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.

Native Soil

Swales can be located over any soil type, but HSG A and B soils are best for achieving water balance objectives. Facilities should be located in portions of the site with the highest native soil infiltration rates. Where infiltration rates are less than 15 mm/hr (hydraulic conductivity less than 1x10-6 cm/s) an underdrain is recommended. Native soil infiltration rate at the proposed facility location and depth should be confirmed through in-situ measurements of hydraulic conductivity under field saturated conditions.

Wellhead Protection

Facilities receiving road or parking lot runoff should not be located within year 2 year time-of-travel wellhead protection areas (see local drinking water source protection plan).

Available Space

Reserve open space of about 5 to 20% of the size of the contributing drainage area. A width of at least 2 metres is needed

Site Topography

Contributing slopes should be between 1 to 5%. Swale longitudinal slopes may range from 0.5 to 6% (this prevents ponding while providing residence time and preventing erosion). On slopes steeper than 3%, check dams should be used.

Water Table

Maintaining a separation of 1 m between the elevations of the base of the practice and the seasonally high water table, or top of bedrock is recommended. Lesser or greater values may be considered based on groundwater mounding analysis. See STEP LID Planning and Design Guide wiki page, Groundwater, for further guidance and spreadsheet tool.

Pollution Hot Spot Runoff

To protect groundwater from possible contamination, runoff from pollution hot spots (i.e. (e.g., vehicle fueling, servicing and demolition areas, outdoor storage and handling areas for hazardous materials and some heavy industry sites) should not be treated by swales designed for infiltration. Facilities designed with an impermeable liner (filtration only) can be used to treat runoff from hot spots.

Proximity to Underground Utilities

Designers should consult local utility design guidance for the horizontal and vertical clearance between storm drains, ditches and surface water bodies. Utilities running parallel to the grass swale should be offset from the centerline of the swale. Generally, underground utilities below the bottom of the swale are not a problem.

Karst

Swales designed for infiltration are not suitable in areas of known or implied karst topography.

Setback from Buildings

Should be set back a minimum of 4m from building foundations.

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.

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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.

Construction Considerations

Properly installed enhanced swales should abide by the following recommendations:

• Grass swales should be clearly marked before site work begins to avoid disturbance during construction.
• No vehicular traffic, except that specifically used to construct the facility, should be allowed within the swale site.
• Any accumulation of sediment that does occur within the swale must be removed during the final stages of grading to achieve the design cross section, see above, under the Design section.
• Final grading and planting should not occur until the adjoining areas draining into the swale are stabilized. Flow should also not be diverted into the swale until the banks are stabilized.
• Preferably, the swale should be planted in the spring so that the vegetation can become established with minimal irrigation.
• Installation of erosion control matting or blanketing to stabilize soil during establishment of vegetation is highly recommended.
• If sod is used, it should be placed with staggered ends and secured by rolling the sod. This helps to prevent gullies

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.

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

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  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

Check dams are small dams or weirs constructed across a drainage ditch, swale, or channel to lower the speed of concentrated flows for a certain design range of storm events and to promote infiltration.

Check dams:

• may be constructed of any resilient and waterproof material, including: rock gabions, earth berms, coarse aggregate or rip-rap, concrete or metal. Stone used in check dams should have minimum median diameter 25 - 75 mm.
• for enhanced swales may be up to 0.6 m in height; the maximum design depth of ponded water should be ≤ 0.6 m.
• designed for higher flow velocities should have spillways incorporated into their profile, to direct water to the centre of the swale.
• are usually installed between 10 - 20 m along the swale. The spacing of dams should not exceed the horizontal distance from the toe of the upstream dam to the same elevation on the downstream dam.
• should have energy dissipation and erosion control measures installed in the 1 - 2 m downstream. Examples include large aggregate or turf reinforcement

Sizing and spacing of check dams

Check dams are a feature of enhanced grass 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 a specified period (often 48 - 72 hours). The infiltration may be through the turf grass or other plants directly into the native soil or some soil amendments may be proposed in the design.
2. The gradient between the top of the lower check dam and the bottom of the upper one is called "compensation gradient" which is the future or final effective gradient of the swale. It is formed when material carried by flowing water fills the check dams to spillway level.
3. Dams are usually installed between 10-20 m along the swale. The spaces between check dams can be determined according to the compensation gradient and the effective height of the dams. 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.
4. The compensation gradient of enhanced swales must be < 1 % (0.5 % preferred).

