Rain garden

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Business parking lot that drains to a rain garden. A sunken curb retains asphalt, yet lets water flow off the edges.

A rain garden is a planted depression that is designed to absorb rainwater runoff from impervious urban areas like roofs, driveways, walkways, and compacted lawn areas. This reduces rain runoff by allowing stormwater to soak into the ground (as opposed to flowing into storm drains and surface waters which causes erosion, water pollution, flooding, and diminished groundwater).^[1] Rain gardens can cut down on the amount of pollution reaching creeks and streams by up to 30%.^{[citation needed]}

Native plants are recommended for rain gardens because they generally don't require fertilizer and are more tolerant of one's local climate, soil, and water conditions. The plants — a selection of wetland edge vegetation, such as wildflowers, sedges, rushes, ferns, shrubs and small trees — take up excess water flowing into the rain garden. Water filters through soil layers before entering the groundwater system. Root systems enhance infiltration, moisture redistribution, and diverse microbial populations^[2] involved in biofiltration. Also, through the process of transpiration rain garden plants return water vapor into the atmosphere. A more wide-ranging definition covers all the possible elements that can be used to capture, channel, divert, and make the most of the natural rain and snow that falls on a property. The whole garden can become a rain garden, and all of the individual elements that we deal with in detail are either components of it, or are small-scale rain gardens in themselves.

The concept of a rain garden began in the 1990s in the state of Maryland. They are now one of the fastest growing areas of interest for home landscapes.^{[citation needed]}

[edit] Mimicking natural systems

Before an area is developed, a natural groundwater filtering process takes place. Rainwater flows into low places, where native plants soak up and transpire a small portion of the water. The rest percolates into the ground. In a natural environment such as this, streams and creeks are fed by cool groundwater at a fairly constant rate. This water is buffered by groundwater storage capacity, ion exchange with substrates, and microbial processes within soil. Unfortunately, in most urban environments, the water system no longer works this way. Rain gardens can mimick some of this natural system.

Rain gardens increase infiltration, decrease surface run-off from roofs, roads, and paved areas, and reduce the risk of flash flooding. Not all subsurface water percolates down to the ground water. Plant transpiration, often accelerated by urban heat island effects, speeds evaporation that frees water storage capacity within surface soil even as water continues percolating from saturated soil below. This is particularly true where mulch or debris inhibit direct evaporation from a soil surface. Root and microbial exudates, eg. saccharides, can raise soil's volumetric water holding capacity and retention coefficients for many contaminants. All this promotes natural biofiltration processes.

Surface run-off not absorbed in the rain garden slows significantly—due to the swale and vegetative barrier—which reduces sediment load and pollution downstream. Because water moves slower in the ground than it does over the urban hardscape, rain gardens mitigate peak flow more than just by reducing the volume of water reaching the outlet.

[edit] Mitigating the impact of urban development

In developed areas, the natural depressions are filled in. The surface of the ground is leveled or paved, and water is directed into storm drains. This causes several problems. First of all, streams that are fed by storm drains are subjected to sudden surges of water each time it rains^[3][4][5], which contributes to erosion and flooding. Also, the water is warmer than the groundwater that normally feeds a stream, which upsets the delicate system. Warmer water cannot hold as much dissolved oxygen (DO). Many fish and other creatures in streams are unable to live in an environment with fluctuating temperatures. Finally, a wide variety of pollutants^[6] spill or settle on land surfaces between rain events. The initial rinse from each runoff event can wash this accumulation directly into streams and ponds.

Excess water from an expanding area or increasing development density is cumulative. Flooding results from ever smaller events requiring upgrades of drainage infrastructure. Areas compacted by heavy equipment during past construction activities remain less permeable long after vegetation is reintroduced. Both groundwater recharge and subsurface flow paths are disrupted. Strategies to retain water and soil at their source can slow this harmful cascade.

Rain gardens may be located near a drainpipe from a building’s roof (with or without rain barrels), although if there’s a basement, a French drain may be used to direct the rainwater to a location farther from the building. Normally, a rain garden—or a series of rain gardens—is the endpoint of drainage, but sometimes it can be designed as a pass-through system where water will percolate through a series of gravel layers and be captured by a drain under the gravel and carried to a storm water system. Rapid pass through systems reduce peak discharge and extend hydraulic lag time of the discharge —reversing urbanization's major hydraulic impact. However, rapidly drained systems do not achieve pollution removal rates that more slowly percolating rain gardens do^[7].

