Rain garden

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%.

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 1990's in the state of Maryland. They are now one of the fastest growing areas of interest for home landscapes.

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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.

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.

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 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.

Some broader context for this technology

This is part of a renaissance of new technologies for Sustainable urban drainage systems (SuDs), emerging as engineers, architects, and development planners discover the functional power of more ecologically, and hydrologically integrated technologies that professionals considered privative during a time of industrial modernization. Challenges of real human induced ecological collapses, desertification, and the many facets of global climate disruption are redefining notions of progress.

Inclusion of rain gardens as a legally recognized Best Management Practice (BMP) by the US Environmental Protection Agency (EPA) was a major paradigm shift at the time. The term "rain garden" explicitly distinguishes this BMP from conventional detention ponds, infiltration basins, "NURP" ponds, vegetated swales and increasingly concrete- or gravel-lined conveyance systems engineered for severe storms in the USA. These systems might not be recognized as swales or ponds by people from other parts of the world.

Popular and legislated demand for "rain gardens" can lead contractors to incorrectly label swales, and steep rock-lined retention basins as rain gardens. The spread of this new technology, old as its origins globally, may temporarily outpace technological comprehension of design professionals educated during a period of strong non-biological bias in the civil engineering discipline of the United States. The physical reinforcement of soil by plants, bioengineering, is accepted by construction professionals, but the vital role of plants in the hydrological performance of rain gardens is less understood.

Adjusting biases of large engineering firms toward deep, high volume, rapidly-drained garden designs with as much mulch area as plant area requires rigorous research to quantify negative impacts these choices have on intended rain garden functions of contaminant retention and water purification so that these factor into economic analysis. Unlike models used for flood management design, optimizing retention of net non-point source pollution involves continuous simulation models that account for stochastic processes, such as local weather. While within the capability of a personal computer, these are not yet ubiquitous tools among civil engineers accustomed to reducing risk of worst-case scenarios.

Phytoremediation, green roofs and rain gardens are part of another paradigm shift as Ecohydrological Engineering emerges as a profession and Environmental Engineering reaches a status not before enjoyed in the Civil Engineering community of the USA. Engineering to ensure sustainability in the full ecological context is worth big money where it once was considered unprofitable. Perceived scarcity of healthy water, air, and ecosystems may raise their universally recognized financial import to that of fuels.

Creating a rain garden

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.

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 recieve 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.

Plant Selection

Functional plant traits vary among species and ecotypes, but all plants must transpire to actively grow and flower or fruit. Generally, more flowers and more fruit require more water, but it is most vital that plants survive.

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. Brooklyn Botanical Garden has regional lists of good rain garden plants for the USA. (See reference, below.) When planting a rain garden, it’s often 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. However, excessive fertilization reduces a soil's retention of nutrients e.g. nitrates that can leach to groundwater. Native plants well suited to extracting all they need from local soil can be good candidates in places that must be a nutrient trap. Vegetatively invasive native plants can serve useful roles in rain gardens provided they do not proliferate to exclude other desired plants, or disrupt the aesthetic garden design or adjacent lawn. Avoid use of invasive exotic plants in any landscape situation.

Plants must require minimal maintenance to survive, and be compatability with adjacent land use. Trees under powerlines, or that ubheave sidewalks when soils become moist, or whose roots seek out and needed clog drainage tiles can cause expensive damage. Other landscape considerations still apply.

Transpiration rates can be worth considering. Submerged plants don't transpire to air. Water readily evaporates through stomata as sun warms leaves exposed to moving air, while ponded water remains cool. Arenchyma tissues facilitate oxygen diffusion to submerged roots of many facultative aquatic plants. Without this many plants are forced to restrain transpiration as inflow of oxygen depleted water--either by heating in a surface pond, or by respiring microbes in the ground--suffocates plants. This is counterproductive if such plants shade faster transpiring plants.

