Rain garden
[1] Rain gardens rely on plants and natural or engineered soil medium to retain stormwater and increase the lag time of infiltration, while remediating and filtering pollutants carried by urban runoff.
[2] Rain garden plantings commonly include wetland edge vegetation, such as wildflowers, sedges, rushes, ferns, shrubs and small trees.
Root systems enhance infiltration, maintain or even augment soil permeability, provide moisture redistribution, and sustain diverse microbial populations involved in biofiltration.
[6] 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.
[7] Some de facto rain gardens predate their recognition by professionals as a significant LID (Low Impact Development) tool.
[citation needed] 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.
[8][9][10] Redirected stormwater is often warmer than the groundwater normally feeding a stream, and has been linked to upset in some aquatic ecosystems primarily through the reduction of dissolved oxygen (DO).
[2] 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.
[14] 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.
[15] Additionally, adding a rain garden to a site that experiences excessive rainwater runoff mitigates the water quantity load on public stormwater systems.
[16] Bioretention facilities are primarily designed for water management, and can treat urban runoff, stormwater, groundwater, and in special cases, wastewater.
Environmental benefits of bioretention sites include increased wildlife diversity and habitat production and minimized energy use and pollution.
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.
[20] 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 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.
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.
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.
[13] However, a drywell placed at the lowest spot can become clogged with silt prematurely, turning the garden into an infiltration basin and defeating its purpose as a bioretention system.
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.
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.
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.
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.
[26] Typical rain garden plants are herbaceous perennials and grasses, which are chosen for their porous root structure and high growth rate.
[28] 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.
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.
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.
[29] The National Science Foundation, the United States Environmental Protection Agency, 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.
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.
