Vegetated filter strips
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.
Vegetated filter strips (buffer strips and grassed filter strips) are 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.
Originally used as an agricultural treatment practice, filter strips have evolved into an urban SWM practice. Vegetation may be comprised of a variety of trees, shrubs and native plants to add aesthetic value as well as water quality benefits.
With proper design and maintenance, filter strips can provide relatively high pollutant removal. Maintaining sheet flow into the filter strip through the use of a level spreading device (e.g.gravel diaphragm) is essential.
Using vegetated filter strips as pretreatment practices to other best management practices 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 best management practices. 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 - 5 %. Though steeper slopes increase the likelihood of erosion, incorporation of multiple level spreaders in series or terraces can counteract this.
Soils[edit]
Filter strips are a suitable practice on all soil types. If soils 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 8 to 15% by weight or 30 to 40% by volume.
Flow Path Length Across Impermeable Surface[edit]
A limiting design factor is that the maximum flow path length across the impermeable surface should be less than 25 metres. This is because runoff flowing as sheet flow over an impermeable surface tends to concentrate after 25 metres [2]. Once runoff from an impervious surface becomes concentrated, a swale design should be used instead of a vegetated filter strip [1].
Design[edit]
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.
Pretreatment[edit]
- A gravel diaphragm at the top of the slope is always recommended.
- When filter strip slopes are greater than 5%, a series of level spreaders (e.g. 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[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 of sand and gravel at the toe of the slope for shallow ponding of runoff. 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. [3].
Soil Amendments[edit]
If 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 content of 8 to 15% by weight or 30 to 40% by volume.
Landscaping[edit]
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 [4].
- 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.
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.
Gallery[edit]
Unmown wildflower meadow roadside filter strip/verge in Delft
Filter strips are a common practice in rural road cross sections
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[edit]
| 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[5] |
| 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.
| 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.
| Location | Runoff reduction | |
|---|---|---|
| 2 - 5 m | 8 - 15 m | |
| Guelph, ON[6] | 20 % | 62 % |
| California, USA[7] | 40 % | 70 % |
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 [1]. 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).
| 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 [7] and the BMP Retrofit Pilot Study [8]. These studies concluded that concentration reductions consistently occur for TSS and total heavy metals and frequently for dissolved metals. Nutrients concentrations remained generally unchanged. When vegetation cover on the filter strip is below 80% water quality performance declines.
- ↑ 1.0 1.1 1.2 Barrett, M., Lantin, A., Austrheim-Smith, S. 2004. Stormwater pollutant removal in roadside vegetated buffer strips. Transportation Research Record. No. 1890, pp. 129-140.
- ↑ Claytor, R. and T. Schueler. 1996. Design of Stormwater Filtering Systems. Center for Watershed Protection. Ellicott City, MD.
- ↑ 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.
- ↑ Philadelphia Water Department (PWD). 2007. Philadelphia Stormwater Management Guidance Manual. Philadelphia, PA.
- ↑ 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.
- ↑ 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.
- ↑ 7.0 7.1 Barrett, M. 2003. Roadside Vegetated Treatment Sites (RVTS) Study Final Report, Report # CTSW-RT-03-028. California Department of Transportation. Sacramento, CA.
- ↑ California Department of Transportation (Caltrans). 2004. BMP Retrofit Pilot Program, Final Report, CTSW-RT-01-050. Sacramento, CA.