Grassy Swales

Grassy swales, like vegetated swales, are long, narrow, channels that are used to filter and infiltrate stormwater from parking lots, sidewalks, streets, and other impervious surfaces.  Grassy swales are landscaped solely with a grass seed mix, and may be mown occasionally, depending on aesthetic and stormwater filtering requirements.

Specifications

Grassy swales are designed to have a gentle slope between 0.5% and 5%.  An underdrain may be required for slopes less than 1.5% if significant amounts of standing water remain after a storm event.  Side slopes may not exceed 4 horizontal to 1 vertical, and should be minimized to the extent possible.  Maximum surrounding ground slopes shall be 10% to reduce the velocity of incoming water.  The minimum length for grassy swales is 20 feet.

Facility storage depth may range from 6 to 12 inches.  A freeboard (6 to 12 inches) is required if the adjacent area may be damaged in the case of flooding. The minimum bottom width is 2 feet for private swales, with a minimum total width of 6 feet.

If stormwater will enter the swale through a pipe or other point source, an inlet flow spreader is required; for long swales, additional flow spreaders or check dams are recommended every 12 to 20 feet.  River rock can be used to create an attractive, effective check dam.

If water will enter the facility as sheet flow from a parking lot, sidewalk or other similar impervious surface, a 12 inch minimum flat area is required adjacent to the impervious surface. Tire stops or curbs with curb cuts should be located 6 inches back from the edge of the swale, and curb cuts should be located at a minimum of 10 feet on center, and be 12 inches wide.  An energy dissipater may be required at curb cuts to prevent erosion.  Periodic cleaning may be required to ensure that curb cuts do not get blocked with debris.

For street swales, steeper side slopes (maximum of 3:1) and a narrower total width are allowed.  Impermeable fabric is required along the street edge to the bottom of the swale, and drainage must be provided away from the street.

Planting Specifications

The minimum plant materials per 200 square feet of facility area are:

• 1 evergreen or deciduous tree to be planted around the perimeter of the swale
• Grass seed or sod to completely cover the swale bottom and side slopes
• Shrubs are optional

For the swale flow path, native grass mixes are preferable, and may be substituted for standard swale seed mixes.  A mix of at least 4 different species, including at least one flowering plant, is required.  Facilities should be designed to require a minimum of mowing.  Click here for recommended seed mixes.

Maintenance Requirements

As needed:

• Mow to keep grass height between 4 and 9 inches tall
• Remove any debris or garbage that is caught in the swale
• Clear the inlet(s) and outlet(s) to allow for proper flow dispersal
• Inspect and repair damaged inlet erosion control devices and check dams
• Water grass as needed until established
• Remove sediment build-up, determine source of sediment, and take measures to prevent it from re-entering the facility
• Take appropriate pest control measures if insects or rodents are inhabiting the swale

Seasonally:

• Inspect swale and reseed bare spots if necessary
• Remove any nuisance and prohibited vegetation (as listed in the Portland Plant list)

Grassy Swales in Sandy

Sandy Meadows Subdivision – Pine, Redwood, and Oak Streets, west of Bornstedt Avenue

Dubarko Road near Tickle Creek

Resources Used

• City of Gresham Department of Environmental Services Stormwater Division. Water Quality Manual.  Summer 2003.
• City of Oregon City.  City of Oregon City Stormwater and Grading Design Standard; Chapter 5 of City Code.
• City of Portland Bureau of Environmental Services. Stormwater Management Manual. July 1, 1999; Revised 2004
• Clean Water Services.  Design and Construction Standards for Sanitary Sewer and Surface Water Management. March 2004.
• Sound Native Plants.  Environmental Conditions Favorable for Establishment of Outplanted/Transplanted Plants.  2000.
• Additional information and resources are available on the City of Portland Bureau of Environmental Services web page