A grassed swale is a graded and engineered landscape feature
appearing as a linear, shallow, open channel with trapezoidal
or parabolic shape. The swale is vegetated with flood tolerant,
erosion resistant plants.
The design of grassed swales promotes the conveyance of storm
water at a slower, controlled rate and acts as a filter medium
removing pollutants and allowing stormwater infiltration.
When properly designed to accommodate a predetermined storm
event volume, a grassed swale results in a significant improvement
over the traditional drainage ditch in both slowing and cleaning
In swales, stormwater is slowed by strategic placement of
[ 446 KB pdf file], new window] that encourage ponding and these
ponds in turn facilitates water quality improvements through
filtration and sedimentary deposition. Collected stormwater
is expected to drain away through the soil within several
hours or days.
Grass Swale with Check Dams.
Note significant channel storage capacity created by check dams.
Notched center allows safe overflow without scour around sides.
(image from Virginia Stormwater Management Program
Here are some additional examples of grassed swales and check dams.
Grassed Swales are an appropriate stormwater management
practice for most regions of North America. Swales are a
low cost low maintenance option to remove sediments, nutrients
and pollutants. They increase stormwater infiltration and
add a visually aesthetic component to a site.
Establishment of grassed swales is a potential solution
wherever stormwater needs to be transported from impervious
surfaces, slowed down and allowed to infiltrate into
Exceptions to their use are in desert like areas where irrigation
would be required for long-term maintenance and in colder
regions with permafrost where infiltration is minimal.
In northern climates without permafrost, design and maintenance
requirements should be modified with respect to a shorter
growing season, management of meltwater and depth of frost
in soil (see details in section on Tips and Wisdom).
Typically grassed swales are used as an environmentally
preferential solution or sometimes as an enhancement to the
more traditional curb and gutter based storm sewer system.
The linear structure of swales favors their use in the treatment
of runoff from highways, residential roadways and common
areas in residential sub-divisions, along property boundaries
and in and around parking lots.
Sound, WA Action Team
Generally grassed swales are used to treat relatively small
drainage areas of five acres or less. In highly urbanized
areas or other highly impervious areas grass swales are not
recommended unless constructed in series or function as pretreatment
for other stormwater management practices.
Advantages of correctly designed grassed swales:
- Trap and remove sediments and other pollutants and
thus improve water quality.
- Reduce peak runoff velocity and promote infiltration.
- Reduce erosion.
- Provide for some groundwater recharge if correctly
- Are favored for use for treating highway and residential
road runoff because of their linear structure. Good
to use in replacement of existing drainage ditches.
- Are best if used in low to moderate density developments.
- Are useful as a means of disrupting impervious pavement
in parking areas.
- Has ponding of water for only a short time with little
increase in water temperature thus useful in watersheds
containing cold-water trout streams.
- Are less expensive to build and maintain (easy to
mow) than a traditional curb and gutter system.
- Function as a linear wetlands
- Reduce peak flows and runoff velocity and promote
- Reduce erosion.
- Are easy to design. Can be built in relatively impervious
soils or in seasonally saturated soils or intersecting
- Trap and remove sediments and other pollutants with
increased efficiency and thus improve water quality.
- Create visually appealing and beneficial habitat
between uplands and surface waters
- Are less expensive to build and maintain than a traditional
curb and gutter system
- Provide effective pretreatment of stormwater passing
through for further processing by additional stormwater
Materials and Installation
- The successful, properly functioning grassed swale is
dependant on good engineering design.
- Refer to the State of Minnesota Stormwater Manual
- 2005, Chapter
12-7: Infiltration Practice Guidance [ 1.2 MB] to find an excellent
discussion on siting and engineering design of infiltration
Best Management Practices in Minnesota.
- Geo-technical testing of soil is recommended to establish
soil porosity and identification of location of close-to-surface
bedrock outcrops that may require re-location of swale.
