What are Greenfield Runoff Rates?
Greenfield runoff rates represent the natural rate at which rainfall drains from undeveloped land. These rates are critical for sustainable drainage design (SuDS) to ensure new developments don't increase flood risk downstream.
This calculator uses the IH124 methodology (Report 124) to estimate peak greenfield runoff rates for various return periods (Q1, Q30, Q100). These values form the baseline discharge limits for your drainage strategy.
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Calculate Runoff Rates
Base Greenfield Rate
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QBAR (litres/second)
Mean annual flood peak flow
Return Period Runoff Rates
| Return Period | Growth Factor | Flow Rate (l/s) |
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Note: Q1 is typically used for water quality design, Q30 for sewer design checks, and Q100 (1 in 100 year + climate change) for major flood risk assessment.
Estimated Attenuation Storage
For 1 in 100 year storm + climate change allowance
Required Storage Volume
cubic metres (m³)
Inflow Volume
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m³
Outflow Volume
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m³
Storage Required
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m³
EA guidance Feb 2016, updated 2024
Post-development impermeable area ratio
💰 Typical Storage Solutions
Underground Crates
£60-80/m³
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Lined Ponds
£30-50/m³
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Permeable Paving
£40-60/m³
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⚠️ Important: This is an indicative estimate based on simplified methods. Detailed design requires full hydraulic modelling using software like MicroDrainage, Flow, or Causeway. Actual storage volumes may vary based on site conditions, drainage strategy, and LPA requirements.
Need detailed drainage design with full hydraulic modelling?
Get Expert Drainage Strategy from Aegaea →Site Summary
Location
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Site Area
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SAAR
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SOIL SPR
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