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 allows rainwater runoff from impervious urban areas like roofs, driveways, walkways, and compacted lawn areas the opportunity to be absorbed. 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 involved in biofiltration.^[2] 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.

[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, which contributes to erosion and flooding.^[3][4][5] 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 spill or settle on land surfaces between rain events.^[6] 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 provide significant partial solutions 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 was also much more cost effective than building BMP ponds that could handle 2-, 10-, and 100-year storm events.^[8] 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.^[9]

This is also referred to as Low Impact Development (LID), and is cited by the EPA on their website as a success on the Stormwater Case Studies section of their website.^[10] This webpage has many links to information on Prince George’s County’s literature on implementing 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, as 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 allow water to drain and filter properly, the soil should be replaced. This mixture should typically containing 60% sand, 20% compost, and 20% topsoil. Deep plant roots also create additional channels for storm water to filter into the ground. Sometimes a drywell with a series of gravel layers near the lowest spot in the rain garden will help facilitate percolation. However, a drywell placed at the lowest spot can become clogged with silt prematurely turning the garden into an infiltration basin defeating its purpose. Depression focused recharge of polluted water into wells poses a serious threat and should be avoided. Similarly plans to install a rain garden near a septic system should be reviewed by a qualified engineer. The more polluted the water, the longer it must be retained in the soil for purification. This is often achieved by installing several smaller rain garden basins with soil deeper than the seasonal high watertable. In some cases lined bioretention cells with subsurface drainage are used to retain smaller amounts of water and filter larger amounts without letting water percolate as quickly.

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. Part of a garden that nearly always has standing water is a water garden, wetland, or pond, and not a rain garden. Using the proper terminology ensures that the proper methods are used to achieve the desired results.

[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.

Well planned plantings require minimal maintenance to survive, and are compatible with adjacent land use. Trees under power lines, or that up-heave sidewalks when soils become moist, or whose roots seek out and 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

The examples and perspective in this article deal primarily with the United States and do 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, University of Minnesota 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.

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. Mayor Kay Barnes took the unconventional advice of Lynn Hinkle to bring widespread use of rain gardens to offset the increasing demand on the city’s stormwater infrastructure.

The West Michigan Environmental Action Council has established Rain Gardens of West Michigan as an outreach water quality program.^[12] 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.^[13] 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^[14].

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.^[15]

The city of Portland, Oregon, has established a Clean River Rewards program, to encourage residents to disconnect downspouts from the city’s combined sewer system and create rain gardens. Workshops, discounts on storm water bills, and web resources are offered. ^[16]

The city of Austin, Texas, has established rain gardens as an “Innovative Water Quality Control”, per its Environmental Criteria Manual. ^[17]

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.^[18]

The Alaska Dept. of Fish and Game submitted a grant to the Wasilla Soil and Water Conservation for a rain graden as a memorial for Steve Duncan, [1]</ref> a long time advocate for soil and water conservation who recently passed away from cancer. What was to be a 20 foot by 10 foot Rain Garden, turned into over 100 feet long by 10-12 feet wide. A brass plaque is being placed in the garden to recognized Steves work with the local comunity and to teach kids the importance of water conservation. A story of it aired on Wed. May 20 2009.

[edit] See also

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

[edit] References

[edit] Further reading

• Kraus, Helen, and Anne Spafford. Rain Gardening in the South: Ecologically Designed Gardens for Drought, Deluge & Everything in Between. Eno Publishers: Hillsborough, NC, 2009. ISBN 978-0-9820771-0-8

[edit] External links

• 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)
• Stormwater Tender project — Little Stringybark Creek, Victoria, Australia
• Rain Garden Design Templates for the Chesapeake Bay Watershed
• 10,000 Rain Gardens — Kansas City
• Wisconsin Department of Natural Resources — Rain Gardens
• How to build residential rain gardens and lists of native plants by US regions (from Low Impact Living)