Executive summary WSP has been engaged by Heritage Minerals to update the Dam Break Assessment and Consequence Category Assessment for the proposed new Sandstone 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 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 3 embankment layout — Sunny Day piping failure of the Stage 5 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, Science and Innovation (DESI) [3]. The results are summarised in Table ES.1.
Table ES.1
Summary results of consequence category assessment
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 was assigned as ‘High A’ for Stage 1 and ‘ Extreme ’, for Stages 3 and 5, based on the ANCOLD [2] criteria. According to the DESI [3] criteria, the Dam Break consequence is ‘ High ’ for all stages. The Overtopping/Environmental Spill and Seepage consequence categories were rated as ‘Low’ under both ANCOLD [2] and DESI [3] for all scenarios. 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 Open Cut Pit (OCP) and inundation of the Mundic Red
Project No PS213278 Sandstone Gully TSF Dam Break and Consequence Category Assessments Mount Morgan Gold Mine Heritage Minerals
WSP January 2025 Page iii
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