The principle behind a rain garden is to mimic the natural environment in the built environment. Any type of building almost always results in more impervious area, which in turn increases the amount of run-off. Since run-off carries pollutants into streams, the extra run-off may well mean extra pollution. The rain garden, through controlling water flow, helps to also control pollution.
What Happens to the Water in a Rain Garden?
A rain garden is typically sited so that run-off can enter the garden. In this it differs from a typical landscape area, or island. These would simply take the rain that falls on them, but not any from adjacent paved or vegetated areas.
So a rain garden must be placed to facilitate receipt of run-off. The top of the soil in the rain garden is depressed below adjacent areas, to allow for storage of run-off within the rain garden. Since water will be ponding, the plants that are put in a rain garden need to be the types that will thrive during inundation.
There are two ways the rain garden helps with the water cycle:
- First, it reduces the peak run-off rate by service as a mini-flood-control reservoir. Water stored in the reservoir, both above and below the soil level, cannot leave the area as quickly as it otherwise would have, and thus the peak flow is reduced.
- Second, the rain garden allows the water temporarily stored to seep into the ground, where it will eventually find its way to the water table. Or, plants in the rain garden will take up water through their roots. Also, standing water in the rain garden will evaporate after the storm is over. Thus, the total number of gallons of water that leave the site is reduced.
With these things happening in the rain garden, run-off from rainfall (or snow or ice melt) more closely resembles how it was prior to human alteration of the natural environment into the built environment.
Does a Rain Garden Form a Pond?
The purpose of a rain garden is to remove water from running off, into creeks or storm sewer systems, and allowing it to soak into the ground as it did before construction created more impermeable surfaces. A rain garden forms a temporary pond, one that will hold water for a day or two while it soaks in, is taken up by plants, and evaporates when the weather is right.
In certain soils, such as tight clays, the rain garden holds water longer than in sandy soils. In these instances the rain garden should be designed to hold less water—a shallower depth, so that the plants are not inundated longer than they can tolerate the inundation and not be damaged.
Must a Rain Garden be in a Vegetated Area?
Most rain gardens are in vegetated areas—that is, they are bounded by lawn (or similar grasses or plants) on all sides. However, a rain garden can be sited adjacent to a parking area or even within a parking area. The rain garden will function the same. Having water forming a temporary pond with paving surrounding it is a concern for the eventual undermining of the pavement. For this type of pond, a shorter detention time is beneficial.
Experience has shown, however, that rain gardens should not be put immediately adjacent to buildings. This protects the building foundation from water seepage in that direction, but it also provides a vegetative pathway for water between the building and the rain garden. This is a sort of pre-treatment, with the grass (or other vegetation) removing much of the contamination prior to the run-off reaching the rain garden.
With proper siting, a rain garden can have a major impact on a mini-watershed, removing pollution, and helping the run-off from the built environment to better mimic the natural environment.
Source: “How Does Your Garden Grow? A Reference Guide to Enhancing your Rain Garden” by Prince George's County Department of Environmental Resources
Source: "Rain Gardens: A how-to manual for homeowners" Wisconsin Department of Natural Resources, Publication PUB-WT-776 2003
David A. Todd - David Todd is a civil engineer, a genealogist, a citizen concerned with the environment, and a writer by passion.
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