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.
 Ability of vegetated filter strips to meet SWM objectives
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
===Water balance===
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.
 Volumetric runoff reduction achieved by vegetated filter strips
Location 	Runoff reduction, by length of strip	
2 - 5 m 	8 - 15 m 	
Guelph, ONAbu-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. 	20 % 	62 %
California, USABarrett, M. 2003. Roadside Vegetated Treatment Sites (RVTS) Study Final Report, Report # CTSW-RT-03-028. California Department of Transportation. Sacramento, CA. 	40 % 	70 %
North Carolina, USACarmen, N.B., Hunt, W.F., Andersen, A.R. 2016. Volume Reduction Provided by Eight Residential Disconnected Downspouts in Durham, North Carolina. Journal of Environmental Engineering. 142(10): 05016002. https://ascelibrary.org/doi/10.1061/%28ASCE%29EE.1943-7870.0001107	57 % 	72 %
===Water Quality===
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 Barrett, M., Lantin, A., Austrheim-Smith, S. 2004. Stormwater pollutant removal in roadside vegetated buffer strips. Transportation Research Record. No. 1890, pp. 129-140..
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).
Pollutant removal efficiencies of vegetated filter strips
Total suspended solids (TSS)	20 - 80 %
Total Nitrogen 	20 - 60 %
Total Phosphorus 	20 - 60 %
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 California Department of Transportation (Caltrans). 2004. BMP Retrofit Pilot Program, Final Report, CTSW-RT-01-050. Sacramento, CA.. These studies concluded that concentration reductions consistently occur for TSS and total heavy metals and frequently for dissolved metals. Nutrients concentrations remained generally unchanged. 
A North Carolina field evaluation of stormwater quality treatment performance of vegetated filter strips featuring level spreader concrete structures as inlets found that systems significantly reduced event mean total suspended solids (TSS) concentrations by between 51% (7.6 m long) and 67% (15.2 m long), and pollutant mass by between 73.1% (7.6 m wide) and 88.9% (15.2 m wide) due to substantial runoff volume reductions attributed to infiltration. Nutrient loads were also significantly reduced, with reductions consistently greater for the 15.2 m long filter strip, confirming the importance of length and impervious drainage area to pervious area ratio for pollutant removal (Winston et al., 2011).Winston, R.J., Hunt W.F., Osmond, D.L., Lord, W.G., Woodward, M.D. 2011. Field Evaluation of Four Level Spreader-Vegetative Filter Strips to Improve Urban Storm-Water Quality. Journal of Irrigation and Drainage Engineering. 137(3): pp. 170-182. https://ascelibrary.org/doi/10.1061/%28ASCE%29IR.1943-4774.0000173 
In a recent international research review on processes for improving stormwwater quality treatment of grass swales and vegetated filter strips, Gavric et al. note that while understanding of hydrology and hydraulics of these stormwater control measures is adequate, there are knowledge gaps in understanding water quality treatment processes, particularly for nutrients, traffic associated organic contaminants, and bacteria (Gavric et al., 2019 Gavric.S, Leonhardt, G., Marsalek, J., Viklander, M. 2019. Processes improving urban stormwater quality in grass swales and filter strips: A review of research findings. Science of the Total Environment. v 669. pp. 431-447. https://www.sciencedirect.com/science/article/pii/S0048969719310502?via%3Dihub).
NOTE:Water quality performance declines when vegetation cover on the filter strip falls below 80%.
Category:Performance