Item
DETSI Comment
How comment has been addressed
Relevant Sections in
Supporting Information
Report
Regarding TSF Design
12
Provision of the certified CCA is required to be submitted with the formal submission Clarification that ‘stage 2’ included an assessment of overtopping and seepage assessments within the preliminary CCA
A revised CCA and Dam Break Assessment (Rev C September 2025) has been provided The failure scenarios are described in Section 3.1 of the revised CCA and DBA. Key outcomes relevant to this comment: are: • An embankment overtopping was deemed as non-credible for the proposed Upper Mundic Gully TSF, given the storage design that allows containment of a 120- hr PMP storm volume. After such an
Appendix C
13
Section 4.3.2 in full. Refer to 4.3.2.2.6 and 4.3.2.2.9 specifically Full DBA and CCA provided in Appendix C
extreme storm, a subsequent storm of a sufficient magnitude to cause catastrophic damage or failure of the dam is unlikely to occur prior to the water level being drawn down. Therefore, the environmental spill/overtopping consequence has been based on a more frequent event following a PMP storm, which is considered to be a credible failure scenario. Note, this scenario is not expected to sufficiently erode the embankment to induce a breach. It is expected that any spills would be largely attenuated by the OCP and contained in the downstream drainage infrastructure and Dee River. This has been undertaken for both Stage 1 and Stage 2 A seepage assessment has been conducted for both Stage 1 and Stage 2 with results presented in 4.3.2.2.9
•
•
14
Appendix C states WSP understands that Heritage Minerals plans to construct another tailings dam, Sandstone Gully TSF, within Sandstone Gully and the OCP... It is recommended that this dam break assessment be revisited in further studies to include the final proposed Sandstone Gully TSF footprint... to evaluate risks and potential for a cascade failure. Based on this, it is assumed Sandstone Gully TSF can interact hydraulically with Upper Mundic Gully TSF and the potential cumulative or cascading risk posed by future interactions has not been
Section 4.3.9 in the supporting report describes the scenario and potential risks. 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
Section 4.3.9
Powered by FlippingBook