- Be certain that local authorities have been approached
- An understanding of requirements (Some cities have
manuals or fact sheets available)
- Permits and inspections required
- Siting and Design
- A suggested
design process [ 2.7
MB] includes the following steps:
- Compute water quality treatment volume (WQV) for
surfaces to be treated as required by local authorities
and/or permitting agencies
- Determine the swales dimensions ( bottom width,
depth, length and slope) required to store the WQV
in a shallow ponding depth (18” maximum depth)
- Compute the WQV drawdown time to ensure it is less
than 24 hours
- Compute the 2 and 10 year frequency storm event
- Check the 2 year storm velocity for potential to
cause erosion, adjust swale geometry if necessary
and reevaluate WQV design parameters
- Check the 10 year storm for depth and velocity,
adjust swale geometry if necessary and reevaluate
WQV and 2 year design parameters
- Provide minimum of 6” freeboard (height of
channel sides above water surface) above 10 year
stormwater surface profile
- A reference to find needed design calculations is: Design
of Stormwater Filtering Systems (1996) by Claytor
and Schueler; Center for Watershed Protection and
Chesapeake Bay Research Consortium; Ellicott City
and Solomons, MD.
Additional references to help understand complexities
of design can be found in a number of municipal, state and non-governmental organizations
who provide Best Stormwater Management information.
These are offered only as a means of emphasizing and
confirming the complexity of national stormwater infiltration
issues and that requirements are typical throughout
- To slow water flow in order to prohibit erosion of
swale, check dams may have to be designed and added
to grassed swales. Check dams not only slow water velocity
but encourage ponding of water on the upstream side
of each dam, hereby encouraging infiltration of stormwater;
their design criteria:
- Should be a maximum of 18 inches in height and
height should not exceed half the height of swale
- Spacing will depend on both the longitudinal slope
of the swale and height of swale combining to provide
desired WQV behind the dams.
- Anchoring of dams is necessary to prevent washout.
Each side of the dam needs to extend two – three
feet into the swale wall on both sides.
- The downstream side of check dams need to be protected
from scour with sufficiently sized rip rap placed
over geotechnical fabric.
- Check dams should be notched at their top to allow
passage of two year design storm and a six inch freeboard
to the top of the swale to handle a 10 year design
- Check dams can be made from wood (pressure treated
6”x 6” or 8”x 8” or water
resistant logs e.g. cedar) placed on top of a rock
base. Concrete, gabions or rip rap (sufficiently
sized to withstand flow of water) are used successfully
- Additional design considerations for structural components
of Grassed swales:
- At the point of inflow into the swale a pretreatment
and sediment collection forebay can be designed.
It should be able to contain 25% of the WQV. The
installation of a rip rap pretreatment area contained
by a check dam built on a stone base will serve the
required function. [view
- At the end of a dry grassed swale the installation
of a check dam can be combined with a pea gravel
filled basin that leads excessive water to the underdrain
system. Output from this underdrain must be managed.
- Culverts can be used to maintain swale connectivity
where a road is planned to cross the swale. The culvert
must be sized sufficiently large to accept the swales
design flow for the10 year storm. Rip rap should be
placed at the culvert entrance to reduce erosion.
- In cold climates culverts should have a minimum
diameter of 18 inches and a minimum slope of one
percent to help keep water moving and prevent blockages.
- Wintertime maintenance is a must-do to avoid potential
flooding. In areas that exceed frost depths greater
than five feet and/or snow depths over eight feet
need to posses a portable steamer to clear blocked
- Accurate grading is essential to construction of a
properly functioning grassed swale.
- Machinery used for excavation and grading should not
be driven over the swale site since compaction of soil
is likely to occur.
- The swale site should be protected from storm water
runoff that will cause erosion and sedimentation during
construction. Final grading and planting should not occur
until the adjoining areas draining into the swale are
- Any accumulation of sediments that does occur must
be removed during the final stages of grading.
- In dry swales the bottom should be tilled to produce
a highly porous surface.
- Installation of erosion control matting or blanketing
to stabilize soil during establishment of vegetation
is highly recommended.
