Experimental Rain Garden
In October 2009, EPA finished upgrading a major parking lot at its Region 2 laboratory in Edison, New Jersey, with low impact development (LID) features including a rain garden. EPA plans to use this effort as a demonstration project to study the ability of the rain garden to accept stormwater runoff.
The Region 2 laboratory needed a 110-vehicle-capacity parking lot to provide safe and secure parking. The project began as a building and facilities improvement project under the Office of Administration and Resources Management (OARM). EPA’s Office of Research and Development (ORD) recognized the project as an opportunity for research and contributed funding that helped to install instrumentation to evaluate the ability of rain gardens to accept stormwater runoff and lessen the effects of peak stormwater runoff on receiving streams, including stream bank erosion and negative effects on aquatic plant and animal life. ORD is evaluating rain gardens as part of a long-term research project examining multiple stormwater management practices.
Rain Garden Benefits
Rain gardens provide several benefits:
- Vegetation and soils within the rain garden use physical and biological processes to remove contaminants carried by stormwater runoff.
- Infiltrating stormwater into native, underlying soils helps mimic natural drainage processes and reduces the volume of stormwater runoff.
- Stormwater runoff reduction improves the physical and biological integrity of receiving streams by reducing stream bank erosion and negative effects on aquatic communities.
- Rain gardens improve the aesthetics of an area compared to conventional infrastructure.
The rain garden has three goals:
- Reduce stormwater runoff by reducing impervious surfaces.
- Quantify the hydrologic performance—the ability to accept, store, and infiltrate stormwater—of rain gardens through changes in season and rain garden age.
- Test the effect on hydrologic performance of multiple ratios of impervious surface area to rain garden area.
This bird’s-eye view schematic details the design of the rain garden cells located south of the laboratory’s newly constructed LID parking lot. The rain garden consists of six separate cells, represented by solid green rectangles, that are hydrologically isolated from each other using 3/8 inch-thick plastic sheeting installed to a depth of four feet, represented by yellow lines. The six cells receive stormwater runoff, represented by red arrows, from an impervious section of the parking lot and adjoining sidewalk through curb cuts at the south end of the parking lot. Stormwater runoff from the roof of the adjacent building is collected from multiple downspouts and conveyed beneath the sidewalk in an 8-inch diameter pipe, represented by a thick dashed blue line. A dedicated 4-inch diameter pipe, represented by thin dashed blue lines, distributes the roof runoff upward into each rain garden cell just south of the curb cuts.
The drainage area to all six cells is roughly equal, about 1,200 square meters, but because the rain garden cells are different sizes, they represent different percentages of their drainage areas. The smallest cells are 2 percent of their drainage areas, the medium-sized cells 4 percent, and the largest cells 8 percent. Each cell size is duplicated for statistical purposes, and both cells of each size are planted with the same plants. All cells are equipped with instrumentation for quantifying the timing and size of the depth of the stormwater as it infiltrates through the rain garden during and following storm events. Small observation wells were installed at different depths in the center of each rain garden to quantify the elevation of groundwater by infiltrating water.
EPA will first examine and document the capabilities of the garden without plants. When plants are introduced, the Agency will be able to determine whether the plants and the plant root system have the anticipated beneficial effects on stormwater infiltration rates. A more complete understanding of how rain gardens function will enable EPA to provide national guidelines on rain garden design, construction, maintenance, and monitoring, which local organizations can use to reduce peak flows to receiving waters.
The benefits to the area around the rain garden will include:
- A reduction in runoff volume to the conventional storm sewer system due to stormwater infiltration into rain garden media and underlying native soil.
- Improved runoff quality through physical and biological removal of stormwater contaminants during stormwater infiltration through rain garden systems.
The successful application of bioretention and porous pavement systems at Edison’s parking lot demonstration site, as determined by the results of the research and monitoring effort, will help facilitate:
- Technology transfer to other federal facilities and to municipalities considering adopting green infrastructure to alleviate combined sewer overflow problems.
- A more complete understanding of how rain gardens function, enabling EPA to provide information nationwide on rain garden design, construction, maintenance, and monitoring, which local organizations can use to reduce peak flows to receiving waters.
- The reduction of stormwater peak flows, which will help maintain the function and integrity of receiving streams.
- The continuing supply of high-quality, potable water needed for human life by helping watershed managers use rain gardens and other management tools to help assure that receiving waters meet the “fishable and swimmable” goals that Congress outlined in the Clean Water Act of 1972.
Edison’s parking lot was also upgraded with several types of permeable pavement.