B-7 – Area 7 - Assessment of Environmental Effects 40 Figure 26: TSF3 Drainage Diversion Works (Source: EGL (2022e)). 3.2.5.1 Southern Uphill Diversion Channel Extension A diversion channel will divert surface water from the forested catchment above TSF3, including an unnamed tributary of Ruahorehore Stream, around TSF3 and to the Ruahorehore Stream (Figure 26). 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 of TSF3 to a crest of 177 m RL. The length of the new section of Southern Uphill Diversion Drain is approximately 2,950 m. The uphill diversion drain will be sized for a minimum requirement of a 10 year ARI flow, equal to existing resource consent conditions for clean water diversions (RC971307, RC971309, Condition 4). 3.2.5.2 TSF3 Subsurface Drains Groundwater beneath TSF3 will be collected beneath the base geomembrane and earth liner of the impoundment and the embankment through a series of subsurface drains. Leachate from the material within the embankment, i.e. above the Zone A low permeability liner, will be collected via a series of leachate drains. Groundwater and leachate collected in the drains will be directed to a network of manholes and pumped back to the WTP or the Processing Plant via the perimeter ring main system around TSF1A and 2. Electricity to the leachate and underdrainage pumping systems will be extended from existing infrastructure at the toe of TSF1A.
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