GHD | Oceana Gold (New Zealand) Limited | 12537997 | Gladstone Pit TSF 42 Table 8 Wave run-up estimates Scenario Wave run-up height (m) Freeboard above maximum decant pond level (m) Overtop (y/n?) 1 1.8 2 No 2 1.0 2 No 3 4.6 5 No 5.5.2 Minimum freeboard The maximum tailings elevation is RL 1103.0 m and the minimum pit crest is RL 1107.0 m, meaning the minimum freeboard is 4 m. This freeboard accounts for the following: – Maximum operating pond from water balance: 100,000 m3 – Probable maximum precipitation (PMP): 220,000 m3 – 1 m of additional freeboard for wave run-up 5.6 Dam breach assessment Unlike a traditional embankment TSF, the GOP TSF is an in-pit storage facility with tailings stored below ground without relying on an external embankment. Therefore, no dam breach assessment was completed for the GOP TSF and is further justified based on the following: – There is no credible failure mode of the pit crest on the western side of the GOP TSF based on the stability analysis results in Section 5.3.3.2. – The high walls will be mined relatively flat above the final tailings level. Consequently, the likelihood of a landslide/rockslide failure of the pit wall is significantly reduced. – The minimum freeboard is met as described in Section 5.5.2 which accounts for the PMP, as well as waverun up from a potential landslide-generated wave. This freeboard significantly reduces the likelihood of overtopping and tailings release.
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