Elmhurst, IL Environmental Rules: Stormwater Management (2026)

The DuPage County Countywide Stormwater and Floodplain Ordinance is enacted under 55 ILCS 5/5-1062, the Illinois statute that authorizes certain counties (DuPage, Cook, Will, Lake, Kane, McHenry, and others) to adopt a single, uniform stormwater ordinance binding on all municipalities within their borders. After the disastrous floods of August 1987 and 1991, the DuPage County Board adopted the ordinance in 1992; it has been amended multiple times since, and is codified as Chapter 15 of the DuPage County Code. The ordinance regulates land disturbance, detention/retention, the regulatory floodplain (mapped under FEMA FIRMs and DuPage's own detailed studies), Isolated Waters of DuPage County (iwDPC) and other wetlands, MS4 stormwater discharges under the Illinois EPA NPDES Phase II permit, and soil erosion and sediment control.

Site runoff storage is required whenever a development creates 25,000 square feet or more of net new impervious area. The maximum allowable release rate from the detention facility is 0.10 cfs multiplied by the number of acres disturbed by the development, evaluated against pre-development peak discharges for the 2-year, 24-hour and 100-year, 24-hour critical-duration storms (Illinois State Water Survey Bulletin 75 design rainfall). All construction within the regulatory floodplain requires a stormwater permit, and "Special Management Areas" — riparian environments, wetlands, and high-quality aquatic resources — receive enhanced protection. Either DuPage County Stormwater Management (DCSM) or a "certified community" (a municipality DCSM has authorized to issue its own stormwater permits) administers permits; DCSM remains the ultimate enforcement authority for the ordinance countywide.