GHD | Oceana Gold New Zealand Ltd. | 12552081 | Waihi North Project 31 Figure 11 also shows the locations of the two flow gauges used for water management studies; the OGNZL gauge located upstream of the Ruahorehore Stream confluence and in the vicinity of the processing plant area (Frendrups) and the Ruddock gauge located in Ruahorehore Stream. The Waikato Regional Council gauge at Queen’s Head is also considered in this study and is located in the Ohinemuri River downstream and west of the mapped area shown on Figure 11. 3.3 River Flood Analysis River flood modelling is undertaken for Mataura Stream and Ohinemuri River to identify the risk to proposed infrastructure during storm events and potential changes to catchment hydrology. The Mataura Stream drains from a relatively small catchment allowing a flood model to be developed based on Henderson and Collins (2018) flood statistics for small streams, and this modelling is presented in Section 3.3.1. The Ohinemuri River passing the key areas of mine infrastructure services a larger catchment and the same flood statistics cannot be applied. In this case a Mike212 model with a derived rainfall hydrograph is applied for the purpose of flood flow estimation and is presented in Section 3.3.2. 3.3.1 Mataura Stream A flood model was developed using HEC-RAS3 modelling software to determine a 100 year ARI flooding extent and flooding levels in the Mataura Stream. The results of the analysis were then compared to the location of the proposed WSFA to evaluate flood risk for the site. The HEC-RAS model features a single river reach which represents the section of the Mataura Stream adjacent to the proposed portal infrastructure. Model cross section geometries are based on drone LiDAR surveyed by Aerial Surveys in 2019. The model also features a farm ford which crosses the Mataura Stream close to the proposed portal infrastructure. Manning’s coefficient was assumed to be 0.04 for the main channel and 0.05 for overbanks. The upstream and downstream model boundary conditions were set to normal depth with a slope 0.02 and 0.01 respectively. A 100-year ARI flow estimate of 62 m3/s was derived using NIWA’s NZ Flood Statistics GIS layer (Henderson Collins V2 layer, 2018). The flow in the Mataura Stream adjacent to the portal infrastructure can be scaled from the river reach shown in Figure 19 (NZ REACH 3009169). A climate change factor of 1.2 was applied to the flow estimate as recommended by Hauraki District Council. Table 9 shows a summary of the flood flow estimate. 2 DHI Software. Mike 21 Hydrodynamic Module. 3 US Army Corps of Engineers. Hydrologic Engineering Center's (CEIWR-HEC) River Analysis System (HEC-RAS)
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