— Flood severity: potential flood hazard classification as described in Section 2.5
— Arrival time: time required for the flood wave to reach any depth at any location along the modelled extents.
5.2.2
Summary of simulation results
5.2.2.1
Embankment Stage 1 hydraulic modelling results
The OCP is the primary receptor of breach outflows in all scenarios, with these being heavily attenuated and controlled by its spillway discharge. The relatively smaller outflow volume discharged for Scenario 1 (about 31% of the total impounded volume, as presented in Section 4.1) was conveyed through the site drainage system and released to Dee River, completely contained within the natural channel banks. Minor crossings through unsealed site access roads and within the urban perimeter are expected to be impacted by the incoming flows.
5.2.2.2
Embankment Stage 2 hydraulic modelling results
A dam break for Scenario 2 would primarily impact the site infrastructure immediately downstream to the OCP. The outflow discharge exceeds the spillway capacity and overflows are expected to impact the Process Plant, ROM pad and the No.2 Mill Tailings operational areas. 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 (as per Section 2.5) to be concentrated in these areas. Once the discharge reaches the urban areas, a few houses along the Dee River floodplains are impacted by the flooding extents. It is noted that flooding impacting the buildings located within both the mine lease and urban areas was classified as H4 and below, indicating that these buildings are likely to withstand the damage. 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 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP JUly 2025 Page 21
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