STORM vs WSUD | What Is the Difference?

In Brief

WSUD is the broader design approach. It considers how stormwater, water reuse, infiltration, landscaping and drainage can be integrated across a development.

A STORM assessment is a quantitative treatment assessment. It measures how runoff from roofs, driveways, paving and other impervious areas is treated by nominated measures such as rainwater tanks, raingardens and permeable surfaces.

A STORM result may provide evidence that part of a WSUD strategy achieves the required stormwater-quality performance. It does not necessarily describe the complete water-management response for the site.

In practical terms, WSUD shapes the design, while the STORM assessment tests the treatment performance of selected parts of that design.

Understanding the Difference

The terms STORM and WSUD are sometimes used as though they refer to the same report. This can create confusion during project scoping and council submissions.

They occupy different technical territories:

• WSUD is an integrated approach to managing water within urban development.
• STORM is a modelling and reporting process focused on quantitative stormwater treatment performance.

A project team may use WSUD principles to decide where rainwater tanks, raingardens, permeable surfaces and landscaped treatment areas should be located.

The STORM assessment then examines the runoff-generating catchments connected to those measures and calculates whether the proposed combination reaches the required treatment benchmark.

For a wider explanation of STORM reports and treatment assessments, visit the Certified Energy STORM Knowledge Hub.

What Is a STORM Assessment?

A STORM assessment quantifies how effectively a proposed development treats stormwater runoff.

The assessment commonly considers:

• the total site or development area;
• roof catchments;
• driveways and car parking areas;
• paths, patios and other paving;
• landscaped and permeable areas;
• treated and untreated impervious surfaces;
• rainwater tank capacity;
• roof area connected to each tank;
• rainwater reuse demands;
• raingarden or biofiltration areas;
• permeable paving; and
• the combined treatment performance of the design.

The output is used to show whether the proposed treatment measures achieve the applicable stormwater-quality benchmark.

The assessment has a defined and limited purpose. It does not ordinarily provide a complete site-wide water strategy or detailed drainage design.

What Is WSUD?

Water Sensitive Urban Design is an approach to planning and designing development so that water is managed as an integrated part of the site.

Depending on the project, a WSUD response may consider:

• stormwater quality;
• rainwater harvesting and reuse;
• the reduction of unnecessary impervious area;
• infiltration and permeability;
• landscape integration;
• runoff flow paths;
• the relationship between buildings and drainage;
• water conservation;
• treatment trains;
• maintenance of treatment assets; and
• the effect of development on receiving waterways.

WSUD is therefore broader than a single calculator result.

It influences how the site is arranged and how water-management measures work together across architecture, landscape and engineering.

STORM Measures Performance; WSUD Shapes the Strategy

The simplest way to distinguish the two is to consider the question each one answers.

A STORM assessment asks:

Does the modelled combination of catchments and treatment measures achieve the required stormwater treatment performance?

A WSUD strategy asks:

How should water be managed and integrated across the development?

The STORM question is narrower and quantitative.

The WSUD question is broader and design-led. It may involve quantitative modelling, but it also considers site planning, physical integration, water use, landscaping, maintenance and coordination.

How STORM Fits Within a WSUD Response

A STORM assessment can form one part of a broader WSUD response.

For example, the project team may develop a site strategy that:

• collects roof water in rainwater tanks;
• reuses stored water for toilet flushing or irrigation;
• directs driveway runoff to a raingarden;
• uses permeable paving in selected areas;
• retains landscaped infiltration zones;
• reduces unnecessary hardstand; and
• provides accessible maintenance arrangements.

The STORM assessment can test the treatment performance created by those measures.

It may confirm that the strategy reaches the relevant benchmark, or it may identify that additional treatment is needed.

The numerical result supports the design response, but it does not replace the need to show where the measures are located, how runoff reaches them and how they will be maintained.

Can a STORM Report Be Called a WSUD Report?

The terms should not automatically be treated as interchangeable.

A STORM report usually documents:

• site and impervious areas;
• runoff catchments;
• treatment measures;
• modelling assumptions;
• the treatment score or output; and
• the design commitments required to support that result.

A broader WSUD report may also address:

• the overall water-management concept;
• integration with architecture and landscaping;
• water-conservation measures;
• site permeability;
• treatment sequencing;
• maintenance responsibilities;
• construction-stage considerations;
• drainage coordination; and
• other council-specific requirements.

For a straightforward development, council may accept a concise stormwater treatment report and supporting plan information.

For a more complex project, a calculator output alone may not provide enough information to demonstrate the complete WSUD response.

Can a Project Use WSUD Without a STORM Assessment?

Yes. A project can incorporate WSUD principles without necessarily requiring a STORM assessment.

For example, a development may include:

• rainwater reuse;
• permeable landscape areas;
• reduced hard paving;
• passive irrigation;
• vegetated swales;
• raingardens; or
• site-sensitive drainage arrangements.

Whether quantitative treatment modelling is required depends on the planning controls, development type, application pathway and responsible authority.

