Swales

Bioswale, County Court Boulevard, Brampton

This article is about installations designed to capture and convey surface runoff along a vegetated channel, whilst also promoting infiltration.
For underground conveyance systems which promote infiltration, see Exfiltration trenches.

Overview

Swales are linear landscape features consisting of a drainage channel with gently sloping sides. Underground they may be filled with engineered soil and/or contain a water storage layer of coarse gravel material. Two variations on a basic swale are recommended as low impact development strategies, although using a combination of both designs may increase the benefits:
Bioswales are sometimes referred to as 'dry swales', 'vegetated swales', or 'water quality swales'. This type of BMP is form of bioretention with a long, linear shape (surface area typically >2:1 length:width) and a slope which conveys water and generally contains various water tolerant vegetation,
Enhanced grass swales are a lower maintenance alternative, but generally have lower stormwater management potential. The enhancement over a basic grass swale is in the addition of check dams to slow surface water flow and create small temporary pools of water which can infiltrate the underlying soil.
Grass swales are a relatively common landscape feature already and a great opportunity for retrofit, to reduce flow and improve water quality by encouraging settling and infiltration behind a series of check dams.
Retention swales can be imagined as linear, sloped dry ponds. They typically make relatively small contributions to water volume and quality control than many other BMPs, but they may feature as part of a site-wide treatment train approach.

Take a look at the downloadable Enhanced Grass Swales Factsheet below for a .pdf overview of this LID Best Management Practice:

Swales are an ideal technology for:

• Sites with long, linear landscaped areas, such as parking lots
• Connecting with one or more other types of LID

Types of Swale

Property	Bioswale	Enhanced grass swale
Surface water	Minimal Any surface flow can be slowed with check dams	Ponding is encouraged with check dams
Soil	Filter media required	Amendment preferable when possible
Underdrain	Common	Uncommon
Maintenance	Medium to high	Low
Stormwater benefit	High	Moderate
Biodiversity benefit	Increased with native planting	Typically lower

Planning Considerations

Bioswale with check dams
(vertical scale exaggerated)

Stepped bioretention cells alternative for slopes >6 %
(vertical scale exaggerated)

Enhanced swales and in some cases bioswales include check dams that can be designed to pond substantial volumes of water on the swale surface between flow events, so provide both stormwater conveyance and infiltration functions. For information about constraints to infiltration practices, and approaches and tools for identifying and designing within them see Infiltration. For guidance on infiltration testing and selecting a design infiltration rate see Design infiltration rate.

A linear design (surface area typically >2:1 length:width) is a common feature of swales:

• An absolute minimum width of 0.6 m is required for bioswales to promote healthy plant growth, and to facilitate construction,
• Grassed swales are usually mown as part of routine maintenance, so the cross section will be triangular or trapezoidal in shape with maximum side slopes of 1:3. The minimum width for this type would be 2 m. See Best cross sections

Swales may be graded along longitudinal slopes between 0.5 - 6 %:

• Between 1 - 6 %, check dams are recommended to bring the compensation gradient <1 %.
• Slopes > 6% can accommodate a series of stepped bioretention cells, each overflowing into the next with a spillway.

For a table summarizing information on planning considerations and site constraints see Site considerations.

Design

Distance between dams is determined by equalling the elevation of the crest of each dam, with the elevation of the toe of the upstream dam.
(vertical scale exaggerated)

Pretreatment and inlets

To minimize erosion and maximize the functionality of the swale, sheet flow of surface water should be directed into the side of the BMP. Gravel diaphragms, vegetated filter strips and shallow side slopes are ideal. Alternatively, a series of curb inlets can be employed, where each has some form of flow spreader incorporated. Single point inflow can cause increased erosion and sedimentation, which will damage vegetation and contribute to BMP failure. Again, flow spreading devices can mitigate these processes, where concentrated point inflow is required.

Inspection and Maintenance

Maintenance requirements for enhanced grass swales, and swales is similar to vegetated filter strips and typically involve a low level of activity after vegetation becomes established. Grass channel maintenance procedures are already in place at many municipal public works and transportation departments. These procedures should be compared to the recommendations provided on the Inspection and Maintenance: Enhanced Swales page to assure that the infiltration and water quality benefits of enhanced grass swales are preserved.

Routine roadside ditch maintenance practices such as scraping and re-grading should be avoided at swale locations. Vehicles should not be parked or driven on grass swales. For routine mowing, the lightest possible mowing equipment should be used to prevent soil compaction.

For swales located on private property, the property owner, resident or manager is responsible for maintenance as outlined in a legally binding maintenance agreement. Roadside swales in residential areas generally receive routine maintenance from homeowners who should be advised regarding recommended maintenance activities and ensure they do not build anything within or on the channel of the swale which could result in flooding or pooling on theirs or their neighbours' properties.

Take a look at the Inspection and Maintenance: Enhanced Swales page by clicking below for further details about proper inspection and maintenance practices:

Performance

An evaluation of swales and roadside ditches was published by STEP in 1999. The project page and additional tools are available here.

