A vegetated swale is an open channel conveyance system that reduces runoff velocities, lowers peak flows, and can filter or infiltrate some storm flows when check dams are installed, creating a series of cells. In contrast to ditches, swales are designed with a wide flat bottom, a shallow gradient, and shallow side slopes. Densely vegetated swale cells can be very effective at reducing runoff velocities and trapping pollutants.
Grass Swale
A grass swale is simply planted to turf and does not include permanent check dams. These swales typically do not reduce pollutants adequately enough to act as a stand-alone BMP. Typical specifications for a grass swale include a runoff velocity target of 1 foot per second and the ability to handle the peak discharge from a 2-year design storm and to safely function in larger storms.
Dry Swale
A dry swale can function as a conveyance and treatment BMP when used in tandem with check dams that temporarily retain water in a series of cells. Unlike in a grass swale, the filter bed in the swale must consist of a sandy loam subsoil or soil media mix at a depth of approximately 30 inches (see below for information about soil media mix), which allows the water to filter through and remove some pollutants. Dry swale design may also include an underdrain below the infiltration medium that allows infiltration at the required rate but carries away excess water in the drain tile after it has filtered through the base media. Dry swales are designed to prevent standing water.
Typical Dry Swale Profile
Typical Swale Cross-Section
Wet Swale
A wet swale is a long, linear, shallow wetland treatment system that can function as a conveyance and treatment BMP in situations where the water table is located very close to the surface. As in a dry swale, check dams temporarily retain water in a series of cells. These cells may be planted with emergent or transitional wetland plant species to improve pollutant removal.
Application
A common application for swales is around the perimeters of parking lots to collect and treat runoff from paved surfaces. Check dams and soil amendments may be added to slow the storm water flow and encourage infiltration, thereby increasing pollutant removal. Depending on the land available, a vegetated swale is preferably designed to have a meandering, natural appearance, but it can be a straight channel when space is limited.
Cold Climate Suitability
During winter, vegetated swales will accumulate snow and ice, causing runoff to temporarily find an alternate flow path.
Water Quantity and Water Quality Benefits
A vegetated swale is a terrestrial-based water-quantity and water-quality control process. It has a simple, site-integrated design that slows runoff, thereby promoting infiltration, filtration, storage, and water uptake by vegetation.
A vegetated swale can be a storm-water treatment BMP when used in tandem with check dams. If designed as such, the swale reduces pollutants through infiltration.
Construction
If the native soils are not conducive to infiltration, an "engineered" soil mixture comprised of sand and compost is used to promote rapid infiltration and good plant growth.
Performance Specifications for All Bioretention Media (Minnesota Storm Water Manual 2014)
- The growing media must be suitable for supporting vigorous growth of selected plant species.
- The pH range (soil/water 1:1) is 6.0 to 8.5.
- Soluble salts (soil/water 1:2) should not exceed 500 parts per million.
- All bioretention growing media must have a field-tested infiltration rate between 1 and 8 inches per hour. Growing media with slower infiltration rates may clog over time and may not meet drawdown requirements. Target infiltration rates should be no more than 8 inches per hour to allow for adequate water retention for vegetation and for pollutant removal. The following infiltration rates should be achieved if specific pollutants are targeted in a watershed:
- Total suspended solids: any rate is sufficient; 2 to 6 inches is recommended.
- Pathogens: any rate is sufficient; 2 to 6 inches is recommended.
- Metals: any rate is sufficient; 2 to 6 inches is recommended.
- Temperature: slower rates are preferable (less than 2 inches per hour).
- Total nitrogen (TN): 1 to 2 inches per hour; 1 inch per hour is recommended.
- Total phosphorus (TP): 2 inches per hour.
The following additional bioretention growing media performance specifications are required to receive phosphorus reduction credit:
- Option A: Use bioretention soil with phosphorus content between 12 and 36 milligrams per kilogram when using the Mehlich-3 test.
- Option B: Include a soil amendment that facilitates adsorption of phosphorus.
Guidance for Bioretention Media Composition
- Mix B, enhanced filtration blend, is a well-blended, homogenous mixture of 70% to 85% construction sand, and 15% to 30% organic matter.
- Sand provided should be clean construction sand, free of deleterious materials: AASHTO M-6 or ASTM C-33 washed sand.
- MnDOT specifications 3890 Grade 2-certified compost is recommended for organic matter (this compost shall not contain any biosolid/mixed municipal compost/animal manure components).
- It is assumed this mix will leach phosphorus. When an underdrain is utilized, a soil phosphorus test is needed to receive water quality credits for the portion of storm water captured by the underdrain. The phosphorus index (P-index) for the soil must be low, between 10 and 30 milligrams per kilogram when using the Mehlich-3 test. This is enough phosphorus to support plant growth without exporting phosphorus from the cell.
Maintenance
During establishment, vegetated swales require periodic inspection (especially after rain events larger than 1 inch) and immediate repair of rill or gully erosion to prevent expanded erosion. Vegetation requires more frequent maintenance during establishment for weed control and to ensure maximum vegetation density to reduce soil erosion. Any mowing for weed control should be done at a height of 4 to 6 inches to prevent damage to native seedlings. Sediment accumulation should be checked regularly and removed as needed.
Sedimentation reduces channel capacity, infiltration ability, and proper functioning. Sediment removal, erosion repair, and control of invasive species are recurring maintenance tasks that need to be carried out for the life of the swale.
Adapted from the Minnesota Stormwater Manual, the Georgia Stormwater Manual and from the Center for Watershed Protection.