Drainage Flow Calculator

About the Drainage Flow Calculator

Estimates peak stormwater runoff with the Rational Method from the runoff coefficient, rainfall intensity and catchment area.

How to use this calculator

1. Enter the runoff coefficient, or use the value that matches your surface type.
2. Enter the rainfall intensity for the storm you want to study.
3. Enter the drainage area in the correct units, usually acres or hectares depending on the calculator.
4. Click calculate to compute the runoff flow rate.

The formula explained

This calculator uses the Rational Method, written as \(Q = C i A\), to estimate peak runoff. The result is the flow rate generated by rainfall over a drainage area after accounting for how much water becomes runoff.

• Q = peak runoff flow rate
• C = runoff coefficient, a number from \(0\) to \(1\) that represents how much rain becomes runoff
• i = rainfall intensity
• A = drainage area

Step by step method

1. Choose a runoff coefficient \(C\) based on land cover, such as pavement, grass, or mixed ground.
2. Find the rainfall intensity \(i\) for the design storm and the chosen time of concentration.
3. Measure or enter the drainage area \(A\) in the units expected by the calculator.
4. Multiply \(C\), \(i\), and \(A\) to get \(Q\).

Worked example

Problem. A parking lot has a runoff coefficient of \(C = 0.85\), a rainfall intensity of \(i = 2.4\), and a drainage area of \(A = 10\). Find the runoff flow rate \(Q\).

1. Use the Rational Method, \(Q = C i A\).
2. Substitute the values, \(Q = 0.85 \times 2.4 \times 10\).
3. Compute the result, \(Q = 20.4\).

Tips and common mistakes

• Make sure the units for \(i\) and \(A\) match the calculator, because the flow rate depends on consistent units.
• Do not confuse the runoff coefficient \(C\) with percent grade or slope, since \(C\) only describes how much rainfall becomes runoff.

Frequently asked questions