A rain garden, one example of a low-impact development strategy, is a planted shallow depression (the catchment area). It’s main purpose is to collect runoff flowing from a developed area’s impervious surfaces such as streets, parking lots and rooftops so that it can soak directly into the soil. Rain gardens can be large or small, depending on the capacity of runoff they are intended to hold and infiltrate.
A rain garden may blend into the landscape, serving as a beautiful, planned garden area, while at the same time, retaining and treating collected stormwater. The plants used in a rain garden may include ornamentals, wildflowers and native plants; however, plants must be able to tolerate drought as well as waterlogged conditions.
Rain gardens, which are often planted with native vegetation, improve wildlife habitat and may even be used to relink a fragmented ecosystem. In contrast, a landscape of mowed grass offers little to no value to wildlife. The plants in a well-designed rain garden offer a diverse array of plants providing habitat and a food source for wildlife, birds, butterflies and other pollinators.
Rain gardens utilize the biological, chemical and physical properties of plants, soils and their microbes to remove contaminants and pollutants from runoff through the process of:
- landscaping areas with plants that provide high nutrient uptake
- chemical reactions that take place in the soil
- root zone biological degradation; where microbes present in the soil break down inorganic and organic compounds and assist in the elimination of pathogens
Rain gardens are designed so that the water percolates quickly and will not hold standing water for more than a day or two unless a large storm event occur. Therefore, they do not cause problems with mosquitos.
Similar to a rain garden, a bioretention area is larger and is used to handle the runoff associated with larger areas of impervious surfaces. Thus, they often require more planning and engineering in their implementation. Bioretention areas utilize a soil medium engineered for optimal plant growth and water storage. Bioretention areas typically include a pretreatment area where pollutants and trash are settled and collected. They also control overflow drainage. In areas where water cannot easily percolate through the soil (e.g. clay soils), a perforated pipe may be installed underneath the soil, channeling the water to a better runoff location such as a stormwater inlet.