Swales

This article is about installations designed to capture and convey surface runoff along a vegetated channel.

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 design of both may increase the benefits:
Bioswales are sometimes referred to as 'dry swales', 'vegetated swales', or 'water quality swales'. This type of structure is similar to a bioretention cell but with a long linear shape (surface area typically >2:1 length:width )
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.

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 Swales

Property	Bioswale	Enhanced Grass Swale
Surface water	Minimal Any surface flow can be slowed with check dams	Ponding is encouraged with check dams
Engineered soil	Biomedia required	Amendment preferable when possible
Underdrain	Common	Uncommon
Maintenance	Medium to high	Low
Stormwater benefit	High	Medium
Biodiversity benefit	Increased with native planting	Lower

The fundamental components of a swale are:

• Graded channel
• Planting

Additional components may include:

• Biomedia - an engineered soil mix
• Underdrain with clean out and inspection ports
• Impermeable membrane to prevent infiltration to soils below
• Turf reinforcement to prevent scour
• Check dams

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  Bioswale, County Court Boulevard, Brampton

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Planning Considerations

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

• An absolute minimum width is 0.6 m is required for bioswales to have healthy plant growth, and to facilitate construction.
• Grassed swales are usually mown as part of routine maintenance, so the cross section will be parabolic or trapzoidal in shape. The minimum width for this type is 2 m.

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

• Check dams are required on slopes > 3 %.
• Slopes > 6% can accommodate a series of stepped bioretention cells, each overflowing into the next with a weir.

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  Bioswale with check dams
  (vertical scale exaggerated)

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  Stepped bioretention cells alternative for slopes >6 %

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Design

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 spreading 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.

Check dams

Check dams are a feature of enhanced swales. They promote infiltration and evaporation by promoting limited ponding. To design check dams into a swale:

1. The height of each dam is determined by the depth of ponded water that will infiltrate in 24 hours. The infiltration may be into the native soils, into biomedia, or some other soil amendment may be proposed in the design.
2. Dams are usually installed between 10-20 m along the swale. They are distributed such that the crest of each dam is at approximately the same elevation as the toe of the upstream dam. If the slope along the swale varies, so should the distance between the dams.

The objective of these design recommendations are to maximize the distribution of ponded water along the whole BMP. Detailed design may require iteration of the dam heights and distances along each section of a long swale.

• Dams may be constructed of any resilient and waterproof material, including: rock gabions, earth berms, coarse aggregate or rip-rap, concrete, metal or wood.
• Energy dissipation and erosion control measures should be installed in the 1 - 2 m downstream of each dam. Examples include large aggregate or turf reinforcement

Other components

• Underdrain
• Biomedia

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

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Performance

<h4blue>Bioswales</h4blue> 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

<h4blue>Enhanced grass swales</h4blue>

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Incentives and Credits

In Ontario

City of Mississauga
The City of Mississauga has a stormwater management credit program which includes RWH as one of their recommended site strategies[1].

LEED BD + C v. 4

SITES v.2

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See Also

• Bioretention Cells
• Check Dams
• Trees

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External Links

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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 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.

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.

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.

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.

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.

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 perforated pipe used to assist the draining of soils.

Cleanouts are "a fitting access in a drainage system or venting system that is installed to provide access for cleaning and inspection and that is provided with a readily replaceable air tight cover" (O.Reg 332/12: Building Code, 2022).

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.

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

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.

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.

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.

Abiotic transfer of water vapour to the atmosphere.

The practice of adding organic material, such as mulch or compost to topsoil to improve fertility, and tilling of the native soils to reverse compaction and restore its water retaining capacity.

A broad category of particulate material used in construction, including sand, gravel, crushed stone, slag, recycled concrete and geosynthetic aggregates, and available in various particulate size gradations.

Includes the protection of soil from dislocation by water, wind or other agents.

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

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.

Rainwater harvesting.

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.