| Hydrology and Catchments | Stormwater Management Plan | 2210835-1190907474-109 | 21/06/2022 | 4 Sensitivity: General Figure 2. Post-development catchments. Table 1: Pre- and Post-Development Catchment Descriptions and adopted weighted Curve Numbers (CN) Catchment Pre-development Description Post-development Description 1 Catchment 1 is a steep gully which flows to the Mataura Stream. The Catchment 1 gully is to be filled and developed into the storage rock stack. All surface runoff from the rock stack is to be captured in a collection pond and conveyed offsite for treatment. The gully has natural springs that drain into the Mataura Stream. Refer to the Golder report for how these existing underground springs will be managed. CN = 61 CN=78 2 Catchment 2 is a steep gully which drains to an existing wetland (Boffa Miskell, 2021) area which is currently fed by springs within the gullies. The Catchment 2 gully will be mostly unaffected by the development and continue to feed the existing wetland by way of upgrading the existing culvert crossing underneath the proposed mine access road. CN = 61 CN = 62 3 Catchment 3 is a generally flat, low lying area where runoff sheet flows across the farmland, partly to the existing wetland and the remainder to the Mataura Stream. Catchment 3 is developed into the Surface Facilities Area (SFA). This includes office and maintenance buildings, tanks, roading, parking and washdown facilities. Runoff from the SFA will be conveyed to a detention pond and released to the downstream environment at pre-development rates. CN = 61 CN = 80
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