How Is a STORM Score Calculated? | Treatment Performance

In Brief

The calculation begins with the areas of the proposed development. Roofs, driveways, paths, paving and other impervious surfaces are entered as runoff-generating catchments.

Each catchment is then connected to a proposed treatment measure, such as a rainwater tank, raingarden or permeable surface, or identified as untreated.

The assessment tool estimates how effectively the combined treatment strategy reduces stormwater pollutants relative to the applicable best-practice objectives. The resulting percentage is presented as the project’s STORM or stormwater treatment score.

A score of 100% is generally understood to mean that the modelled design meets the relevant best-practice treatment benchmark. It does not mean that all runoff is removed, that no pollutants remain or that the project has satisfied every drainage and planning requirement.

What the STORM Score Represents

The STORM score is a relative measure of stormwater treatment performance.

It compares the treatment achieved by the proposed development with the reference level established by Victoria’s best-practice stormwater-quality objectives.

Those objectives have traditionally included reductions in the typical urban annual loads of:

• suspended solids;
• total phosphorus;
• total nitrogen; and
• litter.

The score is therefore not a simple percentage of rainwater retained on site. It reflects how the modelled combination of catchments and treatment measures performs against the relevant water-quality benchmark.

The exact calculation is performed by the assessment tool. A project team would not normally calculate the final composite score manually.

For an overview of the broader assessment process, visit the Certified Energy STORM Knowledge Hub.

The Best-Practice Objectives Behind the Score

Stormwater treatment assessments in Victoria have historically been tested against established best-practice pollutant-reduction objectives.

These commonly include:

• 80% retention of the typical urban annual suspended solids load;
• 45% retention of the typical urban annual total phosphorus load;
• 45% retention of the typical urban annual total nitrogen load; and
• 70% reduction of the typical urban annual litter load.

The calculator assesses how the proposed treatment strategy performs relative to these objectives.

A 100% result does not mean that 100% of each pollutant is removed. It indicates that the modelled performance reaches the benchmark represented by the tool.

Step 1: Define the Assessment Area

The calculation first needs a clear assessment boundary.

For a typical development, this may be the complete title boundary or the area affected by the proposed buildings and works. The correct boundary depends on the project and the council requirement.

The total area is then divided into surface types, such as:

• building roofs;
• garages and carports;
• balconies or terraces;
• driveways;
• car parking areas;
• paths and patios;
• conventional paving;
• permeable paving;
• landscaped areas; and
• other pervious surfaces.

The areas entered into the model should reconcile with the total site or assessment area.

If impervious surfaces are omitted or underestimated, the score may not accurately represent the proposed development.

Step 2: Identify the Impervious Catchments

Impervious surfaces generate most of the runoff assessed by the tool.

These surfaces prevent rainfall from infiltrating naturally into the ground and instead direct water towards gutters, pits, drains or adjoining areas.

The assessor identifies the area of each catchment and determines where its runoff will go.

For example:

• the main dwelling roof may drain to a rainwater tank;
• a garage roof may discharge without treatment;
• a shared driveway may drain to a raingarden;
• a courtyard may use permeable paving; and
• an entry path may discharge directly to the drainage system.

This allocation is central to the calculation. A treatment measure can only improve the score for the runoff that actually reaches it.

Step 3: Assign Runoff to Treatment Measures

Once the catchments are defined, the assessor links them to the proposed stormwater treatment measures.

Common measures may include:

• rainwater tanks;
• raingardens;
• biofiltration systems;
• permeable paving;
• infiltration systems; and
• other treatment types accepted by the assessment method and council.

The assessment must reflect a credible physical relationship between the catchment and treatment measure.

For example, a roof can only be counted as draining to a tank where suitable gutters and downpipes can connect that roof to the tank. A driveway can only be assigned to a raingarden where the levels and drainage arrangement allow runoff to reach it.

Step 4: Enter the Treatment Details

Each treatment measure requires information describing its size, connection and intended operation.

The inputs vary according to the type of treatment.

Rainwater Tanks

Relevant inputs commonly include:

• tank storage capacity;
• connected roof catchment;
• number of dwellings or occupants served;
• toilet-flushing demand;
• laundry demand, where applicable;
• garden irrigation demand; and
• the landscaped area receiving irrigation.

Raingardens and Biofiltration

Relevant inputs may include:

• treatment surface area;
• connected impervious catchment;
• treatment type;
• infiltration or discharge assumptions; and
• the arrangement of the treatment within the site.

Permeable Paving

The assessment may consider:

• the permeable pavement area;
• the surface itself;
• any adjoining catchment draining onto it;
• the nominated treatment configuration; and
• site conditions affecting its operation.

The score is only as reliable as the assumptions entered into the model.

Step 5: Account for Untreated Runoff

Not every impervious surface will necessarily drain through a treatment measure.

