Greenfield Runoff Rate Calculator

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Greenfield Runoff Rate Calculator

Estimate natural surface water drainage rates using the IH124 method.

Type 1: Very Permeable (0.15) Type 2: Permeable (0.30) Type 3: Moderately Permeable (0.40) Type 4: Slightly Permeable (0.45) Type 5: Impermeable (0.50)
Q1 (1 Year Event) QBAR (Mean Annual Flood) Q30 (30 Year Event) Q100 (100 Year Event)
Estimated Greenfield Runoff Rate:
0.00 l/s
0.00 l/s per hectare

Understanding Greenfield Runoff Rates

The Greenfield Runoff Rate is the peak rate of water flow from a site in its natural, undeveloped state during a specific rainfall event. For civil engineers and developers, calculating this is a critical step in designing Sustainable Drainage Systems (SuDS).

Why Calculate Greenfield Runoff?

When land is developed, natural soil is replaced with impermeable surfaces like concrete and tarmac. This significantly increases surface water runoff. Local authorities typically require that post-development runoff rates do not exceed the original greenfield rates to prevent downstream flooding. This calculator uses the IH124 method, which is the standard benchmark for small catchments in the UK and Ireland.

Key Factors in the Calculation:

  • Site Area: The total size of the development in hectares. If the site is smaller than 50 hectares, the industry standard is to calculate for 50ha and scale the result down linearly.
  • SAAR: Standard Average Annual Rainfall (measured in mm). This accounts for regional climate variations.
  • SOIL Index: A value representing the permeability of the ground. Values range from 0.15 (sandy, well-drained) to 0.50 (heavy clay, impermeable).
  • Return Period: The statistical frequency of a storm event. QBAR represents the average annual peak flow, while Q100 represents a severe storm expected once every 100 years.

Example Calculation

Imagine a 2-hectare site in a moderately rainy area (SAAR 850mm) with Type 4 soil (0.45). To find the 100-year peak runoff:

  1. Calculate $Q_{BAR}$ using the site characteristics.
  2. Apply the 100-year growth factor (typically 3.19).
  3. If $Q_{BAR}$ is 4.2 l/s per hectare, the total 100-year runoff for 2 hectares would be approximately 26.8 l/s.
function calculateRunoff() { var area = parseFloat(document.getElementById('siteArea').value); var saar = parseFloat(document.getElementById('saar').value); var soil = parseFloat(document.getElementById('soilIndex').value); var factor = parseFloat(document.getElementById('returnPeriod').value); if (isNaN(area) || isNaN(saar) || isNaN(soil) || area <= 0) { alert("Please enter valid positive numbers for all fields."); return; } // IH124 Formula: QBAR = 0.00108 * (AREA^0.89) * (SAAR^1.17) * (SOIL^2.17) // AREA in formula is in km2. 1 hectare = 0.01 km2. // However, for sites < 50ha, standard practice is calculate for 50ha and interpolate. var calcArea = area; var scaleDown = 1.0; if (area < 50) { calcArea = 50; scaleDown = area / 50; } // Area in km2 for the formula var areaKm2 = calcArea / 100; // QBAR in m3/s var qbar = 0.00108 * Math.pow(areaKm2, 0.89) * Math.pow(saar, 1.17) * Math.pow(soil, 2.17); // Convert m3/s to l/s var qbarLs = qbar * 1000; // Apply scaling for small sites var scaledQbar = qbarLs * scaleDown; // Apply Return Period Factor var finalRate = scaledQbar * factor; var unitRate = finalRate / area; // Display Results document.getElementById('resultContainer').style.display = 'block'; document.getElementById('runoffValue').innerHTML = finalRate.toFixed(2) + " l/s"; document.getElementById('unitRate').innerHTML = unitRate.toFixed(2) + " l/s per hectare"; }

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