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Parking Lot Filter Strips

filter strip
Filter strips are gently sloping, vegetated areas adjacent to impervious surfaces. They are intended to reduce impacts of sheet flow and velocity of stormwater and help improve its water quality. Sometimes referred to as vegetated filter strips, grassed filter strips, grassed filters or buffer strips, they help remove sediments, other pollutants and increase infiltration.

Although there are few studies on effectiveness of urban filter strips in pollutant removal, properly designed and constructed, urban filter strips are thought capable of removing a minimum of 35% of solids and 40% of nutrients. Pollutant removal appears to depend on the width of the filter, a 150 feet wide strip being superior to a 75 feet wide strip.

Applications

Originally developed as an agricultural treatment practice, filter strips have now become a common urban stormwater management practice. In an urban setting, filter strips are often utilized to treat stormwater from small parking lots. Other uses include treatment of stormwater from roadways, roof downspouts, pretreatment associated with another stormwater treatment such as a grassed swale or as part of a buffer zone to protect streams.

The advantages of grassed filter strips include:

  • Partial removal of sediments and associated contaminants from stormwater runoff
  • Increased infiltration of stormwater runoff and some deposition of soluble contaminants
  • Relatively inexpensive and simple to design and build
  • Require a relatively low maintenance effort
  • Add a visually appealing green effect to parking lots

The potential for use of grassed filter strips is dependant on recognition of site requirements for this form of a stormwater Best Management Practice:

  1. Grassed parking lot filter strips can be utilized in most regions. However:
    • Impermeable clay soils limit treatment effectiveness
    • To increase permeability:
      • Soil porosity can be increased by amending soil
      • Optional under-drains can be incorporated to improve infiltration and help dry out filter strips after storms
    • Cold climates result in frozen ground that minimizes infiltration and treatment effectiveness. Also the use of sand and salt in parking areas for deicing can negatively impact the functioning of the strip. These conditions must be addressed before proceeding with instillation.
  2. When properly designed they require large areas of land:
    • Rule of thumb –one acre of impervious surface requires a filter strip of 580 feet wide by 75 feet long to be effective
    Therefore filter strips are not good options for ultra-urban areas; likewise they are not good solutions to stormwater retrofits (usually lacking space) or stormwater hot-spots (such as a gas station where gas pollutants would be encouraged to infiltrate with possible contamination of ground water or leaching into nearby streams).
  3. Filter strips can be used to treat small parking lots or drainage areas. To achieve optimal effectiveness of grass filter strips, the length of the stormwater flow path to the strip must not exceed critical distances where characteristics of sheet water flow change (see below in Materials and Instillation section for more information).
  4. Site topography is important. To avoid standing water and encourage treatment, grassed filter strips must have a slight slope of between two and six percent. Steeper slopes will encourage water flows with too great of a velocity.

 

Examples

Proctor City Hall
Proctor, MN

Find out more about this project here and from the City of Proctor.
 


Materials and Installation

Design Considerations:

  • The design of grassed filter strips should be incorporated into overall landscape design and be considered as a component of the sites stormwater management program.
  • Because of the uncertainty of effectiveness of filter strips, native or natural vegetation that probably already facilitates stormwater treatment should not be replaced. Enhancement of native or natural  vegetation  to improve their functioning in stormwater management  is preferred.


