Swales

This article is about installations designed to capture and convey surface runoff along a vegetated channel, whilst also promoting infiltration.
For underground conveyance which promotes 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 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 Swale

Property	Bioswale	Enhanced grass swale
Surface water	Minimal Any surface flow can be slowed with check dams	Ponding is encouraged with check dams } 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 Filter media required Amendment preferable when possible Underdrain Common Uncommon Maintenance Medium to high Low Stormwater benefit High Medium Biodiversity benefit Increased with native planting Lower <panelWarning> Bioswale, County Court Boulevard, Brampton </panelWarning> 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. <panelWarning> Bioswale with check dams (vertical scale exaggerated) Stepped bioretention cells alternative for slopes >6 % </panelWarning> 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: 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. 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 Bioretention: Filter media <panelSuccess> 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) </panelSuccess> 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. Design Location Runoff reduction No underdrain Washington >98 % No underdrain United Kingdom >94 % With underdrain Maryland 46 - 54 % Runoff reduction estimate 85 % <h4blue>Enhanced grass swales</h4blue> Construction The following presents a summary of considerations when planning the construction of a low impact development project. More details can be found in the following reference: The site of the infiltration facility must remain outside the limit of disturbance and blocked from site traffic until construction of the facility begins, to prevent soil compaction by heavy equipment. This area must not be used as the site of sediment basins during construction, as the concentration of fines will reduce post-construction infiltration. This area must not be use as a staging area, for storing materials. To prevent sediment from clogging the surface, stormwater must be diverted away from the facility until the drainage area is fully stabilized. As many infiltration facilities are installed in the road right-of-way or tight urban spaces, considerations of traffic control and utility conflicts must be part of the plans and inspections. Sequencing The following is a typical construction sequence to properly install an infiltration practice: The area should be fully protected by silt fence or construction fencing to prevent compaction by construction traffic and equipment. Installation may only begin after entire contributing drainage area has been either stabilized or flows have been safely routed around the area. The designer should check the boundaries of the contributing drainage area to ensure it conforms to original design. The pretreatment part of the design should be excavated first and sealed until full construction is completed. Excavators or backhoes working adjacent to the proposed infiltration area should excavate to the appropriate design depth. The soil in the bottom of the excavation should be ripped to promote greater infiltration. Any accidental sediment accumulation from construction should be removed at this time. Excavate subsurface water storage reservoir to base elevation, Check base elevation and slope, Fracture/rip bottom and roughen side of the excavation to remove smeared surfaces, Install optional geotextiles (or liner for biofilter); overlapping according to design drawings, Install coarse reservoir gravel, and any void forming structures (e.g. underdrains, infiltration chambers, or wells), Check elevation and slope at top of reservoir, Install choking layer and optional geotextile (typically only over the perforated pipe), Check elevation and slope at top of choking layer, Install filter media with additional 30 cm over finish grade of the filter bed, Thoroughly saturate and allow to settle for at least one week. After this time, tamp manually to check settling is complete. Alternatively, installations made in the fall can be left to settle over the whole winter season at this point, Install temporary erosion and sediment control practices, Conduct all other site construction activities (buildings/servicing etc.) Check condition of bioretention after settling period, remediate any deficiencies, Install curbs and pavements and concrete pretreatment devices, Check elevations of curb cuts and other inlets Install erosion control to all inlets!! Remove excess filter media along with any accumulated construction sediment, Install any surface applied additives, Conduct fine grading to surface of filter bed, checking elevations/slopes/compaction, Apply stone or mulch cover for decorative systems, or turf reinforcement for grassed systems, Install erosion control blankets or matting Plant or lay sod, Saturated system thoroughly to settle filer media particles around the roots of new plants, Irrigated the system as required to establish healthy vegetation cover, Inspect and remediate deficiencies after any significant rainfall within the next 3 months or remainder of the first growing season. Facilities containing media Bioretention Sequencing depends on the design: Full infiltration:Pack 50 mm diameter clear stone to storage design depth, top with 100 mm of the choker course, Partial infiltration:Place design depth of 50 mm diameter clear stone for the infiltration volume on bed and then lay the perforated underdrain pipe over it. Pack more clear stone to 75 mm above the top of the underdrain, top with 100 mm of choker layer. Stormwater planters Place an impermeable liner on the bed with 150 mm overlap on sides. Lay the perforated underdrain pipe, Pack 50 mm diameter clear stone to 75 mm above top of underdrain, top with 100 mm of choker layer; Rain gardens No storage or drainage is required, filter media or amended topsoil is laid onto native soils Media installation Media installed over the choker course in 0.3 m lifts until desired top elevation is achieved. Each lift must be thoroughly wetted and drained before adding the next. Wait three weeks to check for settling, and add additional media and regrade as needed. Prepare planting holes for any trees and shrubs, install vegetation, and water accordingly. Install any temporary irrigation. Plant landscaping materials as shown in the landscaping plan, and water them weekly in the first two months. Lay down surface cover in accordance with the design (mulch, riverstone, or turf). Conduct final construction inspection, checking inlet, pretreatment, bioretention cell and outlet elevations. Remove erosion and sediment controls, only when the entire drainage area is stabilized. Checklists Bioretention construction checklist 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 See Also Bioretention cells Check dams External Links

That potion of the water precipitated onto a catchment area, which 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.

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.

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.

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.

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.

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.

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.

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.

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.

That potion of the water precipitated onto a catchment area, which 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.

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.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.

Ground depression acting as a flow control and water treatment structure, that is normally dry.

Soil particles with a diameter less than 0.050 mm.

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.

Soil or media particles smaller than sand and larger than clay (3 to 60 m)

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.

An underlying bed filled with aggregate or other void-forming fill material that temporarily stores stormwater before infiltrating into the native soil or being conveyed by an underdrain pipe.

Filter fabric that is installed to separate dissimilar soils and provide runoff filtration and contaminant removal benefits while maintaining a suitable rate of flow; may be used to prevent fine-textured soil from entering a coarse granular bed, or to prevent coarse granular from being compressed into underlying finer-textured soils.

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.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

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

Rainwater harvesting.

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.

Credit Valley Conservation