Sustainable urban drainage systems (SuDs) offer a natural approach to managing drainage.
When vegetation and permeable surfaces are replaced by concrete, paving or roofed structures, less surface water is absorbed into the ground and more is directed to the surface water drains.
Instead of channelling surface water through a network of pipes to a watercourse, sustainable systems slow and hold back the water, allowing natural processes to break down pollutants, and release it in a more controlled way.
This reduces the impact of new and existing building developments on the surface water drainage, which in turn minimises the chances of flash flooding in extreme weather conditions.
A swale is a broad, shallow channel with dense vegetation covering the side slopes and bottom. The vegetation in the channel provides filtration and solids removal and reduces stormwater flow velocities as the flow is conveyed through the system.
Vegetated swales (also known as grassy swales or biofiltration swales) are flow-based systems sized to treat the water quality flow rate. The treatment effectiveness is correlated to the residence time of the runoff in the swale.
Depending on the native soils, some infiltration may also occur, which decreases runoff volume and provides additional filtering.
Vegetated swales can be well suited to treat runoff from roads and highways because of their linear nature. Swales can be paired with many other treatment measures, such as rain gardens and infiltration basins.
Swales are highly customizable and can include amended soils or sub-surface infiltration trenches.
Benefits
- Improves water quality by removing sediment, particulate matter, and trace metals.
- Can create habitat and increases biodiversity in the city.
- Infiltration of runoff improves urban hydrology and facilitates groundwater recharge.
- Provides aesthetic amenities.
- Improves air quality.
- Low installation costs.
- Low maintenance requirements.
Limitations
- Limited volume reduction and peak flow attenuation unless designed as a dry bio-infiltration swale.
- Impractical in areas with very flat or very steep topography or with erosive or poorly drained soils.
- Little removal of dissolved pollutants and bacteria.
- Poor pollutant removal and vulnerable to erosion when flow velocities are high.
- Limited to relatively small drainage areas