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How to find an appropriate location for a rain garden

Take a walk around the school grounds during the next rainstorm. Make a note of where puddles are forming, which areas are not draining well, and where runoff is flowing.

Finding a good location for a rain garden involves balancing a number of different factors:

The rain garden should be located in a place where it will receive runoff. Check to make sure runoff flows to your site, or could flow with minor modifications, such as cutting a space out of a curb.

The rain garden must be located far enough from the building to avoid damage to the foundation. If your school has a basement, the rain garden should be located at least 25’ from the building. If there is no basement, the rain garden should be at least 5’ from the building.

Find out where underground utilities are buried. Common utilities will include: water, sewer, electricity, natural gas, telephone, cable, and possibly a separate storm sewer (some older areas have combined sewers, where the stormwater drains directly into the sanitary sewer system). It is often difficult and time consuming to find out exactly where all of the utilities are buried on your site, so start early. Write letters to each of the utilities, requesting plans showing the locations of underground utilities on your property. Have your maintenance staff show you where the utilities enter the building, and try to avoid using this area for rain gardens. Before construction, you can call Miss Utility, who will come to your site and mark the locations of utilities with spray paint, so you can be certain of avoiding them. Stay at least five feet horizontally and one foot vertically from any utilities.

Check your soil. Since a lot of earth was moved during the construction of your school and school grounds, it is likely that your soils will be highly compacted, and will not drain well. In order to compensate for this, you should use a special soil mix within the rain garden. You should also install an underdrain at the bottom of the rain garden.

Note underdrain on this cross-section of a rain garden / bioretention area

The underdrain should be a perforated PVC pipe that runs from the bottom of the rain garden out to some discharge point. This could be a grassy area, a wooded area, a stream, or even a parking lot.

Also consider what will happen to runoff during very large storms. The rain garden is designed for small storms. When there is a great deal of rain, the rain garden will fill and eventually overflow. Be aware of where water will go when it overflows the rain garden. Minimize the impact that this overflow will have by directing it toward grassy areas, wooded areas, or existing storm drains.

Avoid trees. Soils near trees tend to have the best drainage, and should not be disturbed.

Avoid areas where there will be heavy foot traffic. Heavy foot traffic will pack down the soil in the rain garden, which will degrade its ability to infiltrate runoff. Place the rain garden somewhere out of the way, where the soil will not be compacted, and the vegetation will not be trampled.

Be sensitive to how other people use of the site. For example, avoid building your rain garden on a favorite picnic spot.
 

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