Bioswales

While few field studies of the pollutant removal capacity of bioswales are available from cold climate regions like Ontario, it can be assumed that they would perform similar to bioretention cells. Bioretention provides effective removal for many pollutants as a result of sedimentation, filtering, plant uptake, soil adsorption, and microbial processes. It is important to note that there is a relationship between the water balance and water quality functions. If a bioswale infiltrates and evaporates 100% of the flow from a site, then there is essentially no pollution leaving the site in surface runoff. Furthermore, treatment of infiltrated runoff will continue to occur as it moves through the native soils.

Volumetric runoff reduction from bioswales

LID Practice	Location	Runoff Reduction*	Reference
Bioswale without underdrain	Washington	98%	Horner et al. (2003)
	Scotland	94%	Jefferies (2005)
Bioswale with Underdrain	Maryland	46 to 54%	Stagge (2006)
Bioretention without underdrain	China	85 to 100%*	Gao, et al. (2018)
	Connecticut	99%	Dietz and Clausen (2005)
	Pennsylvania	80%	Ermilio (2005)
	Pennsylvania	70%	Emerson and Traver (2004)
Bioretention with underdrain
	Ontario	64%	CVC (2020)
	Maryland and North Carolina	20 to 50%	Li et al. (2009)
	North Carolina	40 to 60%	Smith and Hunt (2007)
North Carolina	33 to 50%	Hunt and Lord (2006)
Runoff Reduction Estimate*		85% without underdrain; 45% with underdrain

Click on the citations above to view the performance of these features based upon specifics (i.e. drainage area to size ratio to achieve the reduction mentioned) in the table.

Enhanced grass swales

See Performance section on Enhanced swale page link above for additional information.

Construction

Construction

Galleries

Simple grass swales

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  Grass swale in CVC headquarters parking lot, Mississauga ON

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  Water distributed into a wide grass swale with a rock level spreader

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  Flow in this wide swale is being reduced by taller vegetation on some sections (See Manning's n)

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Bioswales

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  Note the use of stone in the centre of the channel, with mulch cover on the side slopes. Mississauga Road, ON

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  Bioswale in High Point neighborhood, Seattle, WA
  Photo credit: USEPA

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  Streetside swale in Seattle

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  This feature is a bioswale in that the underlying soil has been replaced with engineered filter media. The turf finish simplifies landscape maintenance. Brampton, ON

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  A bioswale located on Country Court Blvd., located in Brampton, ON> Read more about the performance of this feature and associated rpoject, by reading the following Grey to Green Conference Presentation, 2018. Dean Young - STEP, 2018

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  Bioswale located in Downsview Park, Toronto ON. with Check dams.

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  Grassed bioswale with Vegetated filter strips

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  Bioswale located in a parking lot with a 'beehive-styled' overflow, with Curb cuts Inlets at the back of the feature, located in the Burlington ON.

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  Stone-lined bioswale with rock check dams located to helps slow the flow of water as it enter the system from the nearby roads. [[ STEP, 2017

Check dams

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  Enhanced swale with rocky check dams and a metal overflow grate in Northgate Mall parking lot, Seattle. Photo credit: MLSmith

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  Bioswale with rock check dams to slow down the water, encouraging infiltration. Note the biodegradable erosion control blanket still in place. LSRCA headquarters, 2017

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  Concrete check dam with armourstone to protect against erosion. Concrete colour blends well with surrounding stone. This dam extends to the full depth of the swale and has 25 mm ø weep holes below grade.

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  A check dam in action, effectively pooling water.

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  A swale during a rain event, with concrete check dams, armourstone, mulch and Tall grasses to slow down moving water (as shown in the .gif file) to promote infiltration in the feature.

Also see Jen's Pinterest board of check dams

See Also

• Bioswales
• Check dams

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

Emphasis on improving the value of particular aspects of water and related land resources.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

Stormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.

A combination of lot level, conveyance, and end-of-pipe stormwater management practices.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff.

The engineered soil component of bioretention cell or dry swale designs, typically with a high rate of infiltration and designed to retain contaminants through filtration and adsorption to particles.

A perforated pipe used to assist the draining of soils.

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

A body of water smaller than a lake, often artificially formed.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

Deposition of material of varying size, both mineral and organic away from its site of origin by the action of water, wind, gravity or ice.

Settling-out or deposition of particulate matter suspended in runoff.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

A long narrow trench or furrow dug in the ground, as for irrigation, drainage, or a boundary line.

Land owned by private individuals or companies.

Sustainable Technologies Evaluation Program

A shallow excavated surface depression containing prepared filter media, mulch, and planted with selected vegetation.

The attachment of gas, vapour or dissolved matter onto the surface of solid materials.

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

A perforated pipe used to assist the draining of soils.

Credit Valley Conservation

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

Vegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff.

Linear bioretention cell designed to convey, treat and attenuate stormwater runoff. The engineered filter media soil mixture and vegetation slows the runoff water to allow sedimentation, filtration through the root zone, evapotranspiration, and infiltration into the underlying native soil.

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

Lake Simcoe Region Conservation Authority

Structures constructed of a non-erosive material, such as suitably sized aggregate, wood, gabions, riprap, or concrete; used to slow runoff water. Can be employed in practices such as bioswales and enhanced grass swales.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

a top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

A lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts.

Low impact development is a stormwater management and land development strategy applied at the parcel and subdivision scale that emphasizes conservation and use of on-site natural features integrated with engineered, small scale hydrologic controls to more closely mimic pre-development hydrologic functions.

A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.