The Peak Runoff Rate represents the maximum volume of water flowing past a specific point per unit of time during a storm event. This calculation is essential for civil engineers and hydrologists when designing stormwater infrastructure like culverts, ditches, and retention ponds.
The Rational Method Formula
This calculator utilizes the widely accepted Rational Method formula:
Q = C × i × A
Q: Peak discharge (cfs – cubic feet per second)
C: Runoff Coefficient (dimensionless ratio)
i: Average rainfall intensity (inches per hour)
A: Drainage area (acres)
Choosing the Right Runoff Coefficient (C)
The runoff coefficient represents the fraction of rainfall that becomes surface runoff. A higher value indicates more impervious surfaces (like pavement), while a lower value indicates permeable soil and vegetation.
Surface Type
Typical C Value
Concrete / Asphalt
0.70 – 0.95
Industrial / Commercial
0.50 – 0.90
Residential (Flat)
0.25 – 0.40
Lawns / Parks
0.05 – 0.25
Example Calculation
If you have a 2-acre commercial parking lot (C = 0.85) and the design storm rainfall intensity is 3 inches per hour, the calculation would be:
Q = 0.85 × 3 × 2 = 5.1 cfs
document.getElementById('runoffCoefficient').onchange = function() {
var customContainer = document.getElementById('customValueContainer');
if (this.value === 'custom') {
customContainer.style.display = 'block';
} else {
customContainer.style.display = 'none';
}
};
function calculateRunoff() {
var area = parseFloat(document.getElementById('drainageArea').value);
var intensity = parseFloat(document.getElementById('rainfallIntensity').value);
var coefficientSelect = document.getElementById('runoffCoefficient').value;
var cValue;
if (coefficientSelect === 'custom') {
cValue = parseFloat(document.getElementById('customC').value);
} else {
cValue = parseFloat(coefficientSelect);
}
var resultBox = document.getElementById('runoffResult');
var peakRateDisplay = document.getElementById('peakRateVal');
if (isNaN(area) || isNaN(intensity) || isNaN(cValue) || area <= 0 || intensity <= 0 || cValue <= 0) {
alert('Please enter valid positive numeric values for all fields.');
resultBox.style.display = 'none';
return;
}
// The Rational Method: Q = CiA
// In US Customary units: 1.008 * C * i * A is the precise math,
// but Q = CiA is the standard approximation (1 cfs is approx 1 acre-inch per hour)
var peakDischarge = cValue * intensity * area;
peakRateDisplay.innerText = peakDischarge.toLocaleString(undefined, {
minimumFractionDigits: 2,
maximumFractionDigits: 2
});
resultBox.style.display = 'block';
resultBox.scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}