8 Conclusion The DBA results presented in this report provide a visual interpretation of the potential flood inundation and severity, which are required to establish the PAR, PLL and environmental impacts as inputs to a Consequence Category classification. There are though several uncertainties when evaluating hypothetical dam failures. Relevant industry guidelines, references, site-specific data, past assessments and engineering judgment contributed to provide a general understanding of potential impacts in the event of a dam break.
The CCA results for each embankment stage are summarised in Table 8.1.
Table 8.1
Summary results of consequence category assessment
Consequence Category
Guideline
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 Category 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. The Overtopping/Environmental Spill and Seepage Consequence Categories were considered to be ‘ Low ’ for both ANCOLD [2] and DETSI [3]. As discussed in Section 7, 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 38
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