View video "Installing Rain Gardens to Reduce Runoff"
Leaving or creating depressions in the landscape promotes stormwater infiltration and reduces stormwater runoff. Take a walk through a forested area and you'll notice knolls and swales. Shaping the land in this fashion imitates nature by creating contours throughout the landscape, which allows rainwater to soak into the ground—as nature intended. The human tendency is to level the landscape, unaware of the environmental impact of this type of grading. Rain gardens are simply low-lying, vegetated depressions--generally 3 to 6 inches deep--which have absorbent soils that temporarily collect stormwater runoff from impervious surfaces and allow the runoff to slowly percolate into the soil. The depression should be a flat-bottom, saucer shape rather than a bowl shape so that rainwater runoff can sheet out throughout the garden to allow for better infiltration. This generally takes a few hours and shouldn't take more than two days. Rain gardens are attractive landscaping features that function like a natural moist garden, moist meadow, or light forest ecosystem. They can look as informal or as formal as you like. While rain garden dimensions vary, remember, any size rain garden is better than no rain garden.
Rain gardens provide flood control, groundwater recharge, and water-cooling benefits, while the plants, soils, and associated microorganisms remove many types of pollutants—such as excess nutrients, pesticides, oils, metals, and other contaminants—from stormwater runoff. Stormwater pouring off hot roofs, pavement, and other impervious surfaces is temporarily captured, cooled, and allowed to percolate into the ground. Nutrients such as nitrogen and phosphorus, which would otherwise contribute to algae blooms and other problems in the Bay, are instead put to beneficial use by being taken up by the plants in the garden. Some studies show that about 50 percent of such pollution comes from individuals and homeowners, through yard care, yard waste, and chemical pollution from household activities.
Native plant rain gardens also become wildlife oases with colors, fragrances, and the sights and sounds of songbirds and butterflies regularly visiting. Additionally, rain gardens increase groundwater supplies, significant because many people get their water from underground aquifers. The replenishment of groundwater—which is particularly important in times of drought—depends on the absorption of rainwater into the ground.
By creating rain gardens and keeping most of the rain that falls on your site contained on site--the way nature intended—you can help improve water quality in local streams and rivers and ultimately the Chesapeake Bay. Native trees, shrubs, and herbaceous perennials improve the ability of water to filter down and recharge groundwater supplies, unlike turf grass, which tends to form a partially impervious barrier to water infiltration.
Notably, a rain garden is a type of bioretention installation, however, "bioretention" often refers to installations that are designed and engineered to be more complex than home rain gardens in order to mitigate larger amounts of runoff. They are deeper and typically incorporate underdrains.
Source: Chesapeake Ecology Center
Rain Garden Installation
To select a location for your rain garden, begin by observing your yard during a rainfall event. Notice where water is flowing from, and where it is going. Rain gardens should ideally be located between the source of runoff (roofs and driveways) and the runoff destination (drains, streams, low spots, etc.).
For a comprehensive checklist for rain garden installation in Anne Arundel County, visit: http://www.aacounty.org/DPW/Highways/RainGarden.cfm.
Try out Anne Arundel County's updated GIS Mapping Application, made available for public use by the County's Watershed Ecosystem Restoration Services (WERS). It has numerous options, including viewing soil types for any address, square footage of roofs, land parcels, watersheds and subwatersheds, viewing the priority of a specific area (e.g., high, moderate, low) for addressing stormwater issues, in addition to many other features. This is a great tool for students and educators and for citizens who want to learn more and make a difference in their watershed! More information, including instructions on using this tool, can be found on the Watershed Stewards Academy's website.
Maryland Stormwater Design Manual contains detailed information on stormwater mitigation: http://www.mde.state.md.us/Programs/WaterPrograms/SedimentandStormwater/stormwater_design/index.asp.
You can't have a rain garden that is too large. However, any size garden will make a difference, even a small one. The ideal situation is to create a garden that will absorb all the rain that would otherwise flow away from your yard. To calculate the most useful size of a smaller garden, here's how:
• Figure out what kind of soil you have.
• Estimate the area from which your garden will get rain. Multiply width times length of your rooftoop, to get square feet. Add the square feet of paved areas. Remember, though, that different parts of your roof drain to different downspouts—you want to estimate only the square footage that will drain into your rain garden. Don't forget roof overhangs.
• For sandy soil, your rain garden should be 20-30% of the drain area. For example, if your roof and driveway measures 1200 square feet and all the rain from them will be used, your rain garden should be 20 to 30% of that, or 240-360 square feet. (ex: 10' X 24')
• For clay soil, your rain garden should be about 60% of the drain area (Clay absorbs water very poorly; the varieties of rain garden plants that do well in clay take at least three years to get established. Soil replacement may be the best choice in clay soils).
• If you improve your soil drainage and replace your soil with rain garden mix (50-60% sand, 20-30% topsoil, 20-30% compost), your rain garden should generally be about 20-30% of the square footage of your drain area.
Rain gardens for single-family homes will typically range from 150 to 400 square feet. But remember; any size rain garden, even a small one, will contribute to solving local water pollution problems. It will also be a lovely addition to your landscape.
Source: Rain Gardens of West Michigan
Rain Garden Design
The Low Impact Development Center has developed a series of rain garden, or bioretention, design templates that can be used by landscape architects, landscape contractors, garden clubs, and homeowners throughout the Bay. These designs promote the use of rain gardens and Bayscapes by providing a set of easily accessible high quality sustainable and maintainable designs for the landscape industry and citizens. For rain garden design templates, visit: www.lowimpactdevelopment.org/raingarden_design.
"Don't stress too much over it. The rain garden does not have to be perfect to do its job, and it will change over time-that's one of the things that makes it so rewarding: it's a living, dynamic system. Dig a hole, relax, and let nature take its course. Observe and have fun." -Spencer Rowe, Wetland Scientist
New, excellent 2010 guide to rain gardens and other rainscaping techniques! (14 MB)
Native plants for rain gardens, 6-page handout (2.16 MB)