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Rain Garden · Wiki

Synonyme(s): bioretention facilities


General Features

Rain gardens, also called bioretention facilities, are one of a variety of practices designed to increase rain runoff reabsortion 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. Microbes help to break down organic compounds (including some pollutants) and remove nitrogen.

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. Rain gardens also reduce energy consumption by decreasing the load on conventional stormwater infrastructure.

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, and provide significant solutions to important environmental problems that affect many aspects of life.



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 300-400 sq ft (28-37 m2) rain garden on each house's property. 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.
Posted By Gremelin Posted on March 21st, 2022
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