Enhanced swales

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Skeleton schematic illustrating the installation of check dams with centralized, flow concentrating cutouts
The check damsStructures 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. 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 runoffThat potion of the water precipitated onto a catchment area, which 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. along a vegetated channel, whilst also promoting infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface..
For underground conveyance which promotes infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface., see Exfiltration trenches.
For conveyance along planted channels, on both surface and underground, see Bioswales.

Overview

Enhanced swalesA 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. are an ideal technology for:

  • Sloped sites,
  • Cheaply retrofitting and improving the performance of existing grass swalesVegetated, 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..

The fundamental components of an enhanced grassed swaleA 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. are:

Additional components may include:

Planning considerations

When planning a new site, all swalesA 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. 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 swaleA 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. (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 swalesA 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. 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 swalesA 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. 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 damsStructures 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. to enhance all surface BMPS can be safely used to encourage ponding and subsequent infiltration for a day or two.

Design

All swalesA 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. should be designed to meet the following criteria:

  • Minimum residence time of 5 minutes.
  • Maximum flow velocity 0.3 m/s
  • Bottom width between 0.6 - 2.4 m
  • Minimum length 30 m

Maximum depth of flow

  • 50% height of grass for regularly mown swalesA 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., to maximum of 75 mm.
  • 33% height of vegetation for infrequently mown swalesA 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..

Planting Considerations

  • Grasses and herbaceous species with dense root structure cover should be favoured along the bottom of the swaleA 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. for their ability to increase infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface., stabilize soils, retain pollutants and assist with suspended solids.
  • 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 sedimentationDeposition 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. and increasing infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface.. The root structure of this plant material also acts to reduce erosion(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)..
  • Selected grasses or groundcovers for grassed swalesA 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. should be allowed to grow between 75 to 150 mm to assist in filtering suspended solids from stormwaterSurface runoff from at-grade surfaces, resulting from rain or snowmelt events.. 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 swaleA 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. should be planted in the spring so that the vegetation can become established with minimal irrigationHuman application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47.

Modeling

TTT.png

SwaleA 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. element in TTT menu
Weir elements may be incorporated as check damsStructures 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. for detailed design

It is recommended that grass and enhanced grass swales be modelled using the 'SwaleA 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.' element in the TTT. A 'swaleA 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.' has to connect two existing elements within the TTT Bioswales or dry swalesLinear 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., which have amended filter mediaThe 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., should be modelled as bioretention cells. The alternative is to use the 'enhanced swaleVegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff.' within the LIDLow 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. toolbox, but this incorporates fewer design parameters (and doesn't account for infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface.).

A 'swaleA 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.' as a conveyance element in the TTT (key parameters)
General Info
Upstream Node Name of node on the inlet end of the swaleA 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. (higher elevation)
Downstream Node Name of node on the outlet end of the swaleA 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. (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 [3]
Upstream Invert (m) Depth of swaleA 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. invert above node invert at inlet end of the swaleA 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.
Downstream Invert (m) Depth or elevation of the swaleA 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. invert above the node invert at the outlet end of the swaleA 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.
Cross section
Maximum Depth (m) Depth of the swaleA 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.
Bottom Width (m) Bottom width of the trapezoidal swaleA 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.
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 rateThe rate at which stormwater percolates into the subsoil measured in inches per hour. of native (or amended) soil

Parameters for 'enhanced swalesA 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.' in the LIDLow 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. toolbox of the TTT

Surface
BermA 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. 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 LIDLow 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. 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 [3]
Surface slope (%) If the slope > 3%, use Check dams to create temporary ponding, increase infiltrationThe slow movement of water into or through a soil or drainage system.Penetration of water through the ground surface., and slow flow to reduce erosion(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)..
SwaleA 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. 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[4]. The 'Salt Tolerant Mix' is of particular value for low impact developmentA 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. applications alongside asphaltA mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces. 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 gardenA lot level bioretention cell designed to receive and detain, infiltrate and filter runoff, typically used for discharge from downspouts. in a school yard uses stone as both decorative edging and for erosion controlIncludes the protection of soil from dislocation by water, wind or other agents..
This bioswaleLinear 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. in a parking lot uses stone at the inlets and along the bottom of the swaleA 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. to prevent erosion(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)., as the sides are sloped.

For advice on aggregatesA 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. 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 LIDLow 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. practice.

Stone for erosion controlIncludes the protection of soil from dislocation by water, wind or other agents.

AggregatesA 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. 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. [5] However, in some surface landscaped applications there may be a desire to use a rounded aggregateA 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. 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. 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, regular inspection and possible replacement should be scheduled as there is a potential for clogging of this layer to occur.

Stone mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales.

Finer inorganic mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales. 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 mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales. over the top of wood based mulcha top dressing over vegetation beds that provides suppresses weeds and helps retain soil moisture in bioretention cells, stormwater planters and dry swales. has been shown to reduce migration of the underlying layer by around 25% [6]. Inorganic mulches which may be available locally, include:

  • Crushed glass
  • Crushed mussel shells
  • Pea gravel

check dams
  1. 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
  2. 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
  3. 3.0 3.1 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.
  4. 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
  5. 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
  6. 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