Enhanced swales

Skeleton schematic illustrating the installation of check dams with centralized, flow concentrating cutouts. Street runoff would enter the swale through sheet flow or curb cuts and flow across the filter strip on the left side of the image.A note: The following is an "image map", feel free to explore the image with your cursor and click on highlighted labels that appear to take you to corresponding pages on the Wiki.

The check dams are spaced slightly further apart than would be recommended to maximize infiltration capacity i.e. ponding isn't quite continuous between the dams.

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.
  
• For conveyance along planted channels, on both surface and underground, see Bioswales.

Overview

Enhanced Grass Swales are vegetated open channels designed to convey, treat and attenuate stormwater runoff. Simple grass channels or ditches have long been used for stormwater conveyance, particularly for road drainage. Enhanced grass swales incorporate design features such as modified geometry and check dams that improve the contaminant removal and runoff reduction functions of simple grass channel and roadside ditch designs. Bioretention swales (i.e.bioswales, dry swales) incorporate filter media and possibly a perforated pipe underdrain to ensure they drain within the required drawdown time. Where development density, topography and depth to water table permit, swales are a preferable alternative to curb and gutter and storm drains as a stormwater conveyance system.

Enhanced swales are an ideal technology for:

• Sloped sites,
• Cheaply retrofitting and improving the performance of existing grass swales.

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

The fundamental components of an enhanced grassed swale are:

• Graded channel
• Resilient turf grass or other planting
• Check dams, to facilitate short term ponding

Additional components may include:

• Amended soil or filter media to increase infiltration to soils below
• Turf reinforcement, to prevent scour

Planning considerations

When planning a new site, all swales and overground flow paths should be fitted perpendicular to existing contours. See Natural drainage and Existing hydrology.

Best cross sections

Both sections a)triangular and, b)trapezoidal, are constrained within ratios of 8:1 H:V

At lowest flow rates (smallest area) the trapezoidal swale (b) has the greater wetted perimeter; at higher flow rates (greater area) the triangular geometry (a)has a larger wetted perimeter for same area. Both channels modelled using ratios shown in figure above

Enhanced swales aim to both reduce the flow rate and retain a portion of the conveyed water. For these purposes the best x-section is that which maximizes the wetted perimeter for a given area. For a given width and depth, the difference between a triangular and trapzoidal section is small. As shown in the diagrams, under low flow conditions the trapezoidal has greater wetted perimeter, and at higher flows the triangular profile does.

Safety

As shallow grassed swales are a common roadside construction, the Ministry of Transport has created their own guide to maximum flow depth and freeboard. Their advice has been prepared specifically for high risk environments and those stringent constraints should not be applied to all circumstances. In many urban environments the principle of applying check dams to enhance all surface BMPs can be safely used to encourage ponding and subsequent infiltration for a day or two.

Design

See: Enhanced swales: Specifications for further guidance regarding BMP sizing.

All swales should be designed to meet the following criteria:

• Treat drainage areas of <2 hectares
• Minimum residence time of 5 minutes.
• Maximum flow velocity 0.3 m/s
• Bottom width between 0.75 - 3.0 m
• Minimum length 30 m
• Minimum length between checkdams > 5m
• Maximum depth of flow should be 50% height of grass for regularly mown swales, to a maximum of 100 mm, or 33% height of vegetation for infrequently mown swales
• Cross-section shape may be parabolic or trapezoidal, but parabolic is preferable for aesthetics, maintenance and hydraulics.

Cross-section view of a parabolic designed swale. Source: Horry County, Southern California..

Geometry and Site Layout

Minimum planting soil or filter media bed footprint area is based on the design storm runoff volume and effective surface ponding depth behind check dams. Recommended impervious drainage area to pervious facility footprint area ratios (I:P ratios) range from:

• 5:1 on low permeability soils, such as hydrologic soil group (HSG) C and D,
• 20:1 on high permeability soils hydrologic soil group (HSG) Aand D.

Pre-Treatment

Pre-treatment captures sediment before it reaches the filter bed. It is typically necessary unless runoff sediment load is very low (e.g. roof drainage). Pre-treatment options include:

• level spreaders
• stone filter inlets with geotextile fabric and,

catch basins with sump.

Planting Soil & Filter Media

Planting soil or filter media should come pre-mixed from an approved vendor.

