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 (UMG) 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) (ANCOLD, 2012) (ANCOLD, 2012) and the Queensland Department of Environment, Tourism, Science and Innovation (DETSI) (Department of Environment, Tourism, Science and Innovation, 2025). The results are summarised in Table ES.1.
TableES.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)
Significant
Significant
The dam break consequence classification of Upper Mundic Gully TSF is
and
, based on the
ANCOLD (2012)
Low Significant based on DETSI (2025), for Stage 1 and Stage 2 respectively.
The Overtopping/Environmental Spill Consequence Category was Low ANCOLD (ANCOLD, 2012) and DETSI (Department of Environment, Tourism, Science and Innovation, 2025) for all stages. Whereas the Seepage Consequence Category was assessed as for DETSI (Department of Environment, Tourism, Science and Innovation, 2025). 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 (WSP, 2024). WSP understands that Heritage Minerals intends to complete construction of the SSG TSF prior to closure of UMG TSF so that tailings deposition can be moved to SSG TSF, if required. As such, there may be a period during operation where spillway discharge from UMG TSF would need to be contained within the SSG TSF. This will need to be assessed as part of detailed design studies for SSG TSF and hydraulic storage capacities updated to account for this additional volume. Furthermore, it is understood that the closure spillway for UMG TSF is proposed to discharges west to Turner Creek, after placement of a suitable cover system. 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 May 2026 Page v
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