Retrofitting a Suburban Neighborhood with Rain Gardens
Burnsville is a small community of approximately 60,000 people, located along the Minnesota River, roughly 20 minutes from Minneapolis. Originally a rural farming community, Burnsville has grown from 2,700 people in 1960 to become the tenth largest city in the state. With increasing development came community concerns about protecting the lake from sediments and pollutants, primarily phosphorus, carried by urban runoff. Both the Minnesota River and Crystal Lake are major community attractions, from both a natural resource and recreational standpoint.
The Rain Garden Project
|Rain garden during construction.
Leslie Yetka, a water resources specialist for the city of Burnsville, took on the challenge of finding an affordable way to significantly reduce runoff from residential areas that already had curbing and conventional storm sewers in place. Leslie explains, "We didn't have room for traditional treatment approaches such as stormwater ponds, so we needed to focus more on treating the runoff at its source, or basically at each home."
In 2002, she was able to secure funding for a Rain Garden demonstration project - a pilot project that would test the feasibility of retrofitting residential areas with rain gardens that would reduce the volume of runoff, while treating pollutants at the source. The project, a retrofit of a 1980s neighborhood, involved individual designs for each resident-participant's property and close attention to homeowner education and easy maintenance. The gardens were primarily designed to capture street runoff through the installation of curb cuts at each garden. The gardens feature gradual side slopes, limestone retaining walls, and colorful plantings. They were carefully sized to accept the first 0.9 inches of rainfall runoff (at a minimum) from the impervious surfaces in the sub-watershed for each storm event. The City provided $30,000 for the project and the Metropolitan Council, a regional planning agency for the seven-county Twin Cities metro area, contributed an additional $117,000 in the form of a grant.
While the City had been interested in constructing rainwater gardens for some time, questions about their effectiveness remained. To better document their effectiveness, and to assess the likelihood that residents would accept these gardens in their neighborhoods, the pilot project involved a research component called a "paired watershed" study. This approach involved comparing runoff volume for the neighborhood selected to receive the rain gardens with a "control" neighborhood that had very similar initial runoff volumes, but which retained its traditional curb-and-gutter storm drainage system.
The Dakota County Soil and Water Conservation District provided initial technical expertise in evaluating the feasibility and potential of alternative stormwater-treatment practices at a residential-lot level. Leslie then hired Minneapolis-based Barr Engineering Company to help identify which treatment techniques and best management practices would result in the greatest reduction of runoff, given the dollars available for implementation.
The project began with outreach to neighborhoods surrounding Crystal Lake, to determine homeowner interest. Barr Engineering also hosted a series of seminars to explain the threats to water quality in the lake from urban runoff, and how rainwater gardens could help. Existing soils and utilities were also surveyed in these neighborhoods, to determine those most suitable for the introduction of rain gardens. In Spring 2002, after three potential sites had been identified, Leslie helped determine which neighborhood would offer the greatest percentage of voluntary participation in the program. On the street chosen to receive the rainwater gardens, 85 percent of residents agreed to participate by planting the gardens and performing minor maintenance on them, far exceeding the anticipated 30 percent buy-in. Seventeen sites were identified in the treatment watershed: thirteen in the front yards of residents' homes along Rushmore Drive, and four in a backyard swale that drains to Rushmore Drive. Each garden along the street was designed to have a curb cut to capture street runoff. A street one block away with similar runoff volumes was selected to serve as a "control condition" against which effectiveness of the rain gardens would be measured.
An important consideration in the project was the active involvement of homeowners in the design, construction, and maintenance of the rain gardens. This was viewed as critical in obtaining homeowner buy in and ownership of the project — especially because the majority of the gardens were in residents' front yards and would thus be highly visible. To facilitate their involvement, a registered landscape architect from Barr Engineering met with homeowners to explain the choices they could make about the type of rainwater garden that would be created in their yard. There were three options: perennial, wildflower, and shrub gardens. Because plants in a wildflower garden tend to spread quickly without regular maintenance, resulting in an unkempt appearance, most residents chose a combination of the less labor-intensive perennial and shrub gardens, which exhibit a neater appearance.
While the garden designs were being developed, the excavating and grading was sent out for bid. Most of the large companies submitted very high cost estimates, due in part to the fact that the job was small but complex because the gardens were located on roughly 15-foot wide roadway utility easements, which accommodated water, sewer, power, telephone, and cable TV lines. The City awarded the project to a smaller company, whose staff were very excited about the project and completed it for less than the budget ceiling of $50,000.
