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
	2 - 5 m	8 - 15 m
Guelph, ON	20 %	62 %
California, USA	40 %	70 %

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 . 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 %
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.

NOTE When vegetation cover on the filter strip is below 80% water quality performance declines.

The rate at which stormwater percolates into the subsoil measured in inches per hour.

Stormwater Management

The accounting of inflow and outflow of water in a system according to the components of the hydrologic cycle.

Includes the protection of soil from dislocation by water, wind or other agents.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

The portion of water precipitated onto a catchment area, which then flows as surface discharge from the catchment area past a specified point.

Water from rain, snow melt, or irrigation that flows over the land surface.

State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include Source, Conveyance and End-Of-Pipe Controls.

Initial capturing and removal of unwanted contaminants, such as debris, sediment, leaves and pollutants, from stormwater before reaching a best management practice; Examples include, settling forebays, vegetated filter strips and gravel diaphragms.

Total suspended solids

Best management practice. State of the art methods or techniques used to manage the quantity and improve the quality of wet weather flow. BMPs include: source, conveyance and end-of-pipe controls.

a gently sloping, densely vegetated areas that treat runoff as sheet flow from adjacent impervious areas. They function by slowing runoff velocity and filtering out suspended sediment and associated pollutants, and by providing some infiltration into underlying soils.

Soil, sand and minerals washed from land into water, usually after rain. They pile up in reservoirs, rivers and harbors, destroying fish-nesting areas and holes of water animals and cloud the water so that needed sunlight might not reach aquatic plans. Careless farming, mining and building activities will expose sediment materials, allowing them to be washed off the land after rainfalls.