Rain gardens, also called bioretention facilities, are one of a variety of practices designed to increase reabsorption of rain runoff by the soil. They can also be used to treat polluted stormwater runoff. Rain gardens are designed landscape sites that reduce the flow rate, total quantity, and pollutant load of runoff from impervious urban areas like roofs, driveways, walkways, parking lots, and compacted lawn areas. Rain gardens rely on plants and natural or engineered soil medium to retain stormwater and increase the lag time of infiltration, while remediating and filtering pollutants carried by urban runoff. Rain gardens provide a method to reuse and optimize any rain that falls, reducing or avoiding the need for additional irrigation. A benefit of planting rain gardens is the consequential decrease in ambient air and water temperature, a mitigation that is especially effective in urban areas containing an abundance of impervious surfaces that absorb heat in a phenomenon known as the heat-island effect.
Rain garden plantings commonly include wetland edge vegetation, such as wildflowers, sedges, rushes, ferns, shrubs and small trees. These plants take up nutrients and water that flow into the rain garden, and they release water vapor back to the atmosphere through the process of transpiration. Deep plant roots also create additional channels for stormwater to filter into the ground. Root systems enhance infiltration, maintain or even augment soil permeability, provide moisture redistribution, and sustain diverse microbial populations involved in biofiltration.B.C. Wolverton, Ph.D., R.C. McDonald-McCaleb (1986). “Biotransformation of Priority Pollutants Using Biofilms and Vascular Plants.” Journal of the Mississippi Academy of Sciences. Vol. XXXI, pp. 79–89. Microbes help to break down organic compounds (including some pollutants) and remove nitrogen.
Rain gardens are beneficial for many reasons; they improve water quality by filtering runoff, provide localized flood control, create aesthetic landscaping sites, and provide diverse planting opportunities. They also encourage wildlife and biodiversity, tie together buildings and their surrounding environments in integrated and environmentally advantageous ways. Rain gardens can improve water quality in nearby bodies of water and recharge depleted groundwater supply. Rain gardens also reduce the amount of polluted runoff that enters the storm sewer system, which discharges directly to surface waters and causes erosion, water pollution and flooding.University of Rhode Island. Healthy Landscapes Program. “Rain Gardens: Enhancing your home landscape and protecting water quality.” Rain gardens also reduce energy consumption by decreasing the load on conventional stormwater infrastructure.
History The first rain gardens were created to mimic the natural water retention areas that developed before urbanization occurred. The rain gardens for residential use were developed in 1990 in Prince George's County, Maryland, when Dick Brinker, a developer building a new housing subdivision had the idea to replace the traditional best management practices (BMP) pond with a bioretention area. He approached Larry Coffman, an environmental engineer and the county's Associate Director for Programs and Planning in the Department of Environmental Resources, with the idea. The result was the extensive use of rain gardens in Somerset, a residential subdivision which has a rain garden on each house's property.Wisconsin Natural Resources (magazine). “Rain Gardens Made One Maryland Community Famous.” February 2003. This system proved to be highly cost-effective. Instead of a system of curbs, sidewalks, and gutters, which would have cost nearly $400,000, the planted drainage swales cost $100,000 to install. This was also much more cost effective than building BMP ponds that could handle 2-, 10-, and 100-year storm events. Flow monitoring done in later years showed that the rain gardens have resulted in a 75–80% reduction in stormwater runoff during a regular rainfall event.
Some de facto rain gardens predate their recognition by professionals as a significant LID (Low Impact Development) tool. Any shallow garden depression implemented to capture and filter rain water within the garden so as to avoid draining water offsite is at conception a rain garden—particularly if vegetation is planted and maintained with recognition of its role in this function. Vegetated roadside swales, now promoted as "bioswales", remain the conventional runoff drainage system in many parts of the world from long before extensive networks of concrete sewers became the conventional engineering practice in the industrialized world. What is new about such technology is the emerging rigor of increasingly quantitative understanding of how such tools may make sustainable development possible. This is as true for developed communities retrofitting bioretention into existing stormwater management infrastructure as it is for developing communities seeking a faster and more sustainable development path.
