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Rain gardens, also called bioretention facilities, are one of a variety of practices designed to increase rain runoff reabsorption by the soil. They can also be used to treat polluted
stormwater runoff Surface runoff (also known as overland flow) is the flow of water occurring on the ground surface when excess rainwater, stormwater, meltwater, or other sources, can no longer sufficiently rapidly infiltrate in the soil. This can occur when the s ...
. Rain gardens are designed landscape sites that reduce the flow rate, total quantity, and pollutant load of
runoff Runoff, run-off or RUNOFF may refer to: * RUNOFF, the first computer text-formatting program * Runoff or run-off, another name for bleed, printing that lies beyond the edges to which a printed sheet is trimmed * Runoff or run-off, a stock marke ...
from impervious
urban area An urban area, built-up area or urban agglomeration is a human settlement with a high population density and infrastructure of built environment. Urban areas are created through urbanization and are categorized by urban morphology as cities ...
s 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 Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...
, 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 Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow crops, landscape plants, and lawns. Irrigation has been a key aspect of agriculture for over 5,000 years and has been devel ...
. 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 A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (Anoxic waters, anoxic) processes prevailing, especially in t ...
edge vegetation, such as
wildflower A wildflower (or wild flower) is a flower that grows in the wild, meaning it was not intentionally seeded or planted. The term implies that the plant probably is neither a hybrid nor a selected cultivar that is in any way different from the ...
s,
sedges The Cyperaceae are a family of graminoid (grass-like), monocotyledonous flowering plants known as sedges. The family is large, with some 5,500 known species described in about 90 genera, the largest being the "true sedges" genus ''Carex'' wit ...
, rushes,
fern A fern (Polypodiopsida or Polypodiophyta ) is a member of a group of vascular plants (plants with xylem and phloem) that reproduce via spores and have neither seeds nor flowers. The polypodiophytes include all living pteridophytes exce ...
s,
shrub A shrub (often also called a bush) is a small-to-medium-sized perennial woody plant. Unlike herbaceous plants, shrubs have persistent woody stems above the ground. Shrubs can be either deciduous or evergreen. They are distinguished from tree ...
s and small
tree In botany, a tree is a perennial plant with an elongated stem, or trunk, usually supporting branches and leaves. In some usages, the definition of a tree may be narrower, including only woody plants with secondary growth, plants that are ...
s. These plants take up nutrients and water that flow into the rain garden, and they release water vapor back to the
atmosphere An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A ...
through the process of
transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
. Deep plant roots also create additional channels for stormwater to filter into the ground. Root systems enhance
infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...
, maintain or even augment soil permeability, provide moisture redistribution, and sustain diverse microbial populations involved in
biofiltration Biofiltration is a pollution control technique using a bioreactor containing living material to capture and biologically degrade pollutants. Common uses include processing waste water, capturing harmful chemicals or silt from surface runoff, a ...
.B.C. Wolverton, Ph.D., R.C. McDonald-McCaleb (1986)
“Biotransformation of Priority Pollutants Using Biofilms and Vascular Plants.”
''Journal of the Mississippi Academy of Sciences.'' Vol. XXXI, pp. 79-89.
Microbes help to break down organic compounds (including some pollutants) and remove nitrogen. Rain gardens are beneficial for many reasons; they improve water quality by filtering runoff, provide localized
flood control Flood control methods are used to reduce or prevent the detrimental effects of flood waters."Flood Control", MSN Encarta, 2008 (see below: Further reading). Flood relief methods are used to reduce the effects of flood waters or high water level ...
, create aesthetic landscaping sites, and provide diverse planting opportunities. They also encourage wildlife and
biodiversity Biodiversity or biological diversity is the variety and variability of life on Earth. Biodiversity is a measure of variation at the genetic ('' genetic variability''), species ('' species diversity''), and ecosystem ('' ecosystem diversity'') ...
, tie together buildings and their surrounding environments in integrated and environmentally advantageous ways. Rain gardens can improve water quality in nearby bodies of water and recharge depleted
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...
supply. Rain gardens also reduce the amount of polluted runoff that enters the
storm sewer A storm drain, storm sewer ( United Kingdom, U.S. and Canada), surface water drain/sewer ( United Kingdom), or stormwater drain ( Australia and New Zealand) is infrastructure designed to drain excess rain and ground water from impervious su ...
system, which discharges directly to
surface water Surface water is water located on top of land forming terrestrial (inland) waterbodies, and may also be referred to as ''blue water'', opposed to the seawater and waterbodies like the ocean. The vast majority of surface water is produced by pre ...
s and causes
erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...
,
water pollution Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, so that it negatively affects its uses. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. ...
and
flooding A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrolog ...
. University of Rhode Island. Healthy Landscapes Program
“Rain Gardens: Enhancing your home landscape and protecting water quality.”
Rain gardens also reduce energy consumption by decreasing the load on conventional stormwater infrastructure.


