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Grassed Swales

A grassed swale is a graded and engineered landscape feature appearing as a linear, shallow, open channel with trapezoidal or parabolic shape. The swale is vegetated with flood tolerant, erosion resistant plants.

The design of grassed swales promotes the conveyance of storm water at a slower, controlled rate and acts as a filter medium removing pollutants and allowing stormwater infiltration.

When properly designed to accommodate a predetermined storm event volume, a grassed swale results in a significant improvement over the traditional drainage ditch in both slowing and cleaning of water.

In swales, stormwater is slowed by strategic placement of check-dams [ 446 KB pdf file], new window] that encourage ponding and these ponds in turn facilitates water quality improvements through infiltration, filtration and sedimentary deposition. Collected stormwater is expected to drain away through the soil within several hours or days.


Grass Swale with Check Dams.
Note significant channel storage capacity created by check dams.
Notched center allows safe overflow without scour around sides.
(image from Virginia Stormwater Management Program

Here are some additional examples of grassed swales and check dams.

 

Applications

Grassed Swales are an appropriate stormwater management practice for most regions of North America. Swales are a low cost low maintenance option to remove sediments, nutrients and pollutants. They increase stormwater infiltration and add a visually aesthetic component to a site.

Establishment of grassed swales is a potential solution wherever stormwater needs to be transported from impervious surfaces, slowed down and allowed to infiltrate into soils.

Exceptions to their use are in desert like areas where irrigation would be required for long-term maintenance and in colder regions with permafrost where infiltration is minimal.

In northern climates without permafrost, design and maintenance requirements should be modified with respect to a shorter growing season, management of meltwater and depth of frost in soil (see details in section on Tips and Wisdom).

Typically grassed swales are used as an environmentally preferential solution or sometimes as an enhancement to the more traditional curb and gutter based storm sewer system. The linear structure of swales favors their use in the treatment of runoff from highways, residential roadways and common areas in residential sub-divisions, along property boundaries and in and around parking lots.


From: Puget Sound, WA Action Team

Generally grassed swales are used to treat relatively small drainage areas of five acres or less. In highly urbanized areas or other highly impervious areas grass swales are not recommended unless constructed in series or function as pretreatment for other stormwater management practices.

Advantages of correctly designed grassed swales:

  • Dry Swales
    • Trap and remove sediments and other pollutants and thus improve water quality.
    • Reduce peak runoff velocity and promote infiltration.
    • Reduce erosion.
    • Provide for some groundwater recharge if correctly designed.
    • Are favored for use for treating highway and residential road runoff because of their linear structure. Good to use in replacement of existing drainage ditches.
    • Are best if used in low to moderate density developments.
    • Are useful as a means of disrupting impervious pavement in parking areas.
    • Has ponding of water for only a short time with little increase in water temperature thus useful in watersheds containing cold-water trout streams. 
    • Are less expensive to build and maintain (easy to mow) than a traditional curb and gutter system.
  • Wet Swales
    • Function as a linear wetlands
    • Reduce peak flows and runoff velocity and promote infiltration.
    • Reduce erosion.
    • Are easy to design. Can be built in relatively impervious soils or in seasonally saturated soils or intersecting water table
    • Trap and remove sediments and other pollutants with increased efficiency and thus improve water quality.
    • Create visually appealing and beneficial habitat between uplands and surface waters
    • Are less expensive to build and maintain than a traditional curb and gutter system
    • Provide effective pretreatment of stormwater passing through for further processing by additional stormwater management practices.

Swale Cross-sections (click image to enlarge)
From: MPCA stormwater manual 2005 [ 1.7 MB]
-click image to enlarge-

 