Common untreated areas include:

• small roof sections;
• porches and verandahs;
• balconies with separate outlets;
• parts of a driveway that bypass treatment;
• isolated paths; and
• paved areas draining directly to the site outlet.

These surfaces still need to be entered into the assessment.

A high-quality result should not depend on excluding inconvenient catchments from the model. The calculation should represent the actual proposed development, including both treated and untreated runoff.

Step 6: Calculate the Treatment Performance

The assessment tool uses its internal runoff and pollutant-treatment calculations to estimate the performance of each treatment measure.

It then combines the treated and untreated catchments to determine the overall project result.

In practical terms, the final score is influenced by:

• how much impervious area is created;
• how much of that area receives treatment;
• the type of treatment used;
• the size of each treatment measure;
• the catchment connected to each measure;
• the level of rainwater reuse;
• the pollutant-removal performance attributed to the measures; and
• the amount of runoff that remains untreated.

The output is expressed as a percentage relative to the best-practice benchmark.

What Does a Score Below 100% Mean?

A score below 100% generally indicates that the modelled treatment strategy has not yet reached the reference best-practice objective.

This does not necessarily mean that the overall development has been rejected or that every element of the strategy is unsuitable.

It means that, based on the current inputs, further treatment or design refinement may be needed.

Potential responses can include:

• connecting more roof area to a rainwater tank;
• increasing regular rainwater reuse;
• revising the tank capacity;
• adding or enlarging a raingarden;
• increasing permeable paving;
• redirecting driveway runoff to treatment;
• reducing unnecessary impervious area; or
• combining more than one treatment measure.

The most appropriate change depends on which surfaces and treatment measures are limiting the result.

What Does a 100% Score Mean?

A 100% STORM score generally indicates that the modelled treatment strategy meets the benchmark represented by the assessment tool.

It does not mean:

• that all stormwater remains on the site;
• that 100% of every pollutant is removed;
• that no runoff reaches the drainage system;
• that every impervious surface is individually treated;
• that the drainage design has been approved;
• that detention requirements have been met;
• that flood risk has been assessed; or
• that every planning permit requirement is satisfied.

The score relates specifically to the modelled stormwater treatment outcome.

Read What Does a 100% STORM Rating Mean? for a more detailed explanation.

Can a Score Exceed 100%?

Depending on the assessment output, a treatment strategy may perform above the minimum reference benchmark.

This can occur where the development includes substantial treatment relative to its impervious area.

However, a result above 100% should not be pursued at the expense of a practical and coordinated design.

The treatment measures still need to:

• fit within the site;
• receive the catchments assigned to them;
• be reflected in the drawings;
• operate as assumed;
• remain maintainable; and
• be acceptable to the relevant council.

A mathematically strong result is not useful if the modelled strategy cannot be constructed or maintained.

How Rainwater Tanks Affect the Score

Rainwater tanks can improve a STORM score by capturing roof runoff and creating storage capacity through ongoing reuse.

Three inputs work together:

1. the size of the tank;
2. the roof area connected to it; and
3. the demand that regularly draws water from the tank.

A larger tank does not automatically produce a proportionally higher result.

If only a small roof area is connected, the tank may not receive enough runoff. If there is little reuse, the tank may remain full and have limited capacity to capture the next rainfall event.

The tank, catchment and reuse demand should therefore be considered as one treatment system.

Read Can Rainwater Tanks Improve a STORM Rating? for a focused explanation.

How Raingardens Affect the Score

A raingarden can improve the result by treating runoff through filtration and biological processes before the water is discharged or infiltrated.

Its modelled contribution depends on factors such as:

• the treatment area;
• the impervious catchment draining to it;
• the treatment configuration;
• the selected assumptions; and
• whether runoff can physically reach the system.

A small raingarden connected to a very large driveway may not provide enough treatment. Conversely, a reasonably sized raingarden connected to a defined catchment can make a significant contribution.

How Permeable Paving Affects the Score

Permeable paving can reduce the untreated impervious area and may provide treatment through temporary storage, filtration and infiltration.

Its effect depends on:

• the area genuinely designed as permeable;
• the pavement system selected;
• any additional runoff directed onto it;
• the underlying site conditions; and
• the way it is represented in the assessment.

The modelled area should match the paving specification and drawings. Conventional paving should not be counted as permeable solely to improve the assessment result.

Why Two Similar Projects Can Receive Different Scores

Two developments with similar site areas can produce very different results.

This may be due to differences in:

• total roof area;
• driveway and paving area;
• landscaped area;
• the proportion of runoff receiving treatment;
• tank size;
• connected roof catchment;
• number of dwellings and reuse demand;
• raingarden sizing;
• permeable surface area; and
• untreated runoff.

The score responds to the complete relationship between the development surfaces and its treatment strategy, not merely the presence of one sustainability feature.

Common Input Errors That Affect the Score

Unexpected results are often caused by incomplete or inconsistent inputs rather than the assessment tool itself.

