| Hydrology and Catchments | Stormwater Management Plan | 2210835-1190907474-109 | 21/06/2022 | 5 Sensitivity: General Catchment Pre-development Description Post-development Description 4 Catchment 4 has a gully that drains to the Mataura Stream. Toward the lower end of the catchment, the topography flattens across farmland. Catchment 4 remains relatively unchanged bar construction of a new sealed carpark in the lower portion of the catchment. Where the existing topography flattens out, a formalised open channel will be constructed to direct the overland flow path to the Mataura Stream. CN=61 CN = 62 5 Catchment 5 is a partial catchment that drains to a different receiving environment to the other catchments. This catchment drains via overland flow to the south of the site. The lower portion of Catchment 5 will be developed for the proposed magazine storage area. The upper portion of the catchment will be diverted around the hardstand area toward the existing flow path west of the site. CN=61 CN = 69 6 Catchment 6 is a steep hill face which flows partly toward the wetland and across farmland to the Mataura Stream. Catchment 6 will be in part developed to form the rock stack contact pond and mine access road. The upper portion of the catchment is a steep hill face for which run off will be captured via a clean water diversion and conveyed to Catchment 2 where practical. CN=61 CN = 74 The runoff rates for each catchment in for the pre and post development scenarios have been calculated using the SCS method and are shown in Table 2 below. Rainfall for the post-developed scenario adopts the HIRDS RCP8.5 climate change adjusted rainfall depths. Table 2: Catchment Flows Pre-dev ARI Flow (m3/s) 24-hour Post-dev ARI Flow (m3/s) 24-hour Storm Event 2-yr 10-yr 100-yr 2-yr 10-yr 100-yr Rainfall Depth (mm) 152 238 369 162 254 396 Catchment 1 1.19 2.40 4.44 1.95 3.40 5.67 Catchment 2 1.01 2.05 3.78 1.17 2.30 4.21 Catchment 3 0.53 1.07 1.99 0.92 1.60 2.65 Catchment 4 0.71 1.43 2.64 0.81 1.60 2.93 Catchment 5 0.33 0.66 1.22 0.44 0.82 1.44 Catchment 6 0.47 0.94 1.74 0.69 1.24 2.11 Total 4.24 8.55 15.81 5.98 10.96 19.01
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