v>EPA
United States
Environmental Protection
Agency
Off ice of Water
Washington, D.C.
832-F-99-006
September 1999
Storm Water
Technology Fact Sheet
Vegetated Swales
DESCRIPTION
A vegetated swale is a broad, shallow channel with
a dense stand of vegetation covering the side slopes
and bottom. Swales can be natural or manmade,
and are designed to trap particulate pollutants
(suspended solids and trace metals), promote
infiltration, and reduce the flow velocity of storm
water runoff. A typical design is shown in Figurel.
Vegetated swales can serve as part of a storm water
drainage system and can replace curbs, gutters and
storm sewer systems. Therefore, swales are best
suited for residential, industrial, and commercial
areas with low flow and smaller populations.
APPLICABILITY
Vegetated swales can be used wherever the local
climate and soils permit the establishment and
maintenance of a dense vegetative cover. The
feasibility of installing a vegetated swale at a
Provide for scour
protection.
(a) Cross section of swale with check dam.
Notation:
L = Length of swale impoundment area per check dam (ft) (b) Dimensional view of swale impoundment area.
Ds = Depth of check dam (ft)
Ss = Bottom slpe of swale (ft/ft)
W = Top width of check dam (ft)
WB = Bottom width of check dam (ft)
Z1S2 = Ratio of horizontal to vertical change in swale side slope (ft/ft)
Source: NVPDC, 1996.
FIGURE 1 EXAMPLE OF A VEGETATED SWALE
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particular site depends on the area, slope, and
perviousness of the contributing watershed, as well
as the dimensions, slope, and vegetative covering
employed in the swale system.
Vegetated swales are easy to design and can be
incorporated into a site drainage plan. While
swales are generally used as a stand-alone storm
water Best Management Practice (BMP), they are
most effective when used in conjunction with other
BMPs, such as wet ponds, infiltration strips,
wetlands, etc.
While vegetated swales have been widely used as
storm water BMPs, there are also certain aspects of
vegetated swales that have yet to be quantified.
Some of the issues being investigated are whether
their pollutant removal rates decline with age, what
effect the slope has on the filtration capacity of
vegetation, the benefits of check dams, and the
degree to which design factors can enhance the
effectiveness of pollutant removal.
ADVANTAGES AND DISADVANTAGES
Swales typically have several advantages over
conventional storm water management practice,
such as storm sewer systems, including the
reduction of peak flows; the removal of pollutants,
the promotion of runoff infiltration, and lower
capital costs. However, vegetated swales are
typically ineffective in, and vulnerable to, large
storms, because high-velocity flows can erode the
vegetated cover.
Limitations of vegetated swales include the
following:
• They are impractical in areas with very flat
grades, steep topography, or wet or poorly
drained soils.
• They are not effective and may even erode
when flow volumes and/or velocities are
high.
• They can become drowning hazards,
mosquito breeding areas, and may emit
odors.
• Land may not be available for them.
• In some places, their use is restricted by
law: many local municipalities prohibit
vegetated swales if peak discharges exceed
140 liters per second (five cubic feet per
second) or if flow velocities are greater than
1 meter per second (three feet per second).
They are impractical in areas with erosive
soils or where a dense vegetative cover is
difficult to maintain.
Negative environmental impacts of vegetated
swales may include:
• Leaching from swale vegetation may
increase the presence of trace metals and
nutrients in the runoff.
• Infiltration through the swale may carry
pollutants into local groundwater.
Standing water in vegetated swales can
result in potential safety, odor, and
mosquito problems.
DESIGN CRITERIA
Design criteria for implementation of the vegetated
swales are as follows:
Location
Vegetated swales are typically located along
property boundaries along a natural grade, although
they can be used effectively wherever the site
provides adequate space. Swales can be used in
place of curbs and gutters along parking lots.
