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 volume capacity of both assessed stages of the proposed TSF, a flood induced overtopping failure was deemed as non-credible. 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, the initial water level for each stage was selected considering containment of the 120-hour PMP volume. As a conservative assumption, initial water level at 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.
Table 3.2
Summary of considered failure scenarios
Initial water level (1) (RL m)
Downstream weather condition
Embankment
Failure mechanism
Sunny day (no concurrent rainfall)
Stage 1
Piping
358.2
Sunny day (no concurrent rainfall)
Stage 2
Piping
388.6
(1) The water level was defined as the120-hour PMP containment volume.
Project No PS213278 Upper Mundic Gully TSF Stage 1 and 2 DBA and CCA Mount Morgan Gold Mine Heritage Minerals
WSP JUly 2025 Page 11
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