Vegetated Filter Strips

5.4.21 Vegetated Filter Strips

5.4.21.1 BMP Description

A vegetated filter (BMP T9.40 in Volume V, Chapter 7 of the Ecology Manual) strip is flat with no side slopes (see Typical Filter Strip figure in Volume V, Chapter 7 of the Ecology Manual). Contaminated stormwater is distributed as sheet flow across the inlet width of the vegetated filter strip. Runoff treatment is provided by passage of water over the surface and through grass.

5.4.21.2 Performance Mechanism

Filter strips remove pollutants primarily by filtration as stormwater moves through the grass blades. This enhances sedimentation and traps pollutants that adhere to the grass and thatch. Pollutants can also be adsorbed by the underlying soil when infiltration occurs, but the extent of infiltration depends on the type of soil, the density of grass, and the slope of the filter strip.

5.4.21.3 Application and Limitations

The vegetated filter strip is typically used adjacent and parallel to paved areas such as parking lots, driveways, and roadways and may be used to fully or partially satisfy basic or enhanced runoff treatment requirements (see Table II-5.24).

Table II-5.24. Vegetated Filter Strip Applicability.
BMP	MR #5: On-site Stormwater Management		MR #6: Runoff Treatment				MR #7: Flow Control
BMP	List	LID Performance Standard	Basic	Enhanced	Oil Control	Phosphorus	MR #7: Flow Control
Vegetated Filter Strip			X	X^a
Notes: Can be used to meet enhanced runoff treatment requirement as part of a two-BMP treatment train; see Volume III, Section 1.2 in the Ecology Manual.

5.4.21.4 Site Considerations

The maximum lateral and longitudinal slope from adjacent roadway drainage area is 5% and 2%, respectively, and the lateral slope of the basic filter strip itself shall be between 1% and 15%. A gravel or crushed surfacing level spreader is also required between the roadway drainage area and the basic filter strip.

5.4.21.5 Design Information

Refer to BMP T9.40 in Volume V, Chapter 7 of the Ecology Manual for vegetated filter strip design criteria and procedures.

5.4.21.6 Minimum Construction Requirements

Minimum construction requirements associated with filter strips include the following:

Do not put filter strips into operation until areas of exposed soil in the contributing drainage areas have been sufficiently stabilized. Deposition of eroded soils can impede the growth of grass in the filter strip and reduce treatment effectiveness. Erosion and sediment control measures shall remain in place until the filter strip vegetation is.
Avoid compaction of the filter strip areas during construction.

5.4.21.7 Operations and Maintenance

Refer to BMP T9.40 in Volume V, Chapter 7 and Volume V, Appendix V‑A of the Ecology Manual for required maintenance practices for vegetated filter strips.