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.

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To estimate the depth of water that can be infiltrated into a surface within a given time:

$y=f'\times t$

Where:

• y = the depth of ponded water in mm,
• f' = the design infiltration rate in mm/hr (after correction, where required).
• t = time in hrs

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After surveying the longitudinal profile of the swale, the number of check dams for the swale can be calculated by using the following equation:

$Number\ of\ dams={\frac {L\left(S_{i}-S_{e}\right)}{h}}$

Where:

• L: Length of swale (m)
• S_i: Initial existing slope ratio of the swale (rise/run)
• S_e: Desired effective slope of the enhanced swale (between 0.005 - 0.01, rise/run)
• h: The average effective height of the check dams in m (excluding foundations)(suggest you use y, calculated above)

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• The first check dam should be constructed on a stable point in the gully such as a rock outcrop, the junction point of the gully to a road, the main stream or river, lake or reservoir.
• If there is no such stable point, a counter-dam must be constructed. The distance between the first dam and the counter-dam must be at least two times the effective height of the first check dam.
• The points where the ensuing check dams are to be built are determined according to the compensation gradient and the effective height of the check dams.
• The effective height of the second check dam is determined by taking into account the depth of the swale, the depth of the spillway and the maximum height of the check dam.
• In this way, all the other proposed check dam points can be calculated.

When spacing check dams, give preference to the narrowest parts of the swale in order to reduce construction costs. In this case, to establish the compensation gradient between the proposed check dams, proportionately increase the foundation depth of the upper check dam when the space between the lower and upper check dam is extended. When the space is shortened, decrease the foundation depth. As the foundation depth is increased, the total height of the check dam (effective height plus foundation depth) should not exceed the permissible, maximum total height.

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Example Calculation

An enhanced swale of 42 m has an existing longitudinal slope of 6 %. The underlying soil has been amended to infiltrate at 16 mm/hr (after safety correction applied); the regulatory authority requires that ponded water must not remain for over 48 hours. The maximum height of the check dams (h) to prevent extended ponding is:: $h=16\ mm/hr\times 48\ hrs=768\ mm$

But the maximum recommended depth of check dams is 0.6 m (600 mm).

(As an aside, these will drain in ${\frac {600\ mm}{16\ mm/hr}}=37.5\ hrs$)

The current vertical distance along the swale is:: $42\ m\times \left({\frac {6}{100}}\right)=2.52\ m$

The desired effective slope to slow flow is 0.5%. The vertical distance along the swale with compensation gradient is:: $42\ m\times \left({\frac {0.5}{100}}\right)=0.21\ m$

The number of check dams required is:: ${\frac {\left(2.52\ m\times 0.21\ m\right)}{0.6\ m}}=3.85$

This is rounded up to 4 dams, creating 5 ponding zones::

${\frac {42\ m}{5}}=8.4\ m$

So the four check dams will be 0.6 m high and spaced every 8.4 m along the swale. These are fairly close together, but this construction is achievable and the dimensions are not remarkable, given the change in gradient is an order of magnitude.

Gallery

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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

External references

FAO

Performance

Ability of Swales to Meet Stormwater Management Objectives

BMP	Water Balance	Water Quality	Erosion Control
Swale with no underdrain	Partial-based on available storage volume and native soil infiltration rate	Yes-size for water quality storage requirement and maximum flow rate 0.5 m/s	Partial-based on available storage volume and native soil infiltration rate
Swale with underdrain or partial infiltration	Partial-based on available storage volume beneath the underdrain and soil infiltration rate	Yes-size for water quality storage requirement	Partial-based on available storage, native soil infiltration rate and if a flow restrictor is used
Swale with underdrain and impermeable liner or no infiltration	Partial-some volume reduction through evapo-transpiration	Yes-size for water quality storage requirement	Partial-based on available storage volume and if a flow restrictor is used

Water Balance

Recent research indicates that a conservative runoff reduction rate of 10 to 20% can be used depending on whether soils fall in hydrologic soil groups A/B or C/D, respectively. The runoff reduction rates can be doubled if the native soils on which the swale is located have been tilled to a depth of 300 mm and amended with compost to achieve an organic content of between 8 and 15% by weight or 30 to 40% by volume. The main contributing factors that influence runoff reduction rates for swales are:

• Native soil types
• Slope
• Vegetative cover and,
• Length of the swale.

