GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 44 5.2 TSF3 Collection Pond (S7) The location of the proposed collection pond (S7) to service TSF3 is shown on Figure 26. Collection pond S6 replaces the existing collection pond S5 and will receive runoff from TSF1A embankment. For the new ponds, a consideration for the design criteria is that the discharge will be to the Ruahorehore Stream. Based on analysis of flow records, the stream responds quickly to rainfall with peak flows occurring within about 30 minutes of rainfall. As a result, the approach applied for Ohinemuri River discharges (to coincide with peak flows) is not considered appropriate. The collection pond water will be pumped to the WTP for treatment until water quality allows for direct discharge. The pond needs to be of a sufficient volume that the WTP operators have some flexibility to store or treat water, and when the pond is eventually converted to a silt pond, there must be sufficient retention to remove suspended solids before discharge. Taking all factors into account, it is recommend retention a minimum of the 10-year 24 hour storm event is a reasonable basis for design (Figure 27). The collection pond sizing is based on a 24 Ha embankment. A runoff analysis is undertaken using the rational method and NIWA’s High Intensity Rainfall Design System (HIRDS V4) with climate change scenario RCP8.5 for the period of 2031-2050. Runoff is represented through conservatively applying a coefficient of 1.0 based on saturated ground conditions during an extended storm event. The total required volume equates to 56,500 m3 (Figure 27), assuming a pump rate of zero, which is reflective of a pump failure event. A section of the Ruahorehore Stream will need to be realigned to accommodate the new collection ponds at TSF3. The design of this realignment should take into account the maintenance of current stream characteristics as far as practical (slope/cross-section, fish passage etc.). Figure 26 Proposed TSF3 Collection Pond
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