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Board OKs resolution to amend Waldo Road zoning condition; engineering review required before development
Summary
Missoula County commissioners voted to adopt a resolution of intent to amend a zoning condition at 9772 Waldo Road that was imposed after a 2019 rain‑on‑snow sheet‑flow event. The amendment keeps a prohibition on basements and requires engineered stormwater plans that address a 100‑year rain‑on‑frozen‑ground scenario before permits are issued.
Missoula County commissioners on March 3 approved a resolution of intent to amend a special zoning condition tied to Lot 1, Denbiker Subdivision, at 9772 Waldo Road. The condition arises from a severe 2019 rain‑on‑snow event that produced sheet‑flow drainage in the Waldo Road/Highway 93 area.
Planner and floodplain administrator Matt Heimel told the board the property is in a commercial center zoning district but carries a legacy condition: any development must construct the lowest floor and utilities at least 2 feet above the highest adjacent grade and basements were previously prohibited. The applicant hired Newfields to complete a hydrologic analysis to quantify peak discharges from the off‑site drainage basin. Heimel summarized the study’s modeled results: a standard 100‑year, 24‑hour (non‑winter) design storm produced a modeled peak discharge of about 96 cubic feet per second (cfs) at the southeast edge of the property; a 100‑year, 24‑hour rain‑on‑frozen‑ground scenario produced about 111 cfs, roughly a 16% increase.
Heimel said county staff and the Department of Natural Resources and Conservation provided technical review and deemed the modeling reasonable after clarifications. Because there is no specific development plan before the county, staff did not support wholesale removal of the condition. Instead staff recommended amending the condition so that basements remain prohibited and any future deep development must submit engineered drainage and stormwater plans that demonstrate no surface or groundwater inundation of structures. Those plans must evaluate both the county’s standard 100‑year design storm and the larger rain‑on‑frozen‑ground scenario (or another technically reasonable method) and be reviewed by county public works prior to permit approval.
Applicant representatives, including Paul Forstinger and owner Taylor Tillman, said they worked with county staff and consultants for more than a year and agreed to the modified condition. IMAG engineers and WGM consultants indicated engineered mitigation — detention/retention, swales, and site grading — would be used to prevent passing stormwater to neighboring properties and likely improve downstream conditions compared with existing uncontrolled sheet flow.
Planning board review: the consolidated planning board unanimously recommended approval of the zoning‑condition amendment. Heimel said public comments acknowledged past flooding and requested mitigation but did not oppose the amendment.
Outcome and next steps: the board’s adoption of the resolution of intent moves the change to a 30‑day public comment period; final resolution and any permit approvals will require the engineered studies and public works approval described in the amended condition.
Why it matters: the decision preserves public safety protections from subsurface and surface inundation but creates a clear path for development if engineers demonstrate mitigation will prevent adverse impacts under a more conservative storm scenario than standard design storms.
Technical note: staff recorded modeled peak discharges of ~96 cfs (standard 100‑year storm) and ~111 cfs (100‑year rain‑on‑frozen‑ground storm).
