Mill Creek staff propose flexible wetland buffers, site-potential tree‑height riparian approach in critical area update

Planning staff presented a suite of proposed changes to Mill Creek’s Critical Areas Ordinance on Oct. 16, telling the Planning Commission that the city must update its code by year-end under the Growth Management Act and that the changes rely on best available science from state agencies.

Justin Horn, planning staff, reviewed a gap analysis prepared with consultant Facet and said staff is drafting code amendments to Chapters 18.04 and 18.06 of the Mill Creek Municipal Code. Horn said the update covers wetlands, fish and wildlife habitat areas (including riparian buffers), geological hazard assessment requirements, and exemptions/exceptions such as for trails. He said the city must “base all of our regulations on the best available science.”

Wetlands: staff presented three options for buffer methodology. Option 1 (staff-preferred) uses buffer widths that vary by wetland category, land-use intensity and a habitat-function score and allows impact minimization measures and buffer averaging in specified circumstances. Options 2 and 3 are progressively more prescriptive and inflexible, with larger fixed buffers in many cases. Horn explained buffer averaging under Option 1—shrinking a buffer in one place and expanding it elsewhere so the average area remains the same—and noted that most urban uses in Mill Creek are considered high intensity. Staff said habitat-function scoring increases buffer widths for higher-value wetlands and that low‑impact uses (for example, bird-watching trails) may permit a 25% reduction from listed widths. Commissioners asked about administrative complexity; Horn replied that Option 1 places more analysis burden on applicants but preserves flexibility for development feasibility.

Riparian (stream) buffers: staff described state guidance that moves from fixed stream buffers to “riparian management zones” sized by site‑potential tree height. Horn showed the Fish and Wildlife polygon data and staff analysis showing that, under site‑potential tree height, many streams would see buffer increases to about 196 feet while North Creek would shrink from the city’s current 150 feet to roughly 105 feet in places. Staff outlined four implementation approaches: require a site‑specific tree‑height analysis on every application; adopt Fish and Wildlife map polygons as the official map (filling gaps by using adjacent polygon values); set a citywide default (e.g., 196 feet) with named-stream exceptions; or retain the old stream-typing approach (not recommended). Commissioners suggested a hybrid approach—use a default map but allow applicants to submit a site analysis—and staff said the city could adopt that hybrid option.

Trails and mitigation sequencing: staff noted current code allows trails in critical area buffers without mitigation and proposed removing that blanket exemption so new trails would require mitigation. Horn described the state-recommended mitigation sequencing—avoid, minimize, rectify/restore, reduce over time, then compensate—and said mitigation-bank credits and in-lieu fees are lower-priority options. Commissioners asked about cost and availability of mitigation bank credits and whether trail projects such as the North Creek Trail or a future city park could become cost-prohibitive; staff said the city does not currently maintain an in‑city mitigation bank or in‑lieu fee program, but those options could be explored.

Geologic hazards and SEPA exemptions: staff proposed requiring geotechnical assessments that account for climate-driven changes in hydrology, and in some areas requiring boring/test pits or groundwater monitoring. Separately, staff proposed raising the city’s SEPA exemption thresholds to match peer cities (per WAC provisions that allow flexible thresholds). Horn emphasized that raising SEPA exemption thresholds would not exempt projects from critical-area requirements; the proposed change would mainly streamline review for smaller projects outside special subareas. He noted the South Town Center area is being addressed through a planned-action EIS and planned-action ordinance, which preloads SEPA analyses for that subarea.

Process and timing: staff said a complete list of proposed ordinance language will be provided in a staff report before the next Planning Commission public hearing. The commission discussed the statutory deadline under the Growth Management Act and staff’s target for Council adoption in December, while acknowledging state agency review could affect timing.

What commissioners asked for: clearer criteria for reasonable‑use exemptions and redevelopment triggers for nonconforming uses; explicit standards for when minor repairs or small expansions remain allowed; examples of feasible mitigation and approximate costs; and a comparison of peer-city experiences with SEPA exemption threshold increases. Commissioners and staff also discussed outreach (the EIS scoping for South Town Center remains open through Oct. 28).