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McHenry County panel reviews proposed stormwater exemption that ties small developments to existing‑vs‑proposed peak flows
Summary
The Technical Advisory Committee reviewed draft stormwater ordinance amendments that would let small projects avoid detention if new impervious area is limited (1 acre or up to 10%) and the increase in peak flow is ≤0.5 cubic feet per second; members asked staff to run critical‑duration examples and refine online tools and curve‑number tables.
McHenry County’s Technical Advisory Committee spent the bulk of its Dec. 9 meeting examining proposed changes to the county’s stormwater ordinance that would create a structured exemption from detention for certain small or low‑impact developments.
Staff told the committee the central exemption would apply where a site adds up to 1 acre of new impervious surface and the increase in peak runoff — comparing proposed to existing conditions — is no greater than 0.5 cubic feet per second (CFS), provided the project maintains existing drainage patterns. A secondary path under consideration would allow up to a 10% increase in impervious area on a parcel if that same 0.5 CFS limit is met.
The proposal uses a curve‑number approach adopted from standard engineering practice (SCS/TR‑55) to quantify existing versus proposed runoff and to give credit for green infrastructure or native‑plant vegetation that reduces the…
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