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Site Appraisal IH 124 · FSR QBAR

Greenfield Runoff Rate Calculator

IH 124 / FEH greenfield runoff rates for QBAR, Q1, Q30, Q100: the basis for greenfield runoff targets under Defra NSTS S2.

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Project & Site Details

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Range: 0.01 to 200 ha
IH124 is calibrated for catchments ≥ 0.5 km² (50 ha); below 50 ha the rate is linearly scaled from the 50 ha reference and should be treated as indicative
From FEH Web Service or Met Office. UK range: 500-3000 mm
SAAR typically ranges 500–3000 mm for UK sites (FEH)
Hydrological Parameters
FSR/WRAP SOIL index (used in the IH124 QBAR regression) — not SPR(HOST). Select from catchment-wide soil data.
From FEH dataset. UK average = 125
For greenfield: 0.00. Urban adjustment only for comparison purposes.
Indicative regional growth factors — confirm with FEH / ReFH for design
Calculation Inputs
Parameter Value Unit
Calculation Methodology

Step 1: Rural QBAR (IH124 QBAR regression)

QBAR_rural = 0.00108 × AREA^0.89 × SAAR^1.17 × SOIL^2.17

Step 2: Catchment Index (IH124 / FSR method)

CIND = 102.4 × SOIL + 0.28 × (CWI - 125)

Step 3: Continentality Factor (IH124 / FSR method)

For 500 ≤ SAAR ≤ 1100: NC = 0.92 - 0.00024 × SAAR
For 1100 < SAAR ≤ 3000: NC = 0.74 - 0.000082 × SAAR

Step 4: Urban Adjustment (IH124 / FSR method)

If URBAN > 0: QBAR_urban / QBAR_rural = (1 + URBAN)^(2×NC) × [1 + URBAN × {(21/CIND) - 0.3}]
Results: Design Discharge Rates
Return Period AEP Growth Factor Peak Flow (l/s) Specific Rate (l/s/ha)
Key Results Summary
QBAR (mean annual flood)
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1% AEP (100-yr)
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Defra NSTS S2 Greenfield Compliance Check
Requirement Check Status
Assumptions & Limitations
Key Assumptions:
  • SAAR and CWI values sourced from FEH Web Service
  • Soil classification based on FSR/WRAP SOIL index values
  • Regional growth factors are indicative — confirm with FEH / ReFH statistical analysis for design
  • IH124 is calibrated for catchments ≥ 0.5 km² (50 ha); smaller areas are linearly scaled from the 50 ha reference rate
  • Climate-change uplift is not added to greenfield rates (they are the natural limit)
  • Urban fraction = 0 for true greenfield areas
Important:

This calculator provides QBAR (mean annual flood) and design return periods for greenfield sites. Results must be verified by a qualified drainage engineer and validated against site-specific hydrological assessments.

Discharge limit (Defra Non-Statutory Technical Standards, S2):

For greenfield sites, peak discharge for the 1-in-1-year and 1-in-100-year events must not exceed the greenfield rate for the same event. There is no "1 l/s/ha" or "3 l/s/ha" minimum in the national standards; a practical minimum outflow of ~5 l/s is often adopted to avoid blockage-prone controls (engineering judgement).

Export PDF Report

Generate a professional PDF report containing all calculations, results, and compliance assessment. The report will include:

  • Project and site information
  • Input parameters and hydrological data
  • Detailed calculation methodology with equations
  • Design discharge rates for all return periods
  • Defra NSTS S2 greenfield compliance assessment
  • Assumptions and limitations
  • Professional footer and branding
Report Information
PDF Contents:

The generated report is suitable for submission to local planning authorities as part of FRA (Flood Risk Assessment) and SuDS design documentation. All calculations follow IH Report 124 (Marshall & Bayliss, 1994) and the Defra Non-Statutory Technical Standards for SuDS (2015).