Bioretention  - High Density Residential

Bioretention cells, also known as rain gardens, can be easily incorporated into the common areas that are typical of apartment settings. For example, the picture to the right is of a bioretention area used in a landscaped courtyard in Portland, Oregon.¹ The apartment building was part of a redevelopment project, and the building's owner wanted to remove the site's runoff from the combined sewer system. Two large planting beds were designed as rain gardens that receive stormwater from the building's downspouts and the courtyard's paved areas. Other designs can be adapted from commercial, industrial and institutional settings. For example, rain gardens can be used in the parking lot islands of an apartment complex or between parking rows. In these higher density developments, rain gardens serve as a natural and beautifying stormwater management solution. The gardens contain vegetation and specific layers of soil, sand, and organic mulch that infiltrate and filter stormwater runoff from the surrounding impervious surfaces.

This second rain garden example is shown being constructed in a neighborhood in St. Paul, Minnesota.  The completed project, known as the Maria Bates Rain Garden², achieved multiple goals.  It created urban greenspace, improved runoff water quality, protected a newly restored wetland area along the Mississippi River, and promoted environmental stewardship and enjoyment by the neighborhood.  Site design for bioretention in these higher density residential areas differs from that in lower density suburbs. For the Maria Bates project, two vegetated swales were used to redirect stormwater from a residential street to the bioretention cell. The 900 square-foot rain garden was designed to treat all of the runoff from the 1-inch 24-hour storm, while overflow from larger storms discharges to the storm sewer system. Capturing all or most of the first flush from the one-acre 75% impervious drainage area, the cell helps to remove oil and grease, heavy metals, nutrients, and sediment from the runoff water. Future plans include redirecting the runoff from an office building's roof into the grass swales as well.

With their multi-functionality, such community stormwater management systems can receive financial support from various sources. For example, the Upper Swede Hollow Neighborhood Association initiated the Maria Bates rain garden as an offshoot of their Lower Phalen Creek Project, which aims to build watershed stewardship through community based initiatives. Since the garden was also designed to serve as an outdoor classroom, the Community Design Center of Minnesota organized local students to help plant the garden and learn about pollution prevention. After construction, nearly 200 students from a local elementary school learned about native plants, water quality, and erosion control during a workshop at the garden. Financial support from city, state, and federal agencies, as well as local and national charitable organizations, were used to fund the project. 

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