Council hears stormwater action plan for Mill Creek basin; short‑term retrofits identified

Public works staff told the Mill Creek City Council on May 27 that the city’s Stormwater Management Action Plan (SMAP) for the Mill Creek basin is complete and identifies both structural and nonstructural steps to improve receiving‑water quality and meet permit requirements.

“This was required by our NPDES permit,” Public Works staff member Carmody said, explaining that the SMAP focuses on the basin that drains to North Creek and includes sites such as Bothell‑Everett Highway, State Route 96/132nd Avenue, Henry M. Jackson High School and Trillium Boulevard. The basin was selected because it showed the poorest condition in the city’s basin prioritization analysis, including elevated fecal coliform readings and pollutant indicators.

Consultants and staff presented six structural “retrofits” that could be completed in the short term (within the city’s CIP) and two targeted nonstructural measures. Short‑term structural recommendations included underground filter vaults at the SR‑96/132nd intersection (estimated $724,000) and a similarly configured filter vault near Jackson High School (estimated $439,000); conveyance and vegetated‑treatment upgrades along 130th Street (estimated $192,000); and regional water‑quality treatment tied to the Trillium Boulevard rebuild (estimated $492,000). Staff said the current permit requires the city to address water quality for 3.4 acres of existing development and that one or a combination of the recommended retrofits could meet that requirement.

Nonstructural actions the plan recommends include a city‑wide stewardship and public‑education campaign (the city has already piloted a pet‑waste campaign in Heron Park) and expanded water‑quality monitoring beyond the parameters covered by the regional Stormwater Action Monitoring (SAM) program to better measure metals, nutrients and turbidity after retrofits are built.

Staff emphasized opportunistic implementation—coordinating retrofits with planned paving and capital projects to reduce cost—and partnering with private owners (HOAs, school district, apartment owners) for shared solutions. “We want to find ways to partner with other groups… so we can split the cost,” a presenter said.

Councilors asked how the recommended projects would be funded. Staff said placeholders are already in the current CIP for several items, that the Trillium Boulevard retrofit is included in the 2025–26 biennium, and that grant opportunities (state ecology grants and others) could partially offset costs. No formal vote or ordinance was required; staff said they will update the CIP later in 2025 to include the phase‑one projects and pursue grants and partnerships to implement the work.