Rain Gardens: Strategic puddles for stormwater management

Chris Cavett, P.E., City Engineer, and Virginia Gaynor, Naturalist, City of Maplewood, Minnesota
Sherri Buss, RLA, Landscape Architect, Bonestroo & Associates, St. Paul, Minnesota
Presenters, 2005 APWA Congress

In many cities throughout the United States, rain gardens are becoming regarded as useful elements of stormwater infrastructure. The concept originated in the 1990s in Prince Georges County, Maryland, when Larry Coffman, associate director of the County's Department of Environmental Resources, coined the term. He and his team were developing cost-effective, low-impact development methods to infiltrate stormwater. Since then, the concept has been used in a variety of forms in many communities to help manage stormwater runoff.

What are rain gardens?
Rain gardens are shallow depressions that capture stormwater runoff from gutters, driveways, streets, parking lots and other impermeable surfaces, and allow it to soak into the soil. Most rain gardens are planted with native or hardy perennial plants, including both grasses and flowering plants. Typically rain gardens are from six inches to one foot deep, and work best on sandy to sandy loam soils. Rain gardens are usually designed to handle up to the first inch of stormwater runoff—the "first flush" that carries the majority of sediments and pollutants. An overflow should be incorporated to handle the runoff from larger storm events.

Rain gardens may be constructed with underdrain systems using sand or gravel and perforated pipe. This increases the effectiveness of infiltration, or allows stormwater filtration in heavier soils.

Benefits of rain gardens
Communities throughout the Twin Cities are enjoying the benefits of these natural-looking stormwater management tools. Rain gardens:

  • Clean stormwater by filtering sediments and nutrients from the majority of storm events that occur in our region

  • Infiltrate runoff to recharge shallow groundwater and reduce the "flashiness" of stormwater flows to creeks and ponds

  • Provide aesthetic benefits in parking lots, neighborhoods and other settings

  • Provide stormwater management in fully developed areas where land is no longer available for ponds, or provide added benefits where existing stormwater systems do not meet current water quality and quantity management standards

  A small rain garden in Maplewood, Minnesota

The Maplewood, Minnesota rain garden experience
The City of Maplewood pioneered the development of rain gardens in Minnesota. The first rain gardens in Maplewood were built in 1996 as part of a roadway reconstruction project in the Birmingham Street neighborhood. The project was created to provide water quality benefits in a fully-developed neighborhood that had no land remaining to install a traditional stormwater pond. Ken Haider, the City Engineer who initiated the project, called the rain gardens "strategic puddles" that would mimic natural conditions but place the "puddles" in acceptable places.

Rain gardens have been so well received by local residents and the City Council that Maplewood has made them a standard element of street reconstruction projects. Since the Birmingham Street project, rain gardens have been implemented in five additional neighborhoods to manage runoff.

The rain gardens are located in grassed swales along both sides of the street. Runoff sheet-flows into the swales and is infiltrated in the swales and rain gardens. Approximately 10' x 24' in size, each garden is planted with colorful, low-maintenance native and hardy perennial plants. The rain garden system is sized to handle the first inch to inch and one-half of rainfall. This includes approximately 80 percent of the rainfall events in the Twin Cities area in a typical year. Larger storm events are routed through planned overflows to ponds.

The following key elements that have helped make the rain gardens successful in Maplewood:

  • Neighborhood participation. At the beginning of each street reconstruction project, City staff hold neighborhood meetings to discuss rain gardens and answer resident questions and concerns. Each neighborhood is given the option of using rain gardens or a more standard piped stormwater system. In each case so far, neighborhoods have opted for the gardens, and roughly one in three homeowners have rain gardens on their property.

  • Garden options. Each resident chooses a rain garden design for his or her yard. The City offers seven rain garden options—including plantings for sunny areas, shady areas, plantings that attract birds and butterflies, and low-maintenance plantings. Based on resident selections, the City purchases and delivers plants for the gardens. Each resident plants his/her garden on a neighborhood planting day.

  • Resident maintenance and City follow-up. The City informs residents at the pre-project meetings that they will be responsible for rain garden maintenance. This typically involves weeding once or twice each year, and occasional watering during dry weather. Maplewood Nature Center staff provide limited follow-up with residents who have questions, including new residents who "inherit" rain gardens.

  • Continuing innovation. The City has adapted each rain garden project to the conditions and needs of each site. While most neighborhoods have preferred a "rural" street section that allows for sheet drainage from the street to the swales where rain gardens are located, the City has also used curb cuts or ribbon curbs in neighborhoods preferring curbs. In some cases, low-flow diversion structures have been added to the system to remove coarse sediments before stormwater flows to the rain gardens.

City Engineer Chris Cavett reports that the rain garden systems may cost less than standard piped systems, depending on the design used in each neighborhood.

Other rain garden projects in the Twin Cities area
Rain garden projects have been completed in many other residential neighborhoods in the Twin Cities area in recent years. Projects have also been completed to serve commercial and institutional land uses. The H.B. Fuller Company corporate headquarters in Vadnais Heights has used rain gardens to clean runoff from its employee parking lots since 1997.

The City of Wayzata completed a rain garden project to serve a small strip mall site in 2000. The rain garden is a long swale, approximately 15' wide by 600' long that cleans stormwater from the shopping center parking lot. The rain garden was sized to infiltrate a one and one-half inch rainfall, and has successfully handled larger storm events. The compacted soils at the bottom of the swale were replaced with engineered soils to improve infiltration in this rain garden.

The University of Minnesota Landscape Arboretum chose Bonestroo to design a parking lot expansion and rain garden system in 2003. Stormwater sheets from the parking lots into the gardens, which are planted with a wide variety of trees, shrubs, native grasses, and flowering perennials. Approximately 18,000 square feet in area, the gardens filter runoff from parking lots greater than one acre in size. The gardens were sized to handle flows from storm events up to one and one-half inch, with overflows channeled to a constructed pond. The Arboretum is currently adding interpretive material to inform visitors about how the gardens work and identify planting options.

Where do rain gardens work best?
The U.S. Geological Survey scientists in the Twin Cities have been monitoring several rain garden projects to determine their performance in managing water quantity and quality, including the Arboretum project. The research is sponsored by the Metropolitan Council of the Twin Cities area.

Early results indicate that rain gardens work best to provide infiltration benefits in areas of sandy to sandy loam soils, and in relatively flat terrain. Rain gardens in areas of heavy soils may filter sediments and some nutrients, but provide little infiltration. Sediment and nutrient removal are occurring in rain gardens where plantings are successful.

Additional information
More information and photos about City of Maplewood rain garden projects, and links to other rain garden information, can be found at: www.ci.maplewood.mn.us.

The three authors will give a presentation on this topic at the 2005 APWA Congress in Minneapolis. The session is entitled "Right as Rain! Rain Gardens as Part of a Stormwater Management Program" and takes place on Tuesday, September 13, at 1:00 p.m. Chris Cavett, P.E., can be reached at (651) 249-2403 or chris.cavett@ci.maplewood.mn.us; Virginia Gaynor can be reached at (651) 249-2170 or virginia.gaynor@ci.maplewood.mn.us; and Sherri Buss can be reached at (651) 604-4758 or sbuss@bonestroo.com.