GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project G-5 Surveyed cross sections were found to be inconsistent with other data such as LiDAR and aerial imagery and the accuracy was not able to be verified and therefore not used in the project. G-2-3 Hydrometric data Hydrometric data includes flow and water level information obtained from long-term gauges located as shown in Figure 5 and used for the hydrological and hydraulic model validation. Flow and frequency Two data sources were available for the study: the Waikato Regional Council gauge at Queen’s Head downstream of the project area, and the OGNZL Frendrups gauge near the Ruahorehore Stream in the vicinity of the mining infrastructure. This information is summarised below. In the model a 10% flow escalation factor is included in the estimates to account for future climate change with an additional 20% to allow for uncertainties in the flood frequency analyses and to account for flow attenuation along the river channels. This is a conservative approach. Queen’s Head flow gauge Queen’s Head flow gauge is located downstream of the project area, as indicated in Figure 4. The latest Return Period Flow information for Ohinemuri at Queens Head was provided by Waikato Regional Council and is included in Table 1 along with the stage which currently corresponds to those flows. This analysis is based on a WISKI (Water Information System KISTERS) calculation for the period 1984 to 2017 and includes the recent large March 2017 event. The 100 year Annual Recurrence Interval (ARI) peak flow derived from this analysis is 485 m3/s. Table 1 Queen's Head gauge data Return Period Flow (m3/s) Corresponding Level (m) 5 Year 267 5.85 10 Year 320 6.44 20 Year 370 6.89 30 Year 399 7.12 50 Year 436 7.40 100 Year 485 7.71 Frendrups flow gauge The OGNZL flow gauge has a catchment area of approximately 50 km2 and has collected 15 min interval data since 1985. Flow frequency analysis of this gauge undertaken by Hydro Logic NZ Ltd is reproduced in Figure 5 following and summarised in Table 2. Table 2 Frendrups Gauge Data Analysis Return Period Flow (m3/s) 5 Year 80 20 Year 108 100 Year 147
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