The Needham Stormwater Bylaw Working Group discussed and largely defended a proposal to require infiltration systems sized to capture a minimum of 1.5 inches of rainfall (a volumetric requirement) for properties that exceed defined impervious thresholds.

Why it matters: the 1.5-inch standard is intended to improve local stormwater recharge and phosphorus capture, and it is a key technical change that will affect designers, builders and property owners. The group noted that phrasing the requirement as a volumetric capacity (a static captured volume) is important to avoid design workarounds based on short-duration infiltration rates.

Key points from the discussion:
- Thresholds: the working draft distinguishes small and large projects. Properties with more than 4,000 square feet of post-construction impervious surface would have to demonstrate compliance with volumetric recharge plus peak rate attenuation; smaller sites would need the volumetric recharge standard only. The 4,000-sq.-ft. threshold and the 1.5-inch volume were discussed repeatedly.
- Comparisons: members cited nearby municipalities with higher standards (for example, Brookline’s higher figure and Watertown’s eight‑inch example cited in discussion) to justify the proposal as within the local/regional range. The group also noted some towns apply standards only for large‑house or major project reviews.
- Volume and communication: the working group reviewed the point that a difference of a half‑inch is more significant than it sounds; on a 1,000‑square‑foot impervious area, an extra 0.5 inch is roughly 300–400 gallons of additional captured water. Members agreed this kind of concrete example is useful in public outreach.
- Cost implications: participants discussed that adding capacity typically requires additional infiltration chambers or modules; one working estimate cited in the meeting suggested a modular infiltration chamber costs on the order of a few hundred dollars per unit (example figure discussed: about $350 per chamber for material). The group emphasized that much of the cost for small additions would occur in mobilization and crew time rather than raw materials and that small projects may be treated differently in the regulations.
- Where to put technical detail: several members argued that specific design details and prescriptive criteria (for example, how to calculate recharge and soil tests) belong in the regulations rather than in the bylaw, while others wanted to preserve the 1.5‑inch minimum in the bylaw to prevent the figure from changing without a town vote.

Unresolved or follow-up items: the working group will finalize public messaging that explains why 1.5 inches was chosen, provide numeric examples (gallons per 1,000 sq. ft.) for outreach, and ask DPW/contractors for concrete cost estimates to inform residents and the Select Board.

Ending: The group maintained the 1.5‑inch volumetric target for the draft and agreed to refine outreach language that shows the real‑world volume and likely incremental cost for typical projects.