Figure 3.3: Modelled Stage 5 tailings breach
3.3 Background flood As discussed in Section 2.2, the flood day failure only assesses the incremental impact of the failure in relation to a natural flooding event. Given the total water storage capacity of the Stages 1 and 3 embankments, a flood induced overtopping failure was deemed as non-credible. Similarly, for Stage 5, the DFS report [5] includes for a spillway that will convey the PMP event, therefore an overtopping failure was also considered as non-credible for Stage 5. A piping breach could also occur during flooding conditions. However, as a larger number of mine personnel are expected within the failure path under fair-weather conditions (mining operations likely interrupted under rainy conditions), only the Sunny Day scenario was conservatively selected for this assessment. 3.4 Initial conditions ANCOLD [2] recommends that the reservoir initial condition should be taken conservatively at full supply level, regardless of prolonged lower water levels in the dam. However, considering the large freeboard adopted for design as indicated in the DFS report [5], the initial water level for Stages 1 and 3 was selected considering containment of the operational pond (2 m above the tailings beach) in addition to the 120-hour PMP volume. For stage 5, the initial water level was set at the spillway invert elevation (RL 328.5 m), 1.5 m below the crest. As a conservative assumption, the initial water level in the OCP was assumed at its spillway invert (RL 273.1 m) for all scenarios.
3.5 Overview of considered failure scenarios
The failure scenarios considered in the dam break modelling are summarised in Table 3.2.
Project No PS213278 Sandstone Gully TSF Dam Break and Consequence Category Assessments Mount Morgan Gold Mine Heritage Minerals
WSP January 2025 Page 12
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