How to Calculate Drainage - Step I
Step 1. Calculate the Runoff – “Q”
Determine the Area – “A”
Before solving for other variables in the rational method equation, it is best to first determine the size of the area where the runoff is coming from.
There are a few things that you will need to calculate the runoff from your home. Here is a list of materials to gather before you begin:
Paper
Pencil and eraser
Highlighter, crayons, colored pencils, or markers
Calculator
Tape measure
Tables and maps from this article
The first step is to determine the area (in square feet) where the runoff is coming from. The picture below shows a residential lot with a house, driveway, and lawn. The four arrows at the corners of the house represent the location of downspouts.
It is much easier to determine the runoff if we first draw a simplified sketch of the property. This sketch should include the house, walkways, driveways, patios, pool, lawn, flower beds, and any other major landscape features. We will use this sketch to keep track of each drainage area.
Now You Try!
Using your tape measure, paper, and pencil, draw a simple sketch of your property. Make sure to include the features listed above. Be sure to include all dimensions on your drawing. These dimensions will be used in future calculations.
Now that we have a sketch of the home, we need to determine where the water is draining. This can be done by casual inspection. Drawing simple arrows that show the direction of the flow will help determine the drainage zones. The arrows need to go from where the water is originating to where the water is draining. Each location where the water is draining is a separate drainage zone.
The “X” in the picture below represents a low spot in the backyard.
Now You Try!
Draw drainage arrows on your sketch.
We can now determine the drainage zones. Each area where the arrows point to is a separate drainage zone. Multiple parts of your property may drain to a single drainage zone. In our example property, the back-left portion of the roof and the back-left portion of the lawn drain to “X”. Labeling and coloring each drainage zone helps keep everything clear. The picture below shows drainage “Zone A”. Notice that everything that drains to this zone is colored yellow.
Now You Try!
Label and color each of your drainage zones.
Now that we know where the water will drain to, we can begin to calculate how much water will run off to each drainage zone. First we will divide the roof into drainage areas. This house has four downspouts, one at each corner of the house, that equally drain the roof runoff. This house is 2000 square feet (50’ x 40’). Each downspout will drain approximately 500 square feet (2000 sf / 4) of roof runoff. The back-left portion of the lawn is also included in drainage Zone A. This portion of the lawn is 900 square feet (30’ x 30’). The runoff area from the roof and the lawn do not get combined into one total square footage. We need to keep these separate for now because the percentage of water that will drain from the roof differs from the percentage of water that will drain from the grass. Multiple portions of the roof that drain to the same drainage zone can be combined into one total roof square footage. After determining the total surface area for each surface draining to each zone, begin a table to keep track of the runoff. A table like the one shown below can be used.
Drainage Table
Drainage Zone C I (in. / hr) A (sf) Q (gpm)
Zone A Roof 500
Zone A Lawn 900
An electronic version of a drainage calculator can be found at: http://www.ndspro.com/drainage-calculator.
Any Additional Questions?
If you have additional questions, feel free to contact our resident drainage expert at [email protected]. You can also visit www.ndspro.com!
Article contributions made by Dr. Drainage of NDS, Inc. Please feel free to +1 him on Google+ at: https://plus.google.com/116777199852694165395/