Underdrains

Underdrains are recommended for bioswales where native soil infiltration rate <15 mm/h (hydraulic conductivity < 1x10^-6 cm/s), and needed for non-infiltrating designs. They are comprised of a length of perforated pipe embedded near the top of the storage reservoir, with an overlying choker layer of medium-sized aggregate, and structures to provide inspection and maintenance access. Alternatively, the perforated pipe could be installed on the reservoir bottom and connected to an upturned pipe assembly or riser. Another option is to include a flow restrictor (e.g. orifice cap or valve) on the underdrain outlet pipe, to optimize infiltration while meeting the required drainage time.

Perforated Pipe

Continuously perforated pipe should be either smooth interior HDPE or PVC pipe with diameter ≥200 mm to reduce freezing risk and facilitate access by camera and cleaning equipment. Perforated pipe extends length of facility and solid pipe is used to connect to storm drain system.

Access Structures

The use of maintenance holes/vertical standpipes connected to the perforated pipe system are used for inspection and flushing, ensure couplings used for standpipe connections are 45° to facilitate pipe access by camera or cleaning equipment. (See to the right for example of couplings attaching to an underdrain pipe.

Conveyance and Overflow

Swales can be designed to be inline or offline from the drainage system. Inline swales accepts all flow from the drainage area and conveys large event flows through an overflow outlet. Overflow structures must be sized to safely convey large event flows out of the facility. Options include flat, dome or ditch inlet catch basins connected to a storm sewer.

See: Overflow: Gallery for examples.

Monitoring Wells

A vertical standpipe consisting of an anchored 100 to 150 mm diameter pipe with perforations along the length within the reservoir, installed to the bottom of the facility, with a lockable cap. The well allows monitoring of inter-event drainage times with the use of a water level data logger sampler.

Planting Considerations

• Grasses and herbaceous species with dense root structure cover should be favoured along the bottom of the swale for their ability to increase infiltration, stabilize soils, retain pollutants and assist with suspended solids.
• Enhanced grass swales may be planted with sod or seed. Stabilize swale with erosion control blanket if planting with seed. Include a temporary cover crop in native seed mix.
• The plant material on the slopes of grass channels must be capable of withstanding periodic inundation in addition to extended periods of drought. Species include grasses and groundcovers, as well as low shrub species.
• Plants along the exterior of this zone act to slow the flow during stormwater events, reducing sedimentation and increasing infiltration. The root structure of this plant material also acts to reduce erosion.
• Selected grasses or groundcovers for grassed swales should be allowed to grow between 75 to 150 mm to assist in filtering suspended solids from stormwater. Therefore these species are either shorter naturally, or tolerate periodic mowing.
• When grasses grow taller they have a tendency to flatten down from the water flow.
• Fine, close-growing species provide for good soil stabilization.
• Species are salt-tolerant due to the typical location of grass channels along roadways and parking lots.
• Erosion protection such as river stone or riprap will be required to dissipate the energy from incoming concentrated flow.
• The channel must be vegetated immediately after grading. Preferably, the swale should be planted in the spring so that the vegetation can become established with minimal irrigation.

Modeling

Swale element in TTT menu

Weir elements may be incorporated as check dams for detailed design

It is recommended that grass and enhanced grass swales be modelled using the 'Swale' element in the TTT. A 'swale' has to connect two existing elements within the TTT Bioswales or dry swales, which have amended filter media, should be modelled as bioretention cells. The alternative is to use the 'enhanced swale' within the LID toolbox, but this incorporates fewer design parameters (and doesn't account for infiltration).

A 'swale' as a conveyance element in the TTT (key parameters)

General Info
Upstream Node	Name of node on the inlet end of the swale (higher elevation)
Downstream Node	Name of node on the outlet end of the swale (lower elevation)
Manning's Roughness	Lower numbers indicate less surface obstruction and result in faster flow. Suggested range for mown grass (dependent on density) 0.03 – 0.06
Upstream Invert (m)	Depth of swale invert above node invert at inlet end of the swale
Downstream Invert (m)	Depth or elevation of the swale invert above the node invert at the outlet end of the swale
Cross section
Maximum Depth (m)	Depth of the swale
Bottom Width (m)	Bottom width of the trapezoidal swale For a triangular channel, enter 0
Left Side Slope (m/m)	Left side slope (run/rise). Suggested value of 3 or 4 if design permits.
Right Side Slope (m/m)	Right side slope (run/rise). Suggested value of 3 or 4 if design permits.
Seepage (mm/hour)	Infiltration rate of native (or amended) soil

Parameters for 'enhanced swales' in the LID toolbox of the TTT

Surface
Berm height (mm)	This is the height of the curb which constrains the overland sheet flow of water. Where the bottom of the slope discharges directly into another LID facility without impedance, the value is 0.
Surface roughness (Manning’s n)	Lower numbers indicate less surface obstruction and result in faster flow. Suggested range for mown grass (dependent on density) 0.03 – 0.06
Surface slope (%)	If the slope > 3%, use Check dams to create temporary ponding, increase infiltration, and slow flow to reduce erosion.
Swale side slopes (run/rise)	Suggested value of 3 or 4 if design permits.