History

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 ) , demonym = Prince Georgian , ZIP codes = 20607–20774 , area codes = 240, 301 , founded date = April 23 , founded year = 1696 , named for = Prince George of Denmark , leader_title = Executive , leader_name = Angela D. Alsobrooks ...
, when Dick Brinker, a developer building a new housing subdivision had the idea to replace the traditional
best management practices Best or The Best may refer to: People * Best (surname), people with the surname Best * Best (footballer, born 1968), retired Portuguese footballer Companies and organizations * Best & Co., an 1879–1971 clothing chain * Best Lock Corporation, ...
(BMP) pond with a
bioretention Bioretention is the process in which contaminants and sedimentation are removed from stormwater runoff. The main objective of the bioretention cell is to attenuate peak runoff as well as to remove stormwater runoff pollutants. Construction of ...
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 rain garden on each house's property.''Wisconsin Natural Resources'' (magazine)
“Rain Gardens Made One Maryland Community Famous.”
February 2003.
This system proved to be highly cost-effective. Instead of a system of curbs,
sidewalk A sidewalk (North American English), pavement (British English), footpath in Australia, India, New Zealand and Ireland, or footway, is a path along the side of a street, highway, terminals. Usually constructed of concrete, pavers, brick, stone ...
s, 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 “
bioswale Bioswales are channels designed to concentrate and convey stormwater runoff while removing debris and pollution. Bioswales can also be beneficial in recharging groundwater. Bioswales are typically vegetated, mulched, or xeriscaped. They co ...
s”, 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 Sustainable development is an organizing principle for meeting human development goals while also sustaining the ability of natural systems to provide the natural resources and ecosystem services on which the economy and society depend. The ...
possible. This is as true for developed communities retrofitting
bioretention Bioretention is the process in which contaminants and sedimentation are removed from stormwater runoff. The main objective of the bioretention cell is to attenuate peak runoff as well as to remove stormwater runoff pollutants. Construction of ...
into existing stormwater management infrastructure as it is for developing communities seeking a faster and more sustainable development path.


Urban runoff mitigation


Effects of urban runoff

In developed urban areas, naturally occurring depressions where
storm water Stormwater, also spelled storm water, is water that originates from precipitation (storm), including heavy rain and meltwater from hail and snow. Stormwater can soak into the soil ( infiltrate) and become groundwater, be stored on depressed land ...
would pool are typically covered by impermeable surfaces, such as asphalt, pavement, or concrete, and are leveled for automobile use.
Stormwater Stormwater, also spelled storm water, is water that originates from precipitation ( storm), including heavy rain and meltwater from hail and snow. Stormwater can soak into the soil ( infiltrate) and become groundwater, be stored on depressed l ...
is directed into
storm drain A storm drain, storm sewer (United Kingdom, U.S. and Canada), surface water drain/sewer (United Kingdom), or stormwater drain (Australia and New Zealand) is infrastructure designed to drain excess rain and ground water from impervious surfa ...
s which may cause overflows of
combined sewer A combined sewer is a type of gravity sewer with a system of pipes, tunnels, pump stations etc. to transport sewage and urban runoff together to a sewage treatment plant or disposal site. This means that during rain events, the sewage gets dil ...
systems or pollution, erosion, or flooding of waterways receiving the storm water runoff.Kuichling, E. 1889. "The relation between the rainfall and the discharge of sewers in populous districts." ''Trans. Am. Soc. Civ. Eng.'' 20, 1–60.Leopold, L. B. 1968
"Hydrology for urban land planning: A guidebook on the hydrologic effects of urban land use."
''Geological Survey Circular'' 554. United States Geological Survey.
Waananen, A. O. 1969. "Urban effects on water yield" in W. L. Moore and C. W. Morgan (eds), ''Effects of Watershed Changes on Streamflow.'' University of Texas Press, Austin and London. Redirected stormwater is often warmer than the
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...
normally feeding a stream, and has been linked to upset in some aquatic ecosystems primarily through the reduction of
dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...
(DO). Stormwater runoff is also a source of a wide variety of pollutants washed off hard or compacted surfaces during rain events. These pollutants may include
volatile organic compounds Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature. High vapor pressure correlates with a low boiling point, which relates to the number of the sample's molecules in the surrounding air, a t ...
,
pesticides Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and la ...
,
herbicides Herbicides (, ), also commonly known as weedkillers, are substances used to control undesired plants, also known as weeds.EPA. February 201Pesticides Industry. Sales and Usage 2006 and 2007: Market Estimates. Summary in press releasMain page fo ...
,
hydrocarbons In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...
and trace metals.Novotny, V. and Olem, H. 1994. "Water Quality: Prevention, Identification, and Management of Diffuse Pollution." Van Nostrand Reinhold, New York.


