File: WAI-983-080-REP-GT-0013_Rev0.docx This report shall only be read in its entirety. 1.0 INTRODUCTION The tailings at the Waihi Operation are currently stored in two Tailings Storage Facilities (TSFs) known as Storage 1A and Storage 2. Oceana Gold (New Zealand) Limited (OGNZL) proposes to construct another TSF, Storage 3, with the embankment crest at RL155 (in Mine Datum minus 1000 m). Storage 3 is to be formed by a zoned downstream embankment that is constructed predominantly from mine overburden material that is excavated as part of the process of obtaining ore from the Gladstone and Martha Open Pits. The embankment forming Storage 3 is generally referred to as a tailings dam. Engineering Geology Limited (EGL) has been engaged by OGNZL to undertake a dam breach assessment for Storage 3 with the embankment crest at RL155. This report presents the results of the dam breach assessment and an assessment of the Potential Impact Classification (PIC) of Storage 3 in accordance with the most recent version of the New Zealand Dam Safety Guidelines (NZDSG) published in 2015 (Ref. 1). The purpose of this dam breach study is to determine the PIC of the proposed Storage 3 dam and to provide maps showing likely areas of inundation to be included in the Emergency Action Plan (EAP) required by the NZDSG for emergency response planning purposes. There are three possible PIC categories in the NZDSG (Low, Medium, or High). Dam design criteria as well as construction and operational requirements are dependent on the PIC. The hypothetical dam breach scenarios assessed in a dam breach study are not scenarios that are expected to occur and no assessment of probability of occurrence is undertaken for the hypothetical failure modes assumed in this study. 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. 1.1. Dam Breach Guidance The methodology used in this dam breach assessment is in accordance with the NZDSG (Ref. 1) and adopts the recommendations in the Canadian Dam Association (CDA) draft Technical Bulletin on Tailings Dam Breach Analyses (TDBA) published Ref: 9216 OCEANA GOLD (NEW ZEALAND) LIMITED WAIHI OPERATION STORAGE 3 - TAILING STORAGE FACILITY - RL155 DAM BREACH AND POTENTIAL IMPACT CLASSIFICATION ASSESSMENT 22 June 2022
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