Victorian Stormwater Treatment Guide
Rainwater tanks, raingardens, permeable surfaces and other WSUD measures can improve stormwater treatment performance, but their contribution depends on more than simply including them in the design.
A stormwater treatment assessment considers how runoff from roofs, driveways, paving and other impervious areas is managed by the treatment measures connected to them. The presence of a tank, raingarden or permeable surface does not by itself establish that the required treatment outcome will be achieved.
Performance depends on the relationship between catchment area, treatment capacity, rainwater reuse, physical connections and any impervious surfaces that remain untreated. A measure that is appropriately sized and connected can contribute strongly to the assessment. The same measure may contribute much less where it receives only a small catchment or cannot operate as represented in the model.
This guide focuses specifically on how common WSUD measures can support quantitative stormwater treatment performance. For the wider difference between site strategy and modelling, see STORM vs WSUD.
In Brief
What Makes a WSUD Measure Effective?
Connected Catchment
The treatment measure must receive runoff from clearly identified roof, paving or other impervious areas.
Appropriate Capacity
Tank storage, treatment area and other measure dimensions should be proportionate to the catchment being assessed.
Practical Integration
The modelled measure should fit within the project, appear in the drawings and remain accessible for operation and maintenance.
Improving performance is therefore usually a coordination exercise rather than a search for one isolated product or oversized treatment element.
Current Victorian Context
STORM Terminology and BlueFactor Assessments
Melbourne Water’s original STORM Calculator has been replaced by BlueFactor for suitable small-scale Victorian developments. The term STORM remains common in earlier permit conditions, council correspondence and industry searches.
The same practical principle remains relevant: treatment performance depends on the catchments, measures and connections represented in the assessment. The responsible council should confirm whether BlueFactor, MUSIC or another accepted assessment method applies to the project.
Treatment Performance
How Do WSUD Measures Affect the Assessment?
A treatment assessment begins with the surfaces created by the proposed development. Roofs, conventional paving, driveways, parking areas and other impervious surfaces generate runoff that may need to be treated.
The assessment then identifies which catchments are connected to rainwater tanks, raingardens, permeable systems or other accepted measures. The combined performance is influenced by both the effectiveness of those measures and the proportion of the site that remains untreated.
This means that adding one large treatment measure does not always resolve the result. A strong response may instead combine roof-water reuse, treatment of driveway runoff, selected permeable surfaces and a reduction in unnecessary hardstand.
The appropriate combination depends on the project layout, available space and assessment pathway. Measures should only be included where they can be delivered as part of the physical development.
Common Treatment Measures
Which WSUD Measures Can Improve Performance?
Roof-Water Capture
Rainwater Tanks
Capture runoff from connected roof areas and store it for nominated reuse. Performance depends on catchment, storage and recurring demand rather than tank capacity alone.
Landscape Treatment
Raingardens and Bioretention
Filter runoff through a designed treatment area. Their contribution depends on treatment size, connected catchment and an achievable inlet and overflow arrangement.
Surface Treatment
Permeable Paving
Can replace selected conventional paving where the complete surface and sub-base system is designed, specified and accepted as permeable.
Site-Suitable Response
Infiltration Measures
May support treatment where soil, groundwater, building proximity and other site conditions allow infiltration to be relied upon safely.
Runoff Reduction
Reduced Impervious Area
Reducing unnecessary conventional paving can lower the area generating untreated runoff and reduce reliance on added treatment infrastructure.
Combined Response
Complementary Measures
A combination of tanks, landscape treatment and permeable areas may provide a more balanced response than relying on one isolated measure.
Rainwater Reuse
Why Does a Larger Rainwater Tank Not Always Produce a Better Result?
A rainwater tank only receives the runoff generated by the roof area connected to it. Increasing tank capacity may therefore provide limited additional value where the connected roof catchment is small.
Regular reuse is also important. Toilet flushing, laundry use where applicable and landscape irrigation can create storage capacity for future rainfall. A tank with little recurring demand may remain full for longer and have less capacity available to capture subsequent runoff.
The strongest tank response usually coordinates three related inputs:
- a meaningful connected roof catchment;
- storage capacity proportionate to that catchment; and
- realistic and clearly documented reuse demand.
Tank locations, connected downpipes and reuse commitments should remain consistent across the assessment, architectural drawings and supporting documentation.
Bioretention Measures
What Determines the Contribution of a Raingarden?
A raingarden or bioretention system filters runoff through a designed treatment profile. Its assessed contribution depends on the relationship between the catchment directed to it and the available treatment area.
