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McHenry County panel debates raising impervious-area exemption, advances plan for a ‘simplified method’ tool
Summary
County staff presented analyses showing why keeping a half-acre trigger may be safer than a blanket 1-acre exemption and proposed a GIS-based "simplified method" to let landowners estimate runoff without routine engineering. The committee asked staff to refine curve-number tables and bring a draft tool back for review.
Staff member (S1) presented a draft approach to McHenry County’s stormwater ordinance on March 5, proposing a county "simplified method" and asking whether the current half-acre exemption for impervious-area review should be raised to 1 acre for large agricultural parcels.
The staff member framed the discussion around two example analyses — a 2-acre tributary example and a 77-acre parcel — and said the simplified method would let users derive a time of concentration and a rational-method C value from conservative curve-number choices. "The half acre is probably best," the staff member said, arguing that a blanket 1-acre exemption could capture more storm durations and trigger additional flood-risk analysis.
Committee members pressed technical points about converting TR-55 curve numbers into rational-method C-values and whether those conversions are consistent across soil types…
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