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City to adopt 2024 Surface Water Design Manual and modernize stormwater code; NPDES compliance items reported complete or in progress
Summary
Staff recommended early adoption of the 2024 Surface Water Design Manual and a code update to streamline local surface‑water rules; staff also reported all agreed‑order corrective actions with March 31 deadlines were submitted and the city is on track for remaining NPDES requirements.
City staff presented an introduction to the state’s Surface Water Design Manual and recommended the council adopt the 2024 manual earlier than the regulatory deadline so development rules remain consistent for incoming projects. Staff also outlined a code‑update plan to modernize Mill Creek’s surface water code and reactivate a surface water permit process for smaller projects.
Mr. Carmody (staff) explained the manual’s policy and technical history: the manual implements federal and state stormwater requirements (NPDES/Phase II permit), contains five technical volumes, and reflects an evolution from combined sewers and single‑event detention to continuous‑time modeling and low‑impact development (LID). He said Mill Creek currently follows the Department of Ecology manual (the 2019 edition) and that the city must adopt an updated manual by 2027; staff recommended adopting the 2024 manual sooner to maintain a consistent development environment for projects pending in the pipeline.
Staff described several practical results of the 2024 manual and local code changes: updated continuous‑flow hydrologic modeling requirements, clearer LID expectations, more detailed best management practice (BMP) design standards, and revised triggers for parcel‑level versus road projects. Mr. Carmody said the manual is lengthy (five volumes, about 1,350 pages) and noted the city intends to brief the planning commission (required for zoning changes) and seek final adoption in July following public hearings.
Separately, staff (Reese) summarized the city’s NPDES compliance work and the agreed order with the Washington Department of Ecology. Staff reported that all corrective actions with deadlines of March 31, 2025, were submitted on time and the Ecology planner has acknowledged receipt. Three outstanding agreed‑order items remain but are on schedule: 1) a behavioral change pilot program (pet‑waste reduction pilot at Heron Park) with a report due June 1, 2025; 2) a compliance plan due June 30, 2025; and 3) monthly status reports to Ecology (ongoing). Staff described other corrective‑action items completed this year: an updated basin inventory, construction inspection processes, a private‑system O&M inspection form and a screening methodology for illicit discharge detection.
Staff requested direction to continue with code updates, hold the required planning commission hearings (June 17), and target adoption in July so the city has a clear regulatory framework for anticipated development projects. Council did not take immediate regulatory action at the meeting but will receive draft code changes and the planning commission’s recommendation before final adoption.
