Executive summary
WSP Australia Pty Ltd (WSP) has been engaged by Heritage Minerals Pty Ltd (Heritage Minerals) to undertake a Dam Break Assessment and Consequence Category Assessment for the proposed new Upper Mundic Gully Tailings Storage Facility (TSF) at their Mt Morgan Mine. The aim of the study is to estimate the potential flooding impacts that might occur in the event of a catastrophic failure of the proposed facility and assign it a consequence rating.
The following dam break scenarios were identified as critical in terms of consequence and adopted in the assessment:
— Sunny Day piping failure of the Stage 1 embankment layout
— Sunny Day piping failure of the Stage 2 embankment layout
The hydraulic modelling was undertaken using the TUFLOW software package. Resulting flood inundation results maps are provided in Appendix A. The consequence category for failure has been assessed based on the methods outlined by the Australian National Committee of Large Dams (ANCOLD) [1] [2] and the Queensland Department of Environment, Tourism, Science and Innovation (DETSI) [3]. The results are summarised in Table ES.1.
Table ES.1
Summary results of consequence category assessment
Guideline
Consequence Category
Stage 1
Stage 2
ANCOLD – Dam Break
Significant
High B
ANCOLD – Environmental Spill
Low
Low
DETSI – Dam Break
Low
Significant
DETSI – Failure to Contain (Overtopping)
Low
Low
DETSI – Failure to Contain (Seepage)
Low
Low
The dam break consequence classification of Upper Mundic Gully TSF is ‘Significant’ and ‘High B’ , based on the ANCOLD [2] criteria, and ‘ Low ’ and ‘ Significant ’ based on DETSI [3], for Stage 1 and Stage 2 respectively. Overtopping/Environmental Spill and Seepage Consequence Categories were considered to be ‘ Low ’ for both ANCOLD [2] and DETSI [3] for all stages. WSP understands that Heritage Minerals plans to construct another tailings dam, Sandstone Gully TSF, within Sandstone Gully and the OCP, also as part of the re-mine works, according to the ongoing Definitive Feasibility Study (DFS) by WSP [4]. It is recommended that this dam break assessment be revisited in further studies to include the final proposed Sandstone Gully TSF footprint in the hydrodynamic models, to review depths and velocities over its embankment and evaluate risks and potential for a cascade failure. Also, it is expected that the construction of this second TSF will reduce the OCP attenuation capacity, which may impact on the routed outflows and consequently the downstream receptors.
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP JUly 2025 Page iii
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