The size and depth of the rain garden, the infiltration rate of the soil, the size of the uphill drainage area, the amount of impervious or semi-impervious surfaces (rooftops, driveways, sidewalks and mowed turf) in the uphill drainage area, the slope of the site, and the amount, intensity and duration of rainfall all influence rain garden effectiveness in managing rainwater runoff.
To determine rain garden sizing, calculate the drainage area leading to your rain garden. If you are redirecting a gutter downspout to point towards your rain garden, calculate the square footage of roof that drains to that downspout. Or, calculate the square feet of land area that drains to your rain garden. A 5:1 ratio of drainage area to the area of a rain garden is a good starting point for building an effective system according to a USGS study. This ratio translates into a rain garden that is approximately 20 percent of the area that drains towards it. Take the amount of square feet in your drainage area and divide by 5. In other words, if there is approximately 500 square feet of drainage area, the rain garden should be about 100 square feet. The average homeowner rain garden is usually between 100–300 square feet, and the minimum ponding depth is 4″ with an average ponding depth ranging from 4–8″. However, keep in mind that smaller rain gardens are still beneficial.
Ninety percent of storms in the greater St. Louis region are 1.14 inches of rainfall or less. The goal of a rain garden is to capture most of the rainfall from these small storms. They are not expected to handle all of the rainfall from large storms that are typically associated with flooding problems. To increase the amount of rainwater that your rain garden can handle, you can increase the size or depth of your rain garden.
In some cases, there may not be enough room for an optimal/optimum size rain garden (such as buildings in close proximity, narrow lots or small lots). In these situations, designing a series of smaller rain gardens connected by bioswales (an elongated vegetated open channel), adding a bioswale without rain gardens, or digging a deeper rain garden are all options. In addition, you may want to implement other rainwater management strategies such as rainwater harvesting or turf reduction. Implementing more than one rainwater management technique will help you achieve optimum rainwater management on your property.
As the square footage of impervious and poorly draining surfaces increases in your uphill drainage area, so does the volume of rainwater that runs off of the area. Some options in this case include amending the soil surrounding your rain garden to make it more permeable, digging a larger or deeper rain garden in order to accommodate the water, or choosing to build a smaller than optimum rain garden. It is important to recognize that any increase in rainwater management is a plus!
If there is not enough room for a rain garden, consider other strategies to transform turf instead.
Continue on to Decide Which Rain Garden Elements to Include