Vegetated swales can be mown grass or any vegetation type that is stable under stormwater flows.
These systems simultaneously convey and treat stormwater runoff. The typical location for swales is next to impervious surfaces such as parking areas and roads, with the potential to take the place of kerb-gutter-pipe systems.
Their linear nature allows the filtering of sheet flows down channel side slopes and then conveyance and further treatment of contaminants along the base of the swale. Treatment is achieved by reduction in flow velocities across a vegetated surface, providing for filtering of contaminants and increased contact time at the plant-soil-water interface where treatment processes occur. The slowing of stormwater flows in vegetated swales increases the time of concentration for stormwater in the catchment and reduces peak flow. It also provides opportunities for infiltration to groundwater.
Dense vegetation and low velocities ensure reasonable treatment effectiveness in swales. Swales are effective at removing metals and coarse to medium-sized sediments. They may act as a standalone water treatment practice where there is sufficient residence time or may provide primary filtering within a treatment train. They may additionally act as level spreaders onto landscape areas for high flow events.
Swales can be integrated into existing landscape elements through alignment with natural flow paths and integration with planting schemes or natural plant communities. In flat areas, swales may be very wide to form subtle undulating flow paths. The linear nature of swales make them suitable for defining boundaries, separating pedestrians from traffic, forming a boundary to mitigate unwanted views, or forming intentional axes or dominant lines within a landscape. Swales also play an important role in softening the expanse of impervious infrastructure such as within car parks or road medians.
Swales are dynamic environments experiencing both rapid inundation and drought. However, they may represent and include relatively diverse plant communities, similar to those associated with intermittent streams and floodplains.
Challenges and solutions
The table below describes some of the common issues and constraints relating to the implementation of filter strips and swales. In all circumstances, there is a potential design solution, which must be balanced against other objectives for the project.
Slopes steeper than 5% are considered inappropriate for swales and filter strips
Swales can be used on steeper slopes (5-8%) utilising weir structures to provide for grade changes.
Similarly, filter strips can use terracing or mid-slope level spreaders to reinforce slopes and evenly disperse flows.
Erosion may result from filter strips and swales being brought online before vegetation is established or from preferential flow paths
Use erosion control fabrics and check dams, or consider diversion around a swale while vegetation is establishing.
To prevent erosion during operation, utilise check dams and densely plant in off-set rows. Maintain grass in swales at no less than 100 mm high to reinforce channel stability.
Infiltration potential may be undesirable due to groundwater levels and structural soils
Impermeable clay liners or underdrains may be utilised along the base of swales to prevent infiltration to adjacent structural soils or to high groundwater.
High sediment loads can create unsightly sedimentation within swales and at the edges of filter strips
Dense planting and/or a reduced mowing regime may localise sediment accumulation for later removal and prevent its transport to the base of the swale.
A level spreader, in the form of a gravel trench, permeable dish or berm can capture sediment at the transition between the contributing catchment and swale/filter strip where it can be easily maintained.
Insufficient space may be available to accommodate vegetated side slopes
A swale may be designed with steeper reinforced side slopes or with vertical walls, so long as entry points are reinforced and tripping hazards are addressed.
The length of a swale may not be long enough to provide for the designed residence time
Where a swale is a preferred option as a sole treatment practice, additional residence time can be achieved through a permeable base, check dams to detain stormwater volumes, and dense and taller vegetation to slow velocities.
The catchment may only allow defined entry points to swales
Level spreaders can direct flows across the side slopes of a swale to achieve full treatment potential. Where the side slopes are not required for treatment, there can be appropriately reinforced point discharges to the swale at regular intervals.
Continuous swales can prevent access
Pedestrian and vehicular access across swales can be constructed with boardwalks and permeable crossing structures to allow flows to continue through the swale.
Inspection should be carried out every two years. Any issues identified during inspection should be rectified.
Correctly maintained filtering and conveyance practices that utilise an underdrain have a design life of 50 years. If no underdrain is used, design life can exceed 100 years. Lack of maintenance can cause blockage and scour, which may require the replacement of the asset to restore function.