7 Conclusion The Dam Break results for the STSF 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 results are summarised in Table 7.1.
Table 7.1
Summary results of Consequence Category assessment for STSF
Guideline
Consequence Category
ANCOLD [2]
Stage 1
High A
Dam break
Stage 3
Extreme
Stage 5
Extreme
Environmental Spill
All
Low
Stage 1
High A
Overall consequence category
Stages 3 to 5
Extreme
DESI [3]
Stage 1
High
Dam break
Stage 3
High
Stage 5
High
Failure to Contain (Overtopping)
All
Low
Failure to Contain (Seepage)
All
Low
Overall consequence category
All
High
The Dam Break Consequence Category of Sandstone Gully TSF is considered to be ‘High A’ , based on the ANCOLD [2] criteria, for Stage 1; and ‘ High ’ based on the DESI [3]; and ‘ Extreme ’ based on the ANCOLD [2] criteria; and ‘ High ’ based on the DESI [3] for Stage 3 and Stage 5. The Overtopping/Environmental Spill and Seepage Consequence Category is considered to be ‘Low’ for both ANCOLD [2] and DESI [3]. It should be noted that the ANCOLD dam break Consequence Category for Stage 3 of ‘Extreme’ is largely driven by overtopping along a section of the southern perimeter of the OCP and inundation of the Mundic Red Oxide tailings area. It has been assumed that mining operations in the Mundic Red Oxide tailings area would coincide with the Sandstone Gully TSF Stage 3 onwards. PLL and hence the Consequence Category could be revised for Stage 3 if there is a change in Mundic Creek mining schedule. Alternatively, the flow into Mundic Creek could be prevented by raising the southern perimeter of the OPC to avoid overtopping into Mundic Red Oxide tailings area, thereby reducing PLL. The required raising is estimated to be in the order 3.0 m (Stage 3) in height along a section of the southern perimeter of the OCP. For Stage 5, the impacts on Mount Morgan township are significantly greater. Despite efforts to avoid impact to the Mundic Red Oxide tailings area, the consequence is likely to remain classified as Extreme for Stage 5.
Project No PS213278 Sandstone Gully TSF Dam Break and Consequence Category Assessments Mount Morgan Gold Mine Heritage Minerals
WSP January 2025 Page 37
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