Soil Requirements
Vegetated swales should not be constructed in
gravelly and coarse sandy soils that cannot easily
support dense vegetation. If available, alkaline
soils and subsoils should be used to promote the
removal and retention of metals. Soil infiltration
rates should be greater than 0.2 millimeters per
second (one-half inch per hour); therefore, care
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must be taken to avoid compacting the soil during
construction.
Vegetation
A fine, close-growing, water-resistant grass should
be selected for use in vegetated swales, because
increasing the surface area of the vegetation
exposed to the runoff improves the effectiveness of
the swale system. Pollutant removal efficiencies
vary greatly depending on the specific plants
involved, so the vegetation should be selected with
pollution control objectives in mind. In addition,
care should be taken to choose plants that will be
able to thrive at the site. Examples of vegetation
appropriate for swales include reed canary grass,
grass-legume mixtures, and red fescue.
General Channel Configuration
A parabolic or trapezoidal cross-section with side
slopes no steeper than 1:3 is recommended to
maximize the wetted channel perimeter of the
swale. Recommendations for longitudinal channel
slopes vary within the existing literature. For
example, Schueler (1987) recommends a vegetated
swale slope as close to zero as drainage permits.
The Minnesota Pollution Control Agency (1991)
recommends that the channel slope be less than 2
percent. The Storm Water Management Manual for
the Puget Sound Basin (1992) specifies channel
slopes between 2 and 4 percent. This manual
indicates that slopes of less than 2 percent can be
used if drain tile is incorporated into the design,
while slopes greater than 4 percent can be used if
check dams are placed in the channel to reduce flow
velocity.
Flows
A typical design storm used for sizing swales is a
six-month frequency, 24-hour storm event. The
exact intensity of this storm must be determined for
your location and is generally available from the
U.S. Geological Survey. Swales are generally not
used where the maximum flow rate exceeds 140
liters/second (5 cubic feet per second).
Sizing Procedures
The width of the swale can be calculated using
various forms of the Manning equation. However,
this methodology can be simplified to the following
rule of thumb: the total surface area of the swale
should be one percent of the area (500 square feet
for each acre) that drains to the swale.
Unless a bypass is provided, the swale must be
sized both to treat the design flows and to pass the
peak hydraulic flows. However, for the swale to
treat runoff most effectively, the depth of the storm
water should not exceed the height of the grass.
Construction
The subsurface of the swale should be carefully
constructed to avoid compaction of the soil.
Compacted soil reduces infiltration and inhibits
growth of the grass. Damaged areas should be
restored immediately to ensure that the desired level
of treatment is maintained and to prevent further
damage from erosion of exposed soil.
Check Dams
Check dams can be installed in swales to promote
additional infiltration, to increase storage, and to
reduce flow velocities. Earthen check dams are not
recommended because of their potential to erode.
Check dams should be installed every 17 meters (50
feet) if the longitudinal slope exceeds 4 percent.
PERFORMANCE
The literature suggests that vegetated swales
represent a practical and potentially effective
technique for controlling urban runoff quality.
While limited quantitative performance data exists
for vegetated swales, it is known that check dams,
slight slopes, permeable soils, dense grass cover,
increased contact time, and small storm events all
contribute to successful pollutant removal by the
swale system. Factors decreasing the effectiveness
of swales include compacted soils, short runoff
contact time, large storm events, frozen ground,
short grass heights, steep slopes, and high runoff
velocities and discharge rates.
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Conventional vegetated swale designs have
achieved mixed results in removing particulate
pollutants. A study performed by the Nationwide
Urban Runoff Program (NURP) monitored three
grass swales in the Washington, D.C., area and
found no significant improvement in urban runoff
quality for the pollutants analyzed. However, the
weak performance of these swales was attributed to
the high flow velocities in the swales, soil
compaction, steep slopes, and short grass height.
Another project in Durham, NC, monitored the
performance of a carefully designed artificial swale
that received runoff from a commercial parking lot.