Stormwater management systems

Stormwater management occurs on a watershed scale to prevent downstream impacts on urban water quality. A watershed is maintained through the cyclical accumulation, storage, and flow of
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...
. Naturally occurring watersheds are damaged when they are sealed by an impervious surface, which diverts pollutant-carrying stormwater runoff into streams. Urban watersheds are affected by greater quantities of pollutants due to the consequences of anthropogenic activities within urban environments. Rainfall on impermeable surfaces accumulates surface runoff containing oil, bacteria, and sediment that eventually makes its way to streams and groundwater. Stormwater control strategies such as infiltration gardens treat contaminated surface runoff and return processed water to the underlying soil, helping to restore the watershed system. The effectiveness of stormwater control systems is measured by the reduction of the amount of rainfall that becomes runoff ( retention), and the lag time (rate of depletion) of the runoff. Even rain gardens with small capacities for daily infiltration can create a positive cumulative impact on mitigating urban runoff. Increasing the number of permeable surfaces by designing rain gardens reduces the amount of polluted stormwater that reaches natural bodies of water and recharges groundwater at a higher rate. Additionally, adding a rain garden to a site that experiences excessive rainwater runoff mitigates the water quantity load on public stormwater systems. The bioretention approach to water treatment, and specifically rain gardens in this context, is two-fold: to utilize the natural processes within landscapes and soils to transport, store, and filter stormwater before it becomes runoff, and to reduce the overall amount of impervious surface covering the ground that allow for contaminated urban runoff. Rain gardens perform most effectively when they interact with the greater system of stormwater control. This integrated approach to water treatment is called the "stormwater chain", which consists of all associated techniques to prevent surface run-off, retain run-off for infiltration or evaporation, detain run-off and release it at a predetermined rate, and convey rainfall from where it lands to detention or retention facilities. Rain gardens have many reverberating effects on the greater
hydrological Hydrology () is the scientific study of the movement, distribution, and management of water on Earth and other planets, including the water cycle, water resources, and environmental watershed sustainability. A practitioner of hydrology is calle ...
system. In a bioretention system such as a rain garden, water filters through layers of soil and vegetation media, which treat the water before it enters the groundwater system or an underdrain. Any remaining runoff from a rain garden will have a lower temperature than runoff from an impervious surface, which reduces the thermal shock on receiving bodies of water. Additionally, increasing the amount of permeable surfaces by designing urban rain gardens reduces the amount of polluted stormwater that reaches natural bodies of water and recharges groundwater at a higher rate.


Bioretention

The concept of LID (low-impact design) for stormwater management is based on
bioretention Bioretention is the process in which contaminants and sedimentation are removed from stormwater runoff. The main objective of the bioretention cell is to attenuate peak runoff as well as to remove stormwater runoff pollutants. Construction of ...
: a landscape and water design practice that utilizes the chemical, biological, and physical properties of soils, microorganisms, and plants to control the quality and quantity of water flow within a site. Bioretention facilities are primarily designed for water management, and can treat urban runoff, stormwater, groundwater, and in special cases,
wastewater Wastewater is water generated after the use of freshwater, raw water, drinking water or saline water in a variety of deliberate applications or processes. Another definition of wastewater is "Used water from any combination of domestic, industrial ...
. Carefully designed
constructed wetlands A constructed wetland is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development. ...
are necessary for the bioretention of sewage water or
grey water Greywater (or grey water, sullage, also spelled gray water in the United States) refers to domestic wastewater generated in households or office buildings from streams without fecal contamination, i.e., all streams except for the wastewater fro ...
, which have greater effects on human health than the implications of treating urban runoff and rainfall. Environmental benefits of bioretention sites include increased wildlife diversity and habitat production and minimized energy use and pollution. Prioritizing water management through natural bioretention sites eliminates the possibility of covering the land with impermeable surfaces.


Water treatment process

Bioretention controls the stormwater quantity through interception, infiltration, evaporation, and transpiration. First, rainfall is captured by plant tissue (leaves and stems) and in the soil
micropores A microporous material is a material containing pores with diameters less than 2 nm. Examples of microporous materials include zeolites and metal-organic frameworks. Porous materials are classified into several kinds by their size. The recom ...
. Then, water performs
infiltration Infiltration may refer to: Science, medicine, and engineering *Infiltration (hydrology), downward movement of water into soil *Infiltration (HVAC), a heating, ventilation, and air conditioning term for air leakage into buildings *Infiltration (me ...
- the downward movement of water through soil - and is stored in the soil until the substrate reaches its moisture capacity, when it begins to pool at the top of the bioretention feature. The pooled water and water from plant and soil surfaces is then evaporated into the atmosphere. Optimal design of bioretention sites aim for shallow pooled water to reach a higher rate of evaporation. Water also evaporates through the leaves of the plants in the feature and back to the atmosphere, which is a process known as
evapotranspiration Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transp ...
. Stormwater quality can be controlled by bioretention through settling, filtration, assimilation,
adsorption Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a ...
, degradation, and decomposition. When water pools on top of a bioretention feature,
suspended solids Suspended solids refers to small solid particles which remain in suspension in water as a colloid or due to motion of the water. Suspended solids can be removed by sedimentation if their size or density is comparatively large, or by filtration. ...
and large particles will settle out. Dust particles, soil particles, and other small debris are filtered out of the water as it moves downward through the soil and interspersed plant roots. Plants take up some of the nutrients for use in their growth processes, or for mineral storage. Dissolved chemical substances from the water also bind to the surfaces of plant roots, soil particles, and other organic matter in the substrate and are rendered ineffective. Soil microorganisms break down remaining chemicals and small organic matter and effectively decompose the pollutants into a saturated soil matter. Even though natural water purification is based on the design of planted areas, the key components of bioremediation are the soil quality and
microorganism A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in old ...
activity. These features are supported by plants, which create secondary pore space to increase soil permeability, prevent soil compaction through complex root structure growth, provide habitats for the microorganisms on the surfaces of their roots, and transport oxygen to the soil.