- In colder climates with short growing seasons (less
than 5 months) the desired level of establishment of
vegetation may take two to three growing seasons. Careful
maintenance of erosion control practices must continue
for the duration.
- Vegetation Selection and Planting
- Type of vegetation selected must take into account:
- Soils conditions.
- Climate – plants have to be sufficiently hardy
to withstand the most extreme conditions to occur in
the local region.
- Topography- vegetation must be able to withstand
forces created by flowing water.
- Available sunlight
- Selected vegetation must meet the following criteria:
- Have a deep root system or form dense sod to resist
- Be vigorous growers.
- Have a high stem density to help slow water and facilitate
- Be tolerant to flooding and be able to survive and
continue to grow after the inundation period. Water
velocities associated with a two year design storm
should not scour out planted materials nor should plants
leaves matted down by water (will reduce amount of
- If to be used near a road the plants must also be
- Contact local area horticulturist and specialists in
native plantings for recommended species used in grassed
swales. Refer to published
lists [ 75
KB] of salt tolerant plants from a variety of habitats
that are suitable for both dry and wet swales constructed
in northern climes. For additional information on plant
salt tolerance and plants useful in cold climates see Appendix
E [ 2.2
MB] in the Minnesota
Stormwater Manual (2005).
- Some commonly used grasses for dry swales are:
- Park grade Kentucky bluegrass or fescue or Creeping
bentgrass(If regularly mowed)
- Native grasses – switchgrass, big bluestem,
little bluestem, Indian grass or side-oats grama (typically
mowed seasonally). Their use will increase diversity
of animals and their extensive root systems promote
drought tolerance and better filtration. On the negative
side the swale will take a longer time to become established.
The use of cover crops and/or soil stabilization techniques
will be required.
- A mixture of native grasses can be visually appealing
and provide variety of texture and color throughout
- Do not use invasive
species like reed canary grass.
- Planting can be accomplished by the use of sod or by
seeding (preferably hydro-seeding [ 240
- Soil preparation includes:
- Prepare top three inches of soil to provide sufficient
aeration to allow rapid root growth.
- Add appropriate types of fertilizer and apply at
prescribed rate to encourage rapid growth.
- Sod rolls are laid perpendicular to slope to assist
in erosion control. Sod edges should butt against each
other and vertical joints staggered (look like a brick
- The laid sod should then be inspected for gaps and
foreign materials then rolled to ensure root surfaces
are in contact with soil.
- Installation of erosion control matting is strongly
encouraged until vegetation is well established.
- Water well at least two to three times in the first
- Well established sod can be identified by grasping
laid sod and tugging upwards. If the sod layer does
not pull away from the soil layer then the sod can
be considered well established, this should take two
to three weeks.
- With hydro-seeding a well established vegetation
mat will take longer to develop and depend on weather
conditions and frequency of watering.
- Proper maintenance of grassed swales is essential.
A maintenance contract with required maintenance steps
delineated has been a successful approach.
- A recommended program would involve the following:
- Mowing of dried swales as required seasonally to
maintain the desired height of vegetation at three
to four inches. Mowing grass too short and incorrectly
applying lawn chemicals will jeopardize function.
- At least once a year and more often if required:
- Inspection for erosion. Any obvious damage to grass
or to the swales bottom soil bed should be repaired
immediately. Seeded swales will require periodic
re-seeding. Repairs need to conform to original swale
- Remove trash and other debris from all parts of
- If selected grass for swale is not performing select
and plant a new grass.
- For wet swales replant with a revised selection
of wetland plants if current plants are establishing
slowly or failing.
- On as needed basis:
- Till soil at swales bottom if water does not drain
out within 48 hours (swales should drain within 24
- Remove sediment build up when the swales volume
is reduced by 25 percent.
- In colder climates after spring snow melt:
- Any collected sand from winter time sanding must
- Replacement of damaged vegetation should occur
as soon as possible.
