5.1.2 Summary of simulation results The OCP is the primary receptor of breach outflows in all scenarios, with these being heavily attenuated and controlled by the pit spillway discharge. All modelled scenarios indicate that a dam break would primarily impact the township of Mt Morgan and the downstream environment along the Dee River floodplains. The maximum depths and velocities were observed immediately downstream of the OCP and natural flow paths towards Dee River, resulting in the highest hazard risk classification to be concentrated in these areas. — Scenario 1 - the Stage 1 embankment failure impacted site infrastructure, specifically the Process Plant, ROM pad, and No. 2 Mill Tailings operational area — Scenario 2 and 3 - in addition to the impacts observed in Scenario 1, part of the outflow volume also causes the southern crest of the OCP to overtop (overflow depth of ~1.5 m for Stage 3 and ~4.5 m for Stage 5 failures), impacting the Mundic Red Oxide Tailings Area. This additional flow path directed this fraction of the outflows through the mine site and into the Dee River downstream of the township of Mt Morgan. Further downstream and outside of the urban perimeter of Mt Morgan, the results indicate the discharges are mostly contained within the banks of Dee River, satisfying the stopping criteria discussed in Section 2.6.
Project No PS213278 Sandstone Gully TSF Dam Break and Consequence Category Assessments Mount Morgan Gold Mine Heritage Minerals
WSP January 2025 Page 22
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