June 2022 G-01483.84-017-R-Rev0 Because the 31 The two ponds will have the dimensions summarised in Table 14 below. Table 14: Willows Collection Pond dimensions. Willows Collection Pond Impoundment area 0.89 ha Total impoundment volume 18,700 m3 Maximum impounded water depth 3.5 m Embankment length two sections of fill 30 m and 60 m section (total length appox.120) Crest width 5 m Embankment fill volume (excluding foundation stripping) 2,000 m3– 5,000 m3 Total estimated pond cut volume 30,000 m3 Freeboard 0.3 m WRS Toe Seepage Pond Impoundment area 0.04 ha Total impoundment volume 500 m3 Maximum impounded water depth 3 m Embankment length 20 m Crest width 3 m Embankment fill volume (excluding foundation stripping) 250 m3 Neither pond is considered a ‘large dam’ or a ‘classifiable dam’ in accordance with the 2015 NZSOLD guidelines and the 2022 MBIE Building (Dam Safety) Regulations, because they do not retain an impoundment of 20,000 m³ or more and are not 4 m or more in height. However, both embankments will be designed and constructed using the general considerations of dam engineering. Accordingly, the embankments will be designed to impound the reservoirs without excessive seepage, have adequate stability and to incorporate a suitable spillway to control outflows if the capacity of the impoundment is exceeded during a large inflow event. Failure of either pond would have adverse environmental impacts, given the impoundments will contain mine affected water and, if released, would flow into Mataura Stream. Consideration will also be given to the limited design life of both structures, which will be decommissioned after about ten years of operation. 7.3 Willows Collection Pond The WCP footprint includes terraces of the Mataura Stream and the adjacent gently sloping colluvial slopes. The impoundment will incorporate a combination of excavation into the existing slope and construction of a new earth embankment on the lower elevation, river side (Figure 10). Site investigations at the site have included test pits and cored boreholes as shown in (Figure 6). Based on the available subsurface information, the impoundment footprint is underlain by Whiritoa Andesite, incorporating mainly residual soils and highly weathered tuff and andesite. The Whitiroa Andesite is overlain by 3 m to 6 m thickness of alluvium below the
RkJQdWJsaXNoZXIy MjE2NDg3