Vegetated Swales: Effective Stormwater Management Facilities

Plants are increasingly used for the environmental services they provide, such as reducing heat islands, improving streetscapes and decreasing the volumes of water in piped drainage systems. Integrating phytotechnologies, an approach that uses plants to solve environmental problems, into stormwater management infrastructure reduces runoff volumes and flows by allowing water to percolate into the water table.

In this article, you will find basic information on vegetated swales. Three types of swales are covered:

Each type of swale meets different needs, such as stormwater conveyance, treatment, infiltration and retention.

What is a vegetated swale?

A vegetated swale along a road — A vegetated swale in midsummer.

Vegetated swales are shallow, wide, engineered ditches. They are generally located along roads, forming linear retention and infiltration basins. These facilities promote on-site management of surface water (runoff and stormwater).

The plants in swales help reduce volumes and peak flows by increasing the roughness of the channel. Lower flows allow more total suspended solids (TSS) to settle. The plants act as a filter and retain part of the TSS that degrade water quality.

Plants can also absorb heavy metals and nutrients, such as the nitrogen found in stormwater.

Plants provide several benefits that other facilities, such as infiltration trenches, cannot. In particular, they help reduce heat islands, improve the appearance of the infrastructure and filter certain pollutants.

Grassed swales without retention

A grassed swale under construction in the right-of-way of a street in a low-density residential neighbourhood — This grassed swale is equipped with a perforated drain that removes surface water after it infiltrates.

Although grassed swales without retention convey stormwater like ditches do, they stand apart through their ability to treat the pollutants and sediments contained in the water. Swales are wider than ditches and have gentler slopes. Grassed swales also include a filter bed with a perforated drain that efficiently removes stormwater.

Adding plants to the swale slows runoff, which increases sedimentation, infiltration through the soil and evapotranspiration, the transfer of water to the atmosphere through evaporation and plant transpiration during photosynthesis.

Design criteria for grassed swales

To be effective, this type of vegetated swale must meet several design criteria:

Plants must be chosen carefully, since they directly affect runoff velocity. They must also withstand winter conditions.
A parabolic channel shape is preferred, although a trapezoidal shape is also used. For a trapezoidal shape, the bottom width should be more than 0.75 m and less than 3 m, and side slopes should not exceed a 3:1 ratio, to facilitate maintenance and ensure effective pretreatment of the runoff entering at the inlet.
The longitudinal slope may range from 0.5% to 4%, but check dams should be installed when the slope exceeds 3%.
Swales must be designed to convey the water quality flow at velocities below 0.5 m/s. Their capacity must also allow them to convey the design flow, which is typically the 1-in-10-year event.

Swales with permanent retention

Swales with permanent retention combine a constructed wetland with a swale without retention. They differ from swales without retention by their width of 4 to 6 m and by the use of check dams to retain water.

Swales with permanent retention also have no filter bed. Given their size, they can be built in highway medians. The design principles for grassed swales apply to swales with retention.

Note that swales with permanent retention do not significantly reduce runoff volumes. Unlike grassed swales without retention, which normally remain dry, swales with retention maintain a low, continuous flow and do not provide significant storage volume.

Bioretention swales

A bioretention swale built in a street right-of-way — This bioretention swale was built in a low-density neighbourhood.

Bioretention swales differ from other types of swales through the addition of a bioretention component that improves water quality control performance. Water can pond for a short period before infiltrating into the soil. The swale is designed based on the characteristics of the in-situ soil (permeability, soil depth and depth of the water table) and the quality of the runoff.

This type of infrastructure can be built in residential areas, but an overflow system must be provided to ensure that flows do not exceed the water quality flow.

Otherwise, bioretention swales can be used in the same contexts as swales with or without retention. Their design criteria include the criteria set out for swales without retention, in addition to the criteria for bioretention systems.

Typical uses of vegetated swales

The following table summarizes where each type of vegetated swale can be used.

Application	Grassed swale	Swale with permanent retention	Bioretention swale
Parking lot drainage	X		X
Highways	X	X	X
Roads (median or roadside)	X		X
Pretreatment for other stormwater management facilities	X		X
Urban areas			X

Maintaining vegetated swales

A vegetated swale with a check dam in the fall — This vegetated swale is equipped with a check dam.

Depending on the type of swale installed, regular maintenance is required to ensure the infrastructure performs well.

Grassed swales are often installed in the road right-of-way. In this case, maintenance is carried out by the adjacent property owners, and public education campaigns are needed before the system is implemented.

For all types of swales, a spring cleanup is needed to ensure good water percolation. New plants may also need to be planted, especially during the first years after installation.

Advantages and disadvantages of vegetated swales

The main advantages of swales are that they:

Reduce volumes and peak flows (grassed swales and bioretention swales)
Improve runoff quality by treating sediments and pollutants
Contribute to groundwater recharge
Reduce heat islands
Increase the resilience of water management infrastructure
Improve streetscapes
Lower the costs of the drainage system and of water treatment at the plant
Help increase biodiversity

The main disadvantages of swales are that they:

Require regular maintenance
Carry a risk of groundwater contamination
Require a large area (grassed swales and swales with permanent retention)
Must meet several constraints (longitudinal slope, velocity, infiltration, etc.) to be effective

Learn more about other types of stormwater management facilities on the Avizo blog.

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