Enhanced 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.
- For conveyance along planted channels, on both surface and underground, see Bioswales.
Overview[edit]
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[edit]
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[edit]
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[edit]
As shallow grassed swales are a common roadside construction, the Ministry of Transport has created their own guide to maximum flow depth and freeboard[1][2]. 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[edit]
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[edit]
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[edit]
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[edit]
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[edit]
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[edit]
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[edit]
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[edit]
Swales designed for infiltration are not suitable in areas of known or implied karst topography.
Setback from Buildings[edit]
Should be set back a minimum of 4m from building foundations.
Design[edit]
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.
Geometry and Site Layout[edit]
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[edit]
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[edit]
Planting soil or filter media should come pre-mixed from an approved vendor.
Underdrains[edit]
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[edit]
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[edit]
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[edit]
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[edit]
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[edit]
- 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[edit]
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).
| 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 [4] |
| 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 |
| 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 [4] |
| 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[edit]
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[5]. The 'Salt Tolerant Mix' is of particular value for low impact development applications alongside asphalt roadways and paved walkways.
| 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 % |
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[edit]
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. [6] 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[edit]
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% [7]. Inorganic mulches which may be available locally, include:
- Pea gravel
- River rock/beach stone
- Recycled glass
- Crushed mussel shells
Performance[edit]
| 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 |
- ↑ Ontario Ministry of Transportation, & Ontario Ministry for Transportation. (2016). Stormwater Management Requirements for Land Development Proposals. Retrieved February 26, 2018, from http://www.mto.gov.on.ca/english/publications/drainage/stormwater/section8.shtml#controls
- ↑ Drainage and Hydrology Section Transportation Engienering Branch Quality and Standards Division. (1997). MTO Drainage Management Manual. Retrieved from http://www.ontla.on.ca/library/repository/mon/12000/198363.pdf
- ↑ Horry County Government. 2022. Stormwater Engineers - Plans and the Construction Process. https://www.horrycounty.org/Departments/Stormwater/engineers
- ↑ 4.0 4.1 Oregon State Univ., Corvallis. Dept. of Civil, Construction and Environmental Engineering.; Environmental Protection Agency, Cincinnati ONRMRL. Storm Water Management Model Reference Manual Volume I Hydrology (Revised). 2016:233.https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100NYRA.txt Accessed August 23, 2017.
- ↑ Ontario Provincial Standard Specification. (2014). Construction Specification and for Seed and Cover OPSS.PROV 804. Retrieved from http://www.raqsb.mto.gov.on.ca/techpubs/ops.nsf/0/3a785d2f480f9349852580820062910a/$FILE/OPSS.PROV 804 Nov2014.pdf
- ↑ Roger T. Kilgore and George K. Cotton, (2005) Design of Roadside Channels with Flexible Linings Hydraulic Engineering Circular Number 15, Third Edition https://www.fhwa.dot.gov/engineering/hydraulics/pubs/05114/05114.pdf
- ↑ Simcock, R and Dando, J. 2013. Mulch specification for stormwater bioretention devices. Prepared by Landcare Research New Zealand Ltd for Auckland Council. Auckland Council technical report, TR2013/056