Runoff Rate Calculation

Runoff Rate Calculator (Rational Method)

Estimate peak surface water runoff for drainage design

— Select Surface Type (Or enter manually below) — Asphalt / Concrete (Pavement) Roof Surfaces Gravel / Crushed Stone Suburban Residential (Lawns) Woodlands / Forest Parks / Flat Green Space

Calculation Result

Peak Runoff (Q): 0.00 cfs

(cfs = cubic feet per second)

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Understanding the Rational Method (Q = CiA)

The Rational Method is the most widely used formula for calculating peak surface water runoff for small drainage areas (typically under 200 acres). It is essential for civil engineers, urban planners, and landscape architects to size pipes, culverts, and storm drains correctly.

The Components of the Formula:

• Q (Peak Runoff Rate): The maximum volume of water flowing past a point, measured in Cubic Feet per Second (cfs).
• C (Runoff Coefficient): A dimensionless ratio representing the fraction of rainfall that becomes runoff. Impermeable surfaces like asphalt have high C values (0.90+), while permeable surfaces like forests have low values (0.15).
• i (Rainfall Intensity): The rate of rainfall in inches per hour. This is typically determined from IDF (Intensity-Duration-Frequency) curves based on the local climate and the "Time of Concentration."
• A (Drainage Area): The total surface area (in acres) that contributes water to the specific point being measured.

Common Runoff Coefficients (C Values)

Surface Type	Typical C Value
Pavement (Asphalt/Concrete)	0.70 – 0.95
Downtown Business Areas	0.70 – 0.95
Residential (Single Family)	0.30 – 0.50
Lawns, Sandy Soil (Flat)	0.05 – 0.10
Lawns, Heavy Soil (Steep)	0.25 – 0.35

Practical Example

Imagine you are designing a drainage system for a 2-acre shopping center parking lot (Asphalt, C = 0.90). If a 10-year storm event in your area produces a rainfall intensity of 4 inches per hour, the calculation would be:

Q = C × i × A
Q = 0.90 × 4.0 × 2.0
Q = 7.20 cfs

This means your storm drainage pipes must be able to handle a peak flow of 7.20 cubic feet of water every second to prevent flooding.

function calculateRunoff() { var cValue = parseFloat(document.getElementById('runoffC').value); var intensity = parseFloat(document.getElementById('intensity').value); var area = parseFloat(document.getElementById('area').value); var resultDiv = document.getElementById('runoffResult'); var peakFlowSpan = document.getElementById('peakFlow'); // Validation if (isNaN(cValue) || isNaN(intensity) || isNaN(area)) { alert('Please enter valid numerical values for all fields.'); return; } if (cValue 1) { alert('The Runoff Coefficient (C) usually ranges between 0.05 and 0.95.'); } // Calculation: Q = CiA var q = cValue * intensity * area; // Display peakFlowSpan.innerText = q.toFixed(3); resultDiv.style.display = 'block'; // Scroll to result resultDiv.scrollIntoView({ behavior: 'smooth', block: 'nearest' }); }