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Grassed Swale

General

Grassed swales (or vegetated channels) are gently sloping, densely vegetated earthen channels that collect and transport stormwater and reduce the temperature of the water. These channels slow runoff and filter out suspended solids and pollutants while promoting infiltration, retaining runoff for a period of less than 24 hours. Stormwater enters the channel and is slowed by the dense vegetation that grows in the swale. As the runoff’s velocity is lowered, sediments and pollutants are removed by the filtering action of vegetation.

Grassed swales may be used in conjunction with or as an alternative to curb and gutter systems, and may be used as a pretreatment device. They can be used on sites up to 50 acres in size, with the number and length of the swale depending upon the topography of the site and the size of the contributing watershed.

Advantages

  • Relatively low cost
  • Easily to construct and maintain
  • Can be aesthetically pleasing if designed properly
  • Very effective in preventing erosion
  • Capable of carrying large quantities of stormwater

Disadvantages

  • Ineffective in flat areas and areas with very steep slopes
  • Removes a small amount of pollutants
  • Culverts reduce the effectiveness and feasibility of grassed swales
  • Reduced effectiveness with large storm events
  • Effective only as a pretreatment device on highly developed sites as it does not meet the 80% reduction in total suspended solids

Design

Soils

Hydrologic soil groups A, B, and C are suitable with some restrictions, while coarse sands and gravel, by themselves, are generally not recommended because they provide little support for vegetation and have high infiltration rates, providing limited treatment ability. Soils with low permeability are also not recommended, as they do not allow the runoff to infiltrate during the short ponding period.

Dimensions and Slopes

The length and width of swales will depend upon the individual characteristics of the site and must be capable of conveying the runoff from the 10 yr, 24-hour storm event and should also prevent erosion of the channel during this storm event. In general, however, swales should be between 2 and 8 feet wide. Widths greater than eight feet are not suggested, as channelized flow is likely to result.

Swales may provide a shallow ponding area for runoff, with a maximum depth of 18 inches.

The side slopes of the swale should have a horizontal to vertical ratio no greater than 3:1, and generally a ratio of 4:1 or flatter is recommended. These slopes increase the surface area of the channel, make maintenance tasks easier, and improve the safety of the device.

Longitudinal slopes are generally dependent upon the topography of the site, but they should prevent runoff velocities from exceeding 5.0 feet per second. In most cases, swales function best with a longitudinal slope of 1-3%. Slopes less than 1% may cause excessive ponding and sediment deposition, while slopes greater than 4% often result in high velocities. High velocities increase the potential for channel erosion, and may require that stone check dams or erosion matting is installed on such steep slopes. Stone check dams are vertical drops of between 6 and 24 inches that help to reduce the slope of the channel and the velocity of the water. Their use is limited, however, as they often require additional energy dissipating structures and must be spaced at least 50-100 feet apart to prevent erosion of the channel.

Shape

Swales should be designed with a trapezoidal shape. V-shaped are not recommended as they may erode during high flows.

Vegetation

Plant selection will depend upon individual site characteristics such as the length of inundation in the swale and the amount of light available. Native species provide many benefits when compared to other species and are strongly encouraged. However, native species should be selected carefully. Care should also be taken to avoid the use of invasive and exotic species. Whatever the species that is selected, it should be tolerant to inundation, have the ability to form a dense sod, and resist matting. In roadside situations, vegetation should be tolerant of salt. In instances where time is not available for the proper establishment of seed, sodding or temporary seeding is generally preferred.

Vegetation should be maintained between 3 to 8 inches in height, and should extend above the ponding depth at all times. Fertilizer and pesticide use, if necessary, should be applied sparingly and only during dry periods of the year to prevent further runoff pollution.

Maintenance

  • Swales should be inspected periodically during the first year of use and after all major storm events in perpetuity for possible erosion to the channel
  • Trash and other debris should be removed seasonally
  • Stone check dams should be inspected for evidence of bypassing
  • Channelization, barren areas, and low spots within the channel should be repaired and reseeded
  • Accumulated biomass should be removed periodically

Method to Determine Practice Efficiency

Grassed swales are designed as stormwater conveyance channels and provide little treatment ability. As a result, no efficiency is given for this practice.

References