98 17.2 Proposed Water and Sediment Management Clean run-off water from the hills above the TSF will be diverted around the facility to the Ruahorehore Stream. This diversion will be an extension to the existing Southern Uphill Diversion Drain which currently starts behind TSF1A and runs behind the East Stockpile. This drain is set at a level which allows for future potential raising to a crest of RL177. Accordingly, there will be a section of vegetation which will remain between the impoundment level (RL155) and the uphill drain cuts and fills. The length of the new section of Southern Uphill Diversion Drain is approximately 2,950 m. During construction activities, sediment will be managed using standard erosion and sediment control practices that apply to all earthwork activities. Erosion and sediment management and monitoring is set out in OGNZL (2021) and summarised in section 21 of the current report. Once PAF is placed, stormwater runoff will be diverted to the lined perimeter drain and collection ponds as is currently done for TSF1A and 2 sites. The ponds are sized to have sufficient capacity to limit discharge over a spillway to Ruahorehore Stream only in high flow events when dilution is effective. In all other situations the water will be pumped back to the WTP before being discharged to the Ohinemuri River or for use in the Processing Plant until the water quality improves sufficiently to allow direct discharge into adjacent waterways. Collection Pond26 S5 which provides retention of surface water from the northeast part of TSF1A and East Stockpile will be buried by TSF3 works. This pond will be replaced by a new pond at the intersection of the downstream toe of TSF1A and 3. It will be called Collection Pond S6. Collection Pond S5 currently spills excess water to the TSF1A Perimeter Drain which flows to Collection Pond S4. Collection Pond S4 has a spillway for excess flow to the Ruahorehore Stream. To separate the discharges to allow for more efficient management of surface water as rehabilitation is completed, Collection Pond S6 will have its own spillway to the Ruahorehore Stream, rather than spilling to S4. A new collection pond is required to manage the additional embankment runoff area from TSF3. It will be called Collection Pond S7. Its proposed location is the low-lying area at the toe of theTSF3 embankment which is the natural drainage path for water on the site, and it is immediately adjacent to Collection Pond S6. Collection Pond S6 and S7 will both be fully lined with a 1.5 mm HDPE on a 0.6 m earth liner. A forebay and causeway into the main pond will be features of the ponds that will aid in maintenance, as is used in the existing collection ponds. A surface water perimeter drain will direct dirty water runoff to the collection ponds. These drains will be HDPE lined. Realignment of 310 m of Ruahorehore Stream is required to make room for Collection Pond S7 and temporary sediment retention ponds. 26At the time the collection pond design criteria were devised, in 1996, the potential for low pH and elevated trace elements content in the pond discharge water was a key consideration since there had been instances of pond water quality exhibiting these characteristics in earlier years of operation (1993/94). Changes in management approach to rock handling and placement (which includes lime addition and compaction, as well as advancement in rehabilitation) has resulted in improved surface runoff water quality to the existing collection ponds. It is proposed that for new facilities draining directly to the main branch of the Ohinemuri (i.e., the NRS), that the existing 10 year 72 hour duration storm event is still the appropriate design principal. For facilities draining to tributaries of the Ohinemuri (i.e., TSF3 to the Ruahorehore and the WRS to the Mataura Stream) it is proposed that this design principal be adjusted such that overflow events coincide with high stream flows. From GHD (2022A).
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