7
Potential impacts of future deposition expansion
WSP understands that Heritage has plans to increase the tailings storage capacity by constructing an adjacent TSF in Sandstone Gully. A DBA and CCA for the Sandstone Gully TSF have already been undertaken by WSP [31], assigning an overall Consequence Category of Extreme for the final embankment stage. In an expansion scenario, where both TSFs are constructed and used, the results of the DBA and CCA of Upper Mundic Gully TSF may be impacted because of the Sandstone Gully TSF. The current failure extents for both stages of the Upper Mundic Gully TSF indicate a potential for a portion the discharged outflow to interact with the location where the proposed Sandstone Gully TSF is to be constructed, as indicated in Figure 7.1. Flood severity (based on the DV product) at that intersection was rated at the H6 category, which indicate the potential for embankment erosion. In the worst-case scenario, where erosion progresses into a breach initiation, cascade failure of Sandstone Gully TSF would become a credible failure mode, and the overall Consequence Category of Upper Mundic Gully TSF could be elevated to “Extreme” (matching Sandstone Gully TSF).
Figure 7.1
Potential interaction of the Upper Mundic Gully TSF failure discharges with the proposed Sandstone Gully TSF embankment
Additionally, as the Sandstone Gully TSF embankment is proposed to be constructed over a significant portion of the OCP void, the Upper Mundic Gully TSF dam break discharges are expected to be less attenuated within its reduced footprint, resulting in increased spills to the downstream site infrastructure and the urban perimeter of the township of Mt Morgan. 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 further evaluate the risks/credibility of embankment erosion and the potential for a cascade failure and a consequent CCA re-assignment.
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP JUly 2025 Page 37
Powered by FlippingBook