Flood tolerance does not guarantee vibrant growth while inundated. Many swamp trees merely suspend growth during spring floods. Trees generally contribute most when located close enough to tap moisture in the rain garden depression, yet in no position to shade the garden or be inhibited by excessive moisture. That said, shading open surface waters can reduce excessive heating of habitat in recieving waters. Plants tolerate inundation by hot water for less time because heat drives out dissolved oxygen, thus a plant tolerant of early spring floods may not survive summer floods.

A final note on ecotypes is that one wants plants or seed grown in similar conditions to those of the planting site. Just because you choose a species that has been observed growing well under 10cm of water, doesn't guarantee the plants one buys are of that same ecotype. Some facultative wetland plant species produce plants adapted to both wet and dry conditions, while others have seperate ecotypes producing individuals competative in uplands or in marshes.

Mulch

Adding organic mulch around plants is a good idea. Mulch protects establishing plants from rapid desiccation, and otherwise exposed soil from pelting rain drops that collapse pores though which water enters soil. Mulch traps some matter suspended in runoff even before it infiltrates. However, maximum transpiration, and support for soil microbes--responsible for biofiltration--is achieved when actively growing plants cover the soil from varied leaf canopy heights. A living mulch of tiny plants underneath can exclude weeds and increase net transpiration, but all plants must tolerate runoff debris that naturally accumulate in depressions.

Lithic mulch protects soil from rapidly inflowing surface water, and makes sense in particularly arid situations. However separating organic debris-that accumulate from some runoff--from among rocks can be a task. Degrading organic mulches simply add soil structure and bind nutrients which may be harvested with excess compost accumulation in the rain garden. Hardwoods are recomended because they do not float away as readily as softwoods such as pine ^[10].

Winter Processes

In climates where winter soils freeze, dormant plants do not transpire. One may presume rain gardens merely serve as a place to pile snow. However, garden plant structures still serve functions in winter. Windrows can catch drifting snow so it settles preferentially in piles to the side of roads, doorways, and paths.

Conifer trees that maintain most their needles or scales transpire slowly in winter, but continue to intercept significant precipitation, snow, before it reaches the ground. Intercepted snow or ice readily melts and evaporates, or vanishes by direct sublimation by solar radiation in the dry winter air.

Conifer trees in particular re-radiate winter sun such to accelerate snow thaw on their sun-word side. Infrared night photographs used to identify homes with poor insulation in cold climates, often show dormant trees as light blue figures. This is indicating that warmth is conducted through the tree to the air. This can result in ground temperatures beneath groups of trees significantly warmer than in exposed fields, thus such microclimates facilitate early thaw and infiltration is spring. The effect of forestry practices on the distribution of snow accumulation and onset of spring melt has been recognized for more than a decade.

Finally, an observant gardener may notice how snow melts in circles around each tiny plant shoot that protrudes to the surface. As the sun melts snow, shoots provide fairly direct paths past surrounding ice crystals that refreeze water percolating into the snow pack. Perhaps dust on shoot surfaces acts as antifreeze, or adhesion of liquid water to organic surfaces makes it less readily freeze, but the focusing of daily solar energy into water on these preferential flow paths facilitates focused day time infiltration before ambient temperatures melt the bulk of the snow. Each shoot leads to roots which shrink as they dry in winter. Protruding shoots and roots force imperfections in ice lenses that form as water freezes solid within the snow pack or as a hard frost in soil. Being of different material, plant structures expand, contract, and dry at different rates than ice. Whatever the dominant mechanisms it appears that protruding plant shoots-root systems decrease the instance of complete hard frost or ice lens barriers to spring time infiltration. This area of research has significant implication for management of spring time flooding.

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 communities could use it as a resource when planning their own rain garden projects.^[11]

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%.^[12]

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.^[13] Rain Gardens of West Michigan was established as an outreach of the Council as one of its water quality programs.^[14] 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.^[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]

See also

• Bioretention
• Ecohydrology
• Low Impact Development ~and~ *Green Infrastructure
• Microclimate
• Stormwater ~and~ *Surface runoff
• Sustainable urban drainage systems

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.