Design

Stormwater garden design encompasses a wide range of features based on the principles of bioretention. These facilities are then organized into a sequence and incorporated into the landscape in the order that rainfall moves from buildings and permeable surfaces to gardens, and eventually, to bodies of water. A rain garden requires an area where water can collect and infiltrate, and plants can maintain infiltration rates, diverse microorganism communities, and water storage capacity. Because infiltration systems manage storm water quantity by reducing storm water runoff volumes and peak flows, rain garden design must begin with a site analysis and assessment of the rainfall loads on the proposed bioretention system. This will lead to different knowledge about each site, which will affect the choice of plantings and substrate systems. At a minimum, rain gardens should be designed for the peak runoff rate during the most severe expected storm. The load applied on the system will then determine the optimal design flow rate. Existing gardens can be adapted to perform like rain gardens by adjusting the landscape so that downspouts and paved surfaces drain into existing planting areas. Even though existing gardens have loose soil and well-established plants, they may need to be augmented in size and/or with additional, diverse plantings to support a higher infiltration capacity. Also, many plants do not tolerate saturated roots for long and will not be able to handle the increased flow of water. Rain garden plant species should be selected to match the site conditions after the required location and storage capacity of the bioretention area are determined. In addition to mitigating urban runoff, the rain garden may contribute to urban habitats for native
butterflies Butterflies are insects in the macrolepidopteran clade Rhopalocera from the order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The group comprises ...
,
birds Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...
, and
beneficial insects Beneficial insects (sometimes called beneficial bugs) are any of a number of species of insects that perform valued services like pollination and pest control. The concept of ''beneficial'' is subjective and only arises in light of desired outcome ...
. Rain gardens are at times confused with
bioswale Bioswales are channels designed to concentrate and convey stormwater runoff while removing debris and pollution. Bioswales can also be beneficial in recharging groundwater. Bioswales are typically vegetated, mulched, or xeriscaped. They co ...
s. Swales slope to a destination, while rain gardens are level; however, a bioswale may end with a rain garden as a part of a larger stormwater management system. 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 Water garden or aquatic garden, is a term sometimes used for gardens, or parts of gardens, where any type of water feature is a principal or dominant element. The primary focus is on plants, but they will sometimes also house waterfowl, or orn ...
,
wetland A wetland is a distinct ecosystem that is flooded or saturated by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free (Anoxic waters, anoxic) processes prevailing, especially in t ...
, or pond, and not a rain garden. Rain gardens also differ from
retention basin A retention basin, sometimes called a wet pond, wet detention basin, or stormwater management pond (SWMP), is an artificial pond with vegetation around the perimeter and a permanent pool of water in its design. It is used to manage stormwater ...
s, where the water will infiltrate the ground at a much slower rate, within a day or two.


Soil and drainage

Collected water is filtered through the strata of soil or engineering growing soil, called substrate. After the soil reaches its saturation limit, excess water pools on the surface of the soil and eventually infiltrates the natural soil below. The bioretention soil mixture should typically contain 60%
sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class ...
, 20%
compost Compost is a mixture of ingredients used as plant fertilizer and to improve soil's physical, chemical and biological properties. It is commonly prepared by decomposing plant, food waste, recycling organic materials and manure. The resulting ...
, and 20%
topsoil Topsoil is the upper layer of soil. It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs. Description Topsoil is composed of mineral particles and organic matt ...
. Soils with higher concentrations of compost have shown improved effects on filtering groundwater and rainwater. Non-permeable soil needs to be removed and replaced periodically to generate maximum performance and efficiency if used in the bioretention system. The sandy soil (bioretention mixture) cannot be combined with a surrounding soil that has a lower sand content because the clay particles will settle in between the sand particles and form a concrete-like substance that is not conducive to infiltration, according to a 1983 study. Compact lawn soil cannot harbor groundwater nearly as well as sandy soils, because the micropores within the soil are not sufficient for retaining substantial runoff levels. When an area's soils are not
permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwe ...
enough to allow water to drain and filter at an appropriate rate, the soil should be replaced and an underdrain installed. Sometimes a drywell with a series of gravel layers near the lowest spot in the rain garden will help facilitate percolation and avoid clogging at the sedimentation basin. However, a drywell placed at the lowest spot can become clogged with silt prematurely, turning the garden into an
infiltration basin An infiltration basin (or recharge basin) is a form of engineered sump or percolation pond that is used to manage stormwater runoff, prevent flooding and downstream erosion, and improve water quality in an adjacent river, stream, lake or bay. It is ...
and defeating its purpose as a bioretention system. The more polluted the runoff water, the longer it must be retained in the soil for purification. Capacity for a longer purification period is often achieved by installing several smaller rain garden basins with soil deeper than the seasonal high
water table The water table is the upper surface of the zone of saturation. The zone of saturation is where the pores and fractures of the ground are saturated with water. It can also be simply explained as the depth below which the ground is saturated. T ...
. In some cases lined
bioretention Bioretention is the process in which contaminants and sedimentation are removed from stormwater runoff. The main objective of the bioretention cell is to attenuate peak runoff as well as to remove stormwater runoff pollutants. Construction of ...
cells with
subsurface drainage Drainage is the natural or artificial removal of a surface's water and sub-surface water from an area with excess of water. The internal drainage of most agricultural soils is good enough to prevent severe waterlogging (anaerobic condition ...
are used to retain smaller amounts of water and filter larger amounts without letting water percolate as quickly. A five-year study by the U.S. Geological Survey indicates that rain gardens in urban clay soils can be effective without the use of underdrains or replacement of native soils with the bioretention mix. Yet it also indicates that pre-installation infiltration rates should be at least .25 in/hour. Type D soils will require an underdrain paired with the sandy soil mix in order to drain properly. Rain gardens are often located near a building's roof drainpipe (with or without
rainwater tank A rainwater tank (sometimes called a rain barrel in North America in reference to smaller tanks, or a water butt in the UK) is a water tank used to collect and store rain water runoff, typically from rooftops via pipes. Rainwater tanks are device ...
s). Most rain gardens are designed to be an endpoint of a building's or urban site's drainage system with a capacity to percolate all incoming water through a series of soil or gravel layers beneath the surface plantings. A French drain may be used to direct a portion of the rainwater to an overflow location for heavier rain events. If the bioretention site has additional runoff directed from downspouts leading from the roof of a building, or if the existing soil has a filtration rate faster than 5 inches per hour, the substrate of the rain garden should include a layer of gravel or sand beneath the topsoil to meet that increased infiltration load. If not originally designed to include a rain garden onsite, downpipes from the roof can be disconnected and diverted to a rain garden for retrofit stormwater management. This reduces the amount of water load on the conventional drainage system, and instead directs water for infiltration and treatment through bioretention features. By reducing peak stormwater discharge, rain gardens extend
hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counte ...
lag time and somewhat mimic the natural
water cycle The water cycle, also known as the hydrologic cycle or the hydrological cycle, is a biogeochemical cycle that describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly co ...
displaced by
urban development Urban means "related to a city". In that sense, the term may refer to: * Urban area, geographical area distinct from rural areas * Urban culture, the culture of towns and cities Urban may also refer to: General * Urban (name), a list of peop ...
and allow for
groundwater Groundwater is the water present beneath Earth's surface in rock and Pore space in soil, soil pore spaces and in the fractures of stratum, rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit ...
recharge. While rain gardens always allow for restored groundwater recharge, and reduced stormwater volumes, they may not improve
pollution Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, th ...
unless remediation materials are included in the design of the filtration layers.


Vegetation

Typical rain garden plants are herbaceous perennials and grasses, which are chosen for their porous root structure and high growth rate. Trees and shrubs can also be planted to cover larger areas on the bioretention site. Although specific plants are selected and designed for respective soils and climates,Dussaillant et al

Journal of Hydrologic Engineering
plants that can tolerate both saturated and dry soil are typically used for the rain garden. They need to be maintained for maximum efficiency, and be compatible with adjacent land uses. Native and adapted plants are commonly selected for rain gardens because they are more tolerant of the local climate, soil, and water conditions; have deep and variable root systems for enhanced water infiltration and drought tolerance; increase habitat value, diversity for local ecological communities, and overall sustainability once established. Vegetation with dense and uniform root structure depth helps to maintain consistent infiltration throughout the bioretention system. There can be trade-offs associated with using native plants, including lack of availability for some species, late spring emergence, short blooming season, and relatively slow establishment. It is important to plant a wide variety of species so the rain garden is functional during all climatic conditions. It is likely that the garden will experience a gradient of moisture levels across its functional lifespan, so some drought tolerant plantings are desirable. There are four categories of a vegetative species’ moisture tolerance that can be considered when choosing plants for a rain garden. Wet soil is constantly full of water with long periods of pooling surface water; this category includes swamp and marsh sites. Moist soil is always slightly damp, and plants that thrive in this category can tolerate longer periods of flooding. Mesic soil is neither very wet nor very dry; plants that prefer this category can tolerate brief periods of flooding. Dry soil is ideal for plants that can withstand long dry periods. Plantings chosen for rain gardens must be able to thrive during both extreme wet and dry spells, since rain gardens periodically swing between these two states. A rain garden in temperate climates will unlikely dry out completely, but gardens in dry climates will need to sustain low soil moisture levels during periods of drought. On the other hand, rain gardens are unlikely to suffer from intense waterlogging, since the function of a rain garden is that excess water is drained from the site. Plants typically found in rain gardens are able to soak up large amounts of rainfall during the year as an intermediate strategy during the dry season.
Transpiration Transpiration is the process of water movement through a plant and its evaporation from aerial parts, such as leaves, stems and flowers. Water is necessary for plants but only a small amount of water taken up by the roots is used for growth ...
by growing plants accelerates soil drying between storms. Rain gardens perform best using plants that grow in regularly moist soils, because these plants can typically survive in drier soils that are relatively fertile (contain many nutrients). Chosen vegetation needs to respect site constraints and limitations, and especially should not impede the primary function of bioretention. 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 to bioretention sites the most when they are located close enough to tap moisture in the rain garden depression, yet do not excessively shade the garden and allow for evaporation. That said, shading open surface waters can reduce excessive heating of vegetative habitats. Plants tolerate inundation by warm water for less time than they tolerate cold water because heat drives out
dissolved oxygen Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...
, thus a plant tolerant of early spring flooding may not survive summer inundation.


Pollutant removal

Rain gardens are designed to capture the initial flow of stormwater and reduce the accumulation of
toxin A toxin is a naturally occurring organic poison produced by metabolic activities of living cells or organisms. Toxins occur especially as a protein or conjugated protein. The term toxin was first used by organic chemist Ludwig Brieger (1849 ...
s flowing directly into natural waterways through ground filtration. Natural remediation of contaminated stormwater is an effective, cost-free treatment process. Directing water to flow through soil and vegetation achieves particle pollutant capture, while atmospheric pollutants are captured in plant membranes and then trapped in soil, where most of them begin to break down. These approaches help to diffuse runoff, which allows contaminants to be distributed across the site instead of concentrated. The
National Science Foundation The National Science Foundation (NSF) is an independent agency of the United States government that supports fundamental research and education in all the non-medical fields of science and engineering. Its medical counterpart is the National ...
, the
United States Environmental Protection Agency The Environmental Protection Agency (EPA) is an independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon proposed the establishment of EPA on July 9, 1970; it ...
, and a number of research institutions are presently studying the impact of augmenting rain gardens with materials capable of capture or chemical reduction of the pollutants to benign compounds. The primary challenge of rain garden design is predicting the types of pollutants and the acceptable loads of pollutants the rain garden's filtration system can process during high impact storm events. Contaminants may include organic material, such as animal waste and oil spills, as well as inorganic material, such as heavy metals and fertilizer
nutrients A nutrient is a substance used by an organism to survive, grow, and reproduce. The requirement for dietary nutrient intake applies to animals, plants, fungi, and protists. Nutrients can be incorporated into cells for metabolic purposes or excr ...
. These pollutants are known to cause harmful over-promotion of plant and algal growth if they seep into streams and rivers. The challenge of predicting pollutant loads is specifically acute when a rain event occurs after a longer dry period. The initial storm water is often highly contaminated with the accumulated pollutants from dry periods. Rain garden designers have previously focused on finding robust
native plants In biogeography, a native species is indigenous to a given region or ecosystem if its presence in that region is the result of only local natural evolution (though often popularised as "with no human intervention") during history. The term is equi ...
and encouraging adequate biofiltration, but recently have begun augmenting filtration layers with media specifically suited to chemically reduce
redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or ...
of incoming pollutant streams. Certain plant species are very effective at storing mineral nutrients, which are only released once the plant dies and decays. Other species can absorb heavy metal contaminants. Cutting back and entirely removing these plants at the end of the growth cycle completely removes these contaminants. This process of cleaning up polluted soils and stormwater is called
phytoremediation Phytoremediation technologies use living plants to clean up soil, air and water contaminated with hazardous contaminants. It is defined as "the use of green plants and the associated microorganisms, along with proper soil amendments and agronom ...
.


Projects


Australia

* Healthy Waterways Raingardens Program promotes a simple and effective form of stormwater treatment, and aims to raise peoples' awareness about how good stormwater management contributes to healthy waterways. The program encourages people to build rain gardens at home, and has achieved its target is to see 10,000 rain gardens built across Melbourne by 2013. * Melbourne Water's database of Water Sensitive Urban Design projects, including 57 case studies relating to rain gardens/bioretention systems. Melbourne Water is the Victorian State Government agency responsible for managing Melbourne's water supply catchments. * Water By Design is a capacity building program that supports the uptake of Water Sensitive Urban Design, including rain gardens, in South East Queensland. It was established by the South East Queensland Healthy Waterways Partnership in 2005, as an integral component of the SEQ Healthy Waterways Strategy.


