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Newmont Goldcorp ■ <br /> VLF/OSA Closure Stormwater Analysis <br /> NewFields Project No.475.0106.035 <br /> November 8,2019 <br /> 3. STORMWATER MANAGEMENT <br /> A stormwater analysis was completed by NewFields for each facility. HEC-HMS was used to <br /> determine the peak flow rates resulting from the 500-year/24-hour storm event. New <br /> precipitation estimates for the 500-year/24-hour storm were obtained from NOAA. Utilizing the <br /> tools within the HEC-HMS program, the runoff and infiltration estimates were calculated via the <br /> SCS Curve Number Method. All areas with non-native soil in closure conditions were assumed to <br /> have hydrologic soil group (HSG) B. These areas were modeled assuming a closure surface <br /> consisting of growth medium and grasses with no underlying impermeable component (Curve <br /> Number= 71). Aerial imagery was used to distinguish between areas of herbaceous and arboreal <br /> growth in undisturbed land within the watersheds. Hydrologic soil conditions were determined <br /> from the NRCS soils survey map; in these areas, curve numbers for the land use were modified <br /> accordingly. These curve numbers were taken from the Steffens and Associates "Design Report <br /> for Storm Water Management at the Chicago Tunnel Site" dated August 2014 (Steffens and <br /> Associates, Inc., 2014). Stormwater runoff for the two VLFs may increase or decrease depending <br /> on which closure cover alternative is chosen. Table 1 lists each facility's watersheds and <br /> summarizes the channel peak flows and required channel sizes. <br /> Table 1: Summary of Hydrological Analysis <br /> Channel Channel Channel Depth <br /> Closure Peak Flow Bottom with 1ft of <br /> Surface Watershed Channel Name (cfs) Width Freeboard <br /> 500yr,24hr (ft) (ft) <br /> West1 ECOSA Channel 5 135 2 3.5 <br /> West2 ECOSA Channel 6 65 0 3.5 <br /> Northl ECOSA Channel 7 108 0 4.25 <br /> Northeastl ECOSA Channel 1(A) 377 4 5.25 <br /> ECOSA Northeast2 ECOSA Channel 1 (B) 550 4 5.25 <br /> Eastl 332 4 4.75 <br /> ECOSA Channel 2 <br /> East2 332 4 4.75 <br /> South ECOSA Channel 3 81 0 4.25 <br /> Southwest ECOSA Channel 4 199 4 3.25 <br /> WHEX Pit East WHEX Channel 42 2 2.5 <br /> Northl SGOSA Channel 1 462 12 3.5 <br /> SGOSA South2 SGOSA Channel 2 199 12 3.25 <br /> Stack North SGOSA Channel 1 462 12 3.5 <br /> Schist Schist North Schist Island Channel 1 131 4 3 <br /> Island Schist West Schist Island Channel 2 68 4 2.5 <br /> Page 3 <br />