EGL Ref: 9018 22 June 2022 Page 14 WAI-985-000-REP-LC-0006_Rev0.docx This report shall only be read in its entirety. 8.3. Permanent surface water collection systems Surface water drainage systems on the NRS are to be sized for a 100 year ARI flow. 8.4. Northern Rock Stack Collection Pond sizing The NRS Collection Pond is likely to have a capacity greater than 20,000 m3 and have a downstream embankment height greater than 4 m high. This will mean that the collection pond will be classified as a large dam under the Building Act and will require a Building Consent. Design, construction, and operation will need to be in accordance with the New Zealand Dam Safety Guidelines (NZDSG) (Ref. 9). A dam breach assessment will need to be undertaken to determine its Potential Impact Classification (PIC). The PIC sets the design criteria under the NZDSG. It is likely the dam will be assessed as a Low PIC dam. The collection ponds will be sized to manage runoff from a 10 year ARI (72 hour storm) as recommended by GHD (Ref. 10). 9.0 DEVELOPMENT OF THE NORTHERN ROCK STACK 9.1. Design concepts The proposed NRS provides additional area for working stockpiles and long-term disposal of rock as an engineered landform, for the WNP. The proposed facility presented in Drawing 0715 is the maximum potential profile for the proposed footprint. It has a crest elevation of RL173, extending from the lowest existing ground level of RL97 in the TB1 stream (tributary of the Ohinemuri River) which runs through the proposed site. The embankment takes the form of a 76 m high engineered landform. The NRS is formed by progressive placement of fill within the footprint as it is mined from GOP and MOP4. The proposed outer profile as shown in Drawing 0715 is the maximum benched profile that could be rehabilitated. Not all this volume may be required during operation or at closure. The benched slopes target 3h to 1v slopes with benches at 10 m vertical intervals for surface water control. The 3h to 1v slopes are selected for effective rehabilitation in pasture or vegetation. The overall average slope is therefore 3.4h to 1v which is an achievable, geotechnically stable profile, which allows for the management of some materials with weaker strengths. The proposed maximum profile is shown in Drawings 0716 and 0717. The proposed layout of the NRS as shown in Drawing 0715 provides for storage of 7.0 Mm3 of material. This includes the volume of the existing material in the Northern Stockpile. This total volume can accommodate approximately 13.3 Mt of rock, assuming a minimum average dry density of 1.9 t/m3. The NRS facility will provide for storage of PAF and NAF rock. The PAF rock requires a management strategy which mitigates the potential for acid generation. The proposed strategy includes a low permeability NAF earthfill liner, leachate collection drains on top of the liner, a system of subsurface drains beneath the liner to manage natural ground water seepage and that act as a contingency to intercept any seepage through the liner, addition of crushed limestone to the rock to increase its acid
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