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Newmont Goldcorp ■ <br /> VLF/OSA Closure Stormwater Analysis ■ <br /> NewFields Project No.475.0106.035 <br /> November 8,2019 <br /> Channel 3 is 46 cfs and the peak flow reporting to VLF1 4 is 120 cfs. The proposed stormwater <br /> plan can be seen on the Figure 6. <br /> 3.6. Schist Island Backfill <br /> The Schist Island Backfill area consists of four watersheds: Stack Northwest, Schist South, Schist <br /> West, and Schist North. It should be noted that SGOSA Channel 1 runs through watershed Schist <br /> South. <br /> At the north half of the Schist Island backfill, the Schist North watershed reports to Schist Island <br /> Channel 1 and has a peak flow of 131 cfs. The watershed Schist West reports to Schist Island <br /> Channel 2 and has a peak flow of 68 cfs. Both of these channels discharge into an existing <br /> drainage. <br /> At the south half of the Schist Island backfill, the Schist South watershed reports to Schist Island <br /> Channel 3 and has a peak flow of 62 cfs. The watershed Stack Northwest reports to Schist Island <br /> Channel 4 and has a peak flow of 106 cfs. Both of these channels discharge into an existing <br /> drainage. The proposed stormwater plan can be seen on Figure 7. <br /> 3.7. Intermediate Bench Channels <br /> To reduce the potential for erosion on the closure surfaces of the OSA and VLF facilities, bench <br /> channels will be constructed on the side slopes of the reclaimed facilities as shown on Drawings <br /> 010-040 in Appendix C. The bench channels will intercept the stormwater runoff generated <br /> between the benches channels and will convey the stormwater towards the down drains, also <br /> shown on Drawings 010-040 in Appendix C. The down drains, which are robustly armored, will <br /> convey the flows down the slope to the perimeter channels at the edge of each facility. Both the <br /> channels and down drains are sized to convey the 500-yr, 24hr storm event. <br /> Using Flowmaster, NewFields determined facility specific typical channel sizing by completing a <br /> hydrologic analysis to determine the peak runoff from the largest inter-bench watershed for each <br /> facility. Watersheds were further divided between down drains. Down drains are included at <br /> perimeter channel discharge points or where there is a low point in the closure surface grading <br /> along the bench. The resulting channel dimensions were applied to each bench within the facility. <br /> Table 2 summarizes the required channel dimensions for each facility. All bench channels have <br /> 2H:1V side slopes and will tie into the closure surface at a 2H:1V slope.All channel depths include <br /> one foot of freeboard. <br /> Page 7 <br />