Some projects may require another form of assessment, such as BlueFactor or MUSIC modelling. Others may demonstrate their water-sensitive response through plans, specifications and a planning report without a separate STORM output.

Can a Project Achieve a STORM Rating Without a Complete WSUD Strategy?

It may be possible for a project to achieve the required numerical result through a limited set of treatment measures.

For example, a large rainwater tank connected to a substantial roof catchment and regular reuse demand may produce a strong treatment result.

However, the calculator result may not answer broader design questions such as:

• whether unnecessary paving has been reduced;
• whether driveway runoff is managed appropriately;
• whether the treatment measures are integrated with landscaping;
• whether the system is accessible for maintenance;
• whether water reuse has been coordinated with building services;
• whether detention or discharge requirements have been resolved; or
• whether the overall water response is suitable for the site.

A successful numerical result should therefore be supported by a credible and coordinated physical design.

Which WSUD Measures Can Improve a STORM Rating?

Several treatment measures associated with WSUD can contribute to the modelled STORM result.

Rainwater Tanks

Rainwater tanks capture runoff from connected roof areas. Their contribution depends on storage capacity, connected catchment and regular reuse demand.

Toilet flushing, laundry use where applicable, and landscape irrigation can help create storage capacity for later rainfall.

Raingardens

Raingardens can filter runoff from roofs, driveways and other impervious catchments.

The modelled benefit depends on the treatment area, catchment size and physical drainage connection.

Biofiltration Systems

Biofiltration systems use vegetation, engineered filter media and drainage components to remove pollutants from runoff.

They may form part of a landscaped treatment response for suitable projects.

Permeable Paving

Permeable paving allows water to pass through the surface and may support filtration, temporary storage or infiltration.

The area must be genuinely designed and specified as a permeable pavement system.

Infiltration Measures

Suitable infiltration measures may reduce runoff and support treatment where site conditions permit.

Soil, groundwater, slope and building constraints may need to be considered before relying on infiltration.

Reduced Impervious Area

Reducing conventional paving or replacing it with landscaping and permeable surfaces can reduce the amount of runoff requiring treatment.

Example: A Townhouse Development

Consider a three-townhouse development with:

• three dwelling roofs;
• a shared driveway;
• pedestrian paths;
• private courtyards;
• small garden areas; and
• three rainwater tanks.

The WSUD strategy may determine that:

• each dwelling roof is connected to its own tank;
• tank water supplies toilet flushing and garden irrigation;
• part of the driveway drains to a raingarden;
• selected courtyard paving is permeable;
• landscape areas receive passive irrigation; and
• the treatment measures remain accessible for maintenance.

The STORM assessment then measures:

• the roof area connected to each tank;
• the tank capacities and reuse demands;
• the driveway catchment connected to the raingarden;
• the permeable pavement area;
• any untreated impervious surfaces; and
• the combined treatment result.

In this example, WSUD provides the integrated site response. STORM tests whether the treatment components achieve the required performance.

STORM, BlueFactor and WSUD

The original Melbourne Water STORM Calculator has been replaced by BlueFactor for suitable small developments in Victoria.

This changes the current assessment tool but not the basic distinction between modelling and design strategy.

• WSUD remains the broader water-sensitive design approach.
• STORM remains the familiar name associated with the earlier quantitative treatment assessment.
• BlueFactor is the current replacement assessment tool for suitable small developments.

Older permit conditions and council correspondence may continue to request a STORM report or STORM score.

For a new submission, the project team should confirm whether council expects BlueFactor, MUSIC or another accepted assessment format.

How STORM and WSUD Differ From MUSIC

MUSIC is another quantitative modelling tool, not a synonym for WSUD.

It can represent more complex catchments, drainage connections and treatment trains than a basic STORM-style assessment.

The relationship can be summarised as follows:

• WSUD is the integrated design approach.
• STORM or BlueFactor can assess treatment performance for suitable smaller and simpler developments.
• MUSIC can model more complex catchments and connected treatment trains.

Both STORM-style assessments and MUSIC models may be used to provide quantitative evidence supporting a WSUD strategy.

Read STORM vs MUSIC: Which Assessment Does Your Project Need? for the dedicated modelling comparison.

How STORM and WSUD Differ From Drainage Design

Neither the term WSUD nor a STORM assessment should automatically be treated as a complete drainage design.

A drainage design may address:

• stormwater pits;
• pipe sizes;
• surface levels;
• gutter and downpipe connections;
• on-site detention;
• peak flow rates;
• overland flow paths;
• the legal point of discharge; and
• civil construction details.

A STORM assessment focuses on treatment performance.

A WSUD strategy may influence drainage design and landscape integration, but detailed hydraulic or civil documentation may still be required separately.

Why the Difference Matters

Understanding the distinction helps the project team commission the correct service and prepare the right documentation for council.

Confusing STORM and WSUD can lead to several problems:

• a calculator result may be submitted where a broader strategy was requested;
• a general WSUD statement may be provided without quantitative treatment evidence;
• treatment measures may not be coordinated with the drawings;
• drainage or detention requirements may be overlooked;
• the project may receive a request for further information; or
• different consultants may assume that another discipline is resolving the stormwater response.

