The proposed Stage 2 embankment layout is significantly higher than for Stage 1, with the tailings beach elevation positioned above the old spoil dump. In this scenario, displacement and mobilisation of tailings could be expected, with the conical failure surface base elevation forming over the crest of the spoil dump. The estimated cone footprint for Stage 2 is presented in Figure 4.3.
Figure4.3
Failure cone footprint Upper Mundic Gully TSF Stage 2
The outflow volumes calculated for Stage 1 and Stage 2 embankments considers the total water volume ponded above the spoil dump crest. A summary of the total released volume in each scenario is presented in Table 4.2.
Table4.2
Estimated outflow failure discharge for Upper Mundic Gully TSF
Parameter
Stage 1
Stage 2
Unit
Mm 3(1)
Total impounded volume
2.4
10.5
Mm 3
Failure cone volume
-
2.3
Mm 3
Released ponded water volume
0.75
1.3
Mm 3
Total outflow volume
0.75
3.6
Total outflow % of impounded (1) Total impoundment volume includes deposited tailings and decant pond storing the 120-hr PMP event volume. 29 34
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP May 2026 Page 15
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