7 The DBA results presented in this report provide a visual interpretation of the potential flood inundation and severity, which are required to establish the PAR, PLL and environmental impacts as inputs to a Consequence Category classification. There are though several uncertainties when evaluating hypothetical dam failures. Relevant industry guidelines, references, site-specific data, past assessments and engineering judgment contributed to provide a general understanding of potential impacts in the event of a dam break. The CCA results for each embankment stage are summarised in Table 7.1.
Table7.1
Summary results of consequence category assessment
Guideline
Consequence Category
Stage 1
Stage 2
ANCOLD Dam Break
Significant
High B
ANCOLD Environmental Spill
Low
Low
DETSI Dam Break
Low
Significant
DETSI Failure to Contain (Overtopping)
Low
Low
DETSI Failure to Contain (Seepage)
Significant
Significant
The Consequence Category classification of Upper Mundic Gully TSF is Significant and High B , based on the ANCOLD (2012) criteria, and Low Significant based on DETSI (2025), for Stage 1 and Stage 2 respectively. The Overtopping/Environmental Spill Consequence Categories was Low (ANCOLD, 2012) and DETSI (Department of Environment, Tourism, Science and Innovation, 2025). Whereas the Seepage Consequence Category was assessed as for DETSI (Department of Environment, Tourism, Science and Innovation, 2025).
7.1 Triggers for reassessment
The concept design for UMG TSF currently includes allowance to store the design flood (PMF), however, if a spillway is required, the proposed spillway for the UMG TSF will be as follows: Stage 2 spillway discharges south to Sandstone Gully (SSG), noting the concept design also includes storage allowance for the design flood. Closure spillway discharges west to Turner Creek, after placement of a suitable cover system. The timing for construction of SSG TSF is dependent on dewatering and remining of tailings within the TSF footprint. WSP understands that Heritage Minerals intends to complete construction of the SSG TSF prior to closure of UMG TSF so that tailings deposition can be moved to SSG TSF, if required. As such, there may be a period during operation where spillway discharge from UMG TSF would need to be contained within the SSG TSF. This will need to be assessed as part of detailed design studies for SSG TSF and hydraulic storage capacities updated to account for this additional volume, with a corresponding reduction in tailings storage capacity until the final stage when the design flood can be routed through the SSG TSF spillway. The UMG TSF draft CCA will be updated to reflect the above understanding of the spillway configuration and potential direction of any overtopping events.
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP May 2026 Page 36
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