The gardens were planted in fall 2003, with curb cuts installed the following spring to bring them "on line." Barr Engineering and City staff donated their own time one Saturday to help homeowners complete the planting. The installation became a neighborhood block party, with residents making a potluck lunch and then strolling up and down the street to admire other gardens. The cost for each garden was approximately $7500, with about $500 of this going toward plants.
Both the control and treatment watersheds were monitored before and after rainwater garden construction to facilitate statistical comparison of runoff volumes. To ensure that the rain garden neighborhood was comparable to the "control condition" (the neighborhood a block away with traditional curb-and-gutter infrastructure), stormwater and rainfall monitoring began in 2002 and continued through the spring of 2004. Then, both areas were tracked and runoff data were gathered between the summer of 2004 and the fall of 2005, using area-velocity flow meters in the storm sewer pipe at the outlet of each watershed. Water quality data was also gathered.
Results from the comparison were astonishing. In the neighborhood with the rain gardens, runoff volumes were reduced by almost 90 percent. Data from some of the largest rainfall events indicates that the gardens met, and in some cases, exceeded, their original target of capturing the first 0.9 inches of rainfall runoff. In addition, participation by more than 80 percent of the homeowners in the treatment watersheds and the fact that the rain gardens continue to be maintained indicates that sustainable stormwater management strategies can be viewed as an amenity by property owners.
Factors Influencing Success and Lessons Learned
|Rain garden after installation (Photo courtesy of Fred Rozumalski, Barr Engineering)
The Rain Garden demonstration project is unique in Minnesota, and perhaps the country, because it involved fitting rain gardens into a neighborhood that already had a traditional curb-and-gutter design. Also, these gardens are believed to be some of the first designed for attractiveness, high performance, and ease of maintenance in a residential setting.
Several of the strategies employed by Leslie Yetka and her colleagues might be useful for other communities wanting to try a "rain garden retrofit" or similar project and may increase the likelihood of future success. One is the value of trying this approach as a voluntary pilot project and measuring the results. The empirical data on runoff reduction provides powerfull evidence of the effectiveness of such installations. Having this type of data can help other neighborhoods decide to adopt the rain garden approach.
The second strategy is to involve a landscape architect who can collaborate with homeowners on the design of the gardens. With the professional involvement of the landscape architect from Barr Engineering, sound choices were made regarding appropriate, low-maintenance plants that would maintain the well-kept appearance of homeowners' front yards. This meant that the gardens would meet residents' expectations for aesthetics and maintenance with no surprises. According to Leslie, "What the homeowners really cared most about was having a beautifully designed garden to enhance their property, so aesthetics were primary to them. Runoff treatment was secondary, for the most part." Communities wishing to try the rain garden approach are thus encouraged to contact their state's chapter of the American Society of Landscape Architects, which may wish to take on a project like this pro bono, or may be able to provide a referral.
A third factor contributing to success of the Burnsville rain garden project is that it was conducted a neighborhood undertaking - rather than a pilot project limited to just one or two homeowners. Though this was done in order to have a large enough sample for statistical analysis, a side benefit of having many homeowners involved was that the gardens became a social event and experience for the neighborhood, leading to the development of a supportive constituency.
Another aspect of the project that contributed to its success was careful site surveying. The neighborhood's topography, landscaping features, and utility lines were all factored into garden design. The tremendous amount of construction oversight, which Leslie concedes may not be feasible in some cases, allowed the gardens to be constructed exactly as planned. In addition to the necessity for "precision excavating" within the utility corridor in each yard, there was little room for error in elevations, because the gardens would not function as designed if established at too high or too low a ground level. Leslie says a contractor that pays close attention to detail is extremely important. It is also important to check earth grades and soil compaction regularly during construction.
A final aspect contributing to project success was the choice of a small, committed contractor who was excited about the project. Based on this experience, the City would not recommend soliciting bids from larger contractors, but would rather seek to develop a network of small contractors who are interested in sustainable design and committed to quality construction.
Since the project began, the Dakota County Soil and Water Conservation District has conducted home runoff audits on each lot in the study neighborhood to provide homeowners with information on ways to further reduce runoff from their properties, such as redirecting downspouts, installing rain barrels to capture roof runoff, adding gutters in strategic locations, and aerating the lawn to enhance rainwater infiltration.
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