EGL Ref: 9018 22 June 2022 Page 12 WAI-985-000-REP-LC-0006_Rev0.docx This report shall only be read in its entirety. • A delayed and more muted response to rainfall occurs at the foot of the rhyolite dome in the east of the site (WRS07/DH09 and WRS03/WRS03a). Mild to moderate upward vertical hydraulic gradients occur at these locations, which indicates that the upper reaches of the tributary are spring fed. Groundwater levels and gradients typically follow the topography, with a steeper gradient at the hill to the east (0.1), becoming gentler in the alluvial flats below the NRS site (0.03). The shallow groundwater flow direction is approximately west to northwest, following the site topography, towards the perennial tributary (TB1f to TB1a) and the Ohinemuri River. The Ohinemuri River flows roughly north to south past the western side of the NRS site. At this location, the deep aquifer andesite is inferred to flow to the west and is sufficiently deep to be hydraulically separate from the proposed NRS facility. Former gullies and permeable material across the site, including the boulder and cobble alluvium and weathered ignimbrite in the NRS paleochannel, have the potential to act as preferential pathways for groundwater flow, providing a conduit for groundwater discharge to the Ohinemuri River. Elsewhere, depending on the hydraulic gradients and NRS base elevation, seepage may infiltrate deeper in the shallow groundwater system and ultimately discharge to the Ohinemuri River via existing groundwater flow paths. 6.0 SEISMIC HAZARD Estimates of seismic hazard for the site have been provided by GNS Science in 2007 and 2017 (Ref. 7). The 2017 update incorporated the latest knowledge of the Kerepehi Fault System (Ref. 8) and the Hikurangi subduction zone and updated estimates of background seismicity. The tectonic environment and seismic hazard estimates are discussed in Volume 1 (Ref. 1). The design uniform hazard spectra from the probabilistic and deterministic estimates of seismic hazard are shown in Figure 20. The spectra are 5% damped, larger horizontal component acceleration spectra, for Site Class B rock conditions. Peak ground acceleration (PGA) values and corresponding average magnitudes at the base of the embankment are as follows: 150-year return period PGA = 0.10g, Mw = 6.3 84th percentile level for Kerepehi Fault Rupture PGA = 0.23g Mw =7.3 2500-year return period: PGA = 0.27g Mw = 6.6 10,000-year return period: PGA = 0.39g, Mw = 6.9 7.0 FLOOD HAZARD The flood hazard from the Ohinemuri River was assessed by GHD. A 1 in 100 year flow was modelled. This is described in the Tailings Storage and Rock Disposal Natural Hazards and Options Report (Ref. 1). The flood hazard is localised to the incised channel of the Ohinemuri River. No stopbanks are along the Ohinemuri River in the vicinity of site. Flood
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