Clive staff propose technical stormwater changes to restore pre‑law runoff controls

City staff and outside engineers told the Clive City Council on July 10 that a draft rewrite of the city’s post‑construction stormwater manual would adopt new calculation methods intended to comply with a recent state law while preserving local flood‑control levels.

Aaron, a water‑resources engineer with Cooler Engineering Company, told the council the update responds to SF455 and “we found by looking at a big sample of case studies … we would have seen something like 19% increase in storm water leaving those sites” if the city had not updated its methods. Staff said the draft balances two technical changes: using a temporal six‑hour rainfall distribution instead of the older SCS 24‑hour curve, and applying a 25% increase to the precipitation depth used when computing required detention volume to account for observed and projected increases in extreme rainfall.

Why it matters: The changes aim to recreate Clive’s pre‑law level of flood‑control performance while keeping the methodology more defensible and less onerous for developers. Staff said the combined effect of the two revisions would, in most modeled scenarios, produce city‑wide results very similar to the prior standard even if some individual lots show higher release rates or smaller detention volumes.

The technical case and modeling

Aaron and Ray Topah of the project team described dozens of case runs on recent Clive redevelopment and infill sites. The team recommended the temporal six‑hour distribution because it better reflects how rainfall intensity occurs in modern storms, and proposed adding 25% to NOAA Atlas 14 depths for proposed‑condition calculations as a climate‑resilience margin. Ray said the fee‑in‑lieu maintenance estimate was modeled with unit costs: “it was like a $1.50 a square foot” for long‑term upkeep of a dry basin, as an example of how staff is converting basin plan area into an ongoing maintenance obligation.

Fee‑in‑lieu and limited pilot scope

Staff proposed a strictly limited fee‑in‑lieu (frequently heard in the meeting as “fee and loo”) option for flood‑plain constrained parcels that cannot practicably provide on‑site detention. That option would be available only for specific, hardship cases—staff estimated the initial pilot would apply to roughly 40 parcels in the flood plain—and the fee would be set to include construction, design, land value and a 50‑year maintenance allowance so that the city would not be subsidizing long‑term upkeep.

Council questions and next steps

Council members pressed staff on modeling assumptions and whether local builders had been consulted. Staff said they plan to reach out to the Home Builders Association and other stakeholders, to refine the 25% factor with Clive‑specific climate inputs, and to return with updated model runs and a redlined manual for formal consideration. Aaron and staff emphasized they used Clive‑specific Atlas‑14 and watershed modeling rather than simply importing a policy from another region.

What comes next: Staff will revise the draft text, provide additional Clive‑specific modeling results and meet with development stakeholders before the council considers adoption of the manual and any ordinance amendments.