Runoff volumes from impervious surfaces in many urban cities make green roofs necessary to reduce peak volumes to magnitudes that areas available for rain gardens can handle. While some rain garden wash through is acceptable from heavy storms that dilute pollution, depression focused recharge of contaminated runoff is avoided by proper rain garden design. The simplest fail safe for handling polluted runoff is for a garden with one inlet not to accept more volume than it can handle, and not pond to sufficient depth to push water into the water table faster than required for adequate biofiltration.

Rain gardens are beneficial for many reasons: improve water quality by filtering run-off, provide localized flood control, aesthetically pleasing, and provide interesting planting opportunities. They also encourage wildlife and biodiversity, tie together buildings and their surrounding environments in attractive and environmentally advantageous ways, and make a significant contribution to important environmental problems that affect us all.

A rain garden provides a way to use and optimize any rain that falls, reducing or avoiding the need for irrigation. They allow a household or building to deal with excessive rainwater runoff without burdening the public storm water systems. Rain gardens differ from retention basins, in that the water will infiltrate the ground within a day or two. This creates the advantage that the rain garden does not allow mosquitoes to breed.

[edit] History

The first rain gardens were created to mimic the natural water retention areas that occurred naturally before development of an area. 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, the county's Associate Director for Programs and Planning in the Department of Environmental Resources, with the idea.^[8] The result was the extensive use of rain gardens in Somerset, a residential subdivision which has a 300-400 ft² rain garden on each house's property.^[9] 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.^[8] This webpage has many links to information on Prince George's County's literature on implementing Low Impact Development (LID) in a community.

Some de facto rain gardens predate their recognition by professionals as a significant LID tool. Any shallow garden depression implemented to capture and retain rain water within the garden so as to drain adjacent land without running off a property is at conception a rain garden--particularly if vegetation is maintained with recognition of its role in this function. Vegetated roadside swales, now promoted as "bioswales" remain the conventional drainage system in many parts of the world from long before extensive networks of cement sewers became the conventional engineering practice in the USA.

What is globally 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 wealthy developed communities retrofitting bioretention into built stormwater management systems, and for developing communities seeking a faster and more sustainable development path.

[edit] Characteristics

A home rain garden recently planted

A rain garden requires an area where water can collect and infiltrate, and plants to maintain infiltration rates, diverse microbe communities, and water holding capacity. Transpiration by growing plants accelerates soil drying between storms. This includes any plant extending roots to the garden area.

Simply adjusting the landscape so that downspouts and paved surfaces drain into existing gardens may be all that is needed because the soil has been well loosened and plants are well established. However, many plants don't tolerate saturated roots for long and often more water runs off one's roof than people realize. Often the required location and storage capacity of the garden area must be determined first. Rain garden plants are then selected to match the situation, not the other way around.

[edit] Soil and drainage

When an area’s soils are not permeable enough to let water drain and filter properly, the soil in the bottom of the garden is replaced with soil that will help the water to drain, typically containing 60% sand, 20% compost, and 20% topsoil. Deep plant roots create additional channels for storm water to filter into the ground. Sometimes a drywell area with a series of gravel layers may be constructed near the lowest spot in the rain garden to facilitate percolation. However, putting a drywell in the lowest spot washes in maximum silt to clog it prematurely and can make the garden into a rapid infiltration basin without the intended 100% retention of small rain events that rain gardens are designed to achieve. Depression focused recharge of polluted water into wells poses serious ground water pollution threats. Similarly combining septic treatment adjacent to rain gardens warrants careful review by a qualified engineer. Dirtier water must be more completely retained in soil to be purified. This usually means more small rain garden basins and greater required soil depths to the seasonal high watertable. In some cases lined bioretention cells with subsurface drainage are used to retain small events and filter larger ones without letting water percolate deeply on site. If this leachate is not to receive further treatment, the soil media warrants careful attention to achieve water quality goals.

Rain gardens are at times confused with bioswales. Swales slope to a destination, while rain gardens do not; however, a bioswale may end with a rain garden. Drainage ditches may be handled like bioswales and even include rain gardens in series, saving time and money on maintenance. If most the water volume flowing into a garden, flows out again then rain garden may be the wrong term. Similarly, part of a garden that nearly always has standing water is a water garden, wetland, or pond not a rain garden. These semantics clarify where certain rain garden functions are achieved. One combines landscape elements to achieve objectives.

