Vegetative Filtering Systems Four Design Variations
Notes:
Stormwater engineers frequently use open channels or grass swales to convey stormwater runoff. In some cases, open channels can be redesigned to provide significant pollutant removal. It is therefore quite important to define what is meant by open channels, so as to better distinguish the potential differences in pollutant removal potential that various channel designs can have during small storms. In general, these practices are intended for application to smaller sites where the primary design objective is water quality treatment.
Grass channels are designed to meet runoff velocity targets for two very different storm conditions�a water quality design storm and the two year design storm. To meet the water quality criteria, grass channels must have broader bottoms, lower slopes and denser vegetation than most drainage channels.
In a dry swale, the entire water quality volume is temporarily retained by checkdams during each storm. Unlike the grass channel, there is an engineered filter bed in the swale consisting of 30 inches of prepared soil (sandy loam) that is then collected by an underdrain pipe.
The wet swale is usually situated where the water table is located very close to the ground surface. The wet swale essentially acts as a very long and linear shallow wetland treatment system. Like the dry swale, the entire water quality treatment volume is stored and retained within a series of cells in the channel, formed by berms or checkdams.
Filter strips are a slightly different form of vegetative filter in that they require the presence of sheet flow across the entire strip. Once flow concentrates to form a channel, it effectively short-circuits the filter strip.