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Clive council adopts amended stormwater manual, moves to six-hour storm model and a future-conditions modifier
Summary
The council approved amendments to the post-construction stormwater management manual to adopt a six-hour temporal rainfall model and a +0.25 future-conditions modifier; staff and the homebuilders association will continue technical discussions and engineering clarifications, and the resolution passed 5-0.
The Clive City Council on Aug. 14 approved a resolution adopting amendments to the city's post-construction stormwater management manual by a vote of 5-0.
Pete, presenting for city staff, told the council the amendments contain two primary policy changes: switching to a six-hour temporal rainfall model to better reflect current Midwest storm patterns, and adding a "future conditions" modifier of 0.25 to design flows to account for larger and more intense storms expected over time. Pete said the two adjustments "work in concert" and that the net effect for many design examples will be slightly smaller or about the same detention sizing as today's rules.
Staff said they had shared draft language with the Home Builders Association and developers, and that stakeholders had raised questions about the future-conditions modifier's effect on detention sizing. Pete and council members said additional engineering-level clarifications will be developed with stakeholders; staff recommended approval of the amended manual with a plan to continue refining confusing sections by working with engineers and developers.
A council member moved the resolution to adopt the amended manual; the motion was seconded and the clerk posted a recorded vote of five to zero in favor. Staff noted adoption allows the city to move forward with required floodplain and stormwater agreements for upcoming projects and that specific project-level easements and agreements will return to council as needed.
Why it matters: The manual guides how Clive sizes stormwater infrastructure and enforces state and federal requirements; the new temporal model and future-conditions modifier are intended to better match changing storm behavior and planning for long-term performance of infrastructure.
Next steps: Staff will continue stakeholder meetings and bring clarified manual language and project-specific stormwater agreements back to council as required.
