Polk County, IA Environmental Rules: Stormwater Management (2026)
Key Facts
- Design storm
- 1.25-inch rainfall event, 90% of storms
- Channel protection release
- 24-48 hours for 1-yr, 24-hr storm
- Engineer requirement
- Registered Iowa professional engineer
- Maintenance handoff
- At 80% of lots built or sold
- Enforcing agency
- Polk County Public Works
- Violation period
- Each day is a separate violation
Summary
Unincorporated Polk County requires stormwater detention facilities on developments that cannot directly discharge runoff, under the county Zoning Ordinance's Stormwater Runoff standards. Public Works decides detention need at plat, site-plan or grading-permit review, and any required facility must be designed by a professional engineer licensed in Iowa to meet four numeric volume-control standards.
Section 5. Stormwater Detention Required
Developments not able to directly discharge all flows as indicated in Section 2 above shall provide for detention and design the facility as follows:
(A) If stormwater detention is required, the developer or applicant shall construct stormwater detention facilities designed by a registered professional engineer in the State of Iowa and meeting the criteria of this division.
...
Section 6. Design Criteria
(A) Water Quality Volume - To reduce potential increases in downstream erosion and water pollution, practices or techniques shall be employed that capture and treat runoff from a 1.25" rainfall event. This standard addresses approximately 90% of the ... rainfall events that occur in Central Iowa.
Official source re-checked September 7, 2026: the cited page had not changed since it was quoted.
Full Breakdown
Article 8, Division 1 of the Zoning Ordinance makes detention mandatory whenever a site cannot directly discharge all flows under Section 2. Section 5 puts the design duty on a registered Iowa professional engineer and lets the county weigh preliminary plats, historical flooding on the site or watershed, proximity to drainageways, added impervious surface, and future development potential before requiring detention as a condition of plat approval, site-plan approval or a grading or building permit. 25-inch rainfall event, covering roughly 90% of Central Iowa storms; Channel Protection Volume requires extended detention of the 1-year, 24-hour storm released over 24 to 48 hours; Overbank Flood Protection caps post-development peak release for the 2-year storm (and the 5- and 10-year storms) at pre-development levels; and Extreme Flood Protection applies the same natural-conditions cap to the 25-, 50- and 100-year, 24-hour events.
Section 8 puts long-term maintenance on the property owner, though a developer of a cluster or planned development stays on the hook until 80% of units are built or sold, after which the obligation is recorded as a deed restriction running with the land in perpetuity. Section 9 requires the developer's engineer to certify as-built drawings to the county, and lets a county-employed registered engineer inspect facilities during and after construction, with authority to compel correction of anything not built to the certified plans.
Violations & Fines
Noncompliance is enforced as a zoning violation under Article 23: the Zoning Administrator issues written notice describing the violation and a correction deadline, and each day it continues is a separate violation. If a Notice of Violation doesn't bring compliance, a citation follows with up to 30 days to correct. The county can issue a stop-work order, withhold further permits on the property, and seek civil or criminal remedies, injunction or abatement through the courts.
Frequently Asked Questions
When does a Polk County development need stormwater detention?
Who has to design the detention facility?
Who maintains a detention basin after the development is built?
Sources & Official References
Other rules in Polk County
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