102 detailed above, 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 for TSF3 is 155 mRL providing a freeboard of 3.17 m GHD (2022A). Decant water is pumped to the processing plant for re-use or to the WTP. As part of the WTP operation, the storage available in the active TSF is monitored and if necessary, decant water treatment is prioritised to ensure freeboard is maintained. We expect the same management regime to occur at TSF3. GHD (2022A) predict a low probability of incursion into the required minimum 1 m of freeboard where maximum operating water levels are occurring. These volumes assume Priority One excess flows be directed to the active tailings facility. These excess flows27 typically occur during low flow periods where the WTP discharge is constrained by the low flows in the Ohinemuri River. Figure 25: Layout of TSF3, including location of spillway. 27 Excess flows are unable to be discharged under the existing consented regimes due to low flow restrictions in the Ohinemuri River which predominantly occur during the summer months. Excess flows have water sources with little buffering capacity within the water management system. These sources include seepage water, processing plant (and WTP) area runoff, collection pond water and groundwater inflows from the WUG, and RS runoff and seepage (via collection ponds). From GHD (2022A).
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