GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 62 Figure 45 WTP Mean Monthly Excess Flow (Low K Scenario) 6.3.4 TSF Overflow Potential The tailings storage ponds are designed with a freeboard allowance that must contain the probable maximum rainfall event with an additional 1 m contingency above the normal operating levels. The minimum crest level is set to 179.4 mRL for TSF1A, 160.7 mRL for TSF2, 155 mRL for TSF3, and 107 mRL for GOP TSF providing a freeboard of 2.6 m, 3.12 m, 3.17 m and 4.0 m respectively. The site water balance model has been used to test the compliance with these levels with acceptable results. Figure 46 to Figure 49 show the projected live water surface elevations for TSF1A, TSF2, TSF3 and GOP TSF respectively. The peak water levels from the statistical model predict a low probability of incursion into the required 1 m of freeboard where modelled peak water levels exceed the maximum operating water levels at times. These volumes assume Priority One excess flows (refer Section 6.3.3) are directed to the active tailings facility. These excess flows typically occur during low flow periods where the WTP discharge is constrained by low flows in the Ohinemuri River so are unlikely to coincide with potential PMP events. The presented results do not take into account adaptive management procedures (such as prioritisation of decant pumping and treatment and/or increasing cyanide treatment capacity) that could be implemented to maintain compliant water levels and assume maximum pumping rates of 12,000 m3/day to both the processing plant and WTP respectively. These additional volumes could also be accommodated by a corresponding lower tails level or advanced embankment raise if required increasing the available water storage volume. Increased pumping capacity and optimisation of treatment and discharge when the discharge volumes can be maximised could partially alleviate this issue.
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