Bioswales are landscape elements designed to concentrate or remove debris and pollution out of surface runoff water. They consist of a swaled drainage course with gently sloped sides (less than 6%) and filled with vegetation, compost and/or riprap.[1]:19 The water's flow path, along with the wide and shallow ditch, is designed to maximize the time water spends in the swale, which aids the collection and removal of pollutants, silt and debris. Bioswales are also beneficial in groundwater recharge and are effective stormwater mitigation tools. Depending upon the topography of the land, a bioswale may have a meandering or almost straight channel alignment. A bioswale's make-up can be influenced by many different variables, including climate, rainfall patterns, size of the site, budget, and available vegetation that can be planted.

It is important to maintain bioswales to ensure best possible efficiency and effectiveness in removal of pollutants in the stormwater runoff. Planning for these things is an important step, which can include the introduction of filters or large rocks to prevent clogging. Annual maintenance through soil testing, visual inspection, and mechanical testing is also crucial to the health of a bioswale.

A common application is around parking lots, where substantial automotive pollution is settled on the paving and then flushed by the first instance of rain, known as the first flush. The bioswales, or other type of biofilter, can be created around the edges of parking lots to capture and treat the stormwater runoff before releasing it to the watershed or storm sewer.

Two bioswales for a housing development. The foreground one is under construction while the background one is established.

Contaminants addressed

Bioswales work to remove pollutants through vegetation and the soil. As the storm water runoff flows through the bioswale, the pollutants are captured and settled by the leaves and stems of the plants. The pollutants then enter the soil where they decompose or can be broken down by bacteria in healthy soil.[2]

There are several classes of water pollutants that may be collected or arrested with bioswales. These fall into the categories of silt, inorganic contaminants, organic chemicals and pathogens.

  • Silt – How bioswales and plants are constructed slow the conveyance of silt and reduce the turbidity of receiving waters. Filters can be established to capture debris and silt during the process.
  • Organics – Many organic contaminants including Polycyclic aromatic hydrocarbons will volatilize or degrade over time and Bioswales slow the conveyance of these materials into waterways, and before they can affect aquatic life. Although not all organic material will be captured, the concentration of organic material is greatly reduced by bioswales.
  • Pathogens – are deprived of a host or from a nutrient supply long enough for them to become the target of a heterotroph.
  • Common inorganic compounds are macronutrients such as phosphates and nitrates. Principal sources of these nutrients comes from agricultural runoff attributed to excess fertilization. Excess phosphates and nitrates can cause eutrophication in disposal zones and receiving waters. Specific bioswale plants absorb these excess nutrients.
  • Metallic compounds such as mercury, lead, chromium, cadmium and other heavy metals are concentrated in the structures. Unfortunately, these metals slowly poison the surrounding soil. Regular soil removal is required in order to prevent metals from dissolving and releasing back into the environment. Some bioswales are designed to include hyperaccumulator plant species. These plants absorb but do not transform the metals. Cuttings from these plants often decompose back into the pond or are pruned by gardening services that do not know the compost they are collecting is poisonous.

Best locations

Bioswales can be implemented in areas that require stormwater management to regulate the runoff velocity and decontaminate the runoff. Bioswales are created to handle the first flush of pollutants during the event of rain, therefore, locations that have high areas of impervious surface such as roads, parking lots, or rooftops can benefit from additions of bioswales. They can also be integrated into road medians, curb cutouts, sidewalks, or any public space.[3]


Bioswales are useful low-impact development work to decrease the velocity of stormwater runoff while removing pollutants from the discharge. They are extremely beneficial in protecting surface water and local waterways from excessive pollution from stormwater runoff. The longer the runoff stays within the bioswale, the better the pollutant removal outcome. It is also beneficial in removing standing ponds that could potentially attract mosquitos. Bioswales can also be designed to be aesthetically pleasing and attract animals and create habitats. Bioswales can also be beneficial for groundwater recharge.[4]


