Faced with increasing requirements and costs related to stormwater management, a growing number of public works departments are turning to rain gardens to manage runoff. Because properly designed and constructed rain gardens can be beautiful and functional at the same time, residents and businesses increasingly are willing to care for these features on their property. Whether built and maintained by governments or individuals, rain gardens offer a relatively inexpensive and proven means for treating stormwater and reducing runoff's harmful effects.
Rain gardens comprise shallow vegetated depressions designed to collect storm-water runoff and enable it to infiltrate within roughly 24 hours, said Rusty Schmidt, a landscape ecologist in the Minneapolis office of URS Corp. Rapid infiltration is crucial to avoiding mosquitoes and other problems associated with standing water. “We're not making wetlands,” he said.
To assist this process, engineered soils that encourage infiltration may be substituted for a site's native soil, particularly if it consists mostly of clay or is heavily compacted. Another means of facilitating drainage is to install perforated pipe approximately 3 feet below the surface to convey subsurface water to a catch basin or storm sewer. This approach ensures that runoff undergoes filtration before entering the pipe. “The best way to clean water is to run it through soil,” said Schmidt.
Rain gardens also must employ adequate outlets to ensure that excess runoff can exit safely, said Schmidt. For example, a rain garden might include a standpipe to remove water once it reaches a certain height.
Proper plant selection also plays a critical role in ensuring a successful rain garden, said Ted Hartsig, senior soil scientist in the Eudora, Kan., office of Applied Ecological Services Inc. Using plants with deep root systems helps restore a site's ability to infiltrate runoff. “What we're doing is trying to rebuild the natural system of water infiltration into the soil,” said Hartsig. Plants with deep roots create pores and channels underground that facilitate the downward movement of water.
By itself, a single rain garden will not make a big difference in terms of managing stormwater. However, many rain gardens together can result in noticeable improvements, as shown by a recent study by the city of Burnsville, Minn. After monitoring runoff totals for two years in two comparable watersheds of approximately 5 acres each, Burnsville added 17 rain gardens to one of the watersheds and used the other as a control, according to Daryl Jacobson, a water resources specialist for the city. In 2004, the city saw an 82% reduction in stormwater runoff from the watershed with the rain gardens compared to its previous performance. “It was a pretty significant reduction,” said Jacobson.
LEADERS IN THE FIELD
For 10 years, the city of Maplewood, Minn., has been implementing rain gardens as part of its ongoing street reconstruction program. Each year, Maplewood reconstructs approximately 3 to 4 miles of local streets in older neighborhoods that typically lack storm sewers and have limited space for managing stormwater. Rain gardens have enabled Maplewood to treat stormwater runoff in these areas, said assistant city engineer Erin Laberee.
As part of the street reconstruction process, the city builds rain gardens on private property—but in the street right of way—for homeowners who agree to maintain them, said Laberee. To date, the city has constructed approximately 370 rain gardens for homeowners and has built an additional 25 rain gardens on city property, according to horticulturist Virginia Gaynor. Unlike the residential gardens, the city maintains the rain gardens it owns.
Residents can choose from three sizes of rain gardens: 8x16 feet, 10x20 feet, or 12x24 feet. Laberee said that depending on the size, a rain garden usually costs $300 to $500 to install. Rain gardens on city property tend to be significantly larger, depending on the availability of land. Maplewood also has developed seven different rain garden designs featuring low-maintenance plants suitable for Minnesota. Different design styles include gardens for sun or shade, gardens that feature certain plants such as shrubs or daylilies, and gardens that attract butterflies.