[edit] Plant selection

Plants selected for use in a rain garden should tolerate both saturated and dry soil. Using native plants is generally encouraged. This way the rain garden may contribute to urban habitats for native butterflies, birds, and beneficial insects.

Plantings require minimal maintenance to survive, and compatibility with adjacent land use. Trees under powerlines, or that up heave sidewalks when soils become moist, or whose roots seek out and needed clog drainage tiles can cause expensive damage.

Trees generally contribute most when located close enough to tap moisture in the rain garden depression, yet do not excessively shade the garden. That said, shading open surface waters can reduce excessive heating of habitat. Plants tolerate inundation by warm water for less time because heat drives out dissolved oxygen, thus a plant tolerant of early spring flooding may not survive summer inundation.

[edit] Other municipal rain garden projects

This article or section deals primarily with the United States and does not represent a worldwide view of the subject. Please improve this article or discuss the issue on the talk page.

Maplewood, Minnesota has implemented a policy of encouraging residents to install rain gardens. Many neighborhoods had swales added to each property, but installation of a garden at the swale was voluntary. The project was a partnership between the City of Maplewood, U of M, Department of Landscape Architecture, and the Ramsey Washington Metro Watershed District. A focus group was held with residents and published so that other communities could use it as a resource when planning their own rain garden projects.^[10]

In Seattle, a prototype project, used to develop a plan for the entire city, was constructed in 2003. Called SEA Street, for Street Edge Alternatives, it was a drastic facelift of a residential street. The street was changed for a typical linear path to a gentle curve, narrowed, with large rain gardens placed along most of the length of the street. The street has 11% less impervious surface than a regular street. There are 100 evergreen trees and 1100 shrubs along this 3-block stretch of road, and a 2-year study found that the amount of stormwater which leaves the street has been reduced by 98%.^[11]

10,000 Rain Gardens is a public initiative in the Kansas City, Missouri metro area. Property owners are encouraged to create rain gardens, with an eventual goal of 10,000 individual gardens.

The West Michigan Environmental Action Council has begun encouraging rain gardens as a method of reducing the mosqito-borne West Nile virus.^[12] Rain Gardens of West Michigan was established as an outreach of the Council as one of its water quality programs.^[13] Also in Michigan, the Southeastern Oakland County Water Authority has published a pamphlet to encourage residents to add a rain garden to their landscapes in order to improve the water quality in the Rouge River watershed.^[14] In Washtenaw County homeowners can volunteer for the Drain Commission's Rain Garden program, in which volunteers are annually selected for free professional landscape design. The homeowners build the garden themselves as well as pay for landscaping material. Photos of the gardens as well as design documents and drainage calculations are available online^[15].

The city of Atlanta, Georgia, has established a public education project, the Clean Water Campaign (CWC), to encourage residents to learn about stormwater management and to add rain gardens to their properties. They do this through community workshops and an official website.^[16]

In Delaware, several rain gardens have been created through the work of the University of Delaware Water Resources Agency, and environmental organizations, such as the Appoquinimink River Association.^[17]

[edit] External links

• Rain Garden Poster
• Rain garden case study, Burnsville, MN (USA). 2004. Land & Water 48(5).
• Water at the Grass Roots A brief introduction to Low Impact Development and rain gardens
• Details for construction of rain garden with a long plant list from Brooklyn Botanical Garden
• EPA Fact Sheet - Bioretention (rain gardens)
• How to build a rain garden at your home
• Stormwater Tender project
• Filterra Bioretention Systems
• Rain Garden Demo at Mount Evelyn

[edit] See also

• Bioretention
• Ecohydrology
• Low Impact Development
  • Green Infrastructure
• Microclimate
• Stormwater
  • Surface runoff
• Sustainable urban drainage systems
• Water garden

[edit] References

• Dunnett, Nigel and Andy Clayden. Rain Gardens: Sustainable Rainwater Management for the Garden and Designed Landscape. Timber Press: Portland, 2007. ISBN 978-0-88192-826-6
• Prince George’s County. 1993. Design Manual for Use of Bioretention in Stormwater Management. Prince George’s County,MD Department of Environmental Protection.Watershed Protection Branch, Landover, MD.
• Michael L. Clar, Billy J. Barfield, and Thomas P. O’Connor. 2004. Stormwater Best Management Practice Design Guide Volume 2 Vegetative Biofilters. US EPA National Risk Management Research Laboratory.