Materials

Resilient turf grasses are particularly useful in the design of vegetated filter strips, dry ponds and enhanced grass swales. The Ministry of Transportation have standardized a number of grass mixes. The 'Salt Tolerant Mix' is of particular value for low impact development applications alongside asphalt roadways and paved walkways.

Canada #1 Ground Cover (salt tolerant mix)

Common name	Scientific name	Proportion
Tall Fescue	Festuca arundinacea	25 %
Fults Alkali Grass	Puccinellia distans	20 %
Creeping Red Fescue	Festuca rubra	25 %
Perennial ryegrass	Lolium perrenne	20 %
Hard Fescue	Festuca trachyphylla	10 %

This rain garden in a school yard uses stone as both decorative edging and for erosion control.

This bioswale in a parking lot uses stone at the inlets and along the bottom of the swale to prevent erosion, as the sides are sloped.

For advice on aggregates used in underdrains, see Reservoir aggregate.

Stone or gravel can serve as a low maintenance decorative feature, but it may also serve many practical functions on the surface of an LID practice.

Stone for erosion control

Aggregates used to line swales or otherwise dissipate energy (e.g. in forebays) should have high angularity to increase the permissible shear stress applied by the flow of water. However, in some surface landscaped applications there may be a desire to use a rounded aggregate such as 'river rock' for aesthetic reasons. Rounded stones should be of sufficient size to resist being moved by the flow of water. Typical stone for this purpose ranges between 50 mm and 250 mm in diameter. The larger the stone, the more energy dissipation.

• Stone beds should be twice as thick as the largest stone's diameter.
• If the stone bed is underlain by a drainage geotextile, annual inspection and possible replacement should be performed as there is a potential for clogging of this layer to occur.

• 

  Stone lining the ponding zone of this rain garden. Image credit California Native Plant Society

• 

  Coarse angular stone laid onto a geogrid and geotextile. Image from wikimedia commons

Stone mulch

Finer inorganic mulch materials can be of value applied in areas with extended ponding times i.e. in the the centre of recessed, bowl shaped bioretention, stormwater planters, trenches or swale practices. Inorganic mulches resist movement from flowing water and do not float. Applying a thin layer of inorganic mulch over the top of wood based mulch has been shown to reduce migration of the underlying layer by around 25% . Inorganic mulches which may be available locally, include:

• Pea gravel
• River rock/beach stone
• Recycled glass
• Crushed mussel shells

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check dams

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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 gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

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.

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.

Movement of water from one location to another.

Natural or artificial means of intercepting and removing surface or subsurface water (usually by gravity).

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.

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

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.

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 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.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

The upper surface of the zone of saturation, except where the surface is formed by an impermeable body.

Subsurface water level which is defined by the level below which all the spaces in the soil are filled with water; The entire region below the water table is called the saturated zone.

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.

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.

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

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, 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.

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.

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

The natural ground material characteristic of or existing by virtue of geographic origin.

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

A parameter that describes the capability of a medium to transmit water.

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.

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.

The period between the maximum water level and the minimum level (dry weather or antecedent level).

Refers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet.

Refers to a system that when full, stormwater will bypass the practice. Offline systems use flow splitters or bypass channels that only allow the water quality volume to enter the facility. This may be achieved with a pipe, weir, or curb opening sized for the target flow, but in conjunction, create a bypass channel so that higher flows do not pass over the surface of the filter bed.

A system flow of gully inlets, pipes, overland flow paths, open channels, culverts and detention basins used to convey runoff to its receiving waters. City of Toronto 45 Wet Weather Flow Management November 2006

Refers to a system that accepts all of the flow from a drainage area and conveys larger event flows through an overflow outlet.

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

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.

(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).

Surface runoff from at-grade surfaces, resulting from rain or snowmelt events.

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

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

A compacted earthen wall that diverts runoff or creates shallow ponding of runoff. In some cases, runoff ponds behind the berm and gradually flows through it or is infiltrated.

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 mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

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

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 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.

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.

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

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

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