Examples
   

Materials and Installation
  1. The successful, properly functioning grassed swale is dependant on good engineering design.
    • Refer to the State of Minnesota Stormwater Manual - 2005, Chapter 12-7: Infiltration Practice Guidance [ 1.2 MB] to find an excellent discussion on siting and engineering design of infiltration Best Management Practices in Minnesota.
  2. Geo-technical testing of soil is recommended to establish soil porosity and identification of location of close-to-surface bedrock outcrops that may require re-location of swale.
  3. Be certain that local authorities have been approached for:
    • An understanding of requirements (Some cities have manuals or fact sheets available)
    • Permits and inspections required
  4. Siting and Design
    • A suggested design process [ 2.7 MB] includes the following steps:
      • Compute water quality treatment volume (WQV) for surfaces to be treated as required by local authorities and/or permitting agencies
      • Determine the swales dimensions ( bottom width, depth, length and slope) required to store the WQV in a shallow ponding depth (18” maximum depth)
      • Compute the WQV drawdown time to ensure it is less than 24 hours
      • Compute the 2 and 10 year frequency storm event peak discharges
      • Check the 2 year storm velocity for potential to cause erosion, adjust swale geometry if necessary and reevaluate WQV design parameters
      • Check the 10 year storm for depth and velocity, adjust swale geometry if necessary and reevaluate WQV and 2 year design parameters
      • Provide minimum of 6” freeboard (height of channel sides above water surface) above 10 year stormwater surface profile
    • A reference to find needed design calculations is: Design of Stormwater Filtering Systems (1996) by Claytor and Schueler; Center for Watershed Protection and Chesapeake Bay Research Consortium; Ellicott City and Solomons, MD.

