| Proposed Development – Surface Water Management | Stormwater Management Plan | 2210835-1190907474-109 | 21/06/2022 | 8 Sensitivity: General Table 5. Key Culverts at SFA. Excludes culverts to be installed at WRS and Mine access road. Culvert ID Design Storm AEP Approximate Length (m) Minimum Size (m) Justification SW-C2A 1% 50 1.5 100-yr design storm due to potential impact to SFA and overland flow route could lead to additional water in detention pond. SW-C3A 1% 27 1.2 Conveyance to detention pond is for 100-yr AEP flows. Alternate overland flow route is north to Mataura Stream. SW-C4A 1% 10 1.2 100-yr design storm due to potential impact to SFA and overland flow route would restrict pedestrian access to car park. SW-C4B 1% 15 1.2 100-yr design storm due to potential impact overland flow route being close to private property. Overland flow route to be reviewed at detailed design Figure 4: Typical scour protection at inlets and outlets. 4.5 Ponds There are two ponds proposed at the site. One serves as a collection pond for the rock stack contact water, portal platform and mine access road, and the second as a detention pond for the SFA. A 100-year flood assessment of the Mataura Stream (Figure 19, GHD Water management Study, 2021) indicates that some overbank flooding occurs however does not pose a flood risk to the proposed site. WRC Flood Hazard Maps do not indicate any flooding in the downstream vicinity of the site. 4.5.1 Rock stack collection pond Refer to the GHD Water Management Study for further details on the proposed rock stack pond. The pond philosophy is summarised below. Due to potential contaminants from the waste rock extracted from the tunnelling process, any surface runoff from the Willows rock stack, portal site and access road will be conveyed to a collection pond by lined open drains. Rock stack contact water will be treated prior to discharge. In addition to the surface runoff, seepage water will be collected at the base of the rock stack via a bunded catchment then conveyed to the larger rock stack pond by either lined open channel or piped system. Based on the GHD proposal, minimal surface run off will enter the receiving environment in storms less than a 10-year event. The pond is sized to contain the full post-development run off volume for a 10-year, 24-hour storm event. Events greater than the 10-year storm will overflow to the Mataura Stream via an emergency spillway. The spillway concept is detailed in Section 4.5.4, refer GHD Water Management Study for contaminates management of this water.
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