Enhanced swales

File:Dry enhanced swale w-road.png|600px|thumb|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. 
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poly 29 1746 21 2358 2505 522 2143 401 Salt Tolerant Grass Mix 
poly 2084 811 2076 916 2603 1064 2614 967 2614 943 Check Dams
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poly 961 2459 2837 604 3102 651 1429 2463 Berms
poly 18 2432 938 2467 2747 694 2466 600 Grasses
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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.
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. 
===Infiltration===
Enhanced swales include check dams that can be designed to pond substantial volumes of water on the swale surface between flow events, so provide both stormwater conveyance and infiltration functions. 
For information about constraints to infiltration practices, and approaches and tools for identifying and designing within them see Infiltration.
===Best cross sections===
 
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 freeboardOntario 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#controlsDrainage 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. 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. Check the Erosion and Sediment Control Guide
===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.
For a table summarizing information on planning considerations and site constraints see Site considerations.
==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.
===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. 
For sizing of grass swales, see the following suggested equations to calculate their capacity / retention: Retention swales
===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.
thumb|400px
===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.
==Inspection and Maintenance==
Maintenance requirements for enhanced grass swales, and swales is similar to vegetated filter strips and typically involve a low level of activity after vegetation becomes established. Grass channel maintenance procedures are already in place at many municipal public works and transportation departments. These procedures should be compared to the recommendations provided on the Inspection and Maintenance: Enhanced Swales page to assure that the infiltration and water quality
benefits of enhanced grass swales are preserved. 
Routine roadside ditch maintenance practices such as scraping and re-grading should be avoided at swale locations. Vehicles should not be parked or driven on grass swales. For routine mowing, the lightest possible mowing equipment should be used to prevent soil compaction. 
For swales located on private property, the property owner, resident or manager is responsible for maintenance as outlined in a legally binding maintenance agreement. Roadside
swales in residential areas generally receive routine maintenance from homeowners who should be advised regarding recommended maintenance activities and ensure they do not build anything within or on the channel of the swale which could result in flooding or pooling on theirs or their neighbours' properties. 
Take a look at the Inspection and Maintenance: Enhanced Swales page by clicking below for further details about proper inspection and maintenance practices:
==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==
==Materials==
===Check dams===
==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)Sustainable Technologies Evaluation Program. Effectiveness of Retrofitted Roadside Biofilter Swales - County Court Boulevard, Brampton. Technical Brief. https://sustainabletechnologies.ca/app/uploads/2020/11/CC-Bioswale-Tech-brief-2018-FINAL.pdf. https://sustainabletechnologies.ca/app/uploads/2020/11/CC-Bioswale-Tech-brief-2018-FINAL.pdf
Grass Swale 	Sweden 	40 to 55%, 	Rujner et al. (2016)Rujner, H., Leonhardt, G., Perttu, A.M., Marsalek, J. and Viklander, M. 2016. Advancing green infrastructure design: Field evaluation of grassed urban drainage swales. Modélisation/Models-Contrôle à la source/Source control. http://documents.irevues.inist.fr/bitstream/handle/2042/60477/3B7P03-124RUJ.pdf
Grass Swale 	Seoul, Korea 	40 to 75%, 	Rujner et al. (2016)Rujner, H., Leonhardt, G., Perttu, A.M., Marsalek, J. and Viklander, M. 2016. Advancing green infrastructure design: Field evaluation of grassed urban drainage swales. Modélisation/Models-Contrôle à la source/Source control. http://documents.irevues.inist.fr/bitstream/handle/2042/60477/3B7P03-124RUJ.pdf
Grass Swale 	Maryland 	59% 	Davis et al. (2012)Davis, A.P., Stagge, J.H., Jamil, E. and Kim, H. 2012. Hydraulic performance of grass swales for managing highway runoff. Water research, 46(20), pp.6775-6786. http://www.jstagge.com/assets/papers/Hydraulic%20performance%20of%20grass%20swales%20for%20managing.pdf
Grass Swale 	Los Angeles 	52.5%, 	Ackerman and Stein (2008)Ackerman, D. and Stein, E.D. 2008. Evaluating the effectiveness of best management practices using dynamic modeling. Journal of Environmental Engineering, 134(8), pp.628-639. https://www.researchgate.net/profile/Eric-Stein-2/publication/228910558_Evaluating_the_Effectiveness_of_Best_Management_Practices_Using_Dynamic_Modeling/links/0912f509278915fc77000000/Evaluating-the-Effectiveness-of-Best-Management-Practices-Using-Dynamic-Modeling.pdf
Grass Swale 	Various Locations 	40% 	Strecker et al.(2004)Strecker, E., Quigley, M., Urbonas, B., Jones, J. 2004. State-of-the-art in comprehensive approaches to stormwater. The Water Report. Issue 6. August 15,2004.
