After you have located your potential water sources you’ll want to quantify the amount of water they’ll be sending into your rain garden.
First you will need to decide the size of rainfall you’d like to size your rain garden to capture.
This amount will vary depending on the frequency and intensity of the rainfall, and the type of soils, found in your area.
Also things like purpose, budget, and site conditions will shape your Design Volume.
Here in the Chicagoland area I typically size my rain gardens for a 1” rain.
This 1″ refers to the depth of rain that falls across the entire landscape, which is typically how rainfall is measured.
This size allows my gardens to catch on average 87% of the total rainfall by volume.
This can be seen in the graphs below.
The infrequent larger rain storms will not all get captured, but by using this sizing I design a rain garden that is very effective while also being cost efficient.
And bigger isn’t necessarily better.
Small rain gardens have a proportionally larger impact on pollutants as most pollutants are carried off in the “first flush,” or the first few minutes of a good rain as they are washed off the surfaces.
One thing to keep in mind before we begin.
The more accurate your measurements are, the more accurate your design will be.
But there are a lot of variables at play so the last thing you should do is get too focused on your measurements.
A year with either higher or lower than average rain will play a much larger role in the success of your rain garden than will getting your measurements down to the inch.
Below I will cover calculating downspouts and runoff; for sump pump calculations go to Part 2.
So let’s start with the easiest source to quantify; downspouts.
You will want to know the area of the roof that is being collected by the gutters and into the downspout.
But don’t get out your ladder just yet.
You can find the length and width of the roof section by measuring the distances on the ground while walking along the roof.
Multiply the length by the width to get your area.
Then multiply the area by the amount of rainfall you’d like to capture (1”=.083).
This number is your volume and is in cubic feet.
Next let’s look at runoff.
Typically your runoff will come from grass and/or pavement.
With both surfaces you will want to find the area of the surface first.
For pavements such as driveways or patios it can sometimes be difficult to decipher where the water is running off.
I like to use a hose with a spray nozzle attached to the end of it to sprinkle the pavement and see what water is draining where.
Once you have found your drainage boundaries you will want to multiply the length by the width to get your area.
Then you will need to multiply the area by your desired rainfall captured (1”=.083).
This gives you the runoff volume for this area.
But you are not done yet.
As runoff travels across a surface some of the water is lost through infiltration, evaporation, and surface tension.
This water that is lost can be calculated into your volume by using the runoff coefficient of the surface.
There are very detailed and specific runoff coefficients but this simplified list should get the job done for the average rain garden.
You use this chart by finding the surface your runoff is running off and then multiply the volume you have already calculated by the number on the right.
This will give you your Design Volume.
For example, if you have a roof that has a volume of 300 cubic feet coming off of it, you will multiply it by anywhere from .75-.95 depending on the material and slope, I usually use .90 for the typical asphalt shingle low-slope roof.
So 300 x .9 = 270 cubic feet for your Design Volume.
Side note: 1 cubic feet equals 7.48 gallons. I prefer cubic feet, but gallons is something we are more familiar with.
For Sump Pump Calculations continue on to Part 2.