Vegetated filter strips

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

Overview[edit]

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[edit]

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[edit]

The flow path length across the vegetated filter strip should be at least 5 m to provide substantial water quality benefits [1].

Topography[edit]

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[edit]

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.[2].
Once runoff from an impervious surface becomes concentrated, a swale design should be used instead of a vegetated filter strip [1].

Design[edit]

Minimum length of filter strips[3]
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 150 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[edit]

Berms[edit]

  • 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. [4].

Soil Amendments[edit]

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 %.

Inspection and Maintenance[edit]

Maintenance requirements for vegetated filter strips are similar to enhanced grass swales and typically involve a low level of activity after vegetation becomes established. Routine inspection is important to ensure that dense vegetation cover is maintained and inflowing runoff does not become concentrated and create ruts or low points, making the practice redundant (especially as a pretreatment device used for other BMPs). Vehicles should not be parked or driven on filter strips at any time. For routine mowing of grassed filter strips, the lightest possible mowing equipment should be used to prevent soil compaction over the life of the practice, opt for push mowers if possible.


Take a look at the Inspection and Maintenance: Vegetated Filter Strips page by clicking below for further details about proper inspection and maintenance practices:

1 Veg Filter Strips banner.png


Landscaping[edit]

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 [5].

  • 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[6]. 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[edit]

Modeling[edit]

TTT.png

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[7]
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[edit]

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[edit]

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[8] 20 % 62 %
California, USA[9] 40 % 70 %
North Carolina, USA[10] 57 % 72 %

Water Quality[edit]

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 [11]. 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 [9] and the BMP Retrofit Pilot Study [12]. 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).[13]

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 [14]).

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

Life Cycle Costs[edit]

To learn about Life Cycle Costs associated with this practice (i.e. Pre-construction, Excavation, Materials & Installation, Project Management, Overhead, Inspection and Maintenance, Rehabilitation and other associated costs), visit the Vegetated Filter Strip: Life Cycle Costs page to view a cost estimate for a 500 m2 filter strip. Alternatively you can use the STEP's Low Impact Development Life Cycle Costing Tool (LID LCCT) to generate cost estimates customized to your own LID stormwater design project specifications. Take a look at the Vegetated Filter Strip: Life Cycle Costs page by clicking below for further details:

Construction Breakdown VegStrip Full Infil.PNG

References[edit]

  1. 1.0 1.1 Barrett, M., Lantin, A., Austrheim-Smith, S. 2004. Stormwater pollutant removal in roadside vegetated buffer strips. Transportation Research Record. No. 1890, pp. 129-140.
  2. Claytor, R. and T. Schueler. 1996. Design of Stormwater Filtering Systems. Center for Watershed Protection. Ellicott City, MD.
  3. http://www.dnrec.delaware.gov/swc/Drainage/Documents/Sediment%20and%20Stormwater%20Program/Functional%20Equivalents/3.06.2.9.%20Sheet%20Flow_FEQ%20JUL%202016.pdf
  4. Cappiella, K., T. Schueler, and T. Wright. 2006. Urban Watershed Forestry Manual, Part 2. Conserving and Planting Trees at Development Sites. Center for Watershed Protection. Prepared for United States Department of Agriculture, Forest Service.
  5. Philadelphia Water Department (PWD). 2007. Philadelphia Stormwater Management Guidance Manual. Philadelphia, PA.
  6. Ontario Provincial Standard Specification. (2014). Construction Specification and for Seed and Cover OPSS.PROV 804. Retrieved from http://www.raqsb.mto.gov.on.ca/techpubs/ops.nsf/0/3a785d2f480f9349852580820062910a/$FILE/OPSS.PROV 804 Nov2014.pdf
  7. Oregon State Univ., Corvallis. Dept. of Civil, Construction and Environmental Engineering.; Environmental Protection Agency, Cincinnati ONRMRL. Storm Water Management Model Reference Manual Volume I Hydrology (Revised). 2016:233. https://nepis.epa.gov/Exe/ZyPURL.cgi?Dockey=P100NYRA.txt Accessed August 23, 2017.
  8. 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.
  9. 9.0 9.1 Barrett, M. 2003. Roadside Vegetated Treatment Sites (RVTS) Study Final Report, Report # CTSW-RT-03-028. California Department of Transportation. Sacramento, CA.
  10. Carmen, 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
  11. Barrett, M., Lantin, A., Austrheim-Smith, S. 2004. Stormwater pollutant removal in roadside vegetated buffer strips. Transportation Research Record. No. 1890, pp. 129-140.
  12. California Department of Transportation (Caltrans). 2004. BMP Retrofit Pilot Program, Final Report, CTSW-RT-01-050. Sacramento, CA.
  13. 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
  14. 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