Story County, IA Environmental Rules: Stormwater Management (2026)

Section 88.05(4) of the Land Development Regulations sets post-construction stormwater standards for unincorporated Story County. Two triggers require a Stormwater Management Plan: any development, including common development, disturbing one acre or more, and any development that divides land into more than four development lots. Once triggered, the plan must be signed and certified by an Iowa-licensed professional engineer and show pre- and post-development land use, soils, topography at two-foot contours, watercourses, wetlands, floodplains, wells and septic systems.

The county sets three numeric design criteria, calculated using the Iowa Stormwater Management Manual: the site must manage the water-quality volume produced by a 1.25-inch rainfall through infiltration; provide 24 hours of extended detention for the channel-protection volume of the 1-year, 24-hour storm; and limit post-development peak runoff for storms from the 5-year through the 100-year, 24-hour event to the lesser of pre-settlement or existing-condition rates. Low Impact Development practices are required wherever feasible.

Exemptions apply to agricultural land (except where Iowa Code § 335.2 requires otherwise), county, state or federal road maintenance and construction, and maintenance of development that predates the ordinance so long as the footprint doesn't grow. Where infiltration is infeasible, such as poorly draining Group D soils or high groundwater, the site must instead provide water-quality treatment through alternative BMPs, retention ponds, sand filters, vegetated swales, targeting an 80 percent reduction in total suspended solids. Plans are reviewed by the Planning and Development Department alongside the underlying zoning, grading, subdivision or commercial site-plan application.