Mount Prospect, IL Environmental Rules: Stormwater Management (2026)
Key Facts
- Design storm for detention
- 100-year rainfall, any and all durations
- Maximum release rate
- No greater than 0.20 cfs per acre
- Minimum restrictor size
- Two inch diameter
- Basin freeboard
- Minimum of one foot
- Parking lot ponding depth
- Maximum of twelve inches
- Fee in lieu limit
- Projects under one acre and single-family work only
Summary
In the Village of Mount Prospect, every commercial, industrial and multi-family development must include storm water holding facilities, whatever its size. Detention basins must store the runoff of a 100-year rainfall, the release rate is capped at 0.20 cfs per acre, and detention is barred from the side and rear yards of single-family properties.
A. General: All commercial, industrial and multi-family zoned developments, regardless of size, shall include provisions for storm water holding facilities as required in chapter 15 of this code. This storage may be provided in detention ponds, retention ponds, ponding on parking lots, and in underground pipes and storage chambers. Storm water detention shall not be permitted in the side or rear yards of any single-family residential properties. B. Required Storage Volume: All detention basins shall be designed in accordance with the requirements of the MWRD and this code. The detention volume required shall be that necessary to store the runoff of a 100-year rainfall, for any and all durations, from the fully developed drainage area tributary to the reservoir, less that volume discharged during the same duration at the approved release rate. 1. The maximum allowable release rate of storm water runoff generated at a site shall be restricted to: a. The runoff rate from the subject area in its natural undeveloped state. b. Such release rate shall be based on a runoff rate of no greater than 0.20 cfs per acre.
Full Breakdown
Article VI of Chapter 16 of the Mount Prospect Village Code governs storm sewer and drainage. Section 16.601 requires every development, public or private, to build a storm drainage system of storm sewers, drainage swales, or detention basins that stays separate and independent of the sanitary sewer system. The system has to handle runoff from the site itself and from all areas that naturally flow to it.
Section 16.602 sets the design method. Sewers, streams and channels are sized with the rational method (Q = C x i x A) and Manning's formula. Storm sewers and channels use the 10-year storm curves, while streams and channels use the 100-year curves. The maximum time of concentration to a storm sewer inlet is twenty minutes, and public storm sewers cannot be smaller than twelve inches in diameter. Design velocities for storm sewers run from a minimum of two fps to a maximum of ten fps.
Section 16.603 covers detention itself. The quoted release-rate cap of 0.20 cfs per acre applies alongside the natural undeveloped runoff rate. Runoff coefficients for 100-year storms are 0.95 for impervious areas and 0.50 for pervious areas. The minimum restrictor size is a two inch diameter. Dry-bottom basin side slopes cannot be steeper than four to one, the basin floor needs a slope of not less than two percent, and every basin must provide a minimum of one foot of freeboard. Where the public health and safety dictates, a wet-bottom basin gets a fence of at least four feet.
Parking lots can serve as storage, but water ponding on pavement cannot exceed twelve inches in depth, the plan must designate the area, and no future pavement overlays are allowed there. The property owner must also send the Village a letter accepting all liability for injury or damage from the ponding water.
Section 16.604 requires new public storm sewers to be TV-inspected by the director of public works, and private storm sewers must be televised by the contractor before a certificate of occupancy issues. Section 16.605 requires compensatory storage at one and one-half to one for fill in floodplains. Section 16.606 allows a fee in lieu of detention only for projects under one acre and for single-family homes, additions and garages, and only after the village engineer finds detention ineffective or impractical.
Violations & Fines
Section 16.107 makes the director of community development the enforcing officer for Chapter 16, except for portions assigned to the director of public works, fire chief and village engineer. A person who violates or refuses to comply with the chapter is fined an amount set in appendix A, division III of the Village Code, and each day the violation exists is a separate offense. Owners, contractors, architects and agents who take part can each be found guilty. Section 23.1403 A also declares it a nuisance to obstruct the natural drainage on public or private property.
Frequently Asked Questions
Does a small commercial building in Mount Prospect still need detention?
Can I put a detention basin in the side or rear yard of my single-family lot?
How much water can a Mount Prospect site release downstream?
Who enforces the storm water rules?
Sources & Official References
Other rules in Mount Prospect
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