Sunnyvale hears green stormwater infrastructure update as permit tightens requirements
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Summary
City staff told the Sustainability Commission that the regional Municipal Regional Permit's C.3 rules now apply at 5,000 sq ft of altered impervious surface, that Sunnyvale must find an additional five acres of treatment citywide, and that soil, space, maintenance and funding are major constraints to expanding green stormwater infrastructure.
Emma Hinojosa, the city's environmental programs manager for watershed protection, updated the Sunnyvale Sustainability Commission on Nov. 17 on the city's Green Stormwater Infrastructure (GSI) plan and the municipal regional permit requirements known as C.3.
Hinojosa said the GSI plan guides incorporation of low-impact development and engineered vegetated systems on public and private lands to reduce pollutant loads entering local creeks and San Francisco Bay. "C.3 is applicable to new and redevelopment," she said, describing the rule that now applies when a project alters 5,000 square feet or more of impervious surface. "If you expand beyond 50%, then you are required to treat all the storm water that would fall on your site or project area."
The presentation summarized three types of responses under C.3: source controls (trash enclosures, covers), site design elements (landscaping, permeable surfaces) and treatment systems such as bioretention areas. Hinojosa told commissioners the MRP is a regional National Pollutant Discharge Elimination System (NPDES) permit enforced by the San Francisco Regional Water Quality Control Board and is updated roughly every five years. She said the larger permit also asks cities to identify "additional five acres" of treatment beyond regulated projects; Sunnyvale's built-out character makes that requirement especially challenging.
Commissioners asked how GSI eligibility applies to artificial turf, accessory dwelling units and household-sized measures. Hinojosa said turf can qualify depending on its sub-base and design, ADU-related small projects are often exempt from C.3 numeric treatment requirements but are encouraged to adopt site-design measures, and rainwater harvesting generally does not count toward C.3 treatment targets because the permit requires treatment rather than storage.
On maintenance and effectiveness, Hinojosa said the city runs an annual screening to find opportunities in capital projects and coordinates with county programs and Valley Water. The city also conducts LID monitoring in coordination with San Jose and the county; the monitoring effort is ongoing with completion expected before the current permit term ends in 2027. She added that Sunnyvale's dominant clay soils (soil type D) and high water table limit infiltration-based approaches, and many GSI features need irrigation and active maintenance.
Hinojosa identified two near-term public projects that could yield small amounts of treatment acreage: the Evelyn Multi-Use Trail and the Gale/Galeota safe-routes project, totaling under an acre of projected treatment between them. She said much of the acreage counted toward projected targets comes from private development that must incorporate C.3 treatment when redevelopment meets the threshold.
Next steps documented in the presentation included continued coordination with regional partners on alternative compliance approaches, ongoing annual screening of capital projects with Public Works, and preparation to update local projections when the next permit cycle provides final requirements.
The Commission thanked staff for the briefing and pursued follow-up questions about specific projects, plant palettes, and how the city might incentivize private developers to incorporate GSI.