The project tracked 11 storms and concluded that
particulate concentrations of heavy metals (Cu, Pb,
Zn, and Cd) were reduced by approximately 50
percent. However, the swale proved largely
ineffective for removing soluble nutrients. A
conservative estimate would say that a properly
designed vegetated swale may achieve a 25 to 50
percent reduction in particulate pollutants,
including sediment and sediment-attached
phosphorus, metals, and bacteria. Lower removal
rates (less than 10 percent) can be expected for
dissolved pollutants, such as soluble phosphorus,
nitrate, and chloride. Table 1 summarizes some
pollutant removal efficiencies for vegetated swales.
The effectiveness of vegetated swales can be
enhanced by adding check dams at approximately
17 meter (50 foot) increments along their length
(See Figure 1). These dams maximize the retention
time within the swale, decrease flow velocities, and
promote particulate settling. Structures to skim off
floating debris may also be added to the swales.
Finally, the incorporation of vegetated filter strips
parallel to the top of the channel banks can help to
treat sheet flows entering the swale.
OPERATION AND MAINTENANCE
The useful life of a vegetated swale system is
directly proportional to its maintenance frequency.
If properly designed and regularly maintained,
vegetated swales can last indefinitely.
The maintenance objectives for vegetated swale
systems include keeping up the hydraulic and
removal efficiency of the channel and maintaining
a dense, healthy grass cover. Maintenance activities
TABLE 1 EFFECTIVENESS OF DESIGN
SWALES
Pollutant
Total Suspended
Solids
Oxygen Demanding
Substances
Nitrate
Total Phosphorus
Hydrocarbons
Cadmium
Copper
Lead
Zinc
Median % Removal
81
67
38
9
62
42
51
67
71
should include periodic mowing (with grass never
cut shorter than the design flow depth), weed
control, watering during drought conditions,
reseeding of bare areas, and clearing of debris and
blockages. Cuttings should be removed from the
channel and disposed in a local composting facility.
Accumulated sediment should also be removed
manually to avoid the transport of resuspended
sediments in periods of low flow and to prevent a
damming effect from sand bars. The application of
fertilizers and pesticides should be minimal.
Another aspect of a good maintenance plan is
repairing damaged areas within a channel. For
example, if the channel develops ruts or holes, it
should be repaired utilizing a suitable soil that is
properly tamped and seeded. The grass cover
should be thick; if it is not, reseed as necessary.
Any standing water removed during the
maintenance operation must be disposed to a
sanitary sewer at an approved discharge location.
Residuals (e.g., silt, grass cuttings) must be
disposed in accordance with local or State
requirements.
COSTS
Vegetated swales typically cost less to construct
than curbs and gutters or underground storm
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sewers. Schueler (1987) reported that costs may
vary from $16-$30 per linear meter ($4.90 to $9.00
per linear foot) for a 4.5 meter (15-foot) wide
channel (top width).
The Southeastern Wisconsin Regional Planning
Commission (SEWRPC, 1991) reported that costs
may vary from $28 to $164 per linear meter ($8.50
to $50.00 per linear foot) depending upon swale
depth and bottom width. These cost estimates are
higher than other published estimates because they
include the cost of activities (such as clearing,
grubbing, leveling, filling, and sodding) that may
not be included in other published estimates.
Construction costs depend on specific site
considerations and local costs for labor and
materials. Table 2 shows the estimated capital
costs of a vegetated swale.
Annual costs for maintaining vegetated swales are
approximately $1.90 per linear meter ($0.58 per
linear foot) for a 0.5 meter (1.5-foot) deep channel,
according to SEWRPC (1991). Average annual
operating and maintenance costs of vegetated
swales can be estimated using Table 3.
REFERENCES
1. Minnesota Pollution Control Agency.
1991. Protecting Water Quality in Urban
Areas.
2. Schueler, T. R., 1987. Controlling Urban
Runoff. A Practical Manual for Planning
and Designing Urban BMPs.
