June 2022 G-01483.84-017-R-Rev0 Because the 36 7.4 WRS Toe Seepage Pond 7.4.1 Seepage Collection Captured flows from the WRS underdrains will be collected in a toe seepage pond located directly below the WRS in a narrow gully approximately 200 m upstream from the confluence with Mataura Stream. The WRS toe seepage pond will have a maximum stored volume of 500 m³ and will accept a mix of groundwater flow and surface infiltration from the rock stack (Figure 8). The inflow sources and volumes have been calculated by others (GDH 2022). The flows are assumed to be mine affected and will normally be pumped and piped to the WCP and ultimately will be sent to the WTP for treatment. When the capacity of the pipe is exceeded, an operational spillway on the WRS toe seepage pond embankment will overflow into a channel alongside the lower access road, which will direct flow into the WCP. A small embankment will be constructed to retain the pond. The size of the WRS toe seepage pond is described in Table 14. The impoundment footprint is expected to be fully lined with a synthetic liner. The embankment will be constructed from compacted well graded fill material, expected to be imported. The foundations of the embankment will be stripped of all topsoil and weak or compressible soils prior to placement of the embankment fill, which will be compacted in layers in accordance with the detailed design and specifications. 7.5 Surface Drainage 7.5.1 Catchment Data Catchment data used in the runoff assessment is summarised in Table 15. Table 15: Catchment data used in the assessment. Data Format Comments Aerial photography-Willowsfarm50mm_NZMG tif Provided OGNZL Site digital terrain model (DTM) dxf Provided OGNZL Geotechnical field program results various OGNZL/WSP-Golder WRS, haul and access designs dxf Developed WSP-Golder The project site is a rural farm area. Figure 13 and Figure 14 shows delineated catchment areas for the Stage 1 and Final Stage WRS access road alignment respectively. The catchment above the WRS has been divided into a North and South area to divert the undisturbed catchment surface flows around the WRS and reduce contaminated water volumes. 7.5.2 SCS Rainfall Runoff Method The Waikato Stormwater Management Guideline (WRC 2020a) Waikato Stormwater Runoff Modelling Guideline (WRC 2020b) provide the hydrological design guidelines for the Waihi region. The WRC guidelines recommend the use of the Natural Resources Conservation Service (previously the Soil Conservation Service [SCS]) method for estimating rainfall runoff in the Waikato region. WRC, 2020b provides a summary of the recommended methodology, including region-specific variables to be used in the computations.
RkJQdWJsaXNoZXIy MjE2NDg3