iii File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. OCEANA GOLD (NEW ZEALAND) LIMITED WAIHI OPERATION, NEW ZEALAND STORAGE 3 - TAILING STORAGE FACILITY - RL155 DAM BREACH AND POTENTIAL IMPACT CLASSIFICATION ASSESSMENT EXECUTIVE SUMMARY 1. The purposes of this dam breach assessment are to: a. Assess consequences of a hypothetical breach of the Storage 3 TSF to determine the Potential Impact Classification (PIC). The PIC sets the standards for design, construction, and operation of the dam. Dams with higher potential impacts are designed to be resilient to extreme load conditions associated with natural hazards or unlikely scenarios which may occur in operation. b. Develop maps for the Emergency Action Plan (EAP). The maps are used for planning and managing the unlikely event of a breach. 2. Tailings Storage Facility (TSF) - Storage 3 RL155 will be designed, constructed and operated in accordance with modern standards which are set out in the New Zealand Dam Safety Guidelines (NZDSG). OGNZL is also committed to the recommendations in the recently published Global Industry Standard for Tailing Management (GISTM). Dams that are designed and operated to these standards have a low and acceptable risk of potential failure, and a breach would be highly unlikely to occur. 3. Under the NZDSG there are three Potential Impact Categories (PIC), Low, Medium, and High PIC. Tailings Storage Facility (TSF) - Storage 3 RL155 has been assessed to have a HIGH Potential Impact Classification (PIC). The PIC is based on assessing the incremental consequences of a hypothetical breach of a dam under normal (i.e., sunny day) and rainy day conditions. For Storage 3 RL155 the rainy day condition is the more critical case for the assessment of PIC because a breach under normal sunny day conditions would be largely confined to within the Ohinemuri River channel. The PIC for the rainy day condition is based on the scenario which causes the maximum incremental consequences on top of flood conditions in the receiving rivers and streams. For Storage 3 the receiving watercourse is the Ohinemuri River and its tributaries. Four baseline flood events (1 in 100 year, 1 in 1,000 year, 1 in 10,000 year, and Probable Maximum Flood) were considered to assess which flood scenario has the maximum incremental consequence. A breach of Storage 3 associated with a 1 in 1,000 year return period flood into the Ohinemuri River was found to give the maximum incremental consequences and the associated inundation maps are provided to show the basis of assessment for the PIC. 4. The modelled 1 in 1,000 year flood was found to result in near full stopbank flows in the Ohinemuri River passing Paeroa. The additional flow from the modelled dam breach causes the stopbanks to overtop in Paeroa. The risk of a breach into Paeroa is extremely low as a 1 in 1,000 year flood is unlikely and a breach of the TSFs is highly unlikely. If a flood higher than a 1 in 1,000 year flow occurred then the areas behind the stopbanks would be at risk from overtopping, even without a dam breach. In a 1 in 100 year flood the modelling shows no overtopping of the stopbanks in Paeroa occurs with a dam breach. Maps with the rainy day breach with a 1 in 100 year flood are also provided along with the 1 in 1,000 year maps for emergency planning purposes.
RkJQdWJsaXNoZXIy MjE2NDg3