Improper maintenance can lead to high restoration costs to address inefficient bioswales. An accumulation of large sediments, trash, and improper growth of vegetation can all affect the quality and performance of bioswales. It is beneficial at the planning stages to set apart easements to allow for easier maintenance of biowales, whether it be adequate space to locate machinery or safety to those working. Different types of filters can be used to catch sediments. Grass filter strips or rock inlets can be used to filter sediments and particulates; however, without proper maintenance, runoff could flow away from the bioswales due to blockage. Structural inlets have become more common due to the ease of maintenance, use, and its effectiveness. Avoiding the use of floating mulch and selecting the best fit low-maintenance plants ensure better efficiency in the bioswales.[5] Depending on a community's needs for a bioswale, a four step assessment program can be developed. Visual inspection, capacity testing, synthetic runoff, and monitoring are the four steps that can be used to evaluate performance and maintenance of bioswales.[6]

Routine inspection is required to ensure that the performance and aesthetics of bioswales are not compromised. Time and frequency of inspections vary based on different local governments, but should occur at least once a year. Various aspects of inspection can take place, either visually or mechanically. Visual observation of the vegetation, water, and inlets are all crucial to ensure performance. Some organizations utilize checklists to streamline the visual inspection process.[6]

There are different methods to determine if a bioswale needs maintenance. Bioswales are benchmarked to meet a specific level of infiltration to determine if maintenance is required. A staff gauge is used to measure the infiltration rate. Soil chemistry testing is also required to determine if the soil has a certain off-level of any pollutant. Phosphorus and high levels of salinity in the soil are two common pollutants that should be attended to. Analysis of inflow and outflow pollutant concentration is also another way to determine the performance level of bioswales.[5]

Maintenance can span to three different levels of care. Aesthetic maintenance is required to remove weeds that affect the performance of the other plants and the bioswale itself, clean and remove trash, and maintaining the looks of the vegetation. Partial restoration is needed when the inlet is blocked by sediments or when vegetation needs to be replaced. Full restoration is required when the bioswales no longer filter pollutants adequately and overall performance is severely lacking.[5]


Bioswales experience short, potentially intense, periods of rain, flooding and pollutant loading followed by dry seasons. It is important to take into account how the vegetation selected for the bioswales will grow and understanding what types of plants are considered the best fit.[5]

There are four types of bioswales that can be constructed based on the needs of the location.[7]

  • Low grass bioswales utilizes low growing grass that can be landscape, similar to lawns. These types of bioswales tend to be less effective than vegetated bioswales in treating stormwater runoff and sustaining an adequate collection time.
  • Vegetated bioswales are created with taller growing plants, ornamental vegetations, shrubs, and even trees. These types can also be lined with rocks to slow down the velocity of stormwater runoff that is flowing through bioswales to increase collection time for decontamination. Vegetated bioswales can also include vegetation that is highly useful in removing certain chemicals in runoffs very efficiently.
  • Low water use bioswales are helpful in areas that tend to be drier with hotter climate. Xeriscape bioswales are populated with runoff generally only after rain and storms and stay dry otherwise.
  • Wet bioswales are similar to wetlands in which they retain water for a much longer period of time that allows for infiltration of stormwater instead of simply emptying the water at the end of the bioswale into storm drain inlets.

Bioswales require a certain soil composition that does not contain more than 5% clay. The soil itself before implementation should not be contaminated. Bioswales should be constructed with a longitudinal slope to allow sediments to settle. Maximum slope of bioswales is 3:1. A minimum clearance is required to ensure that other infrastructure would not be damaged. The overfill drain should be located at least 6 inches above the ground plain to allow for maximum concentration time of stormwater runoff in the bioswales. Rocks can also be used to slow down the runoff velocity. The use of filters is important to prevent inlets from becoming blocked by sediments or trash.[3]


Two early examples of scientifically designed bioswales for large scale applications are found in the western US. In 1996 for Willamette River Park in Portland, Oregon a total of 2330 lineal feet of bioswale was designed and installed to capture and prevent pollutant runoff from entering the Willamette River. Intermittent check dams were installed to further abet silt capture, which reduced by 50% suspended solids entering the river system.[8]

