Vegetated filter strips

Grass filter strip draining to vegetated swale. Source: Trinkhaus Engineering

Overview

Gently sloping, densely vegetated areas that are designed to treat runoff as sheet flow from adjacent impervious surfaces. Filter strips function by slowing runoff velocities and filtering out sediment and other pollutants, and by providing some infiltration into underlying soils. Filter strips may be comprised of a variety of trees, shrubs, and vegetation to add aesthetic value as well as water quality benefits. They are best suited to treating runoff from roads and highways, roof downspouts and low traffic parking lots. They are also ideal as pretreatment to another lot level or conveyance practice. Filter strips also provide a convenient area for snow storage and treatment.

With proper design and maintenance, filter strips can provide relatively high pollutant removal. Maintaining sheet flow into the filter strip through the use of level spreaders is essential. Using vegetated filter strips as pretreatment practices to other LID BMPs is highly recommended. They also provide a convenient area for snow storage and treatment, and are particularly valuable due to their capacity for snowmelt infiltration. If used for snow storage, the area should be planted with salt-tolerant, non-woody plant species. Because of the simplicity of filter strip designs, physical changes to the practice are not needed for winter operation.

Planning

Filter strips are best suited for pretreatment of runoff from roads and parking lots prior to it being treated by other LID BMPs. They are also an ideal practice within stream or wetland buffer zones. Filter strips can be used as part of a treatment train approach. Filter strips may also be applied at roof leaders, outfalls, or large parking lots if level spreaders are used to create sheet flow. They are often impractical in densely developed urban areas because they consume a large amount of space. Properly functioning filter strips should not pond water on the surface and do not contribute to stream warming. Thus, filter strips are a good stormwater treatment option for cold water streams that support species sensitive to changes in stream temperature.

Available space

The flow path length across the vegetated filter strip should be at least 5 m to provide substantial water quality benefits .

Topography

Filter strips are best used to treat runoff from ground-level impervious surfaces that generate sheet flow (e.g., roads and parking areas). The recommended filter strip slope is between 1 - 3 %. Though steeper slopes increase the likelihood of erosion, incorporation of multiple level spreaders in series or terraces can counteract this (see below).

Flow path length across impermeable surface

A limiting design factor is that the maximum flow path length across the impermeable surface must be < 25 m, as flow tends to concentrate ≥ 25 m over an impermeable surface..
Once runoff from an impervious surface becomes concentrated, a swale design should be used instead of a vegetated filter strip .

Design

Minimum length of filter strips

The first 3 m of filter must be ≤ 2% in all cases.
Slope of filter strip	Minimum length (m)
< 3 %	5
3 - 8 %	10

While filter strips are a simple technology, proper design requires attention to detail because small problems, such as concentration of inflowing runoff or improper grading, can decrease effectiveness and create nuisance soil erosion or ponding of water conditions.

• The maximum contributing flow path length across adjacent impervious surfaces must < 25 m.
• The impervious surfaces draining to a filter strip must have slopes < 3 %.
• The flow path length across the vegetated filter strip should exceed the maximum flow path length across the impervious surface draining to it.
• The filter strip should have a flow path length of ≥ 5 m; however, some pollutant removal benefits are realized ≥ 3 m.
• Should be graded to provide a 75 to 100 mm elevation drop between pavement edge or flush curb and filter strip surface to dissipate energy, promote settling and allow for accumulation of sediment and debris between cleanings.

Pretreatment

• A level spreader or gravel diaphragm at the top of the slope is always recommended.
• When filter strip slopes are greater than 5%, a series of level spreaders, check dams or gravel diaphragms should be used to help maintain sheet flow.
• Filter strips should drain continuously as sheet flow until reaching a swale, bioretention facility, or other LID practice.

Berms

• When designed as a stand alone water quality BMP (i.e., not pretreatment to another BMP) the vegetated filter strip should be designed with a pervious berm at the toe of the slope for shallow ponding of runoff.
• Media for the berm should consist of 40 % excavated topsoil, 40 % sand, and 20 % fine gravel.
• The berm should be 150 to 300 mm in height above the bottom of the depression and should contain a perforated pipe underdrain connected to the storm sewer,
• Runoff ponds behind the berm and gradually flows through it, into the underdrain connected to the storm sewer system. The volume ponded behind the berm should be equal to the water quality storage requirement. During larger storms, runoff will overtop the berm and flow directly into a storm sewer inlet. .

Soil Amendments

If native soils on the filter strip site are highly compacted, or of such low fertility that vegetation cannot become established, they should be tilled to a depth of 300 mm and amended with compost to achieve an organic matter content of 5 - 15 %.

Landscaping

The context of filter strips is often natural, informal and somewhat informal. Filter strip vegetation can consist of turf grasses, meadow grasses, wildflowers, shrubs, and trees. Trees and shrubs with deep rooting capabilities are recommended for planting to maximize soil infiltration capacity .

• Filter strips used for snow storage and treatment should be planted with non-woody vegetation. Designers should choose vegetation that stabilizes the soil and is salt tolerant where the filter strip will be used for snow storage or to treat road runoff.
• Vegetation at the toe of the slope (where ponding may occur) should be able to withstand both wet and dry soil conditions.
• Whatever the type of vegetation used, it must be densely planted to slow runoff, collect sediment, and allow for infiltration.

Although filter strips are often grassed, alternatives include forested filter strips or multi-zone filter strips, which feature several vegetation zones providing a gradual transition from turf to meadow to shrub and forest. The multi-zone filter strip design can be effective as a buffer zone to an existing natural heritage feature.

Resilient turf grasses are particularly useful in the design of vegetated filter strips, dry ponds and enhanced grass swales. The Ministry of Transportation have standardized a number of grass mixes. The 'Salt Tolerant Mix' is of particular value for low impact development applications alongside asphalt roadways and paved walkways.

Canada #1 Ground Cover (salt tolerant mix)

Common name	Scientific name	Proportion
Tall Fescue	Festuca arundinacea	25 %
Fults Alkali Grass	Puccinellia distans	20 %
Creeping Red Fescue	Festuca rubra	25 %
Perennial ryegrass	Lolium perrenne	20 %
Hard Fescue	Festuca trachyphylla	10 %

Gallery

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  Unmown wildflower meadow roadside filter strip/verge in Delft

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  Filter strips are a common practice in rural road cross sections

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  Filter strips between the bioretention facility and the adjacent parking lot reduce sediment loading to the surface of the bioretention cell. Dennis Ave Health Center, Wheaton, MD. Photo credit: Moreau1

Modeling

Vegetated filter strips are found within the LID toolbox

Surface
Berm height (mm)	This is the height of the curb which constrains the overland sheet flow of water. Where the bottom of the slope discharges directly into another LID facility without impedance, the value is 0.
Surface roughness (Manning’s n)	Lower numbers indicate less surface obstruction and result in faster flow. See Turf for ideas of a good number according to mowing practices.
Surface slope (%)	If the slope > 3%, consider using level spreaders to reduce erosion of the surface under high flow velocities.
Soil (native underlying soil, or amended topsoil)
Thickness (mm)	If topsoil has been amended Absorbent landscapes
Porosity (fraction)	Suggest around 0.4, unless otherwise tested.
Field capacity (fraction)	Will vary according to native or amended topsoil
Wilting point (fraction)
Conductivity (mm/hr)
Conductivity slope
Suction head (mm)
Design drawdown time (hrs)	Will be determined by regulatory authority, often 48 - 72 hours to reduce nuisance from mosquitoes

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 %
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%.

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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 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.

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.

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

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.

The treatment of urban runoff as close to the source area as possible through application of small scale stormwater management practices on individual properties that are linked to downstream conveyance and end-of-pipe practices.

A structural practice that is located within the drainage system where flows are concentrated and are being conveyed along corridor. Include but are not limited to pervious pipes, swales, and other similar systems.

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.

Low Impact Development. A stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

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

Penetration of water through the ground surface.

A vegetated area such as a bog, fen, marsh, or swamp, where the soil or root zone is saturated for part of the year.

Stormwater management following the hierarchical approach: Source Control measures, Conveyance Control measure and End of Pipe treatment to achieve the water quality and water balance target for lot level development of the preferred strategy.

A combination of lot level, conveyance, and end-of-pipe stormwater management practices.

A body of water smaller than a lake, often artificially formed.

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.

(1) The wearing away of the land surface by moving water, wind, ice or other geological agents, including such processes as gravitation creep; (2) Detachment and movement of soil or rock fragments by water, wind, ice or gravity (i.e. Accelerated, geological, gully, natural, rill, sheet, splash, or impact, etc).

The minimum linear distance of water flow across a surface.

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.

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 compacted earthen wall that diverts runoff or creates shallow ponding of runoff. In some cases, runoff ponds behind the berm and gradually flows through it or is infiltrated.

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.

A perforated pipe used to assist the draining of soils.

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 stormwater management strategy that seeks to mitigate the impacts of increased urban runoff and stormwater pollution by managing it as close to its source as possible. It comprises a set of site design approaches and small scale stormwater management practices that promote the use of natural systems for infiltration and evapotranspiration, and rainwater harvesting.

A mixture of mineral aggregates bound with bituminous materials, used in the construction and maintenance of paved surfaces.

The total mass of a pollutant entering a waterbody over a defined time period.

The net amount of something (e.g. chemical, such as phosphorus), calculated as the product of concentration and volume in a given time. Some BMPs significantly reduce loading of pollutants to the environment by reducing volume more so than concentration.

A compacted earthen wall that diverts runoff or creates shallow ponding of runoff. In some cases, runoff ponds behind the berm and gradually flows through it or is infiltrated.

The porosity (n) of a mixture is the ratio of the volume of void-space to the total or bulk volume of the mixture. It is closely related to the concept of void ratio (e) where void ratio is the ratio of the volume of void-space to the volume of solids. n = Volume of voids/Total volume of mixture = e/(1+e)

The period between the maximum water level and the minimum level (dry weather or antecedent level).

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.

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.

Total suspended solids

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.

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.