Common issues include:

• omitting part of the driveway or paving;
• entering an incorrect site area;
• assigning an entire roof to a tank that only receives part of it;
• including reuse connections that are not shown on the plans;
• using an outdated tank capacity;
• counting conventional paving as permeable;
• overstating the area of a raingarden;
• failing to include untreated balconies or paths;
• using superseded architectural drawings; and
• double-counting a catchment across two treatments.

The model inputs should be checked against the current architectural, landscape and hydraulic documentation before the report is issued.

STORM and BlueFactor Scores

Melbourne Water’s original STORM Calculator has been replaced by BlueFactor for suitable small-scale Victorian developments.

The two tools belong to the same practical assessment territory: estimating runoff and pollutant generation and testing the treatment measures proposed for a development.

Existing permits and older council correspondence may continue to refer to a STORM score, while a new project may receive a BlueFactor result.

The terminology and exact output format may differ, so the current council requirement should be confirmed before preparing the submission.

What the Score Does Not Calculate

A STORM score is not a complete measure of every stormwater issue affecting a site.

It does not ordinarily determine:

• the required stormwater pipe diameter;
• pit locations or sizes;
• the legal point of discharge;
• on-site detention storage;
• peak discharge rates;
• overland flow paths;
• flood levels;
• finished surface levels;
• structural design of treatment assets; or
• the complete civil drainage design.

These matters may require separate hydraulic, civil or flood-related documentation.

Practical Considerations for Victorian Projects

Measure the Current Design

The calculation should use the latest architectural and landscape plans. Changes to footprints, driveways or paving can alter the score.

Include All Relevant Surfaces

Small untreated areas may appear insignificant individually but can collectively affect the overall result.

Use Achievable Treatment Measures

The assessment should not rely on a tank, raingarden or permeable area that cannot fit within the proposed development.

Coordinate Rainwater Reuse

Toilet, laundry or irrigation reuse included in the model should be documented in the relevant plans and specifications.

Check the Drainage Path

A catchment should only be connected to a treatment measure where the site levels and drainage arrangement make that connection credible.

Confirm the Required Output

The relevant council may request a STORM result, BlueFactor output, MUSIC model or another form of treatment assessment depending on the project.

How Certified Energy Can Help

Certified Energy can assess the proposed site areas and treatment measures and prepare a stormwater treatment result for suitable Victorian developments.

The assessment process may include:

• measuring roof, driveway, paving and landscape areas;
• identifying treated and untreated catchments;
• reviewing rainwater tank capacities and reuse demands;
• assessing raingardens and permeable surfaces;
• testing different treatment combinations;
• identifying why a result is below the target;
• recommending practical improvements;
• documenting the final modelling assumptions; and
• coordinating the result with the current project plans.

Where the project is too large or complex for a STORM-style assessment, the need for MUSIC modelling or separate civil and hydraulic input should be identified.

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

Frequently Asked Questions

How is a STORM score calculated?

The assessment tool models runoff from the development’s impervious surfaces, applies the performance of connected treatment measures and compares the resulting pollutant reductions with the applicable best-practice objectives.

What information affects the score?

The result is affected by site areas, impervious catchments, untreated surfaces, treatment type, treatment size, connected catchments and rainwater reuse assumptions.

What score is usually required?

A result of 100% is generally used to demonstrate that the modelled strategy meets the benchmark represented by the assessment tool. Council requirements should still be confirmed for the individual project.

Does 100% mean all pollutants are removed?

No. It means the modelled reductions reach the relevant best-practice benchmark. It does not mean that the development removes 100% of every pollutant.

Can the score exceed 100%?

Some outputs may indicate performance above the benchmark. The treatment strategy must still be practical, coordinated with the plans and acceptable to council.

Does a larger tank always improve the score?

No. Tank performance also depends on the roof catchment and regular reuse demand. Additional storage may provide limited benefit when those other inputs remain unchanged.

Can untreated areas be excluded?

No. Relevant impervious areas should be included so the assessment accurately represents the proposed development.

Can a raingarden improve the result?

Yes. Its contribution depends on its treatment area, connected catchment, configuration and whether runoff can physically drain to it.

Does permeable paving improve the score?

It may improve the score where a genuine permeable pavement system is proposed and correctly represented in the assessment.

Does the score assess flooding?

No. Flooding, detention, drainage capacity and discharge are separate matters that may require specialist engineering assessment.

Suggested Internal Links

• STORM Assessments and Reports Knowledge Hub
• What Is a STORM Assessment?
• What Does a STORM Report Include?
• What Does a 100% STORM Rating Mean?
• Can Rainwater Tanks Improve a STORM Rating?
• Common Reasons a Project Fails a STORM Assessment
• How to Improve a STORM Rating
• Do Raingardens Improve a STORM Assessment?
• Permeable Paving and STORM Compliance
• What Information Is Needed for a STORM Assessment?