Huff Distribution Flow Calculator

McHenry County Stormwater Exemption Analysis Tool

Calculate peak discharge and determine detention requirements using ISWS Bulletin 75 data and SCS methodology.

Quick Exemption Analysis

Compare existing vs. proposed conditions to determine if detention is required based on McHenry County thresholds.

  • Pipe threshold: <1.0 CFS = exempt
  • Swale threshold: <2.5 CFS = exempt
  • McHenry County CN methodology
  • PDF report generation
Start Analysis

Data Lookup

Look up precipitation frequencies, curve numbers, and other reference data from ISWS Bulletin 75.

  • Precipitation frequencies (Table 7)
  • SCS Curve Numbers
  • Huff distributions (Table 26)
  • Time of concentration calculator
Lookup Data
McHenry County Methodology
Curve Number Approach
  • Existing conditions: Use Soil Group B (actual)
  • Proposed conditions: Use Soil Group D (conservative)
Flow Reduction Factors
  • Impervious added: 0.85 cfs/acre
  • Cleared area: 0.30 cfs/acre

This is an innovative approach specific to McHenry County. The conservative use of Soil Group D for proposed conditions accounts for potential soil degradation during construction.

Data Source: This tool uses precipitation frequency data from ISWS Bulletin 75 (March 2020), Table 7 for the Northeast Section of Illinois. These estimates represent conditions around 2017 and should be periodically reviewed as recommended in Bulletin 75.