Grassed Swales

Design considerations for proposal review | Swales on commercial sites | Swales in Connecticut

Vegetated swales can be incorporated into a site to help maintain predevelopment hydrology. Properly designed swales will help to infiltrate runoff water from smaller storms, and safely convey runoff from larger storms.

	For more information on drainage channels, visit the following sections of the CT Stormwater Quality Manual: Chapter 4, Section 4.4.1 Chapter 11, Filtering Practices, S8
Grassed drainage channels (above) are not engineered to provide infiltration, and primarily serve to convey water. These types of swales may be found adjacent to large highways. Conveyance swales are designated as a "secondary treatment practice" in the CT DEP Stormwater Quality manual.
	For more information on water quality swales, visit the following sections of the CT Stormwater Quality Manual: Chapter 4, Section 4.4.1 Chapter 11, P5.1
In contrast, a water quality swale (shown above) has an engineered design geared towards infiltration. A special special mixture may be used. In addition, an underdrain may also be used to avoid the potential for long periods of standing water on the surface.

Design Considerations for Proposal Review

• The recommended slope for swales is 1-2%. For steeper slope (up to 5%) check dams are recommended to reduce flow velocity and erosion potential. In areas of steep slopes, swales should run parallel to contours of the landscape. Swales may not be appropriate for highly sloped areas.
  

• If used as a primary practice, pretreatment should be installed to accommodate 25% of the water quality volume (WQV).
  

• Dry swales should have a trapezoidal or parabolic cross-section, with side slopes 3:1 or flatter to facilitate maintenance.
  

• Water quality swales should be designed to hold water no longer than 24 hours. A 30 inch deep soil mix consisting of 50% topsoil and 50% sand should be used for water quality swales. Infiltration rates should be 0.5 inch/hour or greater. If native soils are not conducive to infiltration, an underdrain system should be installed beneath the soil layer to avoid long periods of standing water.
  

• Grasses are typically used in dry swales. Criteria for choosing grasses can be found in Chapter 11, P5.6 in the CT Stormwater Quality Manual.
  

• Outlet protection should be required at the discharge point from dry swales to prevent scour.
  

• Be sure that some provision is made for temporary erosion and sediment control during construction. Since vegetated practices have large amounts of disturbed soil, it is critical that some measures are used until the site is stabilized.
  

• Avoid soil compaction during the installation of any dry swale. Highly compacted soil can be nearly impervious to water, and can lead to unwanted long term ponding.
  

• The bottom of a dry swale should be 2-4 feet above the seasonal high water table.
  

• Wet swales should only be used where the water table is at or near the soil surfaces, or in poorly drained soil types. See CT Stormwater Quality Manual (page 11-P5-6) for design specifications for wet swales.
  

Click on image above to view larger image of a grass drainage channel (pdf).	Click on image above to view larger image of a water quality swale (pdf).

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Swales on Commercial Sites

• Water quality swales are generally designed to be used on fairly small watershed areas (<5 acres). Grass drainage channels can be used on larger watersheds (up to 50 acres in size).
  

• Swales are most applicable to low to moderate density land uses, such residential, small commercial parking lots, and institutional areas. For higher density commercial and industrial land uses, swales may be appropriate for pretreatment in conjunction with other stormwater treatment practices, or for stormwater retrofit applications.
  

• An underdrain is recommended for water quality swales in commercial or other intensive land use areas.

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Local Examples

Are you looking for more local examples of swales? Visit CT NEMO's LID database for Connecticut. This site contains searchable database for LID practices, engineering firms familiar with LID, and construction companies who have installed LID practices.

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