Water quality swales are designed primarily for the prescribed stormwater water quality volume and have incorporated specific features to enhance their stormwater pollutant removal effectiveness. They retard and impound stormwater and convey it downstream at velocities low enough to protect against channel and streambank erosion.
Design Criteria Checklist:
The volume of runoff in the swale is less than four to five cubic feet per second
The slope of the site does not exceed five percent
The distance from high groundwater is greater than two feet
Underlying soils have a percolation rate of at least 0.5 inches per hour
The swale’s slide slopes should be 3:1 (h:v) or flatter. Its bottom surface slope is as close to zero as drainage will permit
Water tolerant, erosion resistant species of grass are used. For swales adjacent to roadways, the species is salt tolerant as well. (Reed canary grass is recommended)
Check dams with stones to prevent scouring are included to promote additional infiltration
TSS REMOVAL RATES 60-80%
Advantages:
~Controls peak discharges by reducing runoff velocity and promoting infiltration (especially for dry swales)
~provides pretreatment by trapping, filtering and infiltrating particulate and associated pollutants
~Generally less expensive than curb and gutter systems, roadside swales provide water quality and quantity control benefits, while reducing driving hazards by keeping stormwater flows away from street surfaces.
Disadvantages:
~Higher degree of maintenance required than for curb and gutter systems
~Roadside swales are subject to damage from off street parking and snow removal.
Maintenance Criteria
- Maintenance criteria is required for the proper operation of water quality swales. Plans for water quality swales should identify owners, parties responsible for maintenance, and an inspection and maintenance schedule.
- Water quality swales should be inspected at least semi-annually, and maintenance and repairs made as necessary. Additional inspections should be scheduled during the first few months to make sure the vegetation becomes adequately established. Repairs and reseeding should be done as required.
- Swales should be mowed at least once per year. Grass clippings should be removed. The grass must not be cut too often or shorter than four inches, in order to maintain the effectiveness of the swale.
- Sediment and debris should be removed manually, at least once per year, before the vegetation is adversely impacted.
- Care should be taken to protect water quality swales from snow removal and disposal practices and off street parking