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Rain gardens, bio-retention cells, and various urban wetlands use plants to treat and reduce stormwater runoff and increase soil infiltration. Natural biological and chemical processes can significantly reduce suspended solids and pollutants like nitrogen, phosphorous, and metals by 30-90%.
Rain gardens are typically smaller and positioned alongside roads and buildings while bioretention cells are often used in larger drainage areas. Multiple gardens and cells can be used for large drainage areas, though proper siting is extremely important to maximize effectiveness. A ‘micro urban wetland’ is a decentralized, modular system that can be adapted to a building, area, or community-level. In addition to collecting and filtering rainwater, as with rain gardens and bioretention cells, a micro urban wetland stores purified water in the ground to be recovered as needed.
Rain gardens should be located in sloped areas of up to 10% at least 10 feet away from building foundations and approximately one-third the size of the area providing the runoff. Bioretention cells should be located in landscaped or natural depressions not exceeding 6% slope and should be sized at approximately 5% of the drainage area, which should not exceed five acres. In addition to proper siting and sizing, sunlight requirements of the plant species used should be taken into consideration to maximize effectiveness and reduce maintenance.
Resilient native species that can survive in soaked soils should be used wherever possible. Salt-tolerant plants should also be used for drainage areas that are salted in winter. Design adjustments and combination with other stormwater best management practices and proper snow removal are necessary to ensure that bio-retention cells continue functioning during cold winters. Regular maintenance is required to ensure continued effectiveness and system longevity, including annual mulching, fertilizing, pruning, and removal of dead vegetation, as well as regular trash removal, weeding, and mowing.
Costs range significantly depending on plants used, landscaping required, and use of underdrains, liners, and outlet structures. In calculating cost savings from utilizing bioretention, it is important to take into account the reduction of conveyance and other conventional stormwater management systems and reduced maintenance, design, and liability costs: a medical office building in Maryland saved $24,000 through using bioretention in place of 570 additional feet of storm drain pipe.
Potential regulatory touchpoints in Boston and Massachusetts include: