History[ edit ] The first rain gardens were created to mimic the natural water retention areas that developed before urbanization occurred. 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. What is 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 developed communities retrofitting bioretention into existing stormwater management infrastructure as it is for developing communities seeking a faster and more sustainable development path. Urban runoff mitigation[ edit ] Further information: Urbanization Effects of urban runoff[ edit ] In developed urban areas, naturally occurring depressions where storm water would pool are typically covered by impermeable surfaces, such as asphalt, pavement, or concrete, and are leveled for automobile use.
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History[ edit ] The first rain gardens were created to mimic the natural water retention areas that developed before urbanization occurred. 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.
What is 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 developed communities retrofitting bioretention into existing stormwater management infrastructure as it is for developing communities seeking a faster and more sustainable development path.
Urban runoff mitigation[ edit ] Further information: Urbanization Effects of urban runoff[ edit ] In developed urban areas, naturally occurring depressions where storm water would pool are typically covered by impermeable surfaces, such as asphalt, pavement, or concrete, and are leveled for automobile use.
Stormwater is directed into storm drains which may cause overflows of combined sewer systems or pollution, erosion, or flooding of waterways receiving the storm water runoff.
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 , pesticides , herbicides , hydrocarbons and trace metals. Urban watersheds are affected by greater quantities of pollutants due to the consequences of anthropogenic activities within urban environments.
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. Increasing the amount 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. 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 surfaces covering the ground that allow for contaminated urban runoff.
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. 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.
Carefully designed constructed wetlands are necessary for the bioretention of sewage water or grey water , 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[ edit ] Bioretention controls the stormwater quantity through interception, infiltration, evaporation, and transpiration.
Then, water performs infiltration - 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.
Bioretention controls the stormwater quality through settling, filtration, assimilation, adsorption , degradation, and decomposition. 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 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[ edit ] A home rain garden recently planted 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.
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.
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 , birds , and beneficial insects.
Rain gardens are at times confused with bioswales. 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 , wetland , or pond, and not a rain garden. Rain gardens also differ from retention basins , where the water will infiltrate the ground at a much slower rate, within a day or two.
Soil and drainage[ edit ] 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. Soils with higher concentrations of compost have shown improved effects on filtering groundwater and rainwater.
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 study. 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.
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. In some cases lined bioretention cells with subsurface drainage are used to retain smaller amounts of water and filter larger amounts without letting water percolate as quickly.
A five-year study by the U. 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. Type D soils will require an underdrain paired with the sandy soil mix in order to drain properly. 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.
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 lag time and somewhat mimic the natural water cycle displaced by urban development and allow for groundwater recharge. While rain gardens always allow for restored groundwater recharge, and reduced stormwater volumes, they may not improve pollution unless remediation materials are included in the design of the filtration layers.
Although specific plants are selected and designed for respective soils and climates,  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. 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. 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.
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.
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 lots of 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 , thus a plant tolerant of early spring flooding may not survive summer inundation. 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. Contaminants may include organic material, such as animal waste and oil spills, as well as inorganic material, such as heavy metals and fertilizer nutrients. 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 and encouraging adequate biofiltration, but recently have begun augmenting filtration layers with media specifically suited to chemically reduce redox 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.
The program encourages people to build rain gardens at home, and has achieved its target is to see 10, rain gardens built across Melbourne by 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 mm deep and has a storage capacity of 2. The 12, rain gardens website provides information and resources for the general public, landscape professionals, municipal staff, and decision makers.
Many neighborhoods had swales added to each property, but installation of a garden at the swale was voluntary. 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.
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