California Department of Transportation
 

Biofiltration Strips

Biostrip

What is This Treatment?

Biofiltration Strips use plants to capture and biologically degrade pollutants carried by stormwater runoff. As an additional benefit, Biofiltration Strips 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 Strips includes grasses, forbs, ground cover, and shrubs.

When to Use This Treatment?

Use Biofiltration Strips at sites consistent with Caltrans policy as outlined in the documents linked at the bottom of this page. Typically right-of-way vegetation is maximized as allowed by specific site conditions, climate, and where stormwater velocities will not result in scour.

Benefits:

  • When properly implemented, Biofiltration Strips are aesthetically pleasing. Due to vegetation, Biofiltration Strips 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.

Limitations:

  • Vegetated Biofiltration Strips should not be used in arid regions. Instead, consider using non vegetated strips that rely upon gravel mulch or similar inert materials to protect disturbed soil areas.
  • Vegetated biofiltration strips must have a minimum vegetative cover of 70% within the first full growing season.
  • Discuss with District Design NPDES Coordinator if Biofiltration Strips are under consideration above if contaminated soils or groundwater plumes, although the infiltration that might occur is usually considered incidental.
  • Slopes should be designed as flat as possible. For new construction, widening, or where slopes are otherwise being modified, embankment (fill) slopes should be 1:4 or flatter (Refer to the Highway Design Manual Chapter Index 304.1 for further discussion of slopes).
  • Biofiltration Strips 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 with 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.

Specifications

Specify Biofiltration areas by combining standard Caltrans 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 Strips are not paid for as a separate item. Biofiltration Strips are built and paid for as part of other standard items, such as Compost (Incorporate) and Erosion Control (Hydroseed). 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 Strips on the project plans. Click the image for a larger PDF version.

Biofiltration Strip

Biofiltration Table

Guidance:

Biofiltration Strip Guidance

For guidance on designing a Biofiltration Strip, please review: