stormwater detention for a property known in the packet as Bluebird edition, concluding the record lacked sufficient demonstration that eliminating detention would avoid adverse downstream impacts.

Applicant Aaron told the commission the parcel sits in the floodplain and that previous reviewers had agreed detention was not required; he said he prepared a drainage report and that his professional judgment was that detention in that floodplain location is not warranted. "In my professional opinion, it's not warranted," Aaron said, adding that he had prepared a detention area but believed it unnecessary.

City reviewer Joe and staff highlighted the risk that withholding detention can change the hydrograph and increase peak flows when multiple parts of a watershed are developed simultaneously. Joe said that while HEC‑RAS or similar modeling is above and beyond typical requirements for small sites, the reviewer’s concern was verifying pre‑development flows and ensuring post‑development discharges do not increase peak flows for downstream properties. "When you hold back that water … that is a what‑if scenario. That's the reason that HEC‑RAS modeling is mentioned," Joe said.

Commissioners asked about the scale of the site and potential future development in the basin; several members voiced concern that repeatedly granting waivers on adjacent small parcels could cumulatively alter downstream peak flows. On a roll‑call vote, the motion to approve the waiver failed with no votes in favor (0–7).

What’s next: the applicant may revise the plan to show detention on the site or provide the additional modeling/calculations the reviewer requested and reapply. Staff offered that formal submittals and modeling results can be provided to the planning office for reviewer confirmation.