A second example of a large scale designed bioswale is at the Carneros Business Park, Sonoma County, California. Starting in 1997 the project design team worked with the California Department of Fish and Game and County of Sonoma to produce a detailed design to channel surface runoff at the perimeter of a large parking area. Surface runoff consists of building roof runoff, parking lot runoff and overland flow from properties to the north of the project site. A total of two lineal miles of bioswale was designed into the project. The purpose of the bioswale was to minimize runoff contaminants from entering Sonoma Creek. The bioswale channel is grass-lined, and nearly linear in form. Downslope gradient is approximately 4% and cross-slope gradient is approximately 6%.[9]

A relatively recent project established was the Seattle, Washington Street Edge Alternatives project, completed in 2001. Rather than using traditional piping, SEA's goal was to create a natural landscape that represented what the area was like before development. The street was 11% more pervious than a standard street and was characterized with evergreen trees and bioswales. The bioswales were planted on graded slopes with wetland and upland plants. Other landscaping also focused on native and salmon-friendly plants. SEA provided a strong benefit for stormwater runoff mitigation that helped continue to protect Seattle's creek ecology. The project street also created a more inviting and aesthetically pleasing site as opposed to hard landscaping.[10]

See also


  1. ^ Loechl, Paul M.; et al. (2003). Design Schematics for a Sustainable Parking Lot (PDF). Champaign, IL: US Army Corps of Engineers, Research and Development Center. Archived from the original (PDF) on 2010-06-02. Construction Engineering Research Laboratory. Document no. ERDC/CERL TR-03-12.
  2. ^ "Bioswales". 2013-11-10. Retrieved 2018-03-28.
  3. ^ a b "Bioswales – National Association of City Transportation Officials". National Association of City Transportation Officials. Retrieved 2018-03-05.
  4. ^ "Bioswales can improve water quality resources". MSU Extension. Retrieved 2018-03-21.
  5. ^ a b c d epa (September 2016). "Operation and Maintenance of Green Infrastructure Receiving Runoff from Roads and Parking Lots" (PDF).
  6. ^ a b J., Erickson, Andrew (2013). Optimizing stormwater treatment practices a handbook of assessment and maintenance. Weiss, Peter T., Gulliver, John S. New York: Springer. ISBN 9781461446248. OCLC 830293149.
  7. ^ Caflisch, Mary; Giacalone, Katie (May 2015). "An Introduction to Bioswales". Clemson University. Retrieved 25 February 2018.
  8. ^ France, Robert L. (2002). Handbook of Water Sensitive Planning and Design. Boca Raton, Florida: CRC Press. ISBN 1-56670-562-2.
  9. ^ Lumina Technologies (1998). Hydrology and biology studies for Carneros Business Park, prepared for the William A. Saks Company pursuant to requirements of the County of Sonoma. Approximately 2000 bioswales are projected to be installed in New York City to protect the city's combined sewer system.
  10. ^ "Street Edge Alternatives — Seattle Public Utilities". Retrieved 2018-03-21.

External links


Bioretention is the process in which contaminants and sedimentation are removed from stormwater runoff. Stormwater is collected into the treatment area which consists of a grass buffer strip, sand bed, ponding area, organic layer or mulch layer, planting soil, and plants. Runoff passes first over or through a sand bed, which slows the runoff's velocity, distributes it evenly along the length of the ponding area, which consists of a surface organic layer and/or groundcover and the underlying planting soil. The ponding area is graded, its center depressed. Water is ponded to a depth of 15 cm (5.9 in) and gradually infiltrates the bioretention area or is evapotranspired. The bioretention area is graded to divert excess runoff away from itself. Stored water in the bioretention area planting soil exfiltrates over a period of days into the underlying soils.

Bybee Bridge

The Bybee Bridge is a bridge over McLoughlin Boulevard (Oregon Route 99E) in southeast Portland, Oregon connecting the Eastmoreland and Sellwood neighborhoods. The bridge is named after James Francis Bybee.

