What is Stormwater? 

Stormwater runoff is rainfall or snow melt that runs off impervious (hard) surfaces such as roads, buildings, and compacted soils. This runoff is collected and conveyed through stormwater systems (usually pipes), or flows as surface runoff directly into our streams, rivers and lakes without being treated.

Stormwater can pose a risk to our water resources. Pollutants (e.g., sediment, nitrates and bacteria) accumulate on impervious surfaces between rainfalls. Runoff from rain then washes these pollutants into streams, rivers and lakes, negatively impacting water quality. Stormwater runoff can also adversely affect infrastructure and environmentally sensitive areas, as well as increase both localized and regional flooding. 

What is Stormwater Management?

Stormwater management is the process of managing and controlling the runoff that comes off of primarily impervious surfaces such as rooftops, roadways, parking lots, etc.  Johnson County requires the management of stormwater runoff through practices that allow more rainfall to infiltrate into the ground or store and slowly release captured runoff.

When is a Stormwater Management Plan Required?

If you are submitting a development application for a subdivision or site plan, or an application for a Conditional Use Permit, then you are likely subject to subchapter 8:3.6. of the Johnson County Unified Development Ordinance. 

Subdivisions and Site Plans 

All development applications for preliminary, combined, or final plat, and Site Plan review are subject to the Stormwater Management regulations and must submit a full stormwater management plan in accordance with Chapter 8:3.6. of the Johnson County Unified Development Ordinance at time of application. 

All stormwater facilities shall be installed in accordance with the approved preliminary plat, prior to approval of the final plat.

Residential subdivisions which propose no more than one additional buildable lot may be eligible to defer stormwater management planning until time of future building permit.  A waiver request form must be submitted to request deferral.   See the Waiver Request section for more information. 

Conditionally Permitted Uses

Conditionally permitted uses for which compliance with the stormwater management regulations is required either as a condition of approval or as required in accordance with subsection 8:1.24, must have an approved stormwater management plan in accordance with Chapter 8:3.6 of the Johnson County Unified Development Ordinance plan prior to any site work commencing. 

All stormwater facilities shall be installed in accordance with the approved plan prior to issuance of any Certificate of Occupancy. 

What is a Stormwater Management Plan?

A stormwater management plan details how the stormwater from a development site will be managed so as to protect people and property, and reduce flooding, erosion, and water pollution.  These plans also contain engineered designs for practices that will be used to capture, store, and release stormwater at a controlled rate such as detention or retention basins, and green infrastructure such as bioretention cells.  Established design criteria must be followed when designing these practices.

Design Criteria

In all cases where a stormwater management plan is required, the plan shall be developed in accordance withe the Iowa Stormwater Management Manual.  Subchapter 8:3.6.C outlines the following stormwater management design criteria: 

1. Water Quality Criteria
Post-development runoff shall be captured and treated by water quality Best Management Practices (BMP), as recognized   by the ISWMM, to prevent or minimize water quality impacts from land development. BMPs shall be designed using the following criteria:

  • All practices shall be designed to capture and treat the Water Quality Volume (WQv). The WQv shall be computed as specified in the ISWMM.
  • This criterion shall be met using practices that are included in the ISWMM. BMPs or combinations of BMPs shall be selected that achieve the highest pollutant load reduction for the pollutants of concern.

2. Channel Protection Storage Volume Criteria

  • The stormwater system shall be designed so that post-development stormwater discharges will not erode natural channels or steep slopes.
  • To protect channels from erosion, infiltration or twenty four (24) hour extended detention of the one (1) year, twenty four (24) hour storm shall be provided as specified in the ISWMM.

3. Overbank Flood Protection Criteria.

  • Overbank flood protection shall be provided by controlling the post-development peak discharge rate from exceeding the 5-year, 24-hour pre-development rate of runoff for a five (5) year to a one hundred (100) year, twenty four (24) hour storm, as specified in the ISWMM.

