Swales are shallow, flat bottomed, vegetated open channels designed to convey, treatand often attenuate surface water runoff. When incorporated into site design, they canenhance the natural landscape and provide aesthetic and biodiversity benefits. They areoften used to drain roads, paths or car parks, where it is convenient to collect distributedinflows of runoff, or as a means of conveying runoff on the surface while enhancingaccess corridors or other open space. Swales can have a variety of profiles, can beuniform or non-uniform, and can incorporate a range of different planting strategies,depending upon the site characteristics and system objectives.
Grassed swales have the capability to reduce runoff volume and improve water quality. Volume reduction occurs primarily through infiltration into the soil, either as the waterflows over the slide slope perpendicular to the roadway into the swale or down thelength of the swale parallel to the roadway. Pollutant removal can occur bysedimentation of solid particles onto the soil surface, filtration of solid particles byvegetation, or infiltration of dissolved pollutants (with stormwater) into the soil (Abidaand Sabourin 2006). When solid particles settle to the soil surface or are captured byfiltration on vegetation, the TSS concentration of the runoff is reduced and overall waterquality is improved as long as the solids do not become resuspended.
Retention ponds are ponds or pools designed with additional storage capacity toattenuate surface runoff during rainfall events. They consist of a permanent pond areawith landscaped banks and surroundings to provide additional storage capacity duringrainfall events. They are created by using an existing natural depression, by excavating anew depression, or by constructing embankments.
Retention ponds can provide both storm water attenuation and water quality treatmentby providing additional storage capacity to retain runoff and release this at a controlled rate. Retention ponds have good capacity to remove urban pollutants and improve thequality of surface runoff.