       From: http://www.metrocouncil.org/environment/Water/BMP/manual.htm
  • Design should be site specific, and take into consideration:
    • the need for a minimum treatment length of 25 feet. Greater lengths (in the direction of flow) provide better treatment. Steeper slopes require longer treatment areas. Studies have shown that there is limited pollutant removal from 25 feet long strips but moderate to high removal of pollutants from strips 150 feet long.
    • That the width of a filter strip should run the width of the impervious area to be treated. If adjoining a natural body of water the strip should run the width of property.
    • That effective treatment by strips requires sheet flow. Surface draining to a strip can be no more than 75 feet in length for impervious surfaces or 150 feet in length for pervious surfaces. Storm water flow over a surface greater than these distances changes from a uniform thickness of sheet flow to rivulets of unequal thickness and greater velocity. Therefore strips collecting stormwater from distances greater than indicated above will be ineffective.
    • That the recommended slope is between two and six percent.
    • That water table and bedrock horizons should be two to four feet below filter strip surface
    • That optimal stormwater treatment for one acre of 100% impervious area requires a 580 wide feet by 75 feet long (top to toe) grassed filter strip.
  • Maintaining sheet flow is critical to success:
    • The top and bottom (toe) of a filter strip should be as flat as   possible to help maintain sheet flow.
  • Collection areas above the filter strip may also require adjustment to maintain sheet flow.
  • Incorporation of a pea gravel level or flow spreader will help break up areas of concentrated flow upstream of filter strip and help maintain sheet flow as water enters filter strip.
  • Filter strips are not designed to accommodate large storms beyond the two to three year storm. To prevent washout damage expected with larger storms, a bypass system to other site stormwater management practices should be included:
  • Grassed filter strips are prone to damage by constant vehicular, pedestrian and animal traffic. Design should accomplish moderation of these negative impacts. 
  • Plant species selected must be chosen (best to consult with a horticulturist or landscape architect familiar with vegetation in your region) to:
    • withstand flowing water.
    • deal with extremes in water supply.
    • produce a dense mat-like growth pattern of fine stems that enhance filtration of pollutants.
    • be compatible with the sites climate, soil, shade and pest conditions.
    • in cold climates be tolerant of salt and sand loading.
Construction:
  • Accurate grading is the priority to ensure sheet flow.
  • Soil stabilization practices such as use of mulch or mats are required to prevent erosion during the development phase of a filter strip.
  • A full growing season should pass to allow establishment of strip before becoming part of the site's stormwater management program.
  • The interruption of filter strips by construction entrances and the like will require buffer techniques such as vehicle tracking pads or silt fences.

Costs (1991 dollars - dated information):

  • Will vary between $13,000 and $30,000 per acre of filter strip constructed depending on whether seed or sod is used. Additional costs are cost of design, which is minimal, plus costs of installing a pea gravel level spreader and an optional berm or required cold climate adjustments.
  • The greatest expense in installing a filter strips is the extensive requirement for land and its cost.
  • Maintenance costs average $350 per acre per year. The amount probably overlaps with an existing landscape maintenance program and is therefore a minimal cost.

Maintenance:

  • A strong commitment to maintenance is required to ensure continued  proper function. Maintenance programs should include:
    • Regular mowing of grass to a height of three to four inches.
    • Annual inspections and necessary repairs to include:
      • Removal  of built up sediments in leveler or flow spreader.
      • Removal of any rills and gullies formed in the vegetation mat.
      • Replant any bare soil patches.
      • Replant with a different turf species if the original species has not established.
    • As needed remove any sediment build-up apparent on vegetated area.

Suggested References: Guidebooks, websites and pamphlets

[ = pdf file; it will be opened in a new window]

