California Department of Transportation

Biofiltration Swales

Biofiltration Swale

What is This Treatment?

Biofiltration Swales use plants in channels to capture and biologically degrade pollutants carried by stormwater runoff. As an additional benefit, Biofiltration Swales also reduce the velocity and volume of stormwater runoff.

Biofiltration is provided by both Biofiltration Strips and Biofiltration Swales. Biofiltration Strips, also known as vegetated buffer strips, are vegetated sections of land over which stormwater flows as sheet flow. Biofiltration Swales are vegetated channels that receive and direct the concentrated flow of stormwater.

Acceptable vegetation for Biofiltration Swales includes grasses, forbs, and ground cover.

When to Use This Treatment?

  • Design Biofiltration Swales consistent with Caltrans policy as outlined in the documents linked at the bottom of this page.


  • When properly implemented, Biofiltration Strips and Swales are aesthetically pleasing. Due to vegetation, Biofiltration Strips and Swales look like a landscaped roadside, which makes these devices more acceptable than Treatment BMPs that make use of concrete-lined vaults.
  • Per the BMP Retrofit Pilot Program Final Report (Caltrans, 2004) Biofiltration Strips and Swales were determined to be highly effective Treatment BMPs in reducing sediment and heavy metals and stormwater runoff volumes.
  • Per the BMP Retrofit Pilot Program Final Report (Caltrans, 2004) Biofiltration Strips and Swales were determined to be very cost effective and among the least expensive Treatment BMP per volume of runoff treated.
  • Biofiltration Strips and Swales are well suited to being part of a “treatment-train” system of BMPs and should be considered whenever placing other BMPs that could benefit from pretreatment, especially Infiltration Basins, Infiltration Trenches, and Wet Basins.


  • Must meet maximum longitudinal slope and maximum slope requirements for Biofiltration Swales. Please refer to the the Biofiltration swale guidance at the bottom of this page and consult your Hydraulics Engineer.
  • Vegetated biofiltration swales must have a minimum vegetative cover of 70% within the first full growing season.
  • Vegetated Biofiltration Swales should not be used in arid regions. Instead, consider using non vegetated swales that rely upon gravel mulch or similar inert materials to protect disturbed soil areas.
  • Discuss with District Design NPDES Coordinator if Strips and Swales are under consideration above if contaminated soils or groundwater plumes, although the infiltration that might occur is usually considered incidental.
  • Biofiltration Strips and Swales are not generally subject to the same setback restrictions as Infiltration Devices; however, if unusual geotechnical conditions exist, or if a Biofiltration Strip is proposed above a retaining wall and the soils are known to be especially erodible or permeable, consult District Geotechnical Design.
  • Nearby fill slopes should be observed for signs that the embankment soils are highly erodible, and the District Landscape Architect should be consulted about soil amendments, use of fiber rolls, or other methods to reduce the potential erosion.

Bioswale Graphic



Specify Biofiltration areas by combining standard Department specifications for Incorporate Compost, Erosion Control (Hydroseed), Rolled Erosion Control Products (Netting), and Liner Plants. For more detailed information, see the examples below.

Plans and Details:

Estimate Information:

Biofiltration Swales are not paid for as a separate item. Biofiltration Swales are built and paid for as part of other standard items, such as Compost (Incorporate) and Erosion Control (Hydroseed). Excavated material should be paid under contract item Ditch Excavation or Roadway Excavation depending on the bottom width per Standard Plan A62A. See the details below for examples of how to identify Biofiltration areas while paying for this work as Compost (Incorporate) and Erosion Control (Hydroseed).

The images below provide an example of how to specify Biofiltration Swales on the project plans. Click the image for a larger PDF version.

Biofiltration Swale

Biofiltration Table

Biofiltration Swale Detail



Ornamental Roadside Vegetated Treatment Sites

Ornamental Roadside Vegetated Treatment Sites (ORVTS) Study. The ORVTS Study was implemented to assess the treatment performance of roadside slopes planted with ornamental vegetation. The ORVTS Study monitored water quality to evaluate the pollutant removal efficiency of roadside slopes planted with ornamental vegetation as opposed to the traditional use of forbs and grass species.

Roadside Vegetated Treatment Sites (RVTS) StuDY

The Roadside Vegetated Treatment Sites (RVTS) Study (Caltrans, 2008) was a water quality monitoring project conducted by Caltrans from 2000 to 2008 to evaluate the pollutant removal efficiency of roadside slopes planted with forbs and grass vegetation. The RVTS Study results showed that roadside slopes planted with standard grasses and forbs resulted in large concentration and load reductions for several constituents of concern for highway runoff.

BMP Pilot Retrofit Program Final Report



Biofiltration Swale Guidance

For swale design criteria please comply with:

Erosion Prediction With RUSLE2 Software

Caltrans RUSLE2 Software