Chicago Stormwater Ordinance: Key Concepts
This tool is based on the City of Chicago Stormwater Management Ordinance Manual. Below are the key performance standards relevant to this calculator. For full details, please consult the official city documentation.
When does the Ordinance Apply?
A project is considered a "Regulated Development" and must meet performance standards if it involves:
- Disturbing a land area of 15,000 sq ft or more.
- Creating a new at-grade impervious surface of 7,500 sq ft or more.
(Reference: Stormwater Manual, Section 1.6)
1. Volume Control (Onsite Retention)
The primary goal of volume control is to reduce the total amount of stormwater runoff leaving a site by promoting infiltration and evapotranspiration. This is achieved by capturing and retaining rainfall from impervious surfaces.
Requirement: Capture one-half inch (0.5") of runoff from all impervious surfaces in accordance with volume control BMPs (like green roofs).
Calculation: Volume (cf) = Total Impervious Area (ft²) × (0.5 inches / 12 inches/ft)
The "green" components of a roof system (soil, mineral wool) provide this retention storage and directly count toward meeting the Volume Control requirement.
(Reference: Stormwater Manual, Section 3.2)
2. Rate Control (Onsite Detention)
The goal of rate control is to reduce the peak discharge rate of stormwater leaving the site to prevent overburdening the city's sewer system. This is achieved by temporarily storing (detaining) runoff and releasing it slowly over time.
Requirement: New developments must limit the flow rate of stormwater leaving a site to a pre-determined "Allowable Release Rate." The storage volume needed to achieve this is calculated using the Modified Rational Method for a 100-year storm event.
Calculation: This tool calculates the maximum required storage by analyzing various storm durations. Required Storage (cf) = (Peak Inflow Rate - Allowable Release Rate) × Storm Duration.
The "blue" or detention components of a Purple Roof (the honeycomb layer) provide this detention storage and count toward meeting the Rate Control requirement.
(Reference: Stormwater Manual, Sections 2.1, 2.4, Appendix B)
Disclaimer
This tool provides estimates for planning purposes only and is not a substitute for a final stormwater management plan prepared by a licensed professional engineer. All designs must be submitted to the City of Chicago for review and approval.