County staff proposes simplified stormwater exemption method, tighter release-rate focus

McHenry County staff presented a draft overhaul to the county Stormwater Management Ordinance on a proposed simplified exemption method that would change how developers qualify for detention waivers and how allowable release rates are calculated. The proposal would align a bright-line threshold from 20,000 square feet to a half acre and allow a simplified exemption for up to a 10% increase in impervious area if downstream impacts remain within prescribed limits.

The draft replaces a narrow "no flood damage" test with a "minimal flood risk" standard and requires projects to maintain existing drainage patterns and discharge to a defined channel. "We're going to change the 20,000 square foot to a half acre," staff said during the presentation, noting that most other ordinance language uses acres. The simplified pathway uses standard downstream assumptions — for example, a 24-inch pipe at a half-percent slope or a 3-foot-wide ditch with 3:1 side slopes — and bench marks flow increases at about 1.1 cubic feet per second for the pipe scenario and about 2.25 cfs for the swale scenario; projects that exceed those thresholds would need detention or a site-specific engineer model to substantiate an exemption.

Staff framed the approach as an attempt to compare existing versus proposed conditions rather than rely on static release numbers written decades ago. The method uses established engineering tools (TR-55 curve-number tables and time-of-concentration calculations) and — if implemented — a GIS-driven, point-and-click interface that would automatically populate inputs for nonengineer users. Staff said the tool could let a homeowner, designer or consultant assess whether a site is trending toward exemption or detention without immediate engineer work.

Commissioners and members raised several practical concerns. In rural areas reliant on drain tile, staff said an engineer would need to document tile size and function for the simplified method to apply. On solar farms, staff said they generally prefer to avoid large detention basins so land could revert to agriculture in the future and noted that panels with native vegetation under them are more likely to be treated as pervious — an approach not yet codified in the draft. Committee members repeatedly asked how the ordinance could address battery-storage facilities and fire risk; one member said, "when they have a fire on these things, they basically can't put them out," and staff described ongoing conversations about containment systems, liners and potential canopy concepts to limit runoff to contaminated areas.

Staff also showed worked examples to illustrate when the simplified test would flag the need for detention and when an engineering model could demonstrate negligible downstream impact. The proposal would shift emphasis from a static per-acre release rate to reductions tied to impervious-area increases (staff used placeholder reduction values of roughly 0.85 cfs per impervious acre and 0.3 cfs per cleared acre), a design intended to incentivize green infrastructure such as buffer strips, rain gardens and rain harvesting over traditional detention basins.

Staff said the draft is an early, rough version that will be refined with technical advisory committee (TAC) feedback and additional modeling. The next procedural steps, as described in the meeting, are continued TAC review, more scenarios to test the 0.1-foot (one-tenth foot) allowance in flood-hazard areas, and follow-up versions brought back to the committee for comment.