EGL Ref: 9216 22 June 2022 Page 2 File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. in 2020 (Ref. 2). The dam breach assessment and its results are also required to meet requirements 2.3 and 2.4 of the Global Industry Standard on Tailings Management (GISTM, Ref. 3) that was published in August 2020. The NZDSG (Ref. 1) identifies three levels of assessment for undertaking dam break assessments (initial, intermediate, and comprehensive). A comprehensive assessment is typically required for dams that have high failure consequences which is the case for Storage 3. A comprehensive tailings dam breach study usually requires the identification and consideration of potential failure modes, assessment of tailings characteristics, estimation of outflow volume, dam breach flood routing, mapping of the extent of flood inundation and evaluation of the peak flood depth, flow velocity, time of flood arrival, time of flood peak, and inundation duration at key locations. The CDA Technical Bulletin on TDBA (Ref. 2) is a guidance document prepared by the CDA for tailings dam breach analyses. It is considered to represent the current best practice and is a referenced document in the GISTM. The GISTM was published in August 2020 (Ref. 3). It provides a framework for safe TSF management while affording operators flexibility as to how best to achieve this goal. Requirements 2.3 and 2.4 in the GISTM cover dam breach analyses. A comprehensive dam breach analysis undertaken in accordance with the NZDSG (Ref. 1) and the CDA Technical Bulletin on TDBA (Ref. 2) will satisfy the GISTM requirements. 1.2. Limitations The modelling undertaken for this dam breach assessment uses source data and assumptions that will have differences from modelling undertaken for the management of the flood hazard in the area. Any comparison of maps must consider the differences. The purposes of the Storage 3 dam breach assessment are specific for the assessment of PIC and emergency action planning. This is different to the purposes of flood hazard modelling. Specifically, the dam breach assessment uses Lidar ground level information which may differ from actual levels of the ground and stopbanks. This means that in an actual stopbank full scenario in Paeroa, the actual overtopping locations may differ from those shown on dam breach maps and these should not be relied on for any other purpose other than assessing the dams PIC and developing an EAP. It is normal to review and update the PIC of a dam, including revising inundation maps to account for new information, every five years or whenever modifications to dams or their operational procedures could result in change to downstream consequences of a potential dam failure. 2.0 WAIHI TAILINGS STORAGE FACILITIES At the Waihi Operation, there are two existing TSFs, Storage 1A and Storage 2. As part of the Life of Mine, Storage 1A is proposed to be raised to RL182, and Storage 2 to RL160.7. Both TSFs are downstream construction, except for the proposed 4.7 m centreline construction raise on Storage 2. As of June, 2021, Storage 1A has a minimum crest level of RL173.6 and Storage 2 has a minimum crest level of RL156.4. The location of Storage 3 relative to other site facilities is shown in Figure 1. A site plan of Storage 1A, Storage 2, and Storage 3 is shown in Figure 2. A breach of Storage 3 could result
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