— Select Surface Type —
Concrete/Asphalt (0.70 – 0.95)
Roofs (0.75 – 0.95)
Gravel (0.40 – 0.60)
Grass/Lawns – Heavy Soil (0.15 – 0.35)
Grass/Lawns – Sandy Soil (0.05 – 0.25)
Commercial Area (0.50 – 0.95)
Residential – High Density (0.40 – 0.75)
Residential – Low Density (0.25 – 0.45)
Agricultural/Farmland (0.05 – 0.60)
Dimensionless value between 0 and 1.
inches per hour
Acres
Estimated Peak Runoff Rate (Q)
0.00
cubic feet per second (cfs)
function toggleRunoffUnits() {
var radios = document.getElementsByName('units');
var selectedUnit = 'imperial';
for (var i = 0; i < radios.length; i++) {
if (radios[i].checked) {
selectedUnit = radios[i].value;
break;
}
}
var labelIntensity = document.getElementById('labelIntensity');
var unitIntensity = document.getElementById('unitIntensity');
var labelArea = document.getElementById('labelArea');
var unitArea = document.getElementById('unitArea');
var resultUnit = document.getElementById('resultUnit');
if (selectedUnit === 'imperial') {
unitIntensity.innerHTML = 'inches per hour (in/hr)';
unitArea.innerHTML = 'Acres';
resultUnit.innerHTML = 'cubic feet per second (cfs)';
} else {
unitIntensity.innerHTML = 'millimeters per hour (mm/hr)';
unitArea.innerHTML = 'Hectares';
resultUnit.innerHTML = 'cubic meters per second (m³/s)';
}
// Clear results when switching units to avoid confusion
document.getElementById('resultDisplay').style.display = 'none';
}
function updateCoefficient() {
var dropdown = document.getElementById('surfaceType');
var cInput = document.getElementById('runoffC');
if (dropdown.value) {
cInput.value = dropdown.value;
}
}
function calculateRunoff() {
// 1. Get input values
var c = parseFloat(document.getElementById('runoffC').value);
var i = parseFloat(document.getElementById('intensity').value);
var a = parseFloat(document.getElementById('drainageArea').value);
var errorDisplay = document.getElementById('errorDisplay');
var resultDisplay = document.getElementById('resultDisplay');
var resultValue = document.getElementById('resultValue');
// 2. Validate inputs
errorDisplay.style.display = 'none';
resultDisplay.style.display = 'none';
if (isNaN(c) || isNaN(i) || isNaN(a)) {
errorDisplay.innerHTML = "Please enter valid numbers for all fields.";
errorDisplay.style.display = 'block';
return;
}
if (c 1) {
errorDisplay.innerHTML = "Runoff Coefficient (C) must be between 0 and 1.";
errorDisplay.style.display = 'block';
return;
}
if (i < 0 || a < 0) {
errorDisplay.innerHTML = "Rainfall intensity and Area must be positive numbers.";
errorDisplay.style.display = 'block';
return;
}
// 3. Determine units and calculate
var radios = document.getElementsByName('units');
var selectedUnit = 'imperial';
for (var k = 0; k < radios.length; k++) {
if (radios[k].checked) {
selectedUnit = radios[k].value;
break;
}
}
var q = 0;
if (selectedUnit === 'imperial') {
// Formula: Q = C * I * A
// Q in cfs, I in in/hr, A in acres
// Note: 1.008 is the exact conversion, but 1.0 is standard for the Rational Method approximation.
q = c * i * a;
} else {
// Formula: Q = (C * I * A) / 360
// Q in m³/s, I in mm/hr, A in hectares
q = (c * i * a) / 360;
}
// 4. Display result
resultValue.innerHTML = q.toFixed(4);
resultDisplay.style.display = 'block';
}
How to Calculate Runoff Rate: The Rational Method
Understanding how to calculate runoff rate is essential for civil engineers, landscape architects, and urban planners. The runoff rate determines how much water flows off a surface during a storm event, which dictates the design of drainage systems, stormwater pipes, and retention ponds.
The most common formula used for estimating peak runoff from small drainage areas (typically less than 200 acres) is the Rational Method.
The Rational Method Formula
The Rational Method equation is simple yet powerful:
Q = C × I × A
Where:
Q = Peak Runoff Rate (Discharge).
C = Runoff Coefficient (a dimensionless factor between 0 and 1).
I = Rainfall Intensity (measure of rain severity over time).
A = Drainage Area (size of the catchment area).
Units of Measurement
Depending on your location, you will use either Imperial or Metric units. The formula varies slightly to account for unit conversion factors.
Imperial Units (US Customary)
Q is measured in cubic feet per second (cfs).
I is measured in inches per hour (in/hr).
A is measured in acres.
Note: The conversion factor is effectively 1.008, but standard practice allows using 1.0 for estimation.
Metric Units (SI)
Q is measured in cubic meters per second (m³/s).
I is measured in millimeters per hour (mm/hr).
A is measured in hectares.
Formula Modification: Q = (CIA) / 360.
Understanding the Runoff Coefficient (C)
The Runoff Coefficient (C) represents the percentage of rainfall that becomes runoff rather than soaking into the ground (infiltration) or evaporating. A higher C value means more runoff.
Surface Type
Runoff Coefficient (C)
Concrete / Asphalt
0.70 – 0.95
Residential (Single Family)
0.30 – 0.50
Industrial Areas
0.50 – 0.90
Parks / Lawns (Sandy Soil)
0.05 – 0.25
Parks / Lawns (Clay Soil)
0.15 – 0.35
Forest / Woodlands
0.05 – 0.25
Steps to Calculate Runoff Rate
Determine the Drainage Area (A): Survey the land to calculate the surface area in acres or hectares.
Select the Runoff Coefficient (C): Identify the surface material (grass, pavement, roof). If the area has mixed surfaces, calculate a weighted average for C.
Determine Rainfall Intensity (I): Consult local Intensity-Duration-Frequency (IDF) curves. You need to know the "Time of Concentration" (time it takes for water to flow from the furthest point to the outlet) to pick the correct intensity.
Apply the Formula: Multiply the three values (and divide by 360 if using metric) to get your peak discharge.
Frequently Asked Questions
What is a weighted Runoff Coefficient?
If your drainage area consists of 50% concrete (C=0.9) and 50% grass (C=0.3), you cannot just pick one. You calculate the weighted average: (0.5 × 0.9) + (0.5 × 0.3) = 0.45 + 0.15 = 0.60.
Why is Rainfall Intensity important?
Rainfall intensity varies based on the storm's duration and return period (e.g., a "100-year storm"). Shorter, more violent storms have higher intensity (I) and generate higher peak runoff rates than long, drizzling rains.
When should I NOT use the Rational Method?
The Rational Method is an approximation best suited for small catchments (typically under 200 acres). For large watersheds or complex hydrological modeling involving flood routing or storage, more advanced methods like the TR-55 or Hydrographs should be used.