Filter strips: Performance
Vegetated filter strips are primarily a practice used to achieve water quality improvements although some infiltration can occur, depending on the soil type and infiltration rate.
|Water balance benefit||Water quality improvement||Erosion control benefit|
|Partial: depending on soil infiltration rate||Partial: depending on soil infiltration rate and length of flow path over the pervious area||Partial: depending on soil infiltration rate|
Research indicates that runoff reduction from vegetated filter strips is a function of soil type, slope, vegetative cover and flow path length across the pervious surface. A conservative runoff reduction rate for vegetated filter strips is 25% for HSG C and D soils and 50% for HSG A and B soils. These values apply to filter strips that meet the design criteria outlined in this section.
|2 - 5 m||8 - 15 m|
|Guelph, ON||20 %||62 %|
|California, USA||40 %||70 %|
Vegetated filter strips can provide moderate pollutant removal from runoff. Research suggests that runoff pollutant concentrations and loads decrease when treated with filter strips and that steady state pollutant levels are typically achieved within 5 m of the pavement edge . Based on a synthesis of performance monitoring studies as of 2000, it was reported that pollutant removal efficiencies of vegetated filter strips are highly variable. For this reason, filter strips should be used in conjunction with other water quality best management practices (e.g., as pretreatment).
|Total suspended solids (TSS)||20 - 80 %|
|Total Nitrogen||20 - 60 %|
|Total Phosphorus||20 - 60 %|
|Total heavy metals||20 - 80 %|
Performance of filter strips has also been evaluated based on the Roadside Vegetated Treatment Sites Study  and the BMP Retrofit Pilot Study . These studies concluded that concentration reductions consistently occur for TSS and total heavy metals and frequently for dissolved metals. Nutrients concentrations remained generally unchanged. When vegetation cover on the filter strip is below 80% water quality performance declines.
- Abu-Zreig, M. Rudra, M. Lalonde. H. Whitely and N. Kaushik. 2004. Experimental investigation of runoff reduction and sediment removal by vegetated filter strips. Hydrologic Processes. 18: 2029-2037.
- Barrett, M. 2003. Roadside Vegetated Treatment Sites (RVTS) Study Final Report, Report # CTSW-RT-03-028. California Department of Transportation. Sacramento, CA.
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- California Department of Transportation (Caltrans). 2004. BMP Retrofit Pilot Program, Final Report, CTSW-RT-01-050. Sacramento, CA.