Vegetated filter strips

==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 Barrett, M., Lantin, A., Austrheim-Smith, S. 2004. Stormwater pollutant removal in roadside vegetated buffer strips. Transportation Research Record. No. 1890, pp. 129-140..
===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.Claytor, R. and T. Schueler. 1996. Design of Stormwater Filtering Systems. Center for Watershed Protection. Ellicott City, MD.. 
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 stripshttp://www.dnrec.delaware.gov/swc/Drainage/Documents/Sediment%20and%20Stormwater%20Program/Functional%20Equivalents/3.06.2.9.%20Sheet%20Flow_FEQ%20JUL%202016.pdf
The first 3 m of filter must be ≤ 2% in all cases.	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===
*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. 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..
===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 %.
==Inspection and Maintenance==
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:
==Landscaping==
The context of filter strips is often natural, 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 Philadelphia Water Department (PWD). 2007. Philadelphia Stormwater Management Guidance Manual. Philadelphia, PA..
 
*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.
==Gallery==
==Modeling==
==Performance==
==Life Cycle Costs==
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:
==References==
Category: Pretreatment