5
Dam break and Consequence Category assessment
WSP has undertaken preliminary dam break and Consequence Category assessment for the proposed Upper Mundic Gully TSF [2] and this is presented in Appendix D. The assessments were undertaken for the two construction stages outlined in Section 7.5 and the results for the consequence category assessment are summarised in Table 5.1.
Table 5.1
Summary of results fo r 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 Consequence Classification of Upper Mundic Gully TSF for Dam Break is:
— Based on the ANCOLD [6] criteria: — Stage 1
Significant
— Stage 2
High B
— Based on DETSI [7] criteria: — Stage 1
Low
— Stage 2
Significant.
The Overtopping/Environmental Spill and Seepage Consequence Categories were considered to be Low for both ANCOLD [6] and DETSI. Heritage plans to construct another tailings storage facility in Sandstone Gully [8], downstream of the Upper Mundic Gully TSF. It is recommended the Upper Mundic Gully dam break assessment be revisited at that stage to include the Sandstone Gully TSF in the hydrodynamic models [8], to review flow depths and velocities and evaluate the risks for a potential cascade failure of the Sandstone Gully TSF. It is also expected that the construction of the Upper Mundic Gully Stage 2 embankment will reduce the Open Cut Pit (OCP) attenuation capacity, which may have an impact on the routed outflows and, consequently, the downstream receptors.
Project No PS213278 Upper Mundic Gully TSF Concept Design to RL 390 m Mount Morgan Project Heritage Minerals
WSP October 2025 Page 7
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