GHD | Oceana Gold (New Zealand) Ltd | 12552081 | Waihi North 93 I.2.2 Cross-section 2 (SEEP/W) Base model set up The model length was 3.6 km, beyond the northwestern crest north of the site and through to the Ruahorehore Stream southeast of TSF3 (Figure I.7 and Figure I.8). The cross-section was aligned perpendicular to the direction of dominant groundwater flow of the primary aquifer. However, the purpose of this assessment was to assess potential excavation inflows and drawdown associated with dewatering to 20 m depth during removal of the sensitive tuff. During this time the groundwater flow regime will change locally so that groundwater flow becomes parallel to the cross-section (towards the excavation) and the use of the model to predict drawdown is considered to be acceptable when considering results within close proximity to the excavation. As discussed in the Rationale (Section 5.4.2 of the main report), this analysis is supported by Theis (1935) for drawdown further away from the excavation (Section I.2.3 below). Parameter inputs, calibration and scenarios Cross-section 2 comprised a transient model, set up using the inputs from the calibrated Cross-section 1 model for hydrogeological properties (Table I.2). Boundary conditions were set to achieve a high groundwater table through the sensitive tuff, as per the existing site conditions for the base model and achieve water the presence of water in the Ruahorehore Stream. As with Cross-Section 1, the geological structure for the model was provided by EGL. In this model, the sensitive tuff thickness was only present to 15 m depth. For the purpose of estimating drawdown and allowing settlement analysis, a more conservative 20 m excavation scenario was assessed (Figure I.9). A constant head boundary was applied to the Ruahorehore Stream for the dewatering scenarios to simulate the expected discharge of dewatered groundwater through the initial construction stage of works. A summary of the model scenarios is provided in Table I.10 and model inputs is provided in Table I.8 and Table I.9. Figure I.7 SEEP/W Cross-section 2 location compared with the primary section (Cross-section 1)
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