Civita, San Diego

Civita is a sustainable, transit-oriented 230-acre (93 ha) master-planned community in the Mission Valley area of the city of San Diego, San Diego County, United States. Located on a former quarry site, the urban-style village is organized around a 19-acre (7.7 ha) community park that cascades down the terraced property.Civita development plans call for 60 to 70 acres of parks and open space, 4,780 residences (including approximately 478 affordable units), an approximately 480,000-square-foot retail center, and 420,000 square feet for an office/business campus.The Civita project is budgeted at $2 billion and being developed by Sudberry Properties, in partnership with the Grant family, which has operated the former quarry since 1937.

Curb extension

A curb extension (or also neckdown, kerb extension, bulb-out, bump-out, kerb build-out, nib, elephant ear, curb bulge, curb bulb, or blister) is a traffic calming measure, primarily used to extend the sidewalk, reducing the crossing distance and allowing pedestrians about to cross and approaching vehicle drivers to see each other when vehicles parked in a parking lane would otherwise block visibility.

A curb extension is an angled narrowing of the roadway and a widening of the sidewalk (pavement or footway in UK usage). This is often accompanied by an area of enhanced restrictions (such as a "no stopping" or "no parking" zone) and the appropriate visual reinforcement. This is achieved using painted road markings (e.g. lines, coloured areas, or chevrons), barriers, bollards, or the addition of pavement or street furniture (e.g. planters, lamp standards, or benches).

Curb extensions are often used in combination with other traffic calming measures such as chicanes, speed bumps, or rumble strips, and are frequently sited to "guard" pedestrian crossings. In these cases the "squeeze" effect of the narrowed roadway shortens the exposed distance pedestrians must walk.

Erosion control

Erosion control is the practice of preventing or controlling wind or water erosion in agriculture, land development, coastal areas, river banks and construction. Effective erosion controls handle surface runoff and are important techniques in preventing water pollution, soil loss, wildlife habitat loss and human property loss.

Impervious surface

Impervious surfaces are mainly artificial structures—such as pavements (roads, sidewalks, driveways and parking lots, as well as industrial areas such as airports, ports and logistics and distribution centres, all of which use considerable paved areas) that are covered by impenetrable materials such as asphalt, concrete, brick, stone—and rooftops. Soils compacted by urban development are also highly impervious.

Index of environmental articles

The natural environment, commonly referred to simply as the environment, includes all living and non-living things occurring naturally on Earth.

The natural environment includes complete ecological units that function as natural systems without massive human intervention, including all vegetation, animals, microorganisms, soil, rocks, atmosphere and natural phenomena that occur within their boundaries. Also part of the natural environment is universal natural resources and physical phenomena that lack clear-cut boundaries, such as air, water, and climate.

List of Kia design and manufacturing facilities

Kia Motors maintains 14 manufacturing facilities in eight countries, and research centres in South Korea, the USA, Japan, and Germany. Countries include the United States, Eastern Europe, China, India, Japan, Mexico and Vietnam.

Meadow Brook (Lackawanna River tributary)

Meadow Brook is a tributary of the Lackawanna River in Lackawanna County, Pennsylvania, in the United States. It is approximately 2.0 miles (3.2 km) long and flows through Dunmore and Scranton. The watershed of the stream has an area of 2.43 square miles (6.3 km2), though it used to be considerably larger. It is designated as a Coldwater Fishery and a Migratory Fishery, but many reaches of the stream have been destroyed by mining or post-mining development impacts. The stream flows through a culvert system for much of its length. However, there are areas where it is in an open concrete channel or has a natural streambed. There are also patches of old-growth forest along the stream in the Forest Hill Cemetery.

Meadow Brook has experienced significant flow loss and what flow it does have mainly consists of intermittent stormwater flows. There used to be springs, seeps, and wetlands at the stream's headwaters. However, a colliery (and later a landfill) was built over that area. The Dunmore Cemetery and the Forest Hill Cemetery are in the stream's vicinity. Meadow Brook is a first-order stream.

