5.3 Dam Break and Consequence Category Assessment A dam break assessment (DBA) and consequence category assessment (CCA) for the proposed Upper Mundic Gully TSF has been undertaken with results summarised in Table 22. A copy of the report is provided in Appendix C. Table 22. Summary of results for 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 The consequence classification of Upper Mundic Gully TSF is ‘Significant’ and ‘High B’ based on the ANCOLD criteria and ‘Low’ and ‘Significant’ based on the DETSI criteria for Stage 1 and Stage 2 respectively. The overtopping/ environmental spill and seepage consequence categories were considered to be “low’ for both ANCOLD and DETSI whilst the seepage consequence category was assessed as Significant. Significant The CCA and DBA for the Upper Mundic Gully TSF have been undertaken on the basis that the Upper Mundic Gully TSF and Sandstone Gully TSF will not be constructed or operated concurrently. The Upper Mundic Gully TSF will be constructed, operated and had rehabilitation commenced prior to any construction of the Sandstone Gully TSF. On that basis, a cascade scenario involving both structures simultaneously does not represent a credible worst-case scenario for the purposes the current assessment, and for the purposes of the Manual, as at the time of assessment the proposed Sandstone Gully TSF is not another dam on site. Should any change to project staging occur that results in both structures being operationally concurrently, the CCA and DBA will be updated accordingly and submitted to the administering authority for review. Failure mechanisms Potential failure modes that could result in an embankment failure were identified as credible for the Upper Mundic Gully TSF, as summarised in Table 23. The failure modes selected for further assessment are based on current industry guidelines and on engineering judgement. Table 23. Summary of Failure Mechanisms Assessed 5.3.1
Failure Mode
Rationale for the selection of failure mode
The design criteria proposed for the Upper Mundic Gully TSF defines the water storage capacity as the greater between the Design Storage Allowance (DSA) and the 120-hour PMP volume for both stages. Considering the significant storage containment allowance, overtopping failure was not considered a credible failure scenario Seepage through the embankment can occur due to chemical breakdown of the liner or defective liner material or construction, and failure of the under liner collection system. Consequently, piping failure driven by a high hydraulic gradient along soil/pipe interfaces in the embankment should be considered.
Overtopping
Piping (internal erosion)
Project number: 25B061
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