Melbourne has an extensive drainage network, but older drainage systems weren’t designed to cope with the larger volumes of water that now flow through our landscape. As more land is built on, more rainwater flows faster off hard surfaces instead of filtering into soils. And with climate change bringing more frequent, severe storms and rising sea levels, flood risk will continue to increase.

Importance of flood models

Flood models simulate the behaviour of stormwater and flood flows through drains and waterways, determining the areas flooded. They don’t create flood risk, but help us better understand the existing risk we face.

The information from flood modelling helps state and local government, developers, businesses and communities understand how best to minimise flood impacts. We also use it to assess land development and subdivision applications, placing conditions on planning permits if needed. This ensures all new developments are designed to protect their occupants from flooding, and that Melbourne’s drainage system functions like it should.

Understanding flood risk helps communities and agencies prepare and take action before and during a flood.

Our flood modelling program

Since the Australian Rainfall and Runoff Guidelines were updated in 2019, we’ve significantly increased our flood modelling budget.

We’ve also committed to updating waterway and stormwater flood models for every catchment across Melbourne, using the best available information to account for the impacts of climate change. Updates will include climate change assumptions forecast to the year 2100, consistent with Victorian State Planning Policy.

Climate change data used

The Australian Rainfall and Runoff Guidelines 2019 set out best practice for flood modelling in Australia, including climate change assumptions. They note that climate change will impact a number of factors considered in flood model development, including:

• rainfall intensity – the focus of current guidance
• rainfall duration
• conditions of the catchment the rain falls onto
• downstream conditions affected by sea level rise.

A planning horizon to 2100 has been selected by Melbourne Water for consistency with Victorian State Planning Policy.

Updating flood models

Updating flood models ensures current information is available to inform flood risk and build community resilience. We now have the technology to do more sophisticated modelling more regularly – in the past it would not have been possible to continually update flood models across Melbourne.

Adopting the recommendations of the independent Maribyrnong River Flood Review, we will review our flood models every 5 years and update them every 10. Any major flood within this timeframe will also trigger a flood modelling review.

October 2022 was the first major flood since the Maribyrnong River modelling was last updated. The flood itself provided an enormous amount of data that was used for our updated model, and will also help us plan and prepare for future floods.

Development and accuracy

Current flood modelling methods are very sophisticated, and use the latest computing and technology to map millions of data points throughout the catchment.

To provide confidence in the Maribyrnong River modelling results, Melbourne Water and external industry experts reviewed the approach and methodology for the hydrology (RORB) and hydraulic (TUFLOW) modelling used to develop the flood mapping, as part of a comprehensive quality assurance process.

• Jacobs Consulting, a global modelling consultant, was engaged to perform the detailed flood modelling work. This was independently peer-reviewed by HARC consulting at key development milestones.
• Both consultancies’ work was also informed by access to an independent academic panel (two leading hydrologists), enabling an additional layer of review and challenge.
• A leading flood modelling specialist, Mark Babister, also considered the model as part of the Pagone Independent Review and commented positively on its integrity at the Victorian Parliamentary Inquiry.