A clear scope should identify whether the project needs:

• a quantitative stormwater treatment assessment;
• a broader WSUD strategy or report;
• BlueFactor or MUSIC modelling;
• a Stormwater Management Plan;
• drainage and detention design; or
• a combination of these services.

Practical Considerations for Victorian Projects

Read the Council Request Carefully

Terms such as STORM report, WSUD response, stormwater treatment assessment and Stormwater Management Plan may imply different deliverables.

Confirm the Accepted Modelling Tool

Establish whether the project should use BlueFactor, MUSIC or another method accepted by the relevant council.

Develop the Strategy Before Finalising the Model

The model should test treatment measures that can physically fit within the architectural and landscape design.

Coordinate the Catchments

Roof, driveway and paved areas should only be assigned to treatment measures where the proposed drainage arrangement makes the connection achievable.

Show the Measures on the Drawings

Rainwater tanks, raingardens and permeable paving used in the assessment should appear consistently in the project documents.

Allow for Maintenance

Treatment assets need practical access and ongoing maintenance. Numerical performance alone does not establish whether a measure will remain effective.

Check for Separate Drainage Requirements

Confirm whether council also requires detention calculations, civil plans, discharge information or flood-related assessment.

Which Service Does Your Project Need?

A focused STORM or BlueFactor assessment may be appropriate where:

• the development is relatively small;
• the catchments are straightforward;
• the treatment measures are uncomplicated;
• council requests a quantitative treatment result; and
• a broader strategy or detailed model is not required.

A broader WSUD response may be appropriate where:

• water management needs to be integrated across the site;
• the design includes several complementary initiatives;
• council requests a written design response;
• landscape and drainage integration is important;
• maintenance commitments need to be documented; or
• the treatment result forms only one part of the submission.

Some developments require both: a WSUD strategy explaining the design and a quantitative assessment demonstrating its treatment performance.

How Certified Energy Can Help

Certified Energy can review the available plans and council correspondence to identify the stormwater treatment assessment required for a Victorian project.

For suitable developments, this may include:

• measuring site and impervious areas;
• identifying runoff catchments;
• reviewing proposed rainwater tanks and reuse demands;
• assessing raingardens and permeable surfaces;
• testing the combined treatment performance;
• identifying practical ways to improve the result;
• documenting the assessment assumptions;
• coordinating treatment commitments with the plans; and
• clarifying where broader WSUD, MUSIC, drainage or engineering input may be required.

The purpose of the assessment is to provide clear evidence of stormwater treatment performance without presenting it as a substitute for a broader water-management or drainage scope.

Explore the STORM Assessment Knowledge Hub or send through the current plans and council request for an initial project review.

Frequently Asked Questions

What is the main difference between STORM and WSUD?

WSUD is the broader approach used to integrate water management into the design of a development. STORM is a quantitative assessment used to measure the treatment performance of selected runoff catchments and measures.

Is STORM part of WSUD?

A STORM assessment can support the stormwater treatment component of a WSUD response. It does not necessarily represent the complete WSUD strategy.

Is a STORM report the same as a WSUD report?

Not always. A STORM report usually focuses on areas, catchments, treatment inputs and the resulting performance score. A WSUD report may cover a broader integrated site response.

Can a project have WSUD without STORM modelling?

Yes. Whether a quantitative assessment is required depends on the project and council requirements. Another modelling method may also be used.

Can a project achieve a STORM rating without good WSUD design?

A project may achieve the numerical benchmark through selected measures, but the result should still be supported by a practical, coordinated and maintainable site design.

Which WSUD measures improve a STORM rating?

Rainwater tanks, raingardens, biofiltration, permeable paving and suitable infiltration measures may contribute where they are properly sized and connected to relevant runoff catchments.

Is BlueFactor the same as WSUD?

No. BlueFactor is an assessment tool for suitable small developments. WSUD is the broader water-sensitive planning and design approach.

Is MUSIC the same as WSUD?

No. MUSIC is a modelling platform that can test more complex catchments and treatment trains. It may be used to support a WSUD strategy.

Does STORM replace drainage design?

No. STORM addresses stormwater treatment performance. Drainage collection, pipe capacity, detention and discharge may require separate civil or hydraulic design.

Can Certified Energy help determine which report is needed?

Yes. The available plans, council request, planning controls and project complexity can be reviewed to identify whether a focused treatment assessment or a broader stormwater scope is appropriate.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• What Is a STORM Assessment?
• When Is a STORM Assessment Required in Victoria?
• What Does a STORM Report Include?
• How Is a STORM Score Calculated?
• STORM vs MUSIC: Which Assessment Does Your Project Need?
• Can Rainwater Tanks Improve a STORM Rating?
• How WSUD Features Help Improve a STORM Rating
• STORM Report vs Stormwater Management Plan
• STORM Assessment vs Drainage Design