Volumetric runoff reduction from enhanced swales

LID Practice	Location	Runoff Reduction*	Reference
Grass Swale	Brampton	15 to 35%,	STEP (2018)
	Sweden	40 to 55%,	Rujner et al. (2016)
	Seoul, Korea	40 to 75%,	Rujner et al. (2016)
	Maryland	59%	Davis et al. (2012)
	Los Angeles	52.5%,	Ackerman and Stein (2008)
	Various Locations	40%	Strecker et al.(2004)
	France	27 to 41%	Barrett et al. (2004)
	Virginia	0%	Schueler (1983)
Runoff Reduction Estimate*		45% on HSG A or B soils; 10% on HSG C or D soils

Water Quality

Research has shown the pollutant mass removal rates of grass swales are variable, depending on influent pollutant concentrations (Bäckström et al., 2006), but generally moderate for most pollutants (Barrett et al., 1998; Deletic and Fletcher, 2006). Median pollutant mass removal rates of swales from available performance studies are 76% for total suspended solids, 55% for total phosphorus, and 50% for total nitrogen (Deletic and Fletcher, 2006). Significant reductions in total zinc and copper event mean concentrations have been observed in performance studies with a median value of 60%, but results have varied widely (Barrett, 2008). Site specific factors such as slope, soil type, infiltration rate, swale length and vegetative cover also affect pollutant mass removal rates. In general, the dominant pollutant removal mechanism operating in grass swales is infiltration, rather than filtration, because pollutants trapped on the surface of the swale by vegetation or check dams are not permanently bound (Bäckström et al., 2006). In a recent international research review on processes for improving stormwwater quality treatment of grass swales and vegetated filter strips, Gavric et al. note that while understanding of hydrology and hydraulics of these stormwater control measures is adequate, there are knowledge gaps in understanding water quality treatment processes, particularly for nutrients, traffic associated organic contaminants, and bacteria (Gavric et al., 2019 ). Designers should maximize the degree of infiltration achieved within a grass swale by incorporating check dams and ensuring the native soils have infiltration rates of 15 mm/hr or greater or specifying that the soils be tilled and amended with compost prior to planting. Several of the factors that can significantly increase or decrease the pollutant removal capacity of swales are provided in the table below:

Factors that Influence the Pollutant Removal Capacity of Swales

Factors that Reduce Removal Rates	Factors that Enhance Removal Rates
Longitudinal slope > 1%	Longitudinal slope < 1%
Measured soil infiltration rate < 15 mm/hr	Measured soil infiltration rate is 15 mm/hr or greater
Flow velocity within channel > 0.5 m/s during a 4 hour, 25 mm Chicago storm event	Flow velocity within channel is 0.5 m/s or less during a 4 hour, 25 mm Chicago storm event
No pretreatment	Pretreatment with vegetated filter strips, gravel diaphragms and/or sedimentation forebays
Side slopes steeper than 3:1 (H:V)	Side slopes 3:1 (H:V) or less

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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.

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

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

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

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.

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.

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.

(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).

Sustainable Technologies Evaluation Program

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 technique of removing pollutants from runoff as it infiltrates through the 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.

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.

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.

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.

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.

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 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.

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.

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.

Abiotic transfer of water vapour to the atmosphere.

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

Decayed organic material used as a plant fertilizer. Compost helps to support healthy plant growth through the slow release of nutrients and the retention of moisture in the soil.

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.

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.

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.

A bioretention BMP featuring an impermeable liner and underdrain that prevents infiltration of runoff into the underlying native soil; provides sedimentation and filtration of urban runoff as it passes through the mulch layer, engineered filter media and vegetation root zone.

A bioretention cell that features an impermeable liner that collects and treats stormwater through sedimentation and filtration only (i.e., no infiltration).

Natural vegetation and vegetative technologies in urban settings such as: urban forests; green roofs; green walls; green spaces; rain gardens; bioswales; community gardens; natural and engineered wetlands and stormwater management ponds; and porous pavement systems. These systems are designed to provide multiple benefits, such as moderate temperatures, clean air and water, and improve aesthetics.

A practice or structural measure that is usually implemented at the beginning of a drainage system or at the lot level, to reduce the volume of runoff and minimize the concentration of pollution in overland flow from private property and prevent pollutants from entering Stormwater runoff or other environmental media, as described by the Ministry of Environment.

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.

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

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.