Rain garden

A concept that began in the 1990s in the state of Maryland, rain gardens are now one of the fastest growing areas of interest for home landscapes.

A rain garden is a planted depression that is designed to take all, or as much as possible, of the excess rainwater run-off from a house or other building and its associated landscape. The plants—a selection of wetland edge vegetation, such as sedges, rushes, ferns, shrubs and trees—absorb the excess water, and through the process of transpiration 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.

Mimicking Natural Systems

Before an area is developed, a natural groundwater filtering process takes place. Rainwater flows into low places, where native wildflowers and grasses soak up some of the water. The rest percolates into the ground over 24-48 hours. In a natural environment such as this, streams and creeks are fed by cool groundwater, which feeds the streams at a constant rate.

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

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, the streams that are fed by storm drains are subjected to sudden surges of water each time it rains, which contributes to erosion. 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. Many fish and other creatures in the streams are unable to live in an environment with fluctuating temperatures.

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 a 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 French drain under the gravel and carried to a storm water system.

Rain gardens are beneficial for many reasons: they lessen the effects of drought, filter some pollutants from run-off, make paved areas more attractive, 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 networks. 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.

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 pond with a bioretention area. He approached Larry Coffman, who headed up the county's associate director for programs and planning with the Department of Environmental Resources, with the idea.

The result was the extensive use of rain gardens in Somerset, which has a 300-400 square foot 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 webpage has many links to information on Prince George's County's literature on implementing Low Impact Development in a community.

Creating a Rain Garden

Rain gardens are sometimes confused with bioswales. Bioswales slope, while rain gardens do not; however, a bioswale may end with a rain garden. Drainage ditches may be handled like bioswales or rain gardens, saving time and money on maintenance.

When an area’s soils are not permeable enough to allow water to 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. The deep roots of the wetland plants create additional channels for the storm water to filter into the ground. Sometimes a drywell area with a series of gravel layers may be constructed in the lowest spot in the rain garden to facilitate percolation.

The plants selected for use in a rain garden should be able to tolerate both saturated and dry soil. Using native plants is generally encouraged. This is so that the rain garden may also contribute to adding urban habitats for native butterflies, birds, and beneficial insects. Brooklyn Botanical Garden has regional lists of good rain garden plants. (See reference, below.) When planting a rain garden, it’s important to use a generous addition of compost or humus in each planting hole. The compost increases the retention of moisture and it increases the aeration of the soil. Adding mulch around the plants is a good idea. Avoid the use of invasive plants in any garden or landscape situation.

Other Municipal Rain Garden Projects

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 communties 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%.

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. Rain Gardens of West Michigan was established as an outreach of the Council as one of its water quality programs. 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.

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.

External links

• From The League of Florida Cities, an article encouraging municipalities that reducing rainwater runoff one yard at a time is a worthwhile effort
• Details for construction of rain garden with a long plant list from Brooklyn Botanical Garden
• An analysis of Tampa Bay and the importance of managing runoff

See also

• Hydrology
• Low Impact Development

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