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, ON	20 %	62 %
California, USA	40 %	70 %
North Carolina, USA	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 . 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.

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).

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 ).

NOTE:Water quality performance declines when vegetation cover on the filter strip falls below 80 %.

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.

The slow movement of water into or through a soil or drainage system.

Penetration of water through the ground surface.

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.

A hard surface area (e.g., road, parking area or rooftop) that prevents or retards the infiltration of water into the soil.

The total surface area upstream of a point on a stream that drains toward that point. Not to be confused with watershed. The drainage area may include one or more watersheds.

A shallow constructed channel, often grass-lined, which is used as an alternative to curb and channel, or as a pretreatment to other measures. Swales are generally characterized by a broad top width to depth ratio and gentle grades.

Vegetated, open channels designed to convey, treat and attenuate runoff. Design variations range from simple grass channels, which are designed primarily for conveyance to more complex treatment and volume reduction designs like enhanced grass swales, and dry swales or bioswales.

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.

Human application of water to agricultural or recreational land for watering purposes. City of Toronto Wet Weather Flow Management November 2006 47

Refers to the conditions in which an organism lives and survives or the conditions in which an organism resides. These conditions can be described as aspects of a “physical”, “social” or an “economic” environment, depending on the perspective perceived by the observer.