- Additions of mulch (added organic matter) may be
required if drainage to swale is from roadway or
parking lot. Deicing salts can damage soil structure
by reducing soils organic matter.
- Maintenance responsibility is normally that of the
- Swales are a relatively cheap alternative to curb
and gutter treatment. Swale maintenance is more frequently
required but these costs are considerably cheaper than
for curb and gutter systems maintenance.
- Swale construction costs have not been well studied.
Costs will differ regionally and with site conditions.
A best estimate has been made of $0.50 per square foot
(2002 dollars). An undated estimate (no older than 2000)
suggests a value of $5.50 per cubic foot of storage provided.
References: Guidebooks, websites and pamphlets
pdf file; it will be opened in a new window]
- 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 in a cold-climate setting.
Download their information on dry
swales [ 2.7
MB] and wet
swales [ 1.9
Storm Water Management in New Development & Redevelopment – Grassed
Swales (2002) by the USEPA; a good review of Grassed
Swale knowledge and practices. A well done summary of
the effectiveness of Grassed Swales as pollutant removal
systems. Strong reference section for pursuit by the
Stormwater Management Handbook (1999) produced by
the Virginia Department of Conservation and Recreation;
an example of a very detailed stormwater management (BMP)
manual. Strong on engineering and construction detail.
Also a useful collection of images depicting grassed
swales; find minimum standards for grassed swales in Chapter 3
[ 741 KB].
- State of Minnesota
Stormwater Manual-2005 is a valuable tool for stormwater
managers. The manual provides details on stormwater management
practices applicable to Minnesota that conserves, enhances,
and restores high-quality water in our lakes, rivers,
streams, wetlands, and ground water.
Go to the chapter on
[ 1.2 MB],
Appendix D-6: Infiltration Practices CADD Details
[ 1.5 MB] and
Appendix E: Minnesota Plant List and Application
[ 2.2 MB]
for details that apply to Grassed Swales.
Tips and Wisdom
Grassed swales have minimal ‘hard structure’ to
be impacted by freeze conditions and snow and therefore a
useful Best Management Practice. Suggested cold climate adjustments
deal with frost depth, snow meltwater volume and a shortened
growing season; considerations include:
- Plowing adjoining areas after snowfall must be carried
out in a judicious manner to prevent damage to the swale’s
edge structure and subsequent erosion. A modest set back
and flag marking of the swale are measures that can help.
- Providing for a five foot setback to prevent frost heave
of roads and parking lots in climate zones where frost
depth exceeds five feet and soils are clay or silt structure.
- Do not use grassed swales in areas of permafrost.
- Salt tolerant plants must be chosen for swales placed
- Short growing seasons are typical of colder climates.
Two seasons may be required for establishment of vegetation.
During this period erosion control practices such as mats
or blankets must protect swale structure.
- Grassed swales can be used for snow storage and are important
in reducing snow melt peak flows through infiltration and
in treating most pollutants associated with snow. However,
extra maintenance will be required to deal with the sand
and de-icing compounds used on roadways and parking lots:
- If swales are to be used for snow storage, plant selections
must be salt tolerant.
- Grassed swales are not recommended for large drainage
- Grassed swales are a cheap alternative to curbs and gutters;
however their efficacy is dependant on careful design and
construction, a thoughtful selection of plants as well
as provision of dependable maintenance over time. Badly
designed grass swales will not remove significant quantities
- If vegetation is not sufficiently established the swale
will not function.
- Given the importance of design and construction, selection
of engineering services and a construction company should
be based on proven, successful past experience with swales
and recommendations from past clients who have had swales
- Wet swales should not be used in high density residential
areas because of potential for mosquito generation and
- Grassed swales are not the best management practice for:
- Ultra urban areas because the areas of pervious surfaces
required for swale development are usually unavailable.
- Stormwater hot spots where land use will generate
stormwater with high levels of contaminants. Good examples
of hot spots would be gas stations and convenience
- In cold weather climates, adjustments to design and maintenance
must occur to deal with high snow loads and significant