About This Tool

Overview

The Huff Distribution Flow Calculator is a web-based tool designed to support stormwater management in McHenry County, Illinois. It calculates peak discharge using the SCS Curve Number method and Huff temporal distributions to determine whether proposed developments require stormwater detention.

This tool implements McHenry County's innovative approach to stormwater exemption analysis, which uses conservative assumptions for proposed conditions to account for potential soil degradation during construction.

Methodology
SCS Curve Number Method

Runoff depth is calculated using the SCS (Soil Conservation Service) Curve Number method:

Q = (P - Ia)² / (P - Ia + S)
where:
Ia = 0.2 * S (initial abstraction)
S = (1000/CN) - 10 (potential maximum retention)
Time of Concentration

Time of concentration is calculated using the Kerby-Kirpich method:

  • Kerby equation for overland flow
  • Kirpich equation for channel flow
  • Minimum Tc = 5 minutes
Peak Discharge

Peak discharge is estimated using the SCS Unit Hydrograph method:

qp = 484 * A / Tp
where A = area (sq mi), Tp = time to peak (hours)
Huff Temporal Distributions

Rainfall is distributed over the storm duration using Huff quartile distributions from ISWS Bulletin 75, Table 26. The quartile is selected based on storm duration:

  • ≤ 6 hours: 1st Quartile
  • 6 - 12 hours: 2nd Quartile
  • 12 - 24 hours: 3rd Quartile (default for design)
  • > 24 hours: 4th Quartile
McHenry County Methodology

McHenry County uses an innovative approach to stormwater exemption analysis that differs from standard practice:

Curve Number Assignment
Existing Conditions: Use Hydrologic Soil Group B
Proposed Conditions: Use Hydrologic Soil Group D (conservative)
Exemption Thresholds
Pipe discharge: < 1.0 CFS increase = exempt
Swale discharge: < 2.5 CFS increase = exempt
Flow Reduction Factors (when detention required)
  • Impervious area added: 0.85 cfs/acre
  • Cleared area (woods/wetlands): 0.30 cfs/acre
Note: This is an innovative approach specific to McHenry County. The conservative use of Soil Group D for proposed conditions accounts for potential soil compaction and degradation during construction activities.
Data Sources
Primary Source

ISWS Bulletin 75 - "Precipitation Frequency for Illinois: An Update for Changing Conditions" (March 2020)

  • Tables 6-15: Precipitation frequencies by section
  • Table 7: Northeast Section (includes McHenry County)
  • Tables 26-28: Huff temporal distributions
Reference Documents
  • USDA NRCS TR-55: Urban Hydrology for Small Watersheds
  • Huff, F.A. (1967): Time Distribution of Rainfall in Heavy Storms
Important: These frequency estimates are based on ISWS Bulletin 75 (2020) using data through 2017. Given the non-stationary nature of precipitation, these estimates should be periodically reviewed and updated as recommended in Bulletin 75.
Version Information
Tool Version: 1.0.0 (Static)
Data Version: ISWS Bulletin 75 (March 2020)
Representative Year: 2017
Region: Northeast Illinois (Section 2)