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…