EGL Ref: 9216 22 June 2022 Page 10 File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. Sources 1 to 16 as shown in Figure A1 in Appendix A. The inflow hydrographs of the natural flows are summarised in Figure A2 in Appendix A. The temporal pattern from Tomlinson and Thompson (1992, Ref. 19) for short duration rainfall events was considered in the routing of natural flows. The model was calibrated against the Flood Frequency Tool (Ref. 20) of National Institute of Water and Atmospheric Research (NIWA), which provides a comprehensive assessment of flood hazard across New Zealand rivers and streams. The peak natural flows of downstream watercourses from the modelling are consistent with the best estimates as given in the NIWA’s Flood Frequency Tool. Analyses were undertaken to determine the incremental consequences of a dam breach with different downstream watercourse flood flows i.e., 1 in 100, 1 in 1,000, 1 in 10,000 AEPs, and the PMF. The greatest incremental consequences were with the 1 in 1,000 AEP flood event. This was adopted as the Rainy Day breach scenario for the PIC assessment. Flood inundation map for the 1 in 1,000 AEP flood event has been prepared for the assessment of incremental effects (refer to Figure A1 in Appendix A). 11.2. Process II 11.2.1. Volume Balance Analysis The volume of tailings and embankment materials eroded and deposited immediately downstream of the Storage 3 embankment in Process II depends on the breach elevation, the profile of the displaced tailings and the topography of the ground downstream of the breach. The volume of deposited materials immediately downstream of the TSF embankment was estimated by comparing the volume balance according to the equation below: V _ + = + _ where: - V _ is the volume of mobilised tailings, which is the sum of eroded tailings in Process I water flood and the liquefied tailings in the Process II mud flood; - is the embankment fill eroded; - _ is the volume of eroded tailings in Process I as calculated in Section 10.4; and - is the volume of materials deposited downstream. The deposition maps for Process II are determined using 3D volumetric modelling with the assumed angle of repose and dam breach parameters as discussed below. 11.2.2. Angle of Repose for Deposited Tailings Materials Historic breaches of TSFs indicate that only a portion of the tailings is released. This is attributed to the fact that unlike water, tailings have shear strength. The released tailings will usually come to rest at an angle which is dependent on the residual strength of the tailings. In addition, observations indicate that the slope of the tailings remaining within the TSF is much steeper than the angle of repose of the breached tailings (Ref. 21).
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