EGL Ref: 8983 23 June 2022 Page 28 WAI-985-000-REP-LC-0004_Rev0.docx This report shall only be read in its entirety. 11.19. Closure Plan The closure plan concepts for Storage 3 are shown in Figure 21 and Drawing 690. The current plan for the closure surface of Storage 3 comprises a perimeter capping layer of ignimbrite rockfill with a 0.5m thick layer of Zone H over the top. The uphill diversion drain along the northern hills will be decommissioned and runoff will be allowed to flow into the impoundment. The eastern section of the drain will remain and form a permanent outlet channel (i.e. spillway) for closure, discharging water to the Ruahorehore Stream. Closure details will be reviewed and advanced and it is recommended they be subject to future approval by Waikato Regional Council and Hauraki District Council. 11.20. Surface Water Runoff from the ground above Storage 3 is intercepted and diverted by the uphill diversion drain. Runoff from the Storage 3 site will initially be managed through a series of diversion channels, earth bunds and SRPs. Initially materials onsite will only be NAF. As the initial embankment is constructed over the Zone A base liner PAF material will be placed. At this point the ability to collect PAF runoff is required. This is likely to be done by controlling the downstream perimeter of the site with a perimeter bund (NAF) required for the embankment construction and the Storage 3 Collection Pond S7 which is to have approximately 90,000m3 storage capacity, be HDPE lined and have pumps to deliver water back to the treatment plant. Runoff from the downstream shoulder of the Storage 3 embankment is to be collected via benches (grassed at final lift to RL155) at approximately 10m vertical intervals. Water can 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 Storage 3 Collection Ponds S6 and S7, which is then pumped to the process plant or to the water treatment plant. Once water quality improves water in the collection ponds can be discharged into the Ruahorehore Stream. The surface runoff pipes or surface drains are likely to be at approximately 200m centres. Pipes are likely to be 220 to 300mm in diameter. A similar piped surface water system on Storage 1A and 2 has operated without incident throughout the operation of the embankment. The same is expected for Storage 3. 12.0 POTENTIAL FAILURE MODES The identification and assessment of potential failure modes for a dam is a routine risk assessment exercise done in accordance with the NZDSG. The failure modes that are identified are not failures that are expected to occur. They are hypothetical failures that
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