GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project G-7 G-3 MIKE 21 model G-3-1 Schematisation overview MIKE 21 software by DHI was used to model the Ohinemuri River study area shown in Figure 6. Catchment hydrology is defined in the model by flow hydrographs representing sub-catchment flows. These flow hydrographs connect to the MIKE 21 2D grid as “Source Points” for the area of interest in order to model the floodplain effects. The Ohinemuri River channel itself is modelled as part of a 2D grid, i.e. there is no 1D model component for the river. The model covers only the upstream catchment in order to allow greater focus on the area of interest. The future scenario model was schematised by addition of the proposed future mining infrastructure to the existing scenario MIKE 21 2D grid. The design contours for the proposed future infrastructure were received from Engineering Geology Limited. The proposed Gladstone Pit, NRS and NRS collection pond were included in the future scenario model. G-3-2 Hydrological model Ohinemuri catchment hydrology is defined using a series of sub-catchments. The catchment boundary and sub-catchments were delineated from GIS analysis of 20 m LINZ contours for the entire catchment area. Based on Flow-Frequency data from Frendrups and Queens Head gauges, the contributing flows are proportionally assigned to each sub catchment based on sub-catchment area. The shape of the flow hydrograph used in the model was developed from analysis of the gauged flow data for several rainfall events from Queen’s Head and Frendrups gauges. The modelled peak flow hydrograph from each sub-catchment is approximated as a triangular distribution with a total duration of 20 hours with 8 hours to the peak as shown in Figure 6. All subcatchments were assumed to peak at the same 8 hour duration even though the sub-catchments are expected to peak at different times during the storm. These hydrographs connect to the MIKE 21 grid at sub-catchment discharge points along the rivers Figure 6 Modelled Peak Flow Hydrograph
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