EGL Ref: 9216 22 June 2022 Page 11 File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. Released saturated tailings have been documented to come to rest at slopes of between 1º to 4º (1V:57H to 1V:14H) measured from the toe of the mobilised tailings back into the impoundment (Refs. 6 & 22). These slope angles depend on the residual shear strength of the tailings after mobilisation and the topography of the downstream area of the dam. Most of the documented breaches of TSFs are on relatively flat ground. For Storage 3 an angle of 1V:30H (i.e., 1.9º) was applied downstream of the dam based on the model of Lucia et al. (1981, Ref. 6). The eroded surfaces of breached embankments, sloping from the upstream shoulder to the downstream shoulder, have been reported to be 1V:7H to 1V:35H (Ref. 6). The slope is dependent on the material used to construct the embankment. Given that material used for the construction of the Storage 3 embankment within a 1V:3H slope of the crest is likely to be mainly rockfill, a 1V:10H grade has been adopted for this study within this extent. Further downstream a flat surface has been assumed. See Figure B1 and C1. 12.0 TDBA RESULTS AND DISCUSSION 12.1. Sunny Day Breach Scenario 12.1.1. Inundated Area The inundation map associated with the Process I water flood and the deposition map associated with the Process II mud flow for the Sunny Day breach scenario are shown in Figures B2 and B3 in Appendix B. The volume of mobilised tailings estimated to be discharged is 0.786 Mm3. Of this 0.240 Mm3 of tailings is estimated to be released as eroded tailings as part of Process I. The volume of Process II material deposited adjacent to the embankment is estimated at 0.937 Mm3 and includes the remaining mobilised tailings and the volume of the eroded embankment. The Process I water flood in the Sunny Day breach scenario will initially discharge into the Ruahorehore Stream then to the Ohinemuri River. Flow will extend up the banks of the Ruahorehore Stream in places however does not affect any houses. Only Baxter Road and the Baxter Road Bridge which is about 3.6 km downstream of Storage 3 are inundated by the flow. Further downstream, the flow will be contained and attenuated within the Ohinemuri River Channel before reaching the Waihi and Paeroa townships. Deposited mud flow (Process II) from the embankment is estimated to spread approximately 300 m from the downstream toe of Storage 3 and will not affect any houses, roads, and bridges. The volumes of tailings and eroded embankment material discharged are summarised in Table 1. The incremental effects are due to the inundation of Baxter Road and the Baxter Road Bridge and are caused by the Process I water flood. 12.1.2. Population at Risk The Population at Risk (PAR) defined in the NZDSG (Ref. 1) is the number of people who would be directly exposed to inundation greater than 0.5 m in depth if they took no action to evacuate. The PAR includes permanent populations (people in houses and buildings) and temporary populations (mine workers, farm workers, people in commercial and retail areas, recreational users, and road and bridge users). The assessment of PAR for the Sunny Day breach scenario is summarised in Appendix B. Baxter Road and the Baxter Road Bridge are likely to be affected by the Process I water flood. The
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