MUSIC Modelling Explained: Council Stormwater Compliance

What is MUSIC and why does council want one?

MUSIC — the Model for Urban Stormwater Improvement Conceptualisation, developed by eWater — is the standard software NSW councils use to assess whether a development’s stormwater treatment strategy meets water quality targets. If your consent conditions or DCP require demonstration of pollutant load reductions, what the council is almost always asking for is a MUSIC model and a summary report.

MUSIC is a continuous simulation tool: rather than analysing a single design storm, it runs years of historical rainfall through your site at six-minute or hourly steps, generating runoff and pollutant loads and routing them through your proposed treatment measures. The output that matters is simple — the percentage reduction in mean annual pollutant load between the developed site without treatment (“source”) and with treatment (“residual”).

The targets you are modelling against

Each council sets its own pollutant reduction targets, usually in its DCP or WSUD guideline. Commonly adopted targets across NSW — which vary by council, so always confirm locally — are reductions in mean annual load of around:

• 90% for gross pollutants
• 85% for total suspended solids (TSS)
• 65% for total phosphorus (TP)
• 45% for total nitrogen (TN)

In practice, total nitrogen is nearly always the binding constraint. Litter baskets and separators dispatch gross pollutants and TSS comfortably, but nitrogen is substantially dissolved and only yields to bioretention, wetlands or engineered media filtration — which is why the TN line in the results table usually decides how big your treatment system must be.

How a MUSIC model is built

Source nodes

The site is split into catchments represented by source nodes — typically urban source nodes subdivided by surface type (roof, road/paving, landscape). Each node carries an area, an impervious fraction and pollutant generation parameters. Councils commonly prescribe these inputs: which rainfall station and period to use, soil parameters and pollutant concentration values. Using the council’s prescribed parameters, not the software defaults, is the first thing an assessing engineer checks.

Treatment nodes

Your WSUD measures are represented by treatment nodes connected downstream of the sources:

• Rainwater tank nodes — model retention and reuse; the reuse demand (toilets, laundry, irrigation) drives the benefit
• Bioretention nodes — defined by filter area, extended detention depth and media properties
• Gross pollutant trap nodes — capture litter and coarse solids
• Generic nodes — used for proprietary devices (cartridge filters, separators), configured with transfer functions or k-C* parameters reflecting the device’s verified performance. Many councils now require SQIDEP verification before accepting proprietary performance claims, and policies on proprietary devices vary by council.

Nodes are linked in the same order water actually flows — tank before rain garden, basket before cartridge vault — so the model mirrors the real treatment train.

MUSIC-link: compliance checking built in

MUSIC-link is an eWater feature that embeds a subscribing council’s specific requirements — rainfall data, source node parameters, node configuration rules and targets — directly into the software. With the council’s profile loaded, MUSIC-link validates the model against those rules and generates a standardised compliance report showing whether each target is met. Many NSW councils now require lodgement of the MUSIC-link report (and often the model file itself) with the DA or CC documentation. Where a council doesn’t subscribe, its published modelling guideline fills the same role and must be followed manually.

What assessors reject — and how to avoid it

• Wrong parameters: default rainfall or pollutant values where the council prescribes its own
• Mismatch with drawings: a 12 m² bioretention node when the landscape plan shows 6 m²
• Unjustified proprietary nodes: generic nodes with optimistic parameters and no verification evidence
• Implausible reuse: tank demand figures that overstate occupancy or irrigation
• Missing bypass logic: treatment claimed on flows the device could never physically treat

A model that survives assessment is one where every number traces to either a council requirement, a drawing or verified device data.

Contrive Consultants designs WSUD systems and prepares MUSIC and DRAINS models accepted by NSW councils, including MUSIC-link compliance reports. Explore our services, call +61 497 848 111, email info@contriveconsultants.com.au or contact us to get your model lodgement-ready.