Water Sensitive Urban Design (WSUD) integrates the urban water cycle: stormwater, water supply, and wastewater, into urban planning and design, with a strong Australian focus on stormwater quality and quantity management.
WSUD Objectives
| Objective | Description |
|---|---|
| Water quality treatment | Reduce pollutant loads (sediment, nutrients, litter) before discharge |
| Flow management | Maintain pre-development flow regime (peak and frequency) to protect waterway health |
| Water conservation | Reuse/harvest stormwater or greywater to reduce potable demand |
| Amenity and liveability | Integrate water features into public space and urban greening |
The Treatment Train Approach
WSUD typically uses a sequence ("treatment train") of measures, each targeting different pollutant sizes/types, rather than a single device:
Source Control → Conveyance Treatment → End-of-Line Treatment
(e.g. rainwater tanks) (e.g. vegetated swales) (e.g. bioretention, wetland)
Common WSUD Elements
| Element | Function |
|---|---|
| Rainwater tanks | Source control: reduces runoff volume and potable demand |
| Vegetated swales | Conveyance with some filtration/infiltration |
| Bioretention systems (rain gardens) | Filtration through engineered soil media and vegetation |
| Constructed wetlands | Settling and biological uptake for larger catchments |
| Permeable pavement | Reduces impervious area, promotes infiltration |
| Gross pollutant traps (GPTs) | Coarse sediment/litter removal (typically pre-treatment) |
Bioretention System Design
Bioretention systems treat stormwater by filtering it through an engineered filter media layer:
$$ \text{Treatment area} \propto \frac{\text{Catchment area} \times \text{Imperviousness}}{\text{Design infiltration/filtration rate}} $$| Layer | Typical Function |
|---|---|
| Extended detention (ponding) | Temporary surface storage above the filter media |
| Filter media | Engineered soil: provides filtration and supports vegetation |
| Transition/drainage layer | Prevents fines migration, conveys treated water to underdrain |
| Underdrain | Collects filtered water to outlet (with or without a raised submerged zone for added nutrient removal) |
Stormwater Quality Targets
Typical Australian best-practice pollutant reduction targets (varies by jurisdiction/guideline, e.g. state EPA or council WSUD guidelines):
| Pollutant | Typical Reduction Target |
|---|---|
| Total Suspended Solids (TSS) | 80–85% |
| Total Phosphorus (TP) | 45–65% |
| Total Nitrogen (TN) | 30–45% |
| Gross pollutants (litter/debris) | 90%+ |
Targets are usually assessed using a continuous simulation model (e.g. MUSIC: Model for Urban Stormwater Improvement Conceptualisation), not a single design storm, since treatment performance depends on the full rainfall record.
Flow Management (Waterway Protection)
Beyond pollutant load, WSUD aims to maintain a pre-development flow regime, since increased frequency of erosive "in-between" flows (not just peak floods) is a major driver of urban waterway degradation.
Integration with Development
| Stage | WSUD Consideration |
|---|---|
| Subdivision/masterplan | Regional/precinct-scale treatment (wetlands, regional bioretention) |
| Individual lot/building | Rainwater tanks, on-lot bioretention, permeable paving |
| Streetscape | Vegetated swales, tree pits with passive irrigation |
Practical Notes
- WSUD performance should be verified with continuous simulation (e.g. MUSIC modelling) against a stormwater quality target, not assumed from a device's generic "typical" removal efficiency: actual performance is highly sensitive to sizing and catchment characteristics.
- Maintenance access and a clear maintenance regime must be designed in from the start: a bioretention system that clogs and is never maintained rapidly loses its treatment function.
- Flow (waterway) protection targets and water quality targets are related but distinct: a system sized only for pollutant removal may still allow erosive flow frequency increases downstream.