EGL Ref: 9216 22 June 2022 Page 4 File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. meaning that failure is highly unlikely. Different failure modes each have different scenarios which lead to failure and different scenarios that may occur during breach. The term ‘potential failure mode’ is not associated with a probability of this event occurring and having potential failure modes is not a reflection of facility safety deficiency (Ref. 3). The potential failure modes and their effects will be considered in the detailed design of proposed Storage 3. A Failure Modes and Effects Analysis (FMEA) for Storage 1A and Storage 2 to their current consented crest heights was conducted at the OGNZL office at Waihi on 15 August 2018 (Ref. 4). The proposed Storage 3 is of similar design as the existing Storage 1A and Storage 2 (i.e., downstream construction). The potential failure modes identified in the FMEA for the existing Storage 1A and Storage 2 are like those associated with the proposed raised Storage 3 and have been adopted in the TDBAs. In the TDBAs, the failure scenarios are selected based on potential failure modes. The failure modes in conjunction with applicable hydrologic conditions in the receiving watercourse constitute the dam failure scenarios selected for a TDBA. For Storage 3, two failure scenarios were considered, each with different initial hydrologic conditions: • Sunny Day breach scenario – This Sunny Day scenario assumes a sudden dam failure occurring during normal hydrologic conditions in the receiving watercourse (i.e., not due to a storm event). There is no triggering storm or flood event and potentially no advanced warning of failure to the downstream areas. The breach scenario is assumed to develop during normal operations from either collapse or overtopping failure modes due to instability of the embankment triggered by strong earthquake shaking, elevated pore pressures in the embankment fill, internal erosion, and/or foundation instability. • Rainy Day breach scenario – This Rainy Day scenario assumes a failure that is triggered by extreme rainfall with a dam breach initiated by overtopping. The downstream incremental impacts are analysed for a range of flood events by comparing the consequences with and without failure of the dam. For the purposes of this study a breach of Storage 3 is assumed to result in the release of both tailings and water. Some tailings are eroded and entrained within the water and flow downstream and some are deposited close to the TSF as a mud type flow. 6.0 POTENTIAL CASCADE FAILURE SCENARIOS Potential cascade failures of dams are commonly considered where dams are in sequence down a watercourse. This is because the failure of one dam can cause extreme loading on the next dam and result in both failing. This can increase the potential damage assessment and result in a higher PIC to be selected for design and greater areas of potential inundation for emergency planning. A breach of Storage 1A into Storage 3 is a potential failure mode, however, the Rainy Day scenario selected already considers a highly improbable scenario of a crest full impoundment volume within Storage 3 (i.e., a volume well above the PMP 72 hour volume) and applies this breach flow at the worst time during peak flood flow in the Ohinemuri River. The assumptions made in the Rainy Day scenario already consider an extreme case. In detailed design further assessment and management of potential cascade failures is recommended. This could involve scenarios where Storage 3 could contain the volume of water released from Storage 1A and situations where emergency spillways are excavated to control outflows from Storage 1A and 3 together.
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