Hi. Dr. Drainage here to teach you how to
calculate and store stormwater runoff. One of the most important elements of designing
a proper drainage system is determining how much stormwater is falling on your property
and what to do with this volume of water. This video was developed to show you how to
do that. This will also provide you with the tools needed to design a custom drainage system
for your property. Let’s get started! Before we begin, there are a few things to
note. 1. Please pause, rewind, and re-watch as many
times as needed 2. Drainage codes vary across the county.
Please check the drainage requirements in your area with your local municipality. They
may use different methods or values and may require that your calculation be stamped by
a licensed civil engineer in your state. Here is a list of things that you will need:
* Paper * A Pen or Pencil
* A Tape measure * And finally, a calculator.
Let’s begin! STEPS
There are 3 steps to follow when calculating drainage:
1. First we need to calculate the storm runoff, or flow. Runoff is given the symbol Q
2. 2nd we convert the storm runoff into a volume of water
3. And finally, we determine how we are going to store the runoff volume
STEP 1 Calculate storm Runoff
I am going to use the Rational Method. There are several methods that can be used to calculate
drainage. The Rational Method is probably the easiest and most widely used method. The
Rational Method’s equation is Q=C x I x A / 96.23; where Q is the storm runoff in
gallons per minute, C is the runoff coefficient, I is the rainfall intensity in inches per
hour, and A is the drainage area in square feet. 96.23 is a conversion factor when you
want the flow rate in gallons per minute and your drainage area is in square feet. Each
of these will be explained in depth shortly. C is the runoff coefficient. This is essentially
the percentage of water that runs off a given surface. For example, if rain is falling at
2 inches per hour and only 1 inch per hour is running off of the surface, the C value
for that surface is 1 divided by 2 or 0.5. Here is a table that shows the C value for
various surfaces. This table is included in the companion calculation guide to this video.
A link to this guide will be available at the end of the video and also on the NDS Homeowner
Drainage Website under the resources tab. I is the rainfall intensity in inches per
hour. I can be calculated in several ways, but in this video we will use the approximate
rainfall values from this 100-year rainfall map. The 100-year rainfall map shows the approximate
amount of rain that will fall in the 100-year storm event.
A is the surface area where the runoff water is coming from, in square feet. The square
footage for ½ of the roof of the house shown here is 40’ by 20’ which equals 800 square
feet. Now let’s put it all together. Again, the
Rational Method equation says that our storm runoff is equal to the runoff coefficient,
“C”, times the rainfall intensity, “I”, times the runoff area, “A”, all divided
by 96.23. The runoff is coming from the roof, so the C value in our equation is 1.0. Let’s
pretend this house is in Atlanta, Georgia. The I value is 3.5 inches per hour, and the
A value is 800 square feet. Multiplying this out we can expect to get 29.10 gallons per
minute of water from this portion of the roof in the 100 year storm event.
Let’s do another example. This time the surface is sand, so the C value is 0.5, the
house is in Las Vegas, Nevada, and the runoff area is 1000 square feet. The expected runoff
here is 7.79 gallons per minute. What if the water is coming from 2 different
surfaces but draining to the same area? You treat them individually, then combine the
results. In this example, water will come from both
the roof and the grass with areas of 800 and 1000 square feet, respectively. We will assume
2 inches of rainfall per hour. The runoff from the roof will be 16.63 gallons per minute
and the runoff from the grass will be 7.27 gallons per minute.
NOW COMBINE The runoff from the roof was 16.63 gallons
per minute and the runoff from the grass was 7.27 gallons per minute. Adding these together
we get 23.90 gallons per minute draining from these areas.
To download the companion calculation guide to this video, which includes all charts and
resources included in this video, click here. A second guide and video which take you through
the steps needed to size your catch basins and pipes are also available. If you’re
interested in learning more about residential stormwater drainage and other NDS drainage
products, visit the NDS Homeowner Drainage Website here.
Questions? Email me at [email protected] If you have any questions or suggestions for
another you would like to see, email me at [email protected] I’m Dr. Drainage.
Talk to you later.