|Vegetated Swale Detail. From City of Portland 2004 Stormwater Management Manual.|
Swales with a gentle slope (less than 4% average) are the most effective at treating stormwater. The maximum slope must be less than 6%. Facility storage depth may range from 2 to 12 inches. A freeboard (6 to 12 inches) may be required if the adjacent area would be damaged by flooding. Maximum side slopes are 3 horizontal to 1 vertical; slopes and depths should be minimized to the extent practical for aesthetic and safety reasons.
Check dams are required every 12 to 20 feet (or a minimum of 2 per swale) to promote ponding, and should be 12 inches in length, the width of the swale, and 3 to 6 inches high. River rock can be used to create an attractive, effective check dam. For privately maintained facilities, the minimum bottom width is 2 feet, with a minimum total facility width of 5 feet. The minimum swale length is 20 feet.
|Street Swale Detail. From City of Portland 2004 Stormwater Management Manual|
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 must be 12 inches wide, located at a minimum of 10 feet on center. An energy dissipater may be necessary at curb cuts to prevent erosion. Periodic cleaning may be required to ensure that curb cuts do not get blocked with debris.
If water will enter the facility through a pipe or other point source, a flow spreader or energy dissipater, such as river rock, is required at the inlet.
For street swales, steeper side slopes (maximum of 2:1) 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.
The minimum plant material quantities per 100 square feet of facility area are:
|Vegetated Street Swale. Barlow Ridge Subdivision along Dubarko Rd. Picture by L. French, 7/14/2005.|
1 evergreen or deciduous tree to be planted around the perimeter of the swale
4 large shrubs
6 shrubs or large grass-like plants
Ground covers at 12 inches on center, or seed, or sod
A minimum of 4 different species of trees and shrubs must be used. Topsoil shall be added to the top 12 inches of the swale, or the soil shall be amended with organic material to support plant growth.
A planting detail for a typical 20 foot long swale with a 2 foot wide bottom is attached (click here).
Click here for a list of recommended native plants. Plants should be chosen based on amount of sunlight the swale will receive, as well as the plant’s ability to withstand both drought and temporary flooding. Other plants may be chosen based on preference.
|Vegetated Parking Lot Swale. Fred Meyer Store. Picture by L. French, 7/25/2006.|
- 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 plants 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
- Inspect and replace diseased or dead plants
- Prune other vegetation as needed
- Remove any nuisance and prohibited vegetation (as listed in the Portland Plant list)
|Curb Cut at Fred Meyer Parking Lot Swale. Picture by L. French, 7/25/2005.|