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.

Are you really a doctor?

my brain hurts!

why use the rational method? you want to calculate volume. the scs curve number gives it directly and more accurately. with the rational method you're calculating peak discharge, so by multiplying with time you're overestimating volume!!

Where to find the 100 year flood maps?

3:12 You actually said "The square footage for one HALF of the roof of the house shown here is 40' x 20' '" AND you drew that dimension. Then why would the square footage be 800 sqft my bald friend? If HALF of the A frame roof that you drew is 800 sqft then the TOTAL should be 1600 sqft. COME ON! And you call yourself a doctor! You'd expect bullshit calculations like that from your nurse not you. Doctor Drainage.

Step in to my office, cause you're fucking fired! And take your nurse with you!

Must be out of date. There is no "resources" tab on the Homeowner Drainage section of the NDS website. C'mon NDS webmaster!