4.  Extreme Flood Protection Criteria

  • Stormwater BMPs shall provide an emergency spillway and/or designated overflow route for the one hundred (100) year, twenty four (24) hour storm, as specified in the ISWMM. The spillway and/or designated overflow route must be designed to safely pass overflows without creating damaging conditions to adjacent or downstream properties and facilities.
Plan Checklist
  • Project narrative describing how stormwater will be addressed.  
  • Stormwater calculations, including break-down of pre- and post-developed land use, drainage areas, curve numbers or runoff coefficients, time of concentration, runoff rates, volumes, velocities, hydrographs, and water surface elevations.  
  • Design checklist for any proposed infiltration practices (e.g. bioretention cell).  
  • Seeding/final stabilization plan and specifications.    
  • Standard drawings and cross sections, with elevations.   
  • Stormwater BMP maintenance and repair plan.  
  • Site plan showing locations of stormwater practices, limits of disturbance, overlay of any sensitive areas, and existing and proposed contours.    
  • A signature block that includes signature lines for the property owner and the PDS Administrator. 

Stormwater Management Materials Required For Development Applications

Stormwater Management Plan

A digital copy of the Stormwater Management Plan must be submitted with the initial Subdivision or Site Plan development application.  For Conditional Use Permits, the timing of the Stormwater Management Plan depends on the use, and is either required at the time of development application, or after the permit is issued but prior to site disturbance. 

Stormwater Maintenance Agreement

Where a stormwater management plan is required, a stormwater maintenance plan is also required.  Subsection 8:3.6.D details these requirements. 

For projects subject to platting, the maintenance agreement must be recorded with the legal documents for final plat. 

For all other projects, the maintenance agreement must be recorded prior to Certificate of Occupancy, Certificate of Completion, or commencement of the use of the site.

Stormwater Management As-Built Plans

Stormwater as-built survey and certification are required for final plat or approval of development applications.  The provided as-built plan must show that the infrastructure is installed in conformance with the approved plans, and must be certified by a Professional Engineer. 

For projects subject to final plat, the as-built stormwater survey must be provided for final plat approval.  For all other projects, the the as-built stormwater survey must be provided prior to Certificate of Occupancy, Certificate of Completion, or commencement of the use of the site.

AutoCAD/GIS Data

AutoCAD/GIS Data must be submitted to include at a minimum all stormwater facilities, conveyances, and easements.  

Request for Waiver

Stormwater Management requirements may be waived if certain conditions listed in chapter 8:3.6.A.6 are satisfied.  Requests must be made prior to filing a development application, and the approved waiver must be included as part of the application materials with the development application.

Stormwater Management Plan Waiver Request Form

Per subsection 8:3.6.A.6, the Zoning Administrator or a duly authorized representative may waive some or all of the requirements of this subsection  if one of the following conditions are met:

a. A stormwater management analysis was previously performed on the property and the associated plan and analysis are adequate for the proposed development or use; or

b. The proposed development or use will not permanently and significantly increase the impervious surface area of the property, negatively impact water quality, or permanently increase soil erosion.

c. The property is subject to the Site Planning requirements in chapter 8:1.26, or stormwater management is required in association with an approved Conditional Use Permit, and both of the following apply:

  • A stormwater runoff analysis, in accordance with the Iowa Stormwater Management Manual, clearly demonstrates that post-developed runoff rates will be decreased or remain unchanged relative to pre-developed runoff rates.
  • All existing and proposed stormwater discharges are and will be authorized under an active National Pollutant Discharge Elimination System (NPDES) permit.

 

STORMWATER MANAGEMENT WAIVER REQUEST FORM

Stormwater Quality Improvement Plan (SWQIP) 

Residential subdivisions which propose no more than one additional buildable lot may be eligible to defer stormwater management planning until time of future building permit.  A waiver request form must be submitted to request deferral. Those developments for which deferral of stormwater management to time of building permit application has been approved, and propose less than 5,000 square feet of impervious rooftop, will submit a Stormwater Quality Improvement Plan (SWQIP) to fulfill stormwater requirements.  SWQIPs are intended to balance traditional stormwater approaches with preservation of sensitive areas and physical site characteristics that naturally lend to stormwater runoff reduction and water quality protection.

Quick Links

Johnson County Soil Health Program

Stormwater Management Waiver Request Form

For More Information or to Report A Concern

Contact: Kasey Hutchinson, Environmental Regulations Coordinator

Phone: 319-356-6083

Email:  [email protected]

Johnson County Administration Building
Planning, Development and Sustainability
913 S Dubuque Street
Iowa City, IA 52240

Additional Information

Resources for Residents
Stormwater Management at Johnson County Facilities