3. Southeastern Wisconsin Regional Planning
Commission, 1991. Cost of Urban
Nonpoint Source Water Pollution Contol
Measures, Technical Report No. 31.
4. U. S. EPA, 1983. Results of the Nationwide
Urban Runoff Program. NTIS PD# 84-18-
5545.
5. U.S. EPA, 1991. A Current Assessment of
Best ManagementPractices: Techniques for
Reducing Nonpoint Source Pollution in the
Coastal Zone.
1.
U.S. EPA, 1992. Storm Water Management
for Industrial Activities: Developing
Pollution Prevention Plans and Best
Management Practices. EPA 832-R92-006,
U.S. EPA, Washington, D.C.
Washington State Department of Ecology.
February, 1992. Storm Water Manual for
the Puget Sound Basin.
ADDITIONAL INFORMATION
Center for Watershed Protection
Tom Schueler
8391 Main Street
Ellicott City, MD21043
City of Durham, North Carolina
Paul Wiebke
Storm Water Department
101 City Hall Plaza
Durham, NC 27701
State of Minnesota
Lou Flynn
Minnesota Pollution Control Agency
520 Lafayette Road North
St. Paul, MN 55155
State of Oregon
Dennis lurries
Oregon Department of Environmental Quality,
Northwest Region
2020 Southwest 4th Avenue, Suite 400
Portland, OR 97201
Southeastern Wisconsin Regional Planning
Commission
Bob Biebel
916 N. East Avenue, P.O. Box 1607
Waukesha, WI53187
Washington State Department of Ecology
Stan Ciuba
Stormwater Unit
P.O . Box 47696
Olympia, WA 98504
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TABLE 2 ESTIMATED CAPITAL COST OF A 1.5- FOOT DEEP, 10-FOOT-WIDE GRASSED SWALES3
Component
Mobilization /
Demobilization-Light
Site Preparation
Clearing1"
Grubbing0
General
Excavationd
Level and Till6
Sites Development
Salvaged Topsoil
Seed, and Mulch'..
Sod9
Subtotal
Contingencies
Total
Unit
Swale
Acre
Yd3
Yd2
Yd2
Yd2
-
Swale
-
Extent
1
0 5
0.25
372
1,210
1,210
1 210
-
1
-
Low
$107
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TABLE 3 ESTIMATED OPERATION AND MAINTENANCE COSTS
Component
Lawn Mowing
General Lawn Care
Swale Debris and Litter
Removal
Grass Reseeding with
Mulch and Fertilizer
Program Administration and
Swale Inspection
Total
Unit Cost
$0.85 71,000 ft2/ mowing
$9. 00 71,000 ft2/ year
$0.10 7 linear foot 7 year
$0.30 7 yd2
$0.15 7 linear foot 7 year,
plus $25 7 inspection
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Swale Size
(Depth and Top Width)
1.5 Foot Depth, One-
Foot Bottom Width,
10-Foot Top Width
$0.14 7 linear foot
$0.18 7 linear foot
$0.10 7 linear foot
$0.01 7 linear foot
$0.15 7 linear foot
$0.58 / linear foot
3-Foot Depth, 3-Foot
Bottom Width, 21 -Foot
Top Width
$0.21 7 linear foot
$0.28 7 linear foot
$0.10 7 linear foot
$0.01 7 linear foot
$0.15 7 linear foot
$ 0.75 / linear foot
Comment
Lawn maintenance area=(top
width + 10 feet) x length. Mow
eight times per year
Lawn maintenance area = (top
width + 10 feet)x length
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Area revegetated equals 1%
of lawn maintenance area per
year
Inspect four times per year
—
Source: SEWPRC, 1991.
The mention of trade names or commercial products does not
constitute endorsement or recommendation for the use by the U.S.
Environmental Protection Agency.
For more information contact:
Municipal Technology Branch
U.S. EPA
Mail Code 4204
401 M St., S.W.
Washington, DC, 20460
MTB
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