June 2022 G-01483.84-017-R-Rev0 Because the 30 7.2 Ponds The objective of the collection ponds will be to prevent mine-affected water from entering the Mataura Stream and to contain stormwater flows up to a 10 year return period. However, the ponds will also be of the smallest stored volume possible to minimise the environmental footprint of each pond, avoiding the surveyed wetland area (including 20m buffer) and DoC stream marginal strip (50 m offset). GHD provided WSP-Golder a graph (Figure 9) for estimating the required collection pond volume for a 7.7 ha total disturbance (includes WRS footprint of approximately 4.9 ha, additional disturbance for a 10 % buffer to account for drainage works, ponds, and the WUG portal access road footprint of approximately 0.7 ha) based on the design 1 in ten year 24 hour storm event, assuming a pump rate of zero, which reflects the case of a pumping failure. This equates to a collection pond size of 18,480 m3. The WCP has a volume of 18,700 m3 and WRS toe pond 500 m3. Figure 9: Pond volume up to 24h storm with C=1 (source: GHD2). 2 Updated Willows Rock Stack Collection Pond, plot provided by email from GHD dated 8 March 2022.
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