5.6 At Source Treatments - WSUD
Water Sensitive Urban Design (WSUD) aims to promote better, more sustainable management of the urban water cycle by decreasing flooding and improving water quality, ultimately improving aquatic systems.
Key principles of WSUD
The Best Practice Environmental Management Guidelines (1999) communicate the key principles of WSUD, which are to:
Protect natural systems - protect and enhance natural water systems within urban developments. The development of water focussed drainage infrastructure promotes the waterways, resulting in it becoming an asset that is to be protected and not exploited. The protected natural system is therefore able to function effectively.
Integrate stormwater treatment into the landscape - use stormwater in the landscape by incorporating multiple use corridors that maximise the visual and recreational amenity of developments. The natural stormwater drainage system can be utilised for its aesthetic qualities within parklands and walking paths, making use of natural topography such as creek lines and ponding areas.
Protect water quality - improve the quality of water draining from urban developments into receiving environment. Through filtration and retention, water draining from urban developments can be treated to remove pollutants close to their source. This approach reduces the effect that polluted water can have upon the environment and protects the natural waterways and environment.
Reduce runoff and peak flows - reduce peak flows from urban development by local detention measures and minimising impervious areas. Local detention and retention enables effective land use for flood mitigation by utilising numerous storage points in contrast to the current practice of utilisation of large retarding basins. This approach subsequently reduces the infrastructure required downstream to effectively drain urban developments during rainfall events.
Add value while minimising development costs - minimise the drainage infrastructure cost of the development. The reduction of downstream drainage infrastructure due to reduced peak flows and runoff minimises the development costs for drainage, whilst enhancing natural features such as rivers and lakes that add value to the properties of the area.
WSUD techniques
The following provides a basic description of WSUD options for development. Technical detail for planning and construction can be sourced from the WSUD Technical Manual.
Litter traps
Litter traps are used for removal of large pollutant matter and are considered to be a primary treatment method in a treatment train approach. Litter traps are normally required to be capable of removing 95% of litter greater than 20mm for a three-month flow. Where significant ecological values exist this requirement is often elevated to litter greater than 5mm for a 6-month flow.
Swales
Swales are linear depression that provide for stormwater collection and conveyance and provide primary and some secondary treatment. Swales are most suited to a slight grade of no more than 1.5% and often designed to fit with the landscape theme.
Vegetation in swale systems can range from grass-lined systems to more densely vegetated depending on the amenity requirement. While swales provide for stormwater conveyance, they also remove gross pollutants, coarse sediment and to some limited degree, nitrogen and phosphorus.
Infiltration system
An infiltration system is normally a shallow excavated trench or pit filled with a fine gravel medium and provides primary and some secondary treatment. Stormwater filtrates from the trench into the surrounding soil where biological treatment takes place (tertiary treatment) and some particulate pollutants are also retained in the trench.
Bioretention systems
Bioretention systems are designed to carry out primary and/or secondary treatment processes of stormwater and retard flows. A bioretention system detains water in a gravel filled trench to allow the biological process to remove dissolved nutrients. This retention or retardation can enable sediments to precipitate out of the water taking along with it some pollutants. A bioretention system would be suitable for this site, and they are often combined with a surface swale.
Porous paving
Porous paving is an alternative to conventional impermeable pavements with many stormwater management benefits. Porus pavements allow water to percolate through to a sub-surface course, from where it infiltrates to the soil.
Rain Gardens
Rain Gardens are landscaping features that may be designed for stormwater quality treatment and can range from individual gardens for houses through to larger treatment systems for larger flows. Similar to an infiltration system, rain gardens are strongly influenced by landscaping objectives. Stormwater runoff can be filtered typically through a sand/organic mulch medium, which subsequently drains to a conventional stormwater conveyance pipe. Rain Gardens can also incorporate aesthetic water/pond features.
Further Information
For specific technical detail on the design and construction of all WSUD techniques please refer to the WSUD Technical Manual.
