McHenry County technical committee weighs shifting stormwater release rates to impervious-area metric

McHenry County Stormwater Technical Advisory Committee members on Oct. 14 spent more than an hour discussing a proposal to change how the county calculates required stormwater "release rates," including whether to base allowable release on percent impervious area rather than the ordinance’s current static project-area method.

The proposal — presented by county staff during agenda item 5 — would scale allowable peak discharge to the amount of proposed impervious surface and give developers credit for green infrastructure and native-vegetation conversions. Staff said the approach could use adjustable reduction factors (an example used in the presentation was a 0.85 multiplier tied to a target curve number of about 40, plus a 0.3 factor for certain cleared-area changes) so that the allowable release rate “reacts” to proposed site design rather than remaining static.

Why it matters: The county’s stormwater release-rate standard determines whether a project must build detention basins, storm sewer upgrades or alternative best-management practices (BMPs). Changing the calculation affects subdivisions, commercial parcels, agricultural leases and large solar installations; it also interacts with MS4 permit reporting and long-term maintenance responsibilities.

Staff outlined the concept and examples. A staff engineer explained that under the current ordinance the required release rate is “static” — the same numeric target regardless of whether a project proposes 20% or 80% impervious cover — and said the proposed method would scale allowable flow based on existing and proposed curve numbers, impervious percentage and documented BMP reductions. The presentation included maps and a table showing sample allowable release rates for 20%, 50% and 80% impervious scenarios and noted the approach would still allow for downstream-capacity checks where known flooding or conveyance limits exist.

Committee members raised several implementation concerns. Jody McCarthy, a committee member, said she worried a countywide, impervious-based standard could “punish” commercial development unless the ordinance includes exemptions or a cap that defaults back to the current standard for very high-impervious sites. Another member said concentrating multiple discharge points into a single outfall — a pattern the current ordinance tried to avoid by checking downstream capacity — remains a practical problem even if the release rate method changes.

Several members pressed staff on agricultural and solar examples. Committee members said solar arrays often include narrow access drives and large areas of native-vegetation seeding (pollinator mixes) and that the right approach should allow credit for those vegetation benefits without forcing permanent detention constructions that would impede future agricultural use or decommissioning. Staff said the example runs showed that native-vegetation conversions reduce required detention volume but do not generally eliminate the need for any detention; they also said the code would need detailed definitions for terms such as “cleared area” or “hydrologically disturbed” to avoid unintended outcomes.

Implementation and enforcement questions dominated the discussion. Committee members asked how the county could monitor and verify long-term performance of distributed BMPs (native plantings, permeable pavers, grassed waterways), noting municipalities and homeowners’ associations already struggle to maintain conventional detention basins. Staff described plans to register BMPs in an asset-management system tied to GIS (staff named Cartograph and Cityworks as example platforms) and to require maintenance plans and periodic documentation (photos, checklists) from owners; staff also said they were exploring public-facing tools so applicants could run a simpletributary-area/curve-number check before filing.

Several practical drafting points were flagged for staff to return with at the next meeting: specific example cases (agriculture, a solar farm, a commercial site and a typical subdivision), clearer definitions of “cleared” or “hydrologically disturbed” areas (including whether forest-to-grass or wetland-to-grass conversions qualify), an approach for small sites that do not have modeling resources (a chart or online calculator), and an option to cap the impervious-based method so that, beyond some threshold, the ordinance would revert to the existing release-rate method or another fixed limit.

Direct quotes from the meeting included a staff participant saying, “I don't want to isolate any one development — I want this to be equal across the board,” and a committee member, Jody McCarthy, saying, “I’m a little concerned about changing it for all of them,” when arguing for narrowly tailored exemptions. Staff also told the group that the change would be intended to “provide greater flexibility while pursuing greater benefits from water quality, flood reduction and infiltration.”

Votes at a glance: The committee recorded two formal motions during the session. The minutes approval motion passed (moved and seconded on the record; unanimous “ayes”), and a separate motion to cancel the November meeting and instead hold the next regular meeting on Dec. 9 passed on a recorded voice vote.

Next steps: Staff was directed to bring back baked examples and draft ordinance language for further review at the December meeting, including (1) worked examples for agricultural, solar, commercial and residential/subdivision cases; (2) recommended definitions for cleared/hydrologically disturbed areas; (3) proposed caps or a hierarchy tying the new method to existing downstream-capacity checks; and (4) options for a user-facing calculator and GIS-based asset tracking for long-term BMP maintenance. The committee did not adopt any ordinance text at the Oct. 14 meeting.

The committee scheduled the next meeting for Dec. 9 to review the examples and draft language.