There is no proposal to construct or operate the Sandstone Gully TSF during the operational life of the Upper Mundic Gully TSF. If it is approved as a feature of the EA, Heritage Minerals will construct the Upper Mundic Gully TSF as part of primary operations for the site. The Sandstone Gully TSF will remain an authorised structure under the EA; however, it will not be constructed as part of primary stages of operation. Any future construction of the Sandstone Gully TSF will require updated assessments to be undertaken, including a reassessment of consequence categories to account for both structures constructed on site. 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 34. Flood severity (based on the depth/velocity product) at that intersection was rated at the H6 category, which indicate the potential for embankment erosion. In the worst-case scenario, if hypothetically both TSFs were in operation (as noted above, this is not planned to occur) 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 34. 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. This consideration is relevant only in the event of concurrent operation of both structures. Again, Heritage Minerals does not intend to operate both structures concurrently, and any risk and impact will be addressed in the future design process required before construction of the Sandstone Gully TSF can proceed.
Project number: 25B061
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