      Additional references to help understand complexities of design can be found in a number of municipal, state and non-governmental organizations who provide Best Stormwater Management information. These are offered only as a means of emphasizing and confirming the complexity of national stormwater infiltration issues and that requirements are typical throughout the nation.
    • To slow water flow in order to prohibit erosion of swale, check dams may have to be designed and added to grassed swales. Check dams not only slow water velocity but encourage ponding of water on the upstream side of each dam, hereby encouraging infiltration of stormwater; their design criteria:
      • Should be a maximum of 18 inches in height and height should not exceed half the height of swale bank.
      • Spacing will depend on both the longitudinal slope of the swale and height of swale combining to provide desired WQV behind the dams.
      • Anchoring of dams is necessary to prevent washout. Each side of the dam needs to extend two – three feet into the swale wall on both sides.
      • The downstream side of check dams need to be protected from scour with sufficiently sized rip rap placed over geotechnical fabric.
      • Check dams should be notched at their top to allow passage of two year design storm and a six inch freeboard to the top of the swale to handle a 10 year design storm.
      • Check dams can be made from wood (pressure treated 6”x 6” or 8”x 8” or water resistant logs e.g. cedar) placed on top of a rock base. Concrete, gabions or rip rap (sufficiently sized to withstand flow of water) are used successfully also. [view images]
    • Additional design considerations for structural components of Grassed swales:
      • At the point of inflow into the swale a pretreatment and sediment collection forebay can be designed. It should be able to contain 25% of the WQV. The installation of a rip rap pretreatment area contained by a check dam built on a stone base will serve the required function. [view image]
      • At the end of a dry grassed swale the installation of a check dam can be combined with a pea gravel filled basin that leads excessive water to the underdrain system. Output from this underdrain must be managed.
    • Culverts can be used to maintain swale connectivity where a road is planned to cross the swale. The culvert must be sized sufficiently large to accept the swales design flow for the10 year storm. Rip rap should be placed at the culvert entrance to reduce erosion.
      • In cold climates culverts should have a minimum diameter of 18 inches and a minimum slope of one percent to help keep water moving and prevent blockages.
      • Wintertime maintenance is a must-do to avoid potential flooding. In areas that exceed frost depths greater than five feet and/or snow depths over eight feet need to posses a portable steamer to clear blocked culverts.
  5. Construction
    • Accurate grading is essential to construction of a properly functioning grassed swale.
    • Machinery used for excavation and grading should not be driven over the swale site since compaction of soil is likely to occur.
    • The swale site should be protected from storm water runoff that will cause erosion and sedimentation during construction. Final grading and planting should not occur until the adjoining areas draining into the swale are stabilized.
    • Any accumulation of sediments that does occur must be removed during the final stages of grading.
    • In dry swales the bottom should be tilled to produce a highly porous surface.
    • Installation of erosion control matting or blanketing to stabilize soil during establishment of vegetation is highly recommended.
      • In colder climates with short growing seasons (less than 5 months) the desired level of establishment of vegetation may take two to three growing seasons. Careful maintenance of erosion control practices must continue for the duration.
  6. Vegetation Selection and Planting
    • Type of vegetation selected must take into account:
      • Soils conditions.
      • Climate – plants have to be sufficiently hardy to withstand the most extreme conditions to occur in the local region.
      • Topography- vegetation must be able to withstand forces created by flowing water.
      • Available sunlight
    • Selected vegetation must meet the following criteria:
      • Have a deep root system or form dense sod to resist scouring.
      • Be vigorous growers.
      • Have a high stem density to help slow water and facilitate sedimentation.
      • Be tolerant to flooding and be able to survive and continue to grow after the inundation period. Water velocities associated with a two year design storm should not scour out planted materials nor should plants leaves matted down by water (will reduce amount of sediment removal).  
      • If to be used near a road the plants must also be salt tolerant.
    • Contact local area horticulturist and specialists in native plantings for recommended species used in grassed swales. Refer to published lists [ 75 KB] of salt tolerant plants from a variety of habitats that are suitable for both dry and wet swales constructed in northern climes. For additional information on plant salt tolerance and plants useful in cold climates see Appendix E [ 2.2 MB] in the Minnesota Stormwater Manual (2005).
    • Some commonly used grasses for dry swales are:
      • Park grade Kentucky bluegrass or fescue or Creeping bentgrass(If regularly mowed)
      • Native grasses – switchgrass, big bluestem, little bluestem, Indian grass or side-oats grama (typically mowed seasonally). Their use will increase diversity of animals and their extensive root systems promote drought tolerance and better filtration. On the negative side the swale will take a longer time to become established. The use of cover crops and/or soil stabilization techniques will be required.
      • A mixture of native grasses can be visually appealing and provide variety of texture and color throughout the seasons.
      • Do not use  invasive species like reed canary grass.
    • Planting can be accomplished by the use of sod or by seeding (preferably hydro-seeding [ 240 KB]).
    • Soil preparation includes:
      • Prepare top three inches of soil to provide sufficient aeration to allow rapid root growth.
      • Add appropriate types of fertilizer and apply at prescribed rate to encourage rapid growth.
      • Sod rolls are laid perpendicular to slope to assist in erosion control. Sod edges should butt against each other and vertical joints staggered (look like a brick wall). [view image]
      • The laid sod should then be inspected for gaps  and foreign materials then rolled to ensure root surfaces are in contact with soil.
      • Installation of erosion control matting is strongly encouraged until vegetation is well established.
      • Water well at least two to three times in the first few weeks.
      • Well established sod can be identified by grasping laid sod and tugging upwards. If the sod layer does not pull away from the soil layer then the sod can be considered well established, this should take two to three weeks.
      • With hydro-seeding a well established vegetation mat will take longer to develop and depend on weather conditions and frequency of watering.
  7. Maintenance
    • Proper maintenance of grassed swales is essential. A maintenance contract with required maintenance steps delineated has been a successful approach.
    • A recommended program would involve the following:
      • Mowing of dried swales as required seasonally to maintain the desired height of vegetation at three to four inches. Mowing grass too short and incorrectly applying lawn chemicals will jeopardize function.
      • At least once a year and more often if required:
        • Inspection for erosion. Any obvious damage to grass or to the swales bottom soil bed should be repaired immediately. Seeded swales will require periodic re-seeding. Repairs need to conform to original swale design.
        • Remove trash and other debris from all parts of the swale.
        • If selected grass for swale is not performing select and plant a new grass.
        • For wet swales replant with a revised selection of wetland plants if current plants are establishing slowly or failing.
      • On as needed basis:
        • Till soil at swales bottom if water does not drain out within 48 hours (swales should drain within 24 hours).
        • Remove sediment build up when the swales volume is reduced by 25 percent.
      • In colder climates after spring snow melt:
        • Any collected sand from winter time sanding must be removed.
        • Replacement of damaged vegetation should occur as soon as possible.
        • Additions of mulch (added organic matter) may be required if drainage to swale is from roadway or parking lot. Deicing salts can damage soil structure by reducing soils organic matter.
    • Maintenance responsibility is normally that of the owner.
  8. Cost
    • Swales are a relatively cheap alternative to curb and gutter treatment. Swale maintenance is more frequently required but these costs are considerably cheaper than for curb and gutter systems maintenance.
    • Swale construction costs have not been well studied. Costs will differ regionally and with site conditions. A best estimate has been made of $0.50 per square foot (2002 dollars). An undated estimate (no older than 2000) suggests a value of $5.50 per cubic foot of storage provided.