  1. Post-Construction Storm Water Management in New Development & Redevelopment - Grassed Filter Strip (2002) by USEPA.
    A good overall review of vegetative filter strip use. Strong reference section for pursuit by the more curious.
  2. Georgia Stormwater Management Manual –Volume 2/3.3.1 Filter Strips (2001).
    A good overview from a technical viewpoint of design. Presents examples of design schematics and required design calculations. [ 260 KB]
  3. New Jersey Stormwater Best Management Practices Manual - Chapter 9.10 - Standard for Vegetative Filters (2004). A very good technical overview of subject matter. Schematics of filter strips and a series of figures (graphs) used to determine Filter Strip length based on the variables of slope, soil type and vegetation type/required total suspended solids removal. [ 250 KB]
  4. The Urban Small Sites Best Management Practices (BMP) Manual (2003) by the Metropolitan Council of Minnesota's Twin Cities offers detailed information on 40 BMPs for stormwater pollution management with comments on their application in a cold-climate setting. [ 360 KB]
  5. Stormwater Practices for Cold Climates by Deb Caraco and Richard Claytor of the Center for Watershed Protection for the USEPA-Region 5 in 1997; focuses on adjustments needed to make traditional stormwater management practices work in cold climates; find information on Filter Strips in the chapter on Infiltration. [ 69 KB]
  6. Understanding environmentally friendly development and design of parking lots associated with such development is a prerequisite to choosing appropriate stormwater management practices, find this background information at
    awarecolorado.org [ 1.1 MB] and nemo.uconn.edu [ 41 KB].
  7. State of Minnesota Stormwater Manual (2005) is a valuable tool for stormwater managers. The manual provides details on stormwater filtration practices [ 890 KB] applicable to Minnesota that conserves, enhances, and restores high-quality water in our lakes, rivers, streams, wetlands, and ground water.
  8. American Rivers 2004. Catching the Rain: A Great Lakes Resource Guide for Natural Stormwater Management. [ 1.5 MB]


Tips and Wisdom

  1. While filter strips appear to be a simple stormwater management solution, their success depends heavily on design and precise grading.  Engineering design and good oversight of grading is best left to those with stormwater management practices expertise and experience.
  2. Commitment to maintenance and repair is vital. At individual sites, several different selected grasses or combinations of grasses may have to tried before the strip functions as expected.
  3. Filter strips can have shrubs and trees dispersed throughout, this practice helps create a visual barrier to a parking lot as well as adding visual appeal and shade.
  4. Pedestrian traffic across a filter strip is undesirable, landscape design should take this problem into account and channel such traffic onto sidewalks away from filter strips.
  5. Filter strips are most effective when coupled with other stormwater BMPs, this is especially true in cold climates or in areas with soils of poor porosity.
  6. In cold climates adjustments that improve the effectiveness of filter strips include:
    • Commitment to a well executed maintenance program with particular attention to spring time inspections and required repair.
    • Reduce the amounts of chloride deicer (detrimental too much of selected vegetation) or sand (clogs the filter strip). Chlorides may also contaminate groundwater and wells.
    • Increase rate of percolation through soil to help compensate for clogging when sand is used and also helps balance the loss of infiltration during winter months.
    • Provide a sufficient set back from pavement to prevent frost heave caused by infiltrated water migrating under pavement surface. Set back restrictions can be overcome by engineering solutions to protect pavement e.g. insulated pavement.
    • Coupling filter strip with other BMPs and utilizing these other practices to deal with the bypassed snow melt. Essentially the filter strip would become seasonally operational.

Limitations

  1. Filter strips are not appropriate in conditions where:
    • sheet flow is difficult to maintain e.g. hilly and large paved areas.
    • land is scarce and expensive.
    • retrofits can not provide sufficient space.
    • high levels of contamination are likely to occur e.g. gas stations.
    • commitment to long term maintenance, repair and regrading can not be established.
    • soils are mostly composed of clay or clay/silt or where the soils do not support dense grass cover. Grass species chosen must withstand high velocity flows and irregular climate conditions.
    • there is permafrost.
  2. Filter strips are functional only if sheet flow can be maintained and inaccurate grading is a primary cause of loss of sheet flow.
  3. Effectiveness of filter strips as a remover of pollutants is debatable and do not provide significant storage through infiltration:
    •  A desired target removal of 80% total suspended solids (TSS) is not achievable by use of filter strips alone.
    • Filter strips do not significantly reduce peak discharge or volume of runoff.
  4. Use of filter strips in cold weather localities requires adjustments to design (for more information see Tips and Wisdom section above) or go directly to Stormwater Practices for Cold Climates-Infiltration section [ 69 KB] or the Filtering BMPs section [ 82 KB].

Necessary adjustments and additions add additional expense. In cold weather climates, filter strips are best coupled with other stormwater management practices.

 

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