EGL Ref: 8983 23 June 2022 Page 24 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. stockpiles are to be for NAF material only. The rhyolite soils of Storage 3 are expected to be NAF. Allowance for a total stockpile volume of approximately 3,000,000 m3 has been made. The stockpiles will require an access road and perimeter drains leading to sediment collection ponds. The existing East Stockpile will require lining as described in Section 11.9. 11.17. Tailings Storage Capacity and Surface Profile The elevation storage curve for the proposed Storage 3 design to RL155 is shown in Figure 15. The storage at RL155 is approximately 7,000,000 m3. Of this volume approximately 5,700,000 m3 will be available for tailings storage, with the rest required for freeboard and storage of supernatant water and extreme rainfall on top of the tailings. It is estimated that tailings will be able to be discharged up to 2.5 m below the crest and the normal operating level of the pond water will be typically at least 3.2 m below the crest. The maximum tailings profile and operating water level is controlled by the need to provide storage for the design flood above the tailings with 1 m freeboard. The design flood is the runoff from a 72-hour probable maximum precipitation (PMP) rainfall event. The PMP volume is approximately 618,000 m3. The decant water will be pumped to the Process Plant for re-use or to the Water Treatment Plant prior to discharge into the Ohinemuri River. It will be necessary to monitor the decant pond water level to ensure that operating water levels are consistent with design assumptions. It will also be necessary to undertake close monitoring of the tailings profile when it nears the maximum design profile to ensure there is enough storage for the design storm (72-hour PMP) with 1 m freeboard. Under normal operation the water in the pond is to be kept away from the crest by targeting a tailings beach of at least 70 m from the crest. This will be managed by discharging tailings from spigots around the embankment crest. This is similar to the current operation on Storage 1A. 11.18. Construction Aspects Storage 3 will require a series of establishment works before tailings can be discharged. The initial works to establish the initial embankment are expected to take 3 to 4 years depending on the sequencing of the work and weather. Figures 16-18 illustrate the various stages of works from site clearance up to the starter embankment with crest at RL135. The works include but are not limited to: • Establishment of initial erosion and sediment controls • Clearing the site of farm fences and trees etc. • Fencing the perimeter • Establishment of an uphill clean water diversion drain and access track • Diversion of the Ruahorehoe Stream adjacent to the collection ponds • Establishment of the Storage 3 topsoil and surplus soil stockpiles
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