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#REDIRECT[[Enhanced grass swales]]
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[[File:Dry enhanced swale.png|thumb|Skeleton schematic illustrating the installation of [[check dams]] with centralized, flow concentrating cutouts]]
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[[File:Wet swale.png|thumb|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.]]
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This article is about installations designed to capture and convey surface runoff along a vegetated channel, whilst also promoting infiltration. <br>
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For underground conveyance which promotes infiltration, see [[Exfiltration trenches]].<br>
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For conveyance along planted channels, on both surface and underground, see [[Bioswales]].
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{{TOClimit|2}}
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==Overview==
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{{textbox|Enhanced swales are an ideal technology for:
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*Sloped sites,
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*Cheaply retrofitting and improving the performance of existing grass swales.}}
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'''The fundamental components of an enhanced grassed swale are:'''
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*Graded channel
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*[[Turf| Resilient turf grass]] or [[#Planting_Considerations|other planting]]
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*[[Check dams]], to facilitate short term ponding
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'''Additional components may include:'''
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*[[Absorbent landscapes| Amended soil]] or [[filter media]] to increase infiltration to soils below
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*[[Turf reinforcement]], to prevent scour
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==Planning considerations==
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When planning a new site, all swales and overground flow paths should be fitted perpendicular to existing contours. See [[Natural drainage]] and [[Existing hydrology]].
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===Best cross sections===
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[[File:Best X-section.png|thumb|Both sections a)triangular and, b)trapezoidal, are constrained within ratios of 8:1 H:V]]
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[[File:Sections_perimeters.png|thumb|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]]
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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.
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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.
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===Safety===
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As shallow grassed swales are a common roadside construction, the Ministry of Transport has created their own guide to maximum flow depth and freeboard<ref>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</ref><ref>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</ref>. 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.
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==Design==
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All swales should be designed to meet the following criteria:
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*Minimum residence time of 5 minutes.
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*Maximum flow velocity 0.3 m/s
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*Bottom width between 0.6 - 2.4 m
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*Minimum length 30 m
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Maximum depth of flow
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{{plainlist|
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*50% height of grass for regularly mown swales, to maximum of 75 mm.
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*33% height of vegetation for infrequently mown swales.
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|}}
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===Planting Considerations===
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*[[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.
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*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.
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*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.
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*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.
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*When grasses grow taller they have a tendency to flatten down from the water flow.
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*Fine, close-growing species provide for good soil stabilization.
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*Species are salt-tolerant due to the typical location of grass channels along roadways and parking lots.
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*Erosion protection such as river stone or riprap will be required to dissipate the energy from incoming concentrated flow.
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*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.
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==Modeling==
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{{:Swales: TTT}}
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==Materials==
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{{:Turf}}
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{{:Stone}}
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[[check dams]]
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