2025-11-14_PERMIT FILE - M2025056 (32)

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JD-8 Mine-Drainage Design Plan 9 4111 flow past the south eastern mine boundary at the culvert exit does not show clearly on the 5 ft topographic contours in Figure 2 but was confirmed by site inspection. 3.3 10-year 24-hour Runoff and Retention for Affected Area As described in Section 2.2, runoff from each of the mine areas was estimated using the "SCS curve number method" for estimating surface runoff volumes (SCS, 1972). Runoff was calculated using hydrologic curve numbers of 85 and 63, obtained from Table 2-2d Runoff Curve Numbers for Arid and Semi-Arid Rangelands of TR-55 (NRCS, 1986) based on hydrologic soil group C3, a vegetative cover of sagebrush with grass understory, and a "Poor" hydrologic condition. Using these parameters, the individual sub-basin calculated runoff value for the 10-year 24-hour design storm is shown in Table 6. Table 6. Site Runoff for 10-yr 24-hour Storm Calculated Using SCS Method Curve Rainfall Runoff Runoff Soil Type Area Hydr. Cover Condition Area Number P Retention 9 Q Group Description (acres) CN (inches) S (inches) (ft3) Bodot/Ustic Sagebrush w/grass 1 C Fair 1.15 63 1.80 5.87 0.06 251 Torriorthents understory Bodot/Ustic Sagebrush w/grass 2 C Poor 1.14 85 1.80 1.76 0.65 2,698 Torriorthents understory 3 Receiving Area 0.57 63 1.80 5.87 0.06 125 Total: 3,074 ft3 Bodot/Ustic Sagebrush w/grass 4 C Fair 1.69 63 1.80 5.87 0.06 369 Torriorthents understory Bodot/Ustic 5 C Sagebrush w/grass Poor 3.16 85 1.80 1.76 0.65 7,479 0 Torriorthents understory 6 Receiving Area 0.18 63 1.80 5.87 0.06