Purpose
Urban runoff mitigation
Urban hydrology In natural environments, most rainfall enters the ground immediately and is absorbed by plants. This process helps to replenish groundwater supplies and reduce the amount of surface runoff. Additionally, vegetation helps to slow down water flow, reducing the risk of flooding and erosion. In suburban areas, impervious surfaces typically only cover 5–10 percent of the land. In contrast, central urban areas are filled with impervious surfaces, which can account for about 90–100 percent of the land cover. In these regions, nearly all rainfall is converted into surface runoff, which frequently overwhelms stormwater management systems. Runoff volumes in urban areas can be two to ten times higher than in an undeveloped area. This dramatic increase in runoff contributes to downstream flooding, erosion, and pollutant transport into aquatic ecosystems. Rain gardens serve as decentralized stormwater controls to help restore more natural hydrologic function in built environments.
Effects of urban runoff In developed urban areas, naturally occurring depressions where storm water would pool are typically covered by impermeable surfaces, such as asphalt, pavement, or concrete, and are leveled for automobile use. Stormwater is directed into storm drains which may cause overflows of combined sewer systems or pollution, erosion, or flooding of waterways receiving the storm water runoff.Kuichling, E. 1889. "The relation between the rainfall and the discharge of sewers in populous districts." Trans. Am. Soc. Civ. Eng. 20, 1–60.Leopold, L. B. 1968. "Hydrology for urban land planning: A guidebook on the hydrologic effects of urban land use." Geological Survey Circular 554. United States Geological Survey.Waananen, A. O. 1969. "Urban effects on water yield" in W. L. Moore and C. W. Morgan (eds), Effects of Watershed Changes on Streamflow. University of Texas Press, Austin and London. Redirected stormwater is often warmer than the groundwater normally feeding a stream, and has been linked to upset in some aquatic ecosystems primarily through the reduction of dissolved oxygen (DO). Stormwater runoff is also a source of a wide variety of pollutants washed off hard or compacted surfaces during rain events. These pollutants may include volatile organic compounds, pesticides, herbicides, hydrocarbons and trace metals.Novotny, V. and Olem, H. 1994. "Water Quality: Prevention, Identification, and Management of Diffuse Pollution." Van Nostrand Reinhold, New York.
Pollutant removal Rain gardens are designed to capture the initial flow of stormwater and reduce the accumulation of toxins flowing directly into natural waterways through ground filtration. Natural remediation of contaminated stormwater is an effective, cost-free treatment process. Directing water to flow through soil and vegetation achieves particle pollutant capture, while atmospheric pollutants are captured in plant membranes and then trapped in soil, where most of them begin to break down. These approaches help to diffuse runoff, which allows contaminants to be distributed across the site instead of concentrated. The National Science Foundation, the United States Environmental Protection Agency, and a number of research institutions are presently studying the impact of augmenting rain gardens with materials capable of capture or chemical reduction of the pollutants to benign compounds.
The primary challenge of rain garden design is predicting the types of pollutants and the acceptable loads of pollutants the rain garden's filtration system can process during high impact storm events. Contaminants may include organic material, such as animal waste and oil spills, as well as inorganic material, such as heavy metals and fertilizer nutrients. These pollutants are known to cause harmful over-promotion of plant and algal growth if they seep into streams and rivers. The challenge of predicting pollutant loads is specifically acute when a rain event occurs after a longer dry period. The initial storm water is often highly contaminated with the accumulated pollutants from dry periods. Rain garden designers have previously focused on finding robust native plants and encouraging adequate biofiltration, but recently have begun augmenting filtration layers with media specifically suited to chemically reduce redox of incoming pollutant streams. Certain plant species are very effective at storing mineral nutrients, which are only released once the plant dies and decays. Other species can absorb heavy metal contaminants. Cutting back and entirely removing these plants at the end of the growth cycle completely removes these contaminants. This process of cleaning up polluted soils and stormwater is called phytoremediation.
Pollutant Removal Performance Rain gardens function by using filter media that collects pollutants from stormwater runoff. Plants also contribute by absorbing pollutants and promoting microbial activity in the root zone. The composition of the soil media and the types of vegetation used greatly affect performance. Media with higher sand content improves infiltration but may decrease nutrient retention. Plants with deep or fibrous root systems improve nitrogen uptake and microbial processing of pollutants.
Studies show that rain gardens with amended soils rich in organic content and deeper media profiles can remove over 60% of total nitrogen under optimal conditions. According to data from the International Stormwater BMP Database, typical removal rates for rain gardens include 80% for total suspended solids (TSS), 50–60% for heavy metals, and significant reductions in nutrients like nitrogen and phosphorus. These findings support rain gardens as an effective best management practice (BMP) for urban stormwater control.
Stormwater management systems