1. Design and Construction /
  2. Vol. 3 New Construction Guidelines /
  3. 1.0 Site and Landscape /
  4. 1.5 Site Engineering

1.5.1 Stormwater Management and Site Drainage

All CPS projects that include construction activity, excavation, or grading must comply with the City of Chicago Regulations for Sewer Construction and Stormwater Management. The City regulates the rate and volume of stormwater allowed to be discharged from regulated developments.

Rate Control Requirements

Projects that are considered regulated developments by DWM typically required rate control measures. Rate control requirements vary for developments that discharge to waters and are not required if the development discharges to Lake Michigan.

Stormwater management systems shall manage the peak rate of discharge from regulated developments, incorporating the maximum permissible release rate. The City of Chicago Stormwater Spreadsheet Tool shall be utilized to determine the permissible release rate; once the release rate is determined, the required detention storage can be calculated.

Requirements to consider during design:

  • A maximum of 400 square feet of contiguous impervious area and 1,500 square feet of contiguous pervious area can drain unrestricted off-site.
  • An overflow path must be provided from the development to the public right-of-way that conveys the 100-year storm while providing freeboard building thresholds.
  • A Minimum of 0.1 feet of freeboard from the high-water level within any detention facility to the overflow invert to the right-of-way must be provided.
  • All detention storage must be contained within the proposed development.
  • Combined sewers are common in the City of Chicago. Sanitary sewer flows (also known as dry weather flows) shall be included in the determination of the maximum allowable release rate from the site.
  • Storm sewers must be designed to convey runoff from storm events up to the five-year event via gravity flow. Runoff from the 100-year event must be contained within the storm sewer system.
  • Off-site flows routed through the detention system must not overflow to the public right-of-way for storms up to the 25-year event.
  • Detention can be stored in underground detention vaults, oversized pipes, and parking lots. Detention strategies shall minimize ponding and the need for underground storage vaults.

See Infiltration Best Management Practices (BMPs) section below and refer to the Chicago Stormwater Management Manual for additional information.

Volume Control Requirements

Volume control is required for all regulated developments. Projects shall reduce runoff volume from a regulated development by one of the following measures:

  • Capture and retain one-half inch of runoff from all impervious surfaces utilizing volume control BMPs. To meet LEED requirements rainwater management shall be planned in a manner best replicating natural site hydrology processes. Retain (i.e infiltrate, evapotranspirate, or collect and reuse) on site the runoff from the developed site for the 80th percentile of regional or local rainfall events using low-impact development (LID) / green infrastructure (GI) practices.
  • Achieve a 15% reduction of impervious surfaces from existing conditions.

The City of Chicago Stormwater Spreadsheet Tool shall be utilized in determining the required volume control. See Infiltration Best Management Practices (BMPs) section below and refer to the Chicago Stormwater Management Manual for additional information.

Infiltration Best Management Practices (BMPs)

Green infrastructure promotes infiltration and evapotranspiration, and it reduces the volume of runoff that is discharged to the city sewer system. BMPs shall be incorporated into development to control rainwater-related impacts in the City of Chicago. Infiltration BMPs can reduce impervious areas and result in a reduction of the required detention volume. Infiltration BMP requirements are as follows:

  • If the sewer system is connected to a combined sewer, the high-water table must be at least three and a half feet below the infiltration surface. If the system is not in a combined sewer area, at least two feet below the bottom of all proposed infiltration BMPs shall be maintained.
  • Underlying soils at the point of infiltration must meet the minimum permeability 0.5 inch per hour. If 0.5 inch per hour cannot be met, BMPs may still be implemented with the use of underdrains.
  • Infiltration BMPs shall be placed in areas with maintenance access, and an Operations and Maintenance Plan (O&M Plan) shall be prepared and implemented by the school. Below is a list of CPS and DWM approved Infiltration BMPs:
  • Permeable Paving (Permeable Surfaces)
    Permeable pavement such as permeable asphalt, permeable pavers, synthetic turf, and permeable play surfaces can reduce the volume of rainwater runoff and pollutants discharged from a site. The subgrade is composed of an open-graded drainage aggregate to create a high permeability rate. The volume created in the void space in the drainage aggregate (porosity of 38%) can be utilized to meet the rate and volume control requirements.
  • Bioinfiltration Systems/Rain Gardens
    Bioinfiltration systems are at-grade facilities that collect rainwater from surrounding impervious areas and promote infiltration and evapotranspiration. These systems pass runoff through a shallow depressed area containing plants, mulch, and a prepared soil. Surface storage and void space in the drainage aggregate (porosity of 38%) and a prepared soil (porosity of 25%) base can be used for rate and volume control requirements. CPS allows a maximum of 12 inches of surface storage in biofiltration systems. The maintenance of these systems shall be discussed with CPS during the design phase.
  • Drainage Swales
    Swales, unlined ditches, and natural channels are superior to conventional storm sewers because they allow water infiltration, remove pollutants, and reduce the peak runoff rate. The void space in the drainage aggregate (porosity of 38%) and the prepared soil (porosity of 25%) base can be used for rate and volume control requirements. The minimum swale slope shall be 0.50%.
  • Infiltration Vaults
    Infiltration vaults are underground rainwater storage facilities with an open bottom, or perforations, to encourage infiltration in areas where permeable subsoil conditions are found. Infiltration vaults can be utilized for volume control and rate control; however, CPS prefers above ground naturalized infiltration facilities when feasible. Vaults vary in material but cast-in-place or precast concrete is required.

See Volume 3 Sections 1.4.5 Natural Amenities, 1.6 Site Materials, and 1.8 Landscaping for additional information.

Detention Systems

The rate control volume provided in detention systems shall be combined with infiltration BMP systems to the largest extent possible. Sometimes underground storage solutions are necessary to limit impacts to programmable open space. Underground storage systems can range from precast concrete vaults, oversized reinforced concrete pipe, oversized corrugated metal pipe (CMP), to prefabricated plastic chamber systems.

Common manufacturers / products that shall be explored include:

  • StormTrap SingleTrap or DoubleTrap concrete module system
  • Contech Engineered Solutions - CMP or ChamberMaxx system
  • Advanced Drainage Systems, Inc. - StormTech Chamber system
  • StormTank - Brentwood Module plastic module system

Erosion/ Stormwater Pollution Prevention Plan (SWPPP)

Temporary and permanent soil erosion and sedimentation control (SESC) measures shall be used to limit water pollution and soil loss. Common temporary SESC measures include silt fence, concrete washouts, stabilized construction entrances, inlet filters, and temporary seeding.

  • A SWPPP shall be created when the site discharges to local waters or city storm sewer but is not necessary when discharging to a combined sewer.
  • A SWPPP conveys the design intent of all SESC measures and inspection requirements to the contractor.
  • The SWPPP plan must conform to the erosion and sedimentation requirements of the 2017 U.S.

Environmental Protection Agency (EPA) Construction General Permit (CGP) or local equivalent, whichever is more stringent. The plan must also describe the measures implemented.

Refer to the Illinois Natural Resources Conservation Service Engineering Index for additional information.