(B) Applicability of site runoff storage requirements (detention). All developments shall comply with the site runoff storage requirements provided in § 164.023 of this subchapter in which:
(1) Manufactured home parks are to be constructed on a parcel larger than one acre in size;
(2) Multi-family homes are to be constructed on a parcel larger than one acre in size;
(3) A parcel is being subdivided into single- family residential lots; and
(4) Nonresidential development.
(C) Exemptions from site runoff storage requirements (detention). Site runoff storage is not required under the following circumstances:
(1) Direct discharge industrial sites where appropriate BMPs are utilized;
(2) Non-industrial direct discharge sites 160 acres or less having the following minimum river frontage:
Site Area | Required Frontage |
0—2 acres | 50 feet |
Up to 5 acres | 100 feet |
Up to 10 acres | 150 feet |
Up to 40 acres | 200 feet |
Up to 80 acres | 350 feet |
Up to 160 acres | 500 feet |
(3) The development is of a nonresidential nature on an existing developed parcel, where new impervious surfaces do not exceed 25,000 square feet in aggregate provided that the design does not further concentrate stormwater flows from the existing condition, and the design provides appropriate best management practices on site;
(4) The development is of a nonresidential nature on an existing agricultural parcel, where new impervious surfaces do not exceed 25,000 square feet in aggregate, which are not exempted under § 164.024;
(5) Roadway developments in rights-of-way under the ownership or control of a unit of local governments when the contiguous area of new roadway construction (excluding previously paved areas) does not exceeds two acres;
(6) Minor subdivision of a single-family residential zoned parcel;
(7) Construction of a single-family residence, an addition to an existing single-family residence, and/or construction of an accessory structure where the cumulative impervious area does not exceed 25,000 square feet on an existing legal lot or parcel;
(8) Single-family structures on legally created agricultural parcels;
(9) Bike trails, pedestrian trails and multi-purpose trails on park district or forest preserve district property when the trail is designed so as to not further concentrate stormwater flows from the existing condition, and when it can be shown by calculations, as required under § 164.023(B), that there will be no net increase in stormwater discharge rates from the proposed construction during the two-year and 100- year rainfall events. BMPs can be utilized to help mitigate discharge rates. In addition, a trail shall not be designed, nor constructed in a manner that could result in impoundment of stormwater onto another property. The trails shall not exceed 12 feet in width to be exempt from the site runoff storage requirements; and/or
(10) A parcel being subdivided into single-family residential lots having a minimum lot size of two and one-half acres or greater. Provided that the exempt subdivision is designed so as to not further concentrate stormwater flows from the existing condition, and when it can be shown that there will be no net increase in stormwater discharge rates from the proposed construction during the two-year and 100- year rainfall events. BMPs can be utilized to help mitigate discharge rates (i.e., roadside ditches).
(11) In solar farm developments, the solar panels will be considered “disconnected” and therefore pervious, if the following criteria are met:
(a) Groundcover vegetation must be maintained in good condition in those areas receiving disconnected runoff, specifically the area under and between the rows of panels. Typically this maintenance is no different than other lawn or landscaped areas. However, care should be taken to avoid soil compaction in these areas.
(b) The total proposed impervious area on the surface of the ground must be less than 25,000 square feet. The total impervious must include any gravel, pavement, asphalt, concrete, panel posts, and so on.
(c) The open space between panels must be equal to or greater than the width of the row of panels. The runoff must sheet flow onto and across the vegetated area to maintain the disconnection. Sheet flow is optimal on sites with gradual slopes (<5%). Level spreaders, terraces, or berms may be used to maintain sheet flow conditions if the average slope is steeper than 5% and <10%. Installations on slopes >10% will require a detailed engineering study to demonstrate the safe and non-erosive conveyance of runoff to the property line or downstream storm water management practice.
(d) For permitting purposes the entire project area is counted toward disturbed area.
(Ord. 10-164, passed 6-17-2010; Ord. 22-24, passed 2-25-2022)