Ranking Pipe Blockage Risk – Elizabeth Street Catchment

Ranking Pipe Blockage Risk

The City of Melbourne engaged Rain Consulting to prepare a Hybrid Flood Management Plan for the Elizabeth Street Catchment, one of Melbourne’s most hazardous urban floodplains. As part of the broader plan, Melbourne Water funded a Blockage Likelihood and Consequence Tool Development Trial.

Above & Beyond Strategies

Predicting Pipe Blockage Consequences

Our engineers considered the following scenarios:

• Is the consequence of blocking a pipe with a high blockage likelihood outside of the flood extent similar to the pipe with the same blockage likelihood, but within the major flood path?
• What is the influence of the distribution of pipes with a similar blockage likelihood within the catchment?
• If a specific pipe gets blocked, are we expecting only a local impact or both local and more cumulative impacts downstream?

The Blockage Tool Development Trial

Rain utilised a GIS-based tool to identify and assess water pipes within and outside probable flood paths to determine the probability of blockage.

• A resulting Blockage Likelihood Score combined physical and environmental risk factors with inputs based on expert logic and available data. Factors included pipe size, pipe size reductions, bend angles, pipe slope, natural terrain slope, tree density, and special conditions.
• A series of scenarios was modelled using TUFLOW. Simulations were undertaken under rainfall intensities of 1%, 5%, and 20% AEP (Annual Exceedance Probability), with a storm duration of 1 hour. In each scenario, pipes with specific blockage scores were selectively blocked to represent varying degrees of blockage likelihood.
• The blockage likelihood categories were considered as Low, Medium and High Likelihood. A further refinement of the Medium Likelihood category allowed for a more granular understanding of blockage consequences.

Key Findings of the Study

• Pipe Blockage has more of an impact on frequent events than extreme events.
• Pipe Maintenance provides important protection when smaller events occur.
• Pipes that block in flood-prone areas generally show more consequences than pipes where there is little or no existing flooding.
• Clusters of blocked pipes have a greater impact than distributed blocked pipes. This could suggest that maintenance closer to major sources of blockage will have an immediate benefit.

Refine ongoing maintenance using the blockage approach.

The QGIS Blockage Likelihood Tool allows council teams to target maintenance where it matters most. This ensures scarce budgets are focused on pipes where failures would pose the most significant safety and property risks, keeping the community safer between storms.

Pipes within the Medium and Low Blockage Likelihood range can be addressed in descending order of blockage score and observed for flooding patterns over time.

Prioritise the highest risk areas early.

By adopting this prioritised approach, asset managers can maximise the efficiency and impact of maintenance efforts while reducing flood risks across the catchment.

The Elizabeth Street Catchment Trial recommends using the QGIS blockage likelihood tool to identify the “highest-risk pipes”. The resulting list can then be used to prioritise maintenance works and to target cleaning before forecasted storms to minimise impacts.  This planning will allow council teams to target maintenance where it matters most.

Rain’s Blockage Assessment Tool is part of a comprehensive Hybrid Flood Management Plan delivered to the City of Melbourne in April 2025. The Tool is on a ‘Short List Roadmap’ of immediate recommended actions.

Flood resilience for the Elizabeth Street Catchment is no longer a distant aspiration. The hybrid approach, the toolkit, the short list of projects, and the blockage tool together present a clear, actionable pathway.