5.4.18 Compost-Amended Vegetated Filter Strips (CAVFS)

5.4.18.1 BMP Description

The compost-amended vegetated filter strip (CAVFS, BMP T7.40 in Volume V, Chapter 7 of the Ecology Manual) is a variation of BMP T9.40: Vegetated Filter Strip that adds soil amendments to the roadside embankment (see Example of a Compost Amended Vegetated Filter Strip (CAVS) figure in Volume V, Chapter 7 of the Ecology Manual). The soil amendments improve infiltration characteristics, increase surface roughness, and improve plant sustainability. Once permanent vegetation is established, the advantages of the CAVFS are higher surface roughness; greater retention and infiltration capacity; improved removal of soluble cationic contaminants through sorption; improved overall vegetative health; and a reduction of invasive weeds. CAVFS have somewhat higher construction costs than BMP T9.40: Vegetated Filter Strip (Volume V, Chapter 7 of the Ecology Manual) due to more expensive materials, but require less land area for runoff treatment, which can reduce overall costs.

5.4.18.2 Performance Mechanism

CAVFS 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.

5.4.18.3 Application and Limitations

CAVFS can be used to meet basic runoff treatment and enhanced runoff treatment objectives (see Table II-5.21). It has practical application in areas where there is space for roadside embankments that can be built to the CAVFS specifications.

Table II-5.21. CAVFS Applicability.

BMP	MR #5: On-site Stormwater Management		MR #6: Runoff Treatment				MR #7: Flow Control
	List	LID Performance Standard	Basic	Enhanced	Oil Control	Phosphorus
CAVFS			X	X

5.4.18.4 Site Considerations

The maximum lateral slope from adjacent roadway drainage area is 5%, and the lateral slope of the CAVFS itself shall be between 2% and 25%. A gravel or crushed surfacing level spreader is also required between the roadway drainage area and the CAVFS.

5.4.18.5 Design Information

The CAVFS design incorporates composted material into the native soils per the criteria in BMP T5.13: Post-Construction Soil Quality and Depth for turf areas. However, the compost shall not contain biosolids or manure. The goal is to create a healthy soil environment for a lush growth of turf. See BMP T7.40 in Volume V, Chapter 7 of the Ecology Manual for detailed design information and guidance for a CAVFS, including information on allowable slopes, level spreader, and soil design.

BMP T7.40 in Volume V, Chapter 7 of the Ecology Manual provides guidance on Runoff Model Representation.

5.4.18.6 Minimum Construction Requirements

Minimum construction requirements associated with CAVFS include the following:

• Do not put CAVFS 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 CAVFS vegetation is established.

• Avoid compaction of the CAVFS areas during construction.

5.4.18.7 Operations and Maintenance

See BMP T7.40 in Volume V, Chapter 7 and Volume V, Appendix V‑A of the Ecology Manual for required maintenance practices for CAVFS.