GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 42 5 COLLECTION PONDS As outlined in Section 2.5, collection ponds and pump systems to date have been sized to contain the 10-year, 72 hour return period event in order to delay any discharge to be approximately coincidental with the peak flood flow in the Ohinemuri River. It is proposed that for new facilities draining directly to the main branch of the Ohinemuri (i.e. the NRS), that the same 10-year 72 hour duration storm event is still the appropriate design principal. For facilities draining to tributaries of the Ohinemuri (i.e., TSF3 to the Ruahorehore and the WRS to the Mataura Stream) it is proposed that this design principal be adjusted such that overflow events coincide with high stream flows. 5.1 Willows Rock Stack Collection Pond The location of the proposed collection pond to service the WRS is shown on Figure 24. As with the existing and proposed NRS and TSF3 collection ponds, the design criterion for the WRS collection pond is that the potential for overflow in a 10-year storm event be avoided until high flow rates within the Mataura Stream are realised. Given the size of the catchment and short time of concentration, a reasonable basis for design is recommended to be retention of the 10-year 24 hour storm event. The collection pond sizing is based on a 7.7 Ha catchment. 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 2031-2050. Runoff is represented through conservatively applying a coefficient of 1.0 based on saturated ground conditions and allowing for residual sub-surface seepage entering the pond. The total required volume equates to 18,480 m3 (Figure 25), assuming a pump rate of zero, which is reflective of a pump failure event. The collection pond water will be pumped to the WTP at Waihi for treatment until the water is of suitable quality to allow direct discharge to the Mataura Stream. Ultimately upon removal of the WRS, the collection pond will be either deconstructed or repurposed.
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