Vegetated Swales
More Detailed Information
- Design Details
- Sample Specifications
- Plant List -Blue Thumb Program plant selector
Design Considerations
Swale Types
Vegetated swales can be applied to most situations. In dry swales, runoff percolates through highly permeable soils or an engineered soil bed. If the underlying soils are permeable, the swale can be designed for volume control (infiltration). If the underlying soils are less permeable, runoff can be returned to the stormwater system via an under drain (biofiltration). Wet swales are used in areas that intersect groundwater levels or in soils that have low percolation rates. Water is stored in shallow pools to settle pollutants and allow vegetation to treat runoff.
Contributing Drainage Area
Typically vegetated swales are designed for small sites (e.g. five acres or less) but they can be applied to larger areas if designed properly (e.g. using vegetated swales in conjunction with other best management practices).
Pretreatment
Depending on the function of the vegetated swale, they can either serve as pretreatment for another BMP or they may require pretreatment. If the swale is designed to retain and infiltrate the runoff generated for smaller rainfall events, consideration should be given to the pretreatment of runoff before it enters the swale (e.g. provide a filter strip upstream of the swale or situate the swale downstream of a water quality pond).
Residence Time
While vegetated swales are typically designed to treat runoff from small storms, they can also be designed to provide volume control if situated in permeable material. By increasing the residence time (the period it takes runoff to travel from one end of the swale to the other) of a dry swale, the practice will be more effective in removing pollutants and infiltrating runoff. Designing the swale with check dams promotes additional storage and infiltration while reducing flow velocities.
Geometry
The main components of a swale’s geometry include: length and longitudinal slope of the swale (design with minimal to no longitudinal slope not to exceed 5 percent), side slopes (minimum 1:3), shape of its cross-section and roughness created by vegetation. The Manning’s equation is a useful tool in determining the swale’s dimensions; however, each swale is unique and all aspects of its geometry should be fitted to site conditions and goals. The geometry of the swale should allow larger storms to safely pass through without scouring or erosion.
Vegitation
Vegetation stabilizes soil, filters pollutants and slows runoff flows increasing residence time. Selected plants should provide dense cover with a root structure that resists erosion and must be able to survive in local soil and microclimate conditions. Dry swale plants must be able to handle periodic inundation while wet swale vegetation may be inundated for longer periods of time. Wetland vegetation is often used in wet swales.
Optional Sand or Gravel Bed
A sand or gravel bed below the vegetated swale should be provided to aerate and drain the planting soil depending on the permeability of the underlying material. If the insitu soils exhibit low permeability, the underdrain may be located in the gravel bed and the vegetated swale will operate as a biofiltration practice.
Outlets/Emergency Overflows
Vegetated swales should be designed to handle larger storms without scouring and erosion. An emergency overflow should be provided to direct excess flow away from the swale and prevent undesirable inundation/ flooding. The overflow should be designed with an energy dissipater to prevent scouring.
Optional Underdrains
An underdrain is a perforated pipe installed in the gravel bed that collects filtered runoff and directs it toward an approved location, such as an existing drainage system.
Depth to Water Table/Bedrock
Dry swales should have a minimum of 2 feet between the bottom of the practice and the seasonally high water table or bedrock. This separation is required to maintain groundwater quality and the hydraulic capacity of the practice. Wet swales may intersect the water table in areas where there is no potential to transport pollutants.
Siting of Facitity
Each site should be considered unique. Vegetated swales should be strategically located to receive runoff from impervious surfaces (sidewalks, roads, parking lots) along the entire length of the swale. If water entering the practice is concentrated, erosion protection (e.g. rip-rap) should be used to dissipate energy and spread the flow across the width of the swale. Vegetated swales are not recommended in areas with steep slopes, sandy soils, concentrated flows, or any other factors that could erode the swale’s banks.
Cold Climates
Swales may be used for snow storage if the vegetation is salt tolerant and the following issues are taken into consideration: swale is located a minimum of three feet above the water table, the swale is located on flat, well-drained soils, and the snow does not contain unusually high concentrations of chlorine.