EGL Ref: 9018 22 June 2022 Page 18 WAI-985-000-REP-LC-0006_Rev0.docx This report shall only be read in its entirety. Runoff from the NRS site will initially be managed through a series of diversion channels, earth bunds and sediment retention ponds. Initially materials onsite will only be NAF. An erosion and sediment control plan will be prepared that will cover these works. As PAF material is placed, the ability to collect PAF runoff and pump it to the treatment plant is required. There is an existing Northern Stockpile silt pond that collects runoff from the Northern Stockpile. This silt pond currently discharges to the TB1 stream. It could be used during development of the NRS to collect runoff for pumping back to the Water Treatment Plant. Ultimately the Northern Collection Pond will store runoff from the NRS and the haul road Workshop Ramp. Runoff from the downstream shoulder of the NRS embankment is to be collected via grassed benches at approximately 10m vertical intervals. Water will be conveyed down to the toe of the embankment to a perimeter drain via buried PVC pipes or surface drains. The perimeter drain will then discharge to the Northern Collection Pond, which is then pumped to the Processing Plant or to the Water Treatment Plant. Once water quality improves in the Collection Pond it can be directly discharged into the Ohinemuri River. 9.8. Northern Rock Stack Collection Pond The NRS Collection Pond will be fully lined with HDPE. It will have its own subsoil drainage and collection sump. The pond will be sized to manage (with pumping) runoff from a 1 in 10 year, 72-hour rain event without discharge. The pond will have a spillway channel which will discharge to the Ohinemuri River. A typical section for the collection pond is shown in Drawing 0761. We recommend that a detailed design report, drawings, and specification is prepared for the NRS Collection Pond because it is likely to meet the definition of a large dam in the Building Act and will need to be designed, constructed and operated in accordance with the NZDSG (Ref. 9). If the dam is a Medium PIC it will need to have a formal dam safety management system and require annual Intermediate Dam Safety Reviews to be undertaken. The Collection Pond requires both resource consent and building consent. 9.9. Northern Rock Stack stockpiles Three stockpiles are shown on Drawing 0715. These are for stockpiling of topsoil and surplus soils from the stripping of the NRS foundations. These materials can be used for construction and rehabilitation of the NRS. Only NAF material is allowed in the stockpiles. Topsoil and rhyolite, dacite and ignimbrite soils that will be stripped from the site are expected to be NAF. Allowance for a total stockpile volume of approximately 184,000 m3 has been made. This is less than the estimated total undercut of the foundations. The rest of the undercut volume will need to be managed within the footprint of the NRS or elsewhere in the Development Site. The stockpiles will require an access road and perimeter drains leading to SRPs.
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