Grass Swale 	France 	27 to 41% 	Barrett et al. (2004)Barrett, M.E. 2008. Comparison of BMP Performance Using the International BMP Database. Journal of Irrigation and Drainage Engineering. September/October. pp. 556-561
Grass Swale 	Virginia 	0% 	Schueler (1983)Schueler, T. 1983. Washington Area Nationwide Urban Runoff Project. Final Report. Metropolitan Washington Council of Governments. Washington, DC.
Runoff Reduction Estimate*	Runoff Reduction Estimate*	45% on HSG A or B soils;
10% on HSG C or D soils	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)Bäckström, M., Viklander, M. and Malmqvist, P.A. 2006. Transport of stormwater pollutants through a roadside grassed swale. Urban Water Journal, 3(2), pp.55-67. https://www.mdpi.com/2073-4441/6/7/1887/htm, but generally moderate for most pollutants (Barrett et al., 1998Barrett, M.E., Walsh, P.M. Malina Jr., J.F. and Charbeneau, R.J. 1998. Performance of Vegetative Controls for Treating Highway Runoff. Journal of Environmental Engineering. November 1998. pp. 1121-1128.; Deletic and Fletcher, 2006Deletic, A., and Fletcher, T.D. 2006. Performance of grass filters used for stormwater treatment – a field and modelling study. Journal of Hydrology. Vol. 317. pp. 261-275.). 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, 2006Deletic, A., and Fletcher, T.D. 2006. Performance of grass filters used for stormwater treatment – a field and modelling study. Journal of Hydrology. Vol. 317. pp. 261-275.). 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, 2008Barrett, M.E. 2008. Comparison of BMP performance using the international BMP database. Journal of Irrigation and Drainage Engineering, 134(5), pp.556-561.). 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., 2006Bäckström, M., Viklander, M. and Malmqvist, P.A. 2006. Transport of stormwater pollutants through a roadside grassed swale. Urban Water Journal, 3(2), pp.55-67. https://www.mdpi.com/2073-4441/6/7/1887/htm). 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 Gavric.S, Leonhardt, G., Marsalek, J., Viklander, M. 2019. Processes improving urban stormwater quality in grass swales and filter strips: A review of research findings. Science of the Total Environment. v 669. pp. 431-447. https://www.sciencedirect.com/science/article/pii/S0048969719310502?via%3Dihub). 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
==Life Cycle Costs==
To learn about Life Cycle Costs associated with this practice (i.e. Pre-construction, Excavation, Materials & Installation, Project Management, Overhead, Inspection and Maintenance, Rehabilitation and other associated costs), visit the Enhanced Swales: Life Cycle Costs page to view a cost estimate. Alternatively you can use the STEP's Low Impact Development Life Cycle Costing Tool (LID LCCT) to generate cost estimates customized to your own LID stormwater design project specifications.
Take a look at the Enhanced Swales: Life Cycle Costs page by clicking below for further details:
==External Links==
Food and Agriculture Organization (FAO) Conservation Guide of the United Nations
==Gallery==
==References==
Category:Infiltration
Category:Calculations
Category:Green infrastructure