Low
Low
It is expected that the outflow will reach areas of MSES, which may cause some impact to the existing ecosystem, but the area of damage is expected to be less than 1 km 2 .
It is expected that the outflow will reach areas of MSES, which may cause some impact to the existing ecosystem, but the area of damage is expected to be less than 1 km 2 .
General environmental harm
Low
Significant
Harm to third-party assets in the failure path is expected to require less than $1 million or in rehabilitation, compensation, repair or rectification costs.
Harm to third-party assets in the failure path is expected to require $1 million or greater (but less than $10 million) in rehabilitation, compensation, repair or rectification costs.
General economic loss or property damage (2)
Overall consequence classification
Low
Significant
(1)
The PLL was calculated without the mining personnel.
(2) General economic loss or property damage does not include the holder’s own mine production, on-site industrial or commercial assets or infrastructure solely for servicing the dam owner.
6.3
Environmental Spill/Failure to contain (Overtopping) Consequence
As discussed in Section 3.1, 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 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.
6.3.1
Population at risk
No PAR is expected for both embankment stages, as overtopping is anticipated to be heavily attenuated in the OCP. It will then be conveyed via the spillway, with part of the flood going southeast and the rest expected to be fully contained within the banks of the Dee River
6.3.2
Potential loss of life
Similarly to PAR, no PLL is expected for both embankment stages.
6.3.3
Environmental considerations
Environmental spills from the Upper Mundic Gully TSF are expected to be heavily attenuated within the OCP, triggering a discharge of the excess water though its spillway. This outflow is expected to be contained within the Dee River banks and the environmental impacts are expected to be low for both embankment stages. Note that the concurrent rainfall event causing the spill is expected to dilute the TSF decant discharge into the OCP, with further dilution anticipated within Dee River from local catchment stormwater runoff.
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP JUly 2025 Page 31
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