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| Michigan Department of Environmental
Quality, 2001. |
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Description
Long narrow grass-lined channels, ditches or median strips designed to filter
and treat storm water from impervious surfaces. Grassed swales and filter strips
increase filtration and provide a long term treatment for pollution at low
costs.
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| US Department of Environmental Quality,
2002. |
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Mechanisms
A reduction in water runoff is achieved through an increase in infiltration
and pollution removal due to greater pervious surface area and decreased runoff
rates. Plant life facilitates the removal of some pollutants through biological
processes.
 |
| New Zealand Water Environment Research
Foundation, 2004. |
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|
Applications
- Useful in reducing runoff contamination from parking lots, paved highways,
roofs, and other impervious surfaces.
- Limited to treatment of a few acres.
- Not recommended for steep slopes.
- Regular maintenance required to maintain health and density of vegetation.
- Not for use with impermeable soils.
- Not for use in highly polluted runoff areas due to the possibility of
groundwater contamination.
- Require large amounts of space relative to impervious area.
- Proper functioning requires great attention to design detail.
- Require maintaining sheet flow.
- Reduce bank erosion and offer animal habitats.
- Useful as sediment filters during construction.
- Relatively low maintenance and cost.
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| Natural Resources Conservation Service,
2003. |
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Detailed Specifications
Design Values*:
- Maximum peak flows of 5 cubic feet per second.
- Hydraulic residence time > 5 min.
- Maximum side slope of 3:1.
- Minimum longitudinal slope of 2%.
- Maximum flow velocity of 0.9 ft/sec.
- Average flow depth ≤ 1 inch.
- Soil permeability of 0.5 in/hr or greater.
- Minimum distance to groundwater of 2-4 feet.
- Minimum of 150 feet to a body of water.
- Install a pea gravel diaphragm at top of slope.
- Install a berm of sand and gravel at bottom of slope.
- Top and bottom of slope should be as flat as possible.
- For more information visit:
US EPA
Grassed Filter Strip Post-Construction Storm Water Management in New Development
& Redevelopment.
Sizing Equations:
- Determine peak flow for a 6-month, 24-hour storm event.
- Manning's equation should be used to determine the length to width ratio.
- Minimum length of 25 feet.
- Minimum width of 15 feet (the greater the width the greater the pollutant
removal).
- Calculate flow velocities from channel dimensions.
- Calculate swale length from the flow velocity and the hydraulic residence
time.
- For more information visit
MDEQ Stormwater Management Guidebook, page 41.
Vegetation:
- Select vegetation that can survive in both wet and dry conditions.
- Turf grass - choose a species best suited to the environment.
- Cattails.
- Any other dense, water tolerant vegetation.
*Note: Local governments may use different design values
than those listed above, be sure to comply with any applicable laws and
regulations.
Plans
City of Portland 2004 Stormwater Management Manual.
NVPDC, 1996 (Cited in United States Environmental Protection Agency, 2002).
Metropolitan Council of Minnesota, 2003.
Claytor & Schueler, 1996 (Cited in Metropolitan Council of Minnesota, 2003).
Claytor & Schueler, 1996 (Cited in Metropolitan Council of Minnesota, 2003).
Center for Watershed Protection, 2000 (Cited in Metropolitan Council of
Minnesota, 2003).
Water Quality
Maintenance
As Needed:
- Maintain grass height at 4-9 inches.
- Remove any debris, garbage, invasive species and sediment buildup.
- Clear the inlet(s) and outlet(s) to allow for proper flow dispersal.
- Inspect and repair damaged inlet erosion control devices and dams.
- Water grass as needed until established and during dry seasons.
- Remove and eliminate sources of sediment build-up.
- Control pest inhabitation.
- Replace grass species with a more climate specific species if necessary.
- Re-grade when necessary.
Seasonally:
- Inspect swale for vegetation loss and reseed bare spots.
- Check for signs of erosion and channelization of flow, and repair when
necessary.
- Reseed bare areas.
Resources
Center for Watershed Protection. 2000. Grassed Filter Strip Fact Sheet.
City of Portland 2004 Storm Water Management Manual. 2004. Grassy Swales.
Claytor, R.A. and Schueler, T.R. 1996. Design of Stormwater Filtering
Systems. Center for Watershed Protection and Chesapeake Research
Consortium.
Historic City of Franklin. January 2002. PTP-05 ACTIVITY: Biofilters: Swales
and Strips.
Journal of Environmental Engineering. February 1998. Characterization of
Highway Runoff in Austin, Texas, Area. ASCE 124 (2): 131-137.
Menerey, Bruce E. Michigan Department of Environmental Quality. August 1999.
Storm Water Management Guidebook Master Document. pp. 41-44.
http://www.deq.state.mi.us/documents/deq-lwm-nfip-SMGCh05.pdf (February
2006).
Michigan Department of Environmental Quality. 2001. Infiltration BMP's.
http://www.michigan.gov/deq/0,1607,7-135-3313_3682_3714-13673--,00.html -
grassed_swale (December 2005).
Metropolitan Council of Minnesota. 2003. Filter Strips. Urban Small Sites
Best Management Practice Manual. pp. 3-213 - 3-219.
http://www.metrocouncil.org/environment/Watershed/BMP/CH3_STFiltFilterStrips.pdf
(February 2006).
Natural Resources Conservation Service: Ohio Lake Erie Buffer Program.
January 2003. Grass Filter Strips – CP 23.
http://www.oh.nrcs.usda.gov/programs/Lake_Erie_Buffer/filter_strips.html
(February 2006).
New Zealand Water Environment Research Foundation. October 2004. On-Site
Stormwater Management Guideline.
http://www.nzwerf.org.nz/publications/sw602/sw602publications/section2.pdf
(February 2006).
The Michigan Department of Transportation. April 1998. Highway Storm Water
Runoff Study.
United States Environmental Protection Agency. August 15, 2002. Grassed
Filter Strip. Post-Construction Storm Water Management in New Development &
Redevelopment.
http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm (January 2006).
United States Environmental Protection Agency. August 15, 2002. Grassed
Swale. Post-Construction Storm Water Management in New Development &
Redevelopment.
http://cfpub.epa.gov/npdes/stormwater/menuofbmps/index.cfm (January 2006).
United States Environmental Protection Agency. September 1999. Vegetated
Swales. Storm Water Technology Fact Sheet. EPA 832-F-99-006.
http://www.epa.gov/owmitnet/mtb/vegswale.pdf (December 2005).
United States Environmental Protection Agency. September 2004. Vegetated
Biofilters. Storm Water Best Management Practice Design Volume 2.
