B-2 – Area 2 - Assessment of Environmental Effects 140 Discharge Receiving Environment Alternative Methods of Discharge Discharge water to land within the sediment retention ponds and decanting earth bunds and to surface water via spillways. . Refer to Section 2 of this AEE. The proposed use of sediment retention ponds and decanting earth bunds is considered best practice construction methodology to reduce the potential for sediment runoff to land and into waterbodies. Sediment retention ponds are designed to allow spill into adjacent waterbodies when flows are sufficiently elevated to provide effective dilution of contaminants. The sediment retention ponds on site will be designed to accommodate a minimum storage volume ratio of at least 3% of the contributing catchment, with an ideal ratio of 3;1, but not exceeding 5:1. Decanting earth bunds will be used to treat sediment laden runoff from areas up to 3000m2. The volumes will be sized at a ratio of 2-3% based on the slope steepness and slope length of the contributing catchment. The spillways have been designed to safely convey 1% AEP events. During events of this magnitude, overflow from the sediment retention and decanting earth bunds can spill flows into the adjacent waterbodies with limited adverse environmental effect due to the higher flows and mixing capacity of the waterbodies. Designing to a large capacity would therefore provide limited additional environmental benefit and was not considered further. Discharge water to surface water from the Main Collection Pond via an emergency spillway. Refer to Section 2 of this AEE. Collection Ponds are commonly used to capture discharges from perimeter drains surrounding mining sites. This technique is currently used by OGNZL at its existing mining facilities at Waihi. Collection pond is designed to allow spill into adjacent waterbody when flows are sufficiently elevated to provide effective dilution of contaminants. Consideration was given to designing larger capacity collection ponds. The current size was considered appropriate as it can accommodate a one in ten year, 24 hour rain event. During events of this magnitude, overflow from the collection ponds can spill flows into the
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