SWALE / BIOSWALE / SHALLOW INFILTRATION BASIN DESCRIPTION
A swale is a linear depressed channel covered with grass or more dense vegetation. A swale is a flat, formed terrain basin for the percolation of runoffs with temporary above-ground storage (DWA-A 138E, 2005). Swales mimic natural drainage processes slowing down runoff velocity, providing evapotranspiration, filtration, and treatment through the root zone, infiltration into the soil. Incorporation of the landscape elements and diverse greenery strategies enhances aesthetic value and creates a distinctive character. There are three types of swells: • Conveyance and attenuation swales obtain runoff and convey it to the destination point, for example, urban wetland. Treatment and Infiltration are not excluded but predefined by the flow intensity and ponding depth (CIRIA, 2015). • Dry swales include a soil filter bed above the underdrain layer filled with gravel. This system provides the additional conveyance volume within the gravel layer. • Wet swales contain a permanent water level at the base, minimum 150 mm, with wetland planting (CIRIA, 2015). This type is convenient for the flat areas, conditions where soil has low infiltration capacity or with the high water table. Urban runoff flows into a swale via curb cuts, trench drain, pipe, gutters or as sheet flow from the adjacent impervious surface. However, the flow concentrating inlets has to contain flow spreaders and pretreatment to prevent a risk of erosion and silting. Vegetated strips at the edge of impervious areas facilitate the runoff pretreatment. Incorporation of the berms and/or check dams perpendicular to the flow path with 10–20 m intervals facilitate settling and infiltration Atlanta Regional Commission, 2016; CIRIA, 2015).
FUNCTION
► Flood control and attenuation of the peak runoff ► Runoff catchment, treatment, infiltration, and conveyance
APPLICATION
► Along low-volume roads, sidewalks, and car parks ► Nature strips and corridors ► Public open spaces, sport fields
BENEFITS
► Pressure reduction on the drainage system ► Urban biodiversity and wildlife habitat ► Moderation of the urban heat island effect ► Groundwater and aquifer recharge ► Enhance of the aesthetic value and appealing landscaping feature
DESIGN CONSIDERATIONS
Trapezoidal or paraboloid shape in cross section for water catchment, convenience in construction and maintenance. The bottom width is 0.5–2.0 m, the depth is 0,4–0,6 m (CIRIA, 2015), the side slope is 2.5:1–4:1 (horizontal: vertical) to resist erosion and provide maintenance access (NACTO, 2018). The longitude slope is 0.5% –6% to assure the water conveyance, the swales with a slope less than 1.5% has to include the underdrain layer (CIRIA, 2015). If the groundwater level is 1m below the base of the swale, the infiltration is allowed (CIRIA, 2015). Overflow elements connected with sewer can be incorporated into design in order to prevent the overload during heavy rainfall events. The recommended water level in swales up to 300 mm with complete filling (DWA-A 138E, 2005).
MAINTENANCE
► Removal of litter, debris, and sediment from forebay and channel ► Regular inspection of structural components, for example, inlets, outlets, and overflows especially after heavy rain events ► Mowing to retain the grass lengths 75–150mm for the dry swales ► Planting management and removal of invasive species ► Restoration of construction levels, repair erosion as required
LIMITATIONS
Swales are not recommended for the areas with a steep slope because plants cannot influence the water flow. Due to the limited space, swales cannot be incorporated into dense urban areas. 3:1 or shallower slope of the graded sides is the most convenient for the maintenance access (NACTO, 2018). A wet swale can have an unpleasant odor and attract mosquitoes.
Figure 4-06: An example of the wet swale ( Source: Author, 2018 based on CIRIA, 2015)
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