Suggested References: Guidebooks, websites and pamphlets

[ = pdf file; it will be opened in a new window]

  1. The Urban Small Sites Best Management Practices (BMP) Manual (2003) by the Metropolitan Council of Minnesota’s Twin Cities offers detailed information on 40 BMPs for stormwater pollution management in a cold-climate setting.
    Download their information on dry swales [ 2.7 MB] and wet swales [ 1.9 MB].
  2. Post-Construction Storm Water Management in New Development & Redevelopment – Grassed Swales (2002) by the USEPA; a good review of Grassed Swale knowledge and practices. A well done summary of the effectiveness of Grassed Swales as pollutant removal systems. Strong reference section for pursuit by the more curious.
  3. Virginia Stormwater Management Handbook (1999) produced by the Virginia Department of Conservation and Recreation; an example of a very detailed stormwater management (BMP) manual. Strong on engineering and construction detail. Also a useful collection of images depicting grassed swales; find minimum standards for grassed swales in Chapter 3 [ 741 KB].
  4. State of Minnesota Stormwater Manual-2005 is a valuable tool for stormwater managers. The manual provides details on stormwater management practices applicable to Minnesota that conserves, enhances, and restores high-quality water in our lakes, rivers, streams, wetlands, and ground water.
    Go to the chapter on Infiltration Practices [ 1.2 MB],
    Appendix D-6: Infiltration Practices CADD Details [ 1.5 MB] and
    Appendix E: Minnesota Plant List and Application
    [ 2.2 MB] for details that apply to Grassed Swales.

Tips and Wisdom
Grassed swales have minimal ‘hard structure’ to be impacted by freeze conditions and snow and therefore a useful Best Management Practice. Suggested cold climate adjustments deal with frost depth, snow meltwater volume and a shortened growing season; considerations include:

  • Plowing adjoining areas after snowfall must be carried out in a judicious manner to prevent damage to the swale’s edge structure and subsequent erosion. A modest set back and flag marking of the swale are measures that can help. 
  • Providing for a five foot setback to prevent frost heave of roads and parking lots in climate zones where frost depth exceeds five feet and soils are clay or silt structure.
  • Do not use grassed swales in areas of permafrost.
  • Salt tolerant plants must be chosen for swales placed along roadways.
  • Short growing seasons are typical of colder climates. Two seasons may be required for establishment of vegetation. During this period erosion control practices such as mats or blankets must protect swale structure.
  • Grassed swales can be used for snow storage and are important in reducing snow melt peak flows through infiltration and in treating most pollutants associated with snow. However, extra maintenance will be required to deal with the sand and de-icing compounds used on roadways and parking lots:
    • If swales are to be used for snow storage, plant selections must be salt tolerant.

 

Limitations

  • Grassed swales are not recommended for large drainage areas.
  • Grassed swales are a cheap alternative to curbs and gutters; however their efficacy is dependant on careful design and construction, a thoughtful selection of plants as well as provision of dependable maintenance over time. Badly designed grass swales will not remove significant quantities of pollutants.
  • If vegetation is not sufficiently established the swale will not function.
  • Given the importance of design and construction, selection of engineering services and a construction company should be based on proven, successful past experience with swales and recommendations from past clients who have had swales installed.
  • Wet swales should not be used in high density residential areas because of potential for mosquito generation and smells.
  • Grassed swales are not the best management practice for:
    • Ultra urban areas because the areas of pervious surfaces required for swale development are usually unavailable.
    • Stormwater hot spots where land use will generate stormwater with high levels of contaminants. Good examples of hot spots would be gas stations and convenience stores.
  • In cold weather climates, adjustments to design and maintenance must occur to deal with high snow loads and significant frost depths.

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