Open section residential roadway drains
to vegetated swales.
Source: LID Center
(click on thumbnail
for enlarged view)

Vegetated swales act as both a stormwater conveyance and treatment system. Pollutant removals in swales are generally the result of filtration through channel and side slope vegetation, infiltration into the swale channel bottom, and microbial activity in the subsurface soils. Monitoring of swales has generally shown significant removals of metals and total suspended solids, primarily through filtration. Nutrient reductions have been variable. Swales often collect sediments and other gross particulate material. These materials may be exported from the swales during large intensity storms. The increase in effluent phosphorus concentrations is likely due to sediment export.  

Shallower sloped, longer, sandy soiled, and densely grassed swales have the highest pollution removal rates.  Shallower slopes and longer channel length increase the amount of time the runoff spends in the swale and encourage infiltration and filtration. Check dams can be added to swales in areas with steep topography to slow conveyance and increase infiltration. The high hydraulic conductivity of sandy soil encourages infiltration. Dense vegetation enhances filtration.