6.3.1 Population at risk No PAR is expected, as overtopping is anticipated to be heavily attenuated in the open pit. 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.
6.3.3 Environmental considerations Environmental spills from the Sandstone 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. 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.
6.3.4 Consequence Classification
6.3.4.1 ANCOLD Table 6.10 presents the Consequence Category assessment for Sandstone Gully according to ANCOLD for the “Environmental spill” Scenario. Table 6.10 Consequence Category assessment for Sandstone Gully TSF– ANCOLD (Environmental spill)
Item
Response
Minor Minimal repairs are expected since there is no risk of a dam break. The repairs are more related to correcting erosion and addressing minor issues in the embankment.
Total infrastructure costs
Minor Some restrictions.
Impact on dam owner’s business
Minor No dwellings, roads or any other third-party assets are expected to be harmed. However, it is expected that the discharged outflows cause minor impacts (less than $1 million in rehabilitation/compensation) to farm supply/use downstream. Minor It is expected that the spills from TSF will discharge in the OCP and will be contained in Dee River banks when spills from the OCP.
Health and social impacts
Environmental impacts
Overall consequence classification
Low
6.3.4.2 DESI
Table 6.11 presents the Consequence Category assessment for Sandstone Gully according to DESI for the “Failure to Contain – Overtopping” Scenario.
Project No PS213278 Sandstone Gully TSF Dam Break and Consequence Category Assessments Mount Morgan Gold Mine Heritage Minerals
WSP January 2025 Page 32
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