Enhanced grass 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

The fundamental components of an enhanced grassed swale are:

• Graded channel
• Resilient turf grass
• Check dams, to facilitate short term ponding

Additional components may include:

• Amended soil, to increase infiltration to soils below
• Impermeable membrane, to prevent infiltration to soils below
• Turf reinforcement, to prevent scour

Planning considerations

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 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

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.

Design

All swales 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

Adding check dams

To calculate the spacing and sizing of check dams, see Check dams: Sizing

Maximum depth of flow

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

Modeling

Swale element in TTT menu

Weir elements may be incorporated as check dams for detailed design

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).

A 'swale' as a conveyance element in the TTT (key parameters)

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
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

Parameters for 'enhanced swales' in the LID toolbox of the TTT

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
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.

The portion of water precipitated onto a catchment area, which then 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.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

A 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.

A 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.

Structures 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.

A 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.

Linear 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.

The 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.

Vegetated open channel, with check dams; designed to convey, treat and attenuate stormwater runoff.

Low 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.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

A 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.

(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).