Estimate the peak water discharge flow rate for a specific area using the Rational Method ($Q = CiA$). This helps in sizing drainage pipes and channels.
Concrete / Asphalt (0.90)
Roofs (0.95)
Grass – Heavy Soil (0.35)
Grass – Sandy Soil (0.15)
Gravel / Packed Earth (0.70)
Residential Area – Mixed (0.50)
Industrial Area (0.80)
Agricultural Land (0.30)
Check local meteorological data for a 10, 25, or 100-year storm event.
Estimated Peak Discharge
Cubic Feet per Second (CFS):0.00
Gallons per Minute (GPM):0.00
Liters per Second (L/s):0.00
Calculation Basis: Based on the Rational Method equation $Q = CiA$, where $Q$ is flow rate, $C$ is the runoff coefficient, $i$ is rainfall intensity, and $A$ is the area in acres.
Understanding Drainage Rates and Runoff
Calculating the drainage rate is a critical step in civil engineering, landscaping, and home construction. It determines how much water your drainage system (gutters, french drains, storm sewers) needs to handle during a rainstorm to prevent flooding.
The Rational Method ($Q = CiA$)
This calculator uses the Rational Method, which is the most common formula for estimating peak runoff from small drainage areas.
C (Coefficient): A dimensionless number representing the surface type. Impervious surfaces like concrete have a high coefficient (near 0.9), meaning 90% of rain runs off. Porous surfaces like sandy grass have low coefficients (0.15), meaning most water soaks in.
i (Intensity): The rate of rainfall in inches per hour. This varies by geographic location and the severity of the storm event (e.g., a "100-year flood" intensity).
A (Area): The size of the land collecting water. While often measured in acres for large projects, our calculator accepts square feet for residential ease.
Why Calculate Flow Rate?
Knowing the Peak Discharge in Gallons per Minute (GPM) or Cubic Feet per Second (CFS) allows you to:
Size Pipes Correctly: A 4-inch PVC pipe handles significantly less water than a 6-inch pipe. If your runoff exceeds the pipe's capacity, water will back up.
Design French Drains: Ensure your trench and aggregate volume can handle the inflow during heavy storms.
Install Sump Pumps: Select a pump with a horsepower rating capable of moving the calculated GPM volume.
Typical Rainfall Intensities
Rainfall intensity varies drastically. For a standard heavy rain, you might use 1 to 2 inches per hour. for extreme storm planning (flash flooding), values can reach 4 to 8 inches per hour depending on your region. Always consult local building codes or NOAA data for precise design criteria.
function calculateRunoff() {
// 1. Get input values
var surfaceC = parseFloat(document.getElementById('surfaceType').value);
var areaSqFt = parseFloat(document.getElementById('drainageArea').value);
var rainIntensity = parseFloat(document.getElementById('rainfallIntensity').value);
// 2. Validate inputs
if (isNaN(surfaceC) || isNaN(areaSqFt) || isNaN(rainIntensity)) {
alert("Please enter valid numbers for Area and Rainfall Intensity.");
return;
}
if (areaSqFt <= 0 || rainIntensity < 0) {
alert("Area and Rainfall Intensity must be positive numbers.");
return;
}
// 3. Perform Calculations
// Convert Square Feet to Acres (1 Acre = 43,560 sq ft)
var areaAcres = areaSqFt / 43560;
// Rational Method Formula: Q (cfs) = C * i * A
// Note: The conversion factor 1.008 is usually dropped in estimation, making it Q = CiA
var qCFS = surfaceC * rainIntensity * areaAcres;
// Convert CFS to Gallons per Minute (1 CFS = 448.831 GPM)
var qGPM = qCFS * 448.831;
// Convert CFS to Liters per Second (1 CFS = 28.3168 L/s)
var qLPS = qCFS * 28.3168;
// 4. Update UI
document.getElementById('resCFS').innerHTML = qCFS.toFixed(3);
document.getElementById('resGPM').innerHTML = qGPM.toFixed(2);
document.getElementById('resLPS').innerHTML = qLPS.toFixed(2);
// Show results box
document.getElementById('drainageResults').style.display = 'block';
}