A small raingarden may not provide sufficient treatment for a large driveway or paved catchment. Increasing the treatment area can improve performance, but only where the larger measure can be accommodated within the landscape and coordinated with access, services and site circulation.
The physical connection is equally important. Runoff assigned to the raingarden in the assessment must be able to reach it through the proposed site arrangement. The drawings should show an achievable inlet, treatment location and overflow response.
The measure should also remain identifiable and accessible after construction so that planting, filter media, inlets and overflow points can be maintained.
Catchment Coordination
Why Do Catchment Connections Matter So Much?
Treatment measures can only contribute to the runoff areas that are credibly connected to them. A large tank cannot treat an unrelated driveway catchment, and a raingarden cannot treat paving that falls or drains in another direction.
Identify every major runoff surface. Roofs, driveways, paths, courtyards and parking areas should be reflected consistently in the assessment.
Assign each treated catchment to an achievable measure. The proposed site and drainage arrangement should support the modelled connection.
Account for untreated impervious area. Paving or roof surfaces that bypass treatment can materially affect the overall result.
Keep the model and drawings aligned. Changes to roof areas, paving or treatment locations may alter the assessment and require review.
Design Refinement
Common Ways to Improve Treatment Performance
Connect More Roof Area
Review whether additional roof catchment can be directed to an existing tank without creating conflict with the proposed design.
Coordinate Regular Reuse
Use realistic internal or external demands that can be documented and delivered as part of the completed project.
Increase Treatment Area
A larger raingarden or bioretention area may improve treatment where sufficient landscape space is genuinely available.
Treat Previously Untreated Paving
Review whether driveway, courtyard or parking runoff can be redirected to an appropriate treatment measure.
Reduce Conventional Hardstand
Removing unnecessary impervious area can reduce the amount of runoff that needs to be treated elsewhere.
Combine Complementary Measures
A coordinated combination may resolve different catchments more effectively than repeatedly enlarging one treatment element.
Design Practicality
The Highest Modelled Score Is Not Always the Best Design
The purpose of refinement is to meet the applicable treatment objective through a credible project response. It is not necessarily useful to maximise the numerical result through measures that are oversized, difficult to construct or unlikely to be maintained.
A very large tank may reduce usable site area or create coordination issues. An oversized raingarden may compete with access, planting or private open space. Permeable paving may be unsuitable where the complete system cannot be properly specified or maintained.
The preferred solution should meet the required performance while remaining proportionate, constructible, documented and consistent with the wider site design.
Technical Boundary
Treatment Performance Does Not Replace Drainage Design
BlueFactor and earlier STORM-style assessments focus on stormwater treatment performance. They do not automatically resolve detailed pipework, pits, detention, discharge, surface levels, overland flow or other civil and hydraulic requirements.
A project can satisfy its treatment benchmark while still requiring separate engineering documentation. The treatment assessment and drainage design should use consistent catchments and proposed measures while remaining technically distinct scopes.
Frequently Asked Questions
WSUD Treatment Measure FAQs
Which WSUD measures can improve stormwater treatment performance?
Rainwater tanks, raingardens, bioretention systems, permeable paving and suitable infiltration measures may contribute. Reducing unnecessary impervious area can also reduce the volume of runoff requiring treatment.
Does increasing rainwater tank capacity always improve the result?
No. The contribution of a tank also depends on its connected roof catchment and recurring reuse demand. Increasing storage alone may provide limited benefit where catchment or reuse is insufficient.
Can driveway runoff be directed to a raingarden?
Potentially, where the site arrangement provides an achievable connection and the raingarden is appropriately designed for the nominated catchment. Detailed levels and drainage coordination may require separate consultant input.
Does permeable paving automatically count as treated area?
Not automatically. The proposed area should use a genuine permeable pavement system and be represented in accordance with the accepted assessment method and project documentation.
What commonly causes a treatment result to fall short?
Common causes include large untreated paved areas, limited roof area connected to tanks, insufficient reuse demand, undersized treatment areas or catchment connections that cannot be achieved within the proposed design.
Should the treatment assessment be updated when the plans change?
Relevant changes to roofs, paving, tank sizes, reuse connections, raingardens or permeable areas should be reviewed because they may alter the catchments and treatment performance represented in the assessment.
Related Guidance
Explore the Connected Stormwater Pathways
Victorian Stormwater Assessment
Does the Proposed Treatment Strategy Need Refinement?
Certified Energy can review the available plans, runoff catchments and proposed WSUD measures to help assess whether the treatment response is suitable and identify practical options where further improvement is required.
Send Your Project Documents
