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How to Calculate Drainage - Introduction

How to Calculate Drainage

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Calculating drainage (also called stormwater runoff) may sound difficult, but it is actually quick and easy to do.  By following the steps outlined in this article, you will be able to determine the amount of water draining from your property and ways that this water can be efficiently stored.

As you work through this guide, keep track of all your work on a separate sheet of paper.  Working slowly and carefully through each step will help you come up with the correct solution to your drainage problem. The equations and methods shown have been simplified.  In all cases, the results will overestimate the runoff and storage requirements.

Please check with your public municipality before beginning any grading or drainage work on your property because they might use different values, different methods, and may require permits or drainage calculations that are reviewed and signed by a licensed civil engineer.   

Runoff Equation        

In most cases, stormwater runoff can be calculated using the Rational Method.  This method can be used for all drainage areas less than 200 acres.  For drainage areas greater than 200 acres, other methods can be used, or the drainage area can be divided into zones less than 200 acres.

The Rational Method equation is: Q = C x I x A where:

Q = Stormwater Runoff (in cubic feet per second)

C = Coefficient of Runoff

I = Rainfall Intensity (in inches per hour)

A = Area of Drainage Zone (in acres)

The equation above can be modified to give you runoff in gallons per minute.  The modified equation is: Q = (C x I x A) / 96.23 where:

Q = Stormwater Runoff (in gallons per minute, gpm)

C = Coefficient of Runoff

I = Rainfall Intensity (in inches per hour)

A = Area of Drainage Zone (in square feet)

Calculating runoff from your property can be done in three steps;

1) Calculate the runoff

2) Calculate the volume of water to be stored

3) Determine how to store the water

Article contributions made by Dr. Drainage of NDS, Inc. Please feel free to +1 him on Google+ at: https://plus.google.com/116777199852694165395/

For more information, please visit www.ndspro.com

Filed under HowToCalculateDrainage NDS YardDrainage DIY StormwaterRunoff