Open Road Park

Open Road Park is among the larger green spaces created in the East Village as a result of community organizing. The site of this park was taken over in 1993 by Open Road, a neighborhood nonprofit that developed the lot into a community garden and playground. Prior to its use as a park, the site was used for many purposes that reflect on the history of the surrounding neighborhood.

Rain garden

One of the wide variety of soil-absorption/filter systems, a rain garden, also called a stormwater garden, is a designed depression storage or a planted hole that allows rainwater runoff from impervious urban areas, like roofs, driveways, walkways, parking lots, and compacted lawn areas, the opportunity to be absorbed. The primary purpose of a rain garden is to improve water quality in nearby bodies of water and to ensure that rainwater becomes available for plants as groundwater rather than being sent through stormwater drains straight out to sea. In fact, it can actually reduce 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) and cut down on the amount of pollution reaching creeks and streams by up to 30%.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. Deep plant roots also create additional channels for stormwater to filter into the ground. Root systems enhance infiltration, maintain or even augment soil permeability, provide moisture redistribution, and sustain diverse microbial populations involved in biofiltration. Microbial populations feed off plant root secretions and break down carbon (such as in mulch or desiccated plant roots) to aggregate soil particles which increases infiltration rates. Also, through the process of transpiration, rain garden plants return water vapor to 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. Thus, the whole garden can become a rain garden, and each component of the whole can become a small-scale rain garden in itself.

Riparian zone

A riparian zone or riparian area is the interface between land and a river or stream. Riparian is also the proper nomenclature for one of the terrestrial biomes of the Earth. Plant habitats and communities along the river margins and banks are called riparian vegetation, characterized by hydrophilic plants. Riparian zones are important in ecology, environmental resource management, and civil engineering because of their role in soil conservation, their habitat biodiversity, and the influence they have on fauna and aquatic ecosystems, including grasslands, woodlands, wetlands, or even non-vegetative areas. In some regions the terms riparian woodland, riparian forest, riparian buffer zone, riparian corridor and riparian strip are used to characterize a riparian zone. The word riparian is derived from Latin ripa, meaning river bank.


Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation. If buried, they may eventually become sandstone and siltstone (sedimentary rocks) through lithification.

Sediments are most often transported by water (fluvial processes), but also wind (aeolian processes) and glaciers. Beach sands and river channel deposits are examples of fluvial transport and deposition, though sediment also often settles out of slow-moving or standing water in lakes and oceans. Desert sand dunes and loess are examples of aeolian transport and deposition. Glacial moraine deposits and till are ice-transported sediments.

Sustainable drainage system

Sustainable drainage systems (also known as SuDS, SUDS, or Sustainable Urban Drainage Systems) are a collection of water management practices that aim to align modern drainage systems with natural water processes. SuDS efforts make urban drainage systems more compatible with components of the natural water cycle such as storm surge overflows, soil percolation, and bio-filtration. These efforts hope to mitigate the effect human development has had or may have on the natural water cycle, particularly surface runoff and water pollution trends. SuDS have become popular in recent decades as our understanding of how urban development affects natural environments, as well as concern for climate change and sustainability, have increased. SuDS often use built components that mimic natural features in order to integrate urban drainage systems into the natural drainage systems or a site as efficiently and quickly as possible.

Swale (landform)

A swale is a shady spot, or a sunken or marshy place. In particular, in US usage, it is a shallow channel with gently sloping sides. Such a swale may be either natural or man-made. Artificial swales are often infiltration basins, designed to manage water runoff, filter pollutants, and increase rainwater infiltration.

Village Building Convergence

The Village Building Convergence (VBC) is an annual 10-day event held every May in Portland, Oregon, United States. The event is coordinated by the City Repair Project and consists of a series of workshops incorporating natural building and permaculture design at multiple sites around the city. Many of the workshops center on "intersection repairs" which aim to transform street intersections into public gathering spaces.

Stormwater management structures
Treatment / Containment
Flow control


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