June 2022 G-01483.84-017-R-Rev0 Because the 35 7.3.3 Lining Given the relatively high permeability of the gravels that underlie the pond footprint, it is anticipated that an artificial liner will be required to avoid seepage of mine affected water into the adjacent wetland and Mataura Stream. A range of suitable proprietary synthetic lining products could be used and a preferred option will be identified during detailed design. Synthetic liners are commonly used in small dams in New Zealand, such as on-farm irrigation storages. The choice of liner will need to take into account the protection against penetration damage, UV protection and design life. A bedding layer of sand to protect against liner damage is anticipated to be placed over the upstream embankment slope and any coarse grained materials exposed in the excavated impoundment. The liner is expected to be anchored into a trench on the embankment crest or in a bench on the cut slopes [not currently shown on the drawings]. 7.3.4 Spillway and Freeboard The WCP is a collection pond that takes inflows from the WRS and WUG portal access road. Surface water inflows have been calculated as described in section 7.5. Seepage volumes piped from the WRS toe seepage pond have been calculated by others (GHD 2022). Water generated from the WUG tunnel developments and requiring treatment is planned to be pumped directly to the WTP bypassing the WCP at Willows Road. Impounded water from the WCP will be pumped via a dedicated pipeline (design by others) located in the Services Trench to the existing WTP for treatment and discharge. The maximum storage volume of the collection pond is 18,700m3 which corresponds to a 1 in ten year 24 hour storm event. Inflows exceeding the capacity of the impoundment will be directed into Mataura Stream via a spillway as shown in Figure 10. The spillway will comprise a channel excavated through natural ground on the north end of the impoundment with a concrete sill preventing downcutting through the channel floor. Nominal width of the spillway is 5 m. Flow depths through the spillway, under the maximum design inflow are expected to be modest (about 0.1 m), though this will need to be confirmed during detailed design when the spillway is dimensioned. Freeboard is the elevation difference between the maximum storage level and crest of the embankment. NZSOLD (2015) suggests a minimum freeboard of 0.9 m. A concrete or rockfill lined channel will be required to convey the expected infrequent spillway overflows down into a relic stream channel on the Mataura Stream terrace. It is also possible that the Mataura Stream will overtop the river terrace during such a flood event. 7.3.5 Earthquake Performance Given the short operational life of the collection pond it is unlikely to experience significant strong earthquake shaking. In addition, the collection pond will operate as a buffer and typically will have reduced impoundment levels much less than the full storage capacity of 18,700 m³. As such it is not believed necessary to design the embankment to withstand strong earthquake shaking as would be typical of a large dam. The embankment will be constructed from compacted well graded fill material on a gravel foundation and is expected to generally withstand earthquake shaking without large scale deformation. In the unlikely event that the embankment suffers damage from earthquake shaking, suitable equipment will be available at short notice to repair any damage. Ultimately upon removal of the WRS, the collection pond will be either deconstructed or repurposed.
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