Calculating surface water drainage - also called stormwater runoff - may sound difficult, but the process is actually straightforward. By following the steps outlined in this article in sequence, you will be able to determine the amount of water draining from your property and ways that this water can be efficiently stored and re-routed.
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.
Important Tip: 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.
In most cases, stormwater runoff can be calculated using the Rational Method. However keep in mind:
If math is not your favorite topic, don't be put off by the title: the calculation is easy.
Here is the rational method equation:
Q = C x I x A
The equation above can be modified further to give you your rainwater runoff in gallons per minute, as outlined below:
Q = (C x I x A) / 96.23
Calculating runoff from your property can be done in three steps:
So let's get started. Within about 60 minutes, we'll have the answers we need.
Before solving for other variables in the rational method equation, ideally we should 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:
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.
Calculating the runoff is easier if we first draw a simplified sketch of the property. Your 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.
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 on the right represents a low spot in the backyard.
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.
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. Our example house:
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.
Examine your property:
Now that we know the area and surface material for everything contributing to each drainage zone, we can move on to the next variable in our drainage equation.
Answer: the Coefficient of Runoff, C, is the average percentage of water that runs off of a given surface material.
A higher percentage of water that falls on concrete will run off than water that falls onto grass or sand.
The Coefficient of Runoff is equal to Runoff / Rainfall, expressed as a decimal.
A table for the average Coefficient of Runoffs is shown below.
In our example property:
Using the Coefficient of Runoff table above:
Each portion of the USA experiences different amounts of rainfall. As mentioned in the opening to this guide, your municipality may require you to use a different rainfall intensity value when determining your runoff. The map included here is a generalized map for a 100-year storm event for the United States. The numbers on the map represent the amount of rain that would fall in 1 hour for a storm that will come (on average) once every 100 years.
To use this map, find your location on the map and follow the line to the perimeter of the map that shows your expected rainfall. If your location falls between two lines, take the average of the rainfalls. Our sample property is in Las Vegas, NV. The expected rainfall in the 100-year storm in Las Vegas, NV is 1.5 inches per hour. We now record this number in our Drainage Table.
Using the 100-year storm map above, find your location and expected rainfall amount. Record this value on you Drainage Table.
Now that we have all the variables filled into our Drainage Table, we are ready to calculate our runoff, Q.
Notice that even though the grass area is nearly 2X that of the roof, the roof runoff is nearly 2X that of the grass area. Here's why: the percentage of water that runs off of the roof is much higher than the percentage of water that runs off the grass.
So we can calculate the runoff for each area contributing to each drainage zone.
Once we have the runoff for each area, we can combine all of the runoff that contributes to each zone, so all the areas contributing to drainage Zone A can be combined, and all the areas contributing to drainage Zone B can be combined, etc.
Another way to calculate the drainage is find a weighted "C" value for the whole area contributing to the low spot.
To calculate the weighted average, you:
So for example, using the value above:
What if water runs off of the roof and then passes over the lawn before reaching the low spot as it does in our example? Answer:
Handy Tip: an electronic version of a drainage calculator can be found at: http://www.ndspro.com/drainage-calculator
Congratulations: you have now completed hardest part. The next step in is to determine the volume of water to be stored.
Each municipality has their specific rules when it comes to determining how much water can be drained off of your property. A method that is commonly used is called the First Flush rule:
The reasoning behind the First Flush rule is that all of the pollutants, fertilizers, oils, and other chemicals that are on the surface will be washed off in the first 15 minutes of the storm. The goal is to prevent this polluted water from entering the public storm system. The public storm systems often drain directly to rivers, lakes, bays, and, oceans. Capturing and storing this polluted water on individual properties prevents our waterways from becoming polluted.
Storing the first 15 minutes of runoff from the 100-year storm event is usually sufficient storage capacity for most storms. If you want additional storage capacity, the runoff time can be increased to any desired length.
To calculate the volume of water that needs to be stored:
Using the table below will help calculate the volume of water to be stored.
Calculate the volume of water to be stored in each zone by multiplying the runoff, Q, from each drainage zone by 15 minutes (or the required time length).
Now that you know the volume of water to be stored, we need to determine how to efficiently store this water. There are two ways that this water can be stored:
Below is a list of advantages and disadvantage of each method.
NDS is the leader in stormwater management and offers a variety of products that can help you efficiently store your stormwater.
NDS Flo-Well and EZ-Drain are two products that can be easily installed by the average homeowner.
NDS Flo-Well takes the place of a traditional gravel dry well. A dry well is essentially a hole that is dug into the ground that is then backfilled with gravel.
The problem with a traditional gravel dry well is that after backfilling with gravel, the only space available to store water is in the space between the gravel.
The space between backfill material is commonly called "void space". The available void space for gravel is usually 30 to 40%.
So for any given dry well, only 30 to 40% of the original hole size can be used to store water.
Using the required storage for each drainage zone, calculate the required number of Flo-Wells using either:
NDS has provided a Flo-Well calculator that can be used to determine the number of Flo-Wells that would be needed for your job.
Your NDS Flo-Well calculator can be found at: http://www.ndspro.com/flo-well-calculator
EZ-Drain is an all-in-one gravel-free French drain.
In comparison, a traditional French drain is a perforated pipe surrounded by gravel and a geotextile fabric. The two major disadvantages of traditional gravel French drains are:
The table below shows a comparison between a traditional gravel French drain and EZ-Drain.
For more information on EZ-Drain, go to: http://www.ndspro.com/drainage-systems/french-drains/ez-drain-french-drain
The table below gives the storage and flow capacities of EZ-Drain.
To store 114.6 gallons, (114.6 / 11.4) = 11 - 7" bundles or (114.6 / 15.8) 8 - 8" bundles are required.
Using the required storage for each drainage zone, calculate the required number of EZ-Drain bundles using either:
NDS has provided an online calculator that can be used to determine the length of trench and quantity of EZ-Drain bundles that are required for your job. This calculator can be found at: http://www.ndspro.com/ezdrain-calculator
So for your home, using the required storage for each drainage zone, calculate the required number of EZ-Drain bundles and Flo-Wells if they are used together. Use a table like the one above to help with the calculations.
Armed with your important information, new skills, additional knowledge, and the hints, tips, and techniques provided in this mini-course, you're now ready to make your plans a reality.
You can purchase Flo-Wells and Ez-Driain from the fine retailers shown on the home page.
If you have any further questions, please feel free to contact our resident drainage expert at firstname.lastname@example.org