Swales

This article is about installations designed to capture and convey surface runoff along a vegetated channel, whilst also promoting infiltration. 
For underground conveyance systems which promote infiltration, see Exfiltration trenches.
==Overview==
Swales are linear landscape features consisting of a drainage channel with gently sloping sides. Underground they may be filled with engineered soil and/or contain a water storage layer of coarse gravel material. Two variations on a basic swale are recommended as low impact development strategies, although using a combination of both designs may increase the benefits:
Bioswales are sometimes referred to as 'dry swales', 'vegetated swales', or 'water quality swales'. This type of BMP is form of bioretention with a long, linear shape (surface area typically >2:1 length:width) and a slope which conveys water and generally contains various water tolerant vegetation,
Enhanced grass swales are a lower maintenance alternative, but generally have lower stormwater management potential. The enhancement over a basic grass swale is in the addition of check dams to slow surface water flow and create small temporary pools of water which can infiltrate the underlying soil.
Grass swales are a relatively common landscape feature already and a great opportunity for retrofit, to reduce flow and improve water quality by encouraging settling and infiltration behind a series of check dams. 
Retention swales can be imagined as linear, sloped dry ponds. They typically make relatively small contributions to water volume and quality control than many other BMPs, but they may feature as part of a site-wide treatment train approach. 
Take a look at the downloadable Enhanced Grass Swales Factsheet below for a .pdf overview of this LID Best Management Practice:
 Types of Swale
Property 	Bioswale 	Enhanced grass swale
Surface water 	MinimalAny surface flow can be slowed with check dams	Ponding is encouraged with check dams
Soil 	Filter media required 	Amendment preferable when possible
Underdrain 	Common 	Uncommon
Maintenance 	Medium to high 	Low
Stormwater benefit 	High 	Moderate
Biodiversity benefit	Increased with native planting 	Typically lower
==Planning Considerations==
Enhanced swales and in some cases bioswales 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. For guidance on infiltration testing and selecting a design infiltration rate see Design infiltration rate.
A linear design (surface area typically >2:1 length:width) is a common feature of swales:
*An absolute minimum width of 0.6 m is required for bioswales to promote healthy plant growth, and to facilitate construction,
*Grassed swales are usually mown as part of routine maintenance, so the cross section will be triangular or trapezoidal in shape with maximum side slopes of 1:3. The minimum width for this type would be 2 m. See Best cross sections
Swales may be graded along longitudinal slopes between 0.5 - 6 %:
*Between 1 - 6 %, check dams are recommended to bring the compensation gradient <1 %.
*Slopes > 6% can accommodate a series of stepped bioretention cells, each overflowing into the next with a spillway.
For a table summarizing information on planning considerations and site constraints see Site considerations.
==Design==
Pretreatment and inlets
To minimize erosion and maximize the functionality of the swale, sheet flow of surface water should be directed into the side of the BMP. Gravel diaphragms, vegetated filter strips and shallow side slopes are ideal. Alternatively, a series of curb inlets can be employed, where each has some form of flow spreader incorporated. Single point inflow can cause increased erosion and sedimentation, which will damage vegetation and contribute to BMP failure. Again, flow spreading devices can mitigate these processes, where concentrated point inflow is required.
==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:
==Performance==
An evaluation of swales and roadside ditches was published by STEP in 1999. The project page and additional tools are available here.
===Bioswales===
Click on the citations above to view the performance of these features based upon specifics (i.e. drainage area to size ratio to achieve the reduction mentioned) in the table.
 
=== Enhanced grass swales===
See Performance section on Enhanced swale page link above for additional information.
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
==Construction==
Construction
==Galleries==
===Simple grass swales===
===Bioswales===
===Check dams===
==See Also==
*Bioswales
*Check dams
==References==
Category:Infiltration
Category:Green infrastructure