GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project G-1 G-1 Introduction G-1-1 Purpose This report describes the hydraulic model GHD has built for OGNZL to define flood inundation potential of the Ohinemuri River in the vicinity of proposed mine development, including the NRS. New mining infrastructure needs to be located or protected to avoid river inundation and potential infrastructure damage. The model of the river was also built to check that proposed mine developments do not result in increased flood potential to the downstream Waihi township. The Ohinemuri River is located in the Hauraki District of the Waikato Region. Although its upper catchment is near the eastern coast, the river flows west past the township of Waihi and through the narrow Karangahake Gorge before joining the Thames/Waihou River near Paeroa. This report provides an overview of the model build process and the model results. G-2 Input information G-2-1 Existing information Previous flood modelling Modelling of the 100 year floodplain in the vicinity of Baxter Road was undertaken by Woodward-Clyde in 1996. Figure 1 below shows the flood extent around the tailings storage facilities predicted previously. There is no known Hauraki District Council modelling for the Ohinemuri River. Figure 1 Previous Modelling of 100 year floodplain Anecdotal evidence suggests no known flooding of the Ohinemuri River above the level of Baxter Road Bridge. However minor flooding of a low point on the left bank immediately upstream of the bridge has been reported in large events by site staff. Flooding of the Ruahorehore Stream, a tributary of the Ohinemuri River south of the Tailings Storage Facilities (TSF’s), has also been noted in large rainfall events. Recent large rainfall events include: 13 April 2017 8 March 2017
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