GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project G-9 Key model parameters for the 2D surface model are summarised in Table 3. Table 3 2D model parameters Parameter Values Floodplain resistance, Mannings M 20 Flooding depth 0.02 m Drying depth 0.01 m Eddy viscosity 0.4 m2/s G-3-4 Model Boundaries The Ohinemuri River downstream water level at the open boundary is set at a constant water level of 79 m in the model. This is equivalent to the base flow water level at this location. The flow hydrographs derived from analysis of the gauged flow data from Queen’s Head and Frendrups gauges are connected to the MIKE 21 2D grid as “Source Points”. The locations of these flow boundaries (Source Points) are shown in Figure 7. G-3-5 Model Limitations The limitations of the model are discussed below. The model uses a conservative approach for the purpose of assessing the flood plain effects on current and future mining infrastructure works. The sub-catchment peak flows are assumed to peak at the same time in the model although the peak flow timings can vary significantly in practise depending on the subcatchment size, topography, etc. The peak flows used in the model are derived from limited long term gauging. These flows have been conservatively increased by 20% to provide some allowance for the uncertainty associated with the limited gauge record. This could be improved if more gauging was available. The Ohinemuri River channel bathymetry is modelled based on the LiDAR surface levels. LiDAR represents the top water level rather than the bottom of the river channel, therefore the model is conservative as there is some loss of river cross sectional area. Some surveyed cross section data was provided for the Ohinemuri River in the downstream end the project area. However these were found to be inconsistent with the LiDAR and aerial imagery, and the accuracy was not able to be verified. Where the river is crossed by roads, the river channel was manually cut into the 2D surface to provide continuation of the river channel. Bridges are not explicitly represented in the model at this stage as there is no evidence of historical overtopping of the bridges, therefore no interaction between the bridge deck and floodwaters. G-3-6 Model verification The model peak flows and volumes at several points along the rivers as shown in Figure 8 were compared with the expected peak flows and volumes to verify the model outputs. The 100 year ARI results are summarised in Table 4. The model peak flows and volumes in the river channels are validated to within +1% (towards downstream end of river channel) to +20% (upstream end). The model results are somewhat conservative at the upstream river channels. A good validation of model peak water level at the Frendrups gauge location was achieved when compared with the river water level data from the Frendrups gauge.
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