EGL Ref: 9216 22 June 2022 Page 8 File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. regression models in Section 10.5.1 and are summarised in Table 2. The volume of released material in Process I was estimated to be the total volume of supernatant water (i.e., 0.24 Mm3) and an equal volume of eroded tailings (i.e., 0.24 Mm3). The breach bottom elevation on the upstream face of the embankment for TDBA Process I, was taken to be the same as the Process II breach geometry shown in Figure B1. An elevation-storage relationship was derived to represent the volume change from the breach bottom elevation to the normal operation decant level. It is shown in Figure 3. The dam breach outflow hydrographs were derived by using HEC-RAS. 10.5.2.2. Rainy Day Breach Scenario For the Rainy Day breach scenario, the potential failure mode is assumed to be overtopping and progressive erosion. The dam breach outflow hydrograph is governed by the erodibility of the dam. Breach parameters were determined by using the regression models in Section 10.5.1 and are summarised in Table 3. Only the Von Thun and Gillete (Ref. 16) and Xu and Zhang (Ref. 17) regressions were considered, as these two regression models account for the effects of embankment erodibility. The volume of release material in Process I was estimated to be the total volume of supernatant water (i.e., 1.194 Mm3) and eroded tailings for Process I (i.e., 0.786 Mm3). The breach bottom elevation on the upstream face of the embankment for TDBA Process I, was taken to be the same as the Process II breach geometry shown in Figure B1. The elevation-storage relationship for the cone of depression shown in Figure 3 was adopted to represent the volume change from the breach bottom elevation to the crest level of the embankment. The dam breach outflow hydrographs were derived by using HEC-RAS. 10.5.2.3. Adopted Outflow Hydrograph A suite of breach outflow hydrographs for the Sunny Day and Rainy Day breach scenarios are shown in Figures 4 and 5, respectively. The maximum breach outflows are associated with the model of Von Thun and Gillete (Ref. 16) for both of the Sunny Day and Rainy Day conditions. The maximum associated outflow hydrographs have been used in the runout analysis to assess the extent of inundation. 11.0 RUNOUT ANALYSIS A two-phase approach was adopted in the TDBAs of Storage 3 for the runout analysis. The runout analysis of Process I water flood including entrained tailings was modelled as a water breach and the runout analysis of Process II of deposited materials was estimated using 3D volumetric modelling. The analyses are discussed in the following sections. 11.1. Process I 11.1.1. Flood Routing Tool The downstream inundation due to the TDBA Process I was modelled by the computer program HEC-RAS 2D (Ref. 11). This 2D flood modelling program was developed to model the hydraulics of water and debris/mudflow over alluvial fans, in channels and flood plains. The model uses the full dynamic wave momentum equation and a central
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