United Kingdom

* The Wildfowl and Wetlands Trust's London Wetland Centre includes a rain garden designed by Nigel Dunnett. *
Islington London Borough Council Islington London Borough Council is the local authority for the London Borough of Islington in Greater London, England. The council was created by the London Government Act 1963 and replaced two local authorities: Finsbury Metropolitan Borough Co ...
commissioned sustainable drainage consultants Robert Bray Associates to design a pilot rain garden in the Ashby Grove development which was completed in 2011. This raingarden is fed from a typical modest domestic roof catchment area of 30m² and is designed to demonstrate how simple and cost effective domestic rain gardens are to install. Monitoring apparatus was built into the design to allow Middlesex University to monitor water volumes, water quality and soil moisture content. The rain garden basin is 300mm deep and has a storage capacity of 2.17m³ which is just over the volume required to store runoff from the roof catchment in a 1 in 100 storm plus 30% allowance for climate change. * The Day Brook Rain Garden Project has introduced a number of rain gardens into an existing residential street in Sherwood, Nottingham


United States

* The 12,000 rain garden campaign for Puget Sound is coordinating efforts to build 12,000 rain gardens in the Puget Sound Basin of Western Washington by 2016. The 12,000 rain gardens website provides information and resources for the general public, landscape professionals, municipal staff, and decision makers. By providing access to the best current guidance, easy-to-use materials, and a network of trained "Rain Garden Mentor" Master Gardeners, this campaign seeks to capture and cleanse over 200 Million gallons of polluted runoff each year, and thereby significantly improve Puget Sound's water quality. *
Maplewood, Minnesota Maplewood is a city in Ramsey County, Minnesota, United States. The population was 42,088 at the 2020 census. Maplewood is ten minutes' drive from downtown Saint Paul. It stretches along the northern and eastern borders of Saint Paul. Maplewo ...
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 The University of Minnesota, formally the University of Minnesota, Twin Cities, (UMN Twin Cities, the U of M, or Minnesota) is a public land-grant research university in the Twin Cities of Minneapolis and Saint Paul, Minnesota, United States. ...
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. * Some local governmental organizations offer local grants for residents to install raingardens. In Dakota County Minnesota, the Dakota County Soil and Water Conservation District offers $250 grants and technical assistance through their Landscaping for Clean Water programhttp://www.dakotaswcd.org/cleanwater_form.html to encourage residents to install residential raingardens. * In
Seattle Seattle ( ) is a seaport city on the West Coast of the United States. It is the seat of King County, Washington. With a 2020 population of 737,015, it is the largest city in both the state of Washington and the Pacific Northwest region o ...
, 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 from 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 99%. * 10,000 Rain Gardens is a public initiative in the
Kansas City, Missouri Kansas City (abbreviated KC or KCMO) is the largest city in Missouri by population and area. As of the 2020 United States census, 2020 census, the city had a population of 508,090 in 2020, making it the List of United States cities by populat ...
metro area. Property owners are encouraged to create rain gardens, with an eventual goal of 10,000 individual gardens. * The
West Michigan West Michigan and Western Michigan are terms for an arbitrary region in the U.S. state of Michigan's Lower Peninsula. Most narrowly it refers to the Grand Rapids- Muskegon-Holland area, and more broadly to most of the region along the Lower Pe ...
Environmental Action Council has established Rain Gardens of West Michigan as an outreach water quality program. 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. In Washtenaw County, homeowners can volunteer for the Water Resources Commissioner's Rain Garden program, in which volunteers are annually selected for free professional landscape design. The homeowners build the gardens themselves as well as pay for landscaping material. Photos of the gardens as well as design documents and drainage calculations are available online. The Washtenaw County Water Resource Commissioner's office also offers yearly in person and online Master Rain Gardener classes to help guide those interested in the rain garden design, building, and upkeep process. * The city of
Portland, Oregon Portland (, ) is a port city in the Pacific Northwest and the largest city in the U.S. state of Oregon. Situated at the confluence of the Willamette and Columbia rivers, Portland is the county seat of Multnomah County, the most populous ...
, has established a Clean River Rewards program, to encourage residents to disconnect downspouts from the city's
combined sewer A combined sewer is a type of gravity sewer with a system of pipes, tunnels, pump stations etc. to transport sewage and urban runoff together to a sewage treatment plant or disposal site. This means that during rain events, the sewage gets dil ...
system and create rain gardens. Workshops, discounts on storm water bills, and web resources are offered. * In
Delaware Delaware ( ) is a state in the Mid-Atlantic region of the United States, bordering Maryland to its south and west; Pennsylvania to its north; and New Jersey and the Atlantic Ocean to its east. The state takes its name from the adjacent ...
, 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. * In New Jersey, the Rutgers Cooperative Extension Water Resources Program has already installed over 125 demonstration rain gardens in suburban and urban areas. The Water Resources Program has begun to focus on using rain gardens as
green infrastructure Green infrastructure or blue-green infrastructure refers to a network that provides the “ingredients” for solving urban and climatic challenges by building with nature.Hiltrud Pötz & Pierre Bleuze (2011). Urban green-blue grids for sustainab ...
in urban areas, such as Camden and Newark to help prevent localized flooding, combined sewer overflows, and to improve water quality. The Water Resources Program has also revised and produced a rain garden manual in collaboration with The Native Plant Society of New Jersey. * According to the Massachusetts Department of Environmental Protection, rain gardens may remove 90% of total suspended solids, 50% of nitrogen, and 90% of phosphorus. * Dr. Allen P. Davis is an environment and civil engineering professor at the University of Maryland, College Park. For the past 20 years, Davis and his team have been studying the effectiveness of rain gardens. For their research, they constructed two rain gardens on campus near the Anacostia River watershed in the Fall of 2001. Much of the runoff from the University of Maryland campus, a member of the Anacostia Watershed Restoration Partnership, ends up in the Anacostia River feeding into the Chesapeake Bay. This research finds rain gardens to be a very effective method of water capture and filtration, encouraging others in the Chesapeake Bay Watershed to implement rain gardens. ** Davis' research showed that rain gardens aid in the capturing and bio-degradation of pollutants such as suspended solids, bacteria, metals, oil, and grease. ** Water quality analyzed at the University of Maryland showed a significant increase in water clarity after rain garden filtration. ** There is a rain garden at the Center for Young Children (CYC) at University of Maryland designed by students from the Department of Plant Science and Landscape Agriculture. The rain garden allows teachers at the CYC to educate future students on sustainability.


China

* At the University of Technology in Xi'an China, a rain garden was built to observe and study over 4 years. This study showed that over 4 years, there were 28 large storm events in Xi'an. Within these 28 storms, the rain garden was able to retain the rainfall from a majority of the storms. Only 5 of these storms caused the rain garden to overflow. * Rain Gardens in this sub-humid loess region of Xi'an China, are Low Impact Developments (LID). * China plans to implement a "
sponge city Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through ...
" program in response to
urban flooding Urban flooding is the inundation of land or property in a built environment, particularly in more densely populated areas, caused by rainfall overwhelming the capacity of drainage systems, such as storm sewers. Although sometimes triggered by event ...
. This program will prioritize the natural environment and will include rain gardens, green roofs, wetlands and more permeable surfaces to slow down storm water retention.


See also

*
Bioswale Bioswales are channels designed to concentrate and convey stormwater runoff while removing debris and pollution. Bioswales can also be beneficial in recharging groundwater. Bioswales are typically vegetated, mulched, or xeriscaped. They co ...
* Climate-friendly gardening *
Constructed wetland A constructed wetland is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development ...
*
Ecohydrology Ecohydrology (from Greek , ''oikos'', "house(hold)"; , ''hydōr'', "water"; and , '' -logia'') is an interdisciplinary scientific field studying the interactions between water and ecological systems. It is considered a sub discipline of hydrology, ...
*
Green roof A green roof or living roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and draina ...
*
Microclimate A microclimate (or micro-climate) is a local set of atmospheric conditions that differ from those in the surrounding areas, often with a slight difference but sometimes with a substantial one. The term may refer to areas as small as a few squ ...
*
Rainwater harvesting Rainwater harvesting (RWH) is the collection and storage of rain, rather than allowing it to run off. Rainwater is collected from a roof-like surface and redirected to a tank, cistern, deep pit (well, shaft, or borehole), aquifer, or a reservoir w ...
*
Runoff footprint A runoff footprint is the total surface runoff that a site produces over the course of a year. According to the United States Environmental Protection Agency (EPA) stormwater is "rainwater and melted snow that runs off streets, lawns, and other s ...
*
Urban runoff Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain , storms and other precip ...
*
Water-energy nexus The water-energy nexus is the relationship between the water used for energy production,Spang, E. S., Moomaw, W. R., Gallagher, K. S., Kirshen, P. H., and Marks, D. H. (2014). "The water consumption of energy production: an international compariso ...


References


Further reading

* Dunnett, Nigel and Andy Clayden. ''Rain Gardens: Sustainable Rainwater Management for the Garden and Designed Landscape''. Timber Press: Portland, 2007. * Liu, Jia, David J. Sample, Cameron Bell and Yuntao Guan. 2014
“Review and Research Needs of Bioretention Used for the Treatment of Urban Stormwater”
Water, 6 (4): 1069–1099
“doi:10.3390/w6041069”
* 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. * * * Kraus, Helen, and Anne Spafford. ''Rain Gardening in the South: Ecologically Designed Gardens for Drought, Deluge & Everything in Between.'' Eno Publishers: Hillsborough, NC, 2009. * Bray, B., Gedge, D., Grant, G., Leuthvilay, L. ''UK Rain Garden Guide''. Published by RESET Development, London, 2012


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
Creating a Rain Garden
Details for construction of a rain garden with a link to a long plant list from Brooklyn Botanical Garden]
Stormwater Tender project
— Little Stringybark Creek, Victoria, Australia
Rain Garden Design Templates for the Chesapeake Bay Watershed

Wisconsin Department of Natural Resources — Rain Gardens

Healthy Waterways Raingardens Program — Melbourne, Victoria, Australia
{{DEFAULTSORT:Rain Garden Sustainable gardening Environmental engineering Waste management concepts Sustainable design Types of garden Hydrology and urban planning Waste treatment technology Landscape architecture Water conservation Stormwater management