Grassed infiltration swales ("swales") are depressions (collection areas) in which stormwater from roofs, streets and driveways is collected following a rain or snow event. Swales are usually found in the front portion of new lots next to the street and sidewalk. Swales are used to collect and dispose of stormwater runoff, and to remove pollutants from stormwater before it reaches underground sources of drinking water. Within a swale there is typically an elevated grate (4" to 6" above grade) that is the cover for a drywell. The drywell receives overflow from the swale in the event of excessive runoff. Stormwater enters swales through cuts in the curb or through catch basins and infiltrates through the grass and soil. Pollutants in the stormwater are removed by the grass and soil in this infiltration process, helping to protect the Aquifer from contamination.
Drainage swales rely on the infiltration process, which is when water "soaks" into the ground and eventually recharges groundwater. The infiltration process requires the soil to have pore spaces for the water to pass through. Some ground materials have larger pore spaces than others, for example gravel is very effective for allowing water to pass through. Clay soils have small pore spaces that limit the amount of water that can pass through.
County Requirements for Construction of Swales
The depth from the swale floor to the top of drywell grate must be no more than 8 inches, but deep enough to accommodate the designed volume of runoff.
Following a storm event the swale should not have standing water for more than 72 hours.
The floor of the swale should be level to evenly distribute water throughout the swale.
The drywell should not be placed near the inlet to the swale. Drywells should be installed according to the approved plans.
Construction Techniques to Increase Swale Performance
Do not drive over swale area prior to construction. Compaction of soils will dramatically reduce infiltration.
Shallow bedrock under the swale area will prevent percolation and infiltration, significantly reducing the performance of the swale.
Importing top soil or sand will create a layered subsurface which is not conducive to good infiltration. On site top soil should be used in swales whenever possible. When off-site top soil/sand is used it should be mixed with 2" of organic matter per 6" of top soil and rototilled into the existing surface. This will greatly increase the infiltration capacity of the swale.
Do not clean concrete truck chutes, masonry tools, and painting equipment in or near swales. These materials will dry and produce a layer of impermeable material that can destroy the infiltration process.
Planting Grass in Swales
Sod is not the only option for establishing grass in swales. Hydroseeding is becoming more and more accepted. Recommendations for seed mixes are moving to grass species that are able to establish in poor soils and have a high drought tolerance. For aesthetic purposes, drought tolerant species are being utilized in areas that do not require turf.
The following recommendations on bed preparation apply to seeding and sod.
Prepare a "fluff" layer of 1-3" of soil into which the sod can root rapidly. The floor of the swale should be level.
Apply a starter fertilizer such as 10-20-10, at the rate of 1 to 2 lb. of phosphorus per 1000 sq ft and mix this thoroughly with the upper 1-4” of soil. Phosphorus is important to establishing roots in newly lain sod because it promotes root growth.
Sod should be lain "butted up," i.e., the pieces are put together as tightly as possible without overlapping. They are also staggered like bricks in a wall. The long axis of the rectangular sod is be lain perpendicular to the slope, helping to reduce erosion.
The sod sod is rolled to firm it against the graded surface and inspected for gaps that need to be closed or for debris that needs to be removed. The sod should be set 1" below the top of the inlet to insure that the growth of grass will not block the inlet.
Sod should be fresh and after being placed should be kept damp for 2-3 weeks to encourage rooting.
Erosion on construction sites occurs when cleared and graded slopes are not protected from the erosive forces of rain fall and runoff. Frequently, there are no measures taken to protect infiltration swales from the deposition of the sediments in runoff. This results in a fine layer of silt that clogs the bottom of the swale.
Slopes that have been graded must be protected from erosion.
Inlets to the swale should be protected form runoff carrying silt eroded form the site.
Drywells in the swale should be protected form sedimentation by placing filter fabric under the grate.
Contacts for More Information:
Design & Construction Criteria for Swales, Drywells, Catch Basins, Etc.
Spokane County Public Works Development Services Section
Inspection of Completed Grassed Bio-infiltration Swales
Spokane County Stormwater Technician