Background and Challenges

Over the past decade, Bagdad and Mangalore have experienced significant flooding, with events estimated between 5% to 20% Annual Exceedance Probability (AEP). These floods have inundated streets and properties, isolating the towns and causing substantial damage. In January 2023, the Southern Midlands Council engaged Flussig Engineers to conduct detailed flood studies and complete a Floodplain Management Study. This included developing hydrologic models for the Bagdad Rivulet catchment and a hydraulic model for the Bagdad and Mangalore townships.

Study Methodology and Key Findings

The study identified parts of Bagdad and Mangalore as communities at flood risk, using typical flood profiles adopted across Tasmania. The models were developed and calibrated against the 2016 flood event, employing ARR2019 hydrological inputs and running the hydraulic model using the InfoWorks ICM engine.

Major outcomes of the study include:

• 1% AEP Flood Event Modelling: Detailed calibration and assessment of the 1% AEP flood event.
• Utilisation of New LiDAR Data: Incorporating new LiDAR data from 2019 to better represent current flood behaviour.
• Local Inflow Representation: Accurate modelling of local inflows from small sub-catchments around Bagdad and Mangalore, using ARR2019 guidelines.
• Road System Crossings: Modelling all local catchments reporting to road system crossings, reflecting current best practices and updated survey data.

The study also involved site visits to assess the condition of local waterways and culvert crossings, leading to significant updates in the model:

• Updated Inflow Representation: Improved accuracy in model inflow representation.
• Inclusion of Road and Rail Culverts: Confirmed through site inspection.
• Refined Grid Size: Reduced from 6m to 1m for enhanced precision.
• Further Routing Considerations: Detailed routing for the Bagdad Rivulet and Horfield Creek waterways.

Detailed Calibration and Design Event Analysis

Expert hydrologists conducted a thorough calibration assessment to estimate rainfall distribution over the local catchments of the Bagdad Rivulet. The revised model was run for various design events, including the Defined Flood Event (DFE) for the Midlands Council (Q100+Climate Change), to estimate the cost of damage for events ranging from 20% AEP to 1% AEP and the PMP event.

Community Engagement and Mitigation Planning

Flussig Engineers held a meeting with Southern Midlands Council representatives to discuss the study’s findings and potential mitigation options. This included identifying mitigation responsibilities for private and public properties and road culvert crossings.

Addressing Community Concerns

Although not identified as a specific flooding issue, the condition of watercourses crossing under the Midland Highway and some local road drainage remain key community concerns. Potential safety hazards from water and sediment on the roadways were highlighted and conveyed to relevant stakeholders, with expected discussions between the Southern Midlands Council and the Department of State Growth to address these issues.

Conclusion

The Flood Inundation Study by Flussig Engineers marks a significant achievement for Bagdad and Mangalore, providing the Southern Midlands Council with vital data and insights for informed flood risk management. This study enhances community resilience and lays the groundwork for effective flood mitigation strategies, ensuring a safer future for these towns.

For further details on the study and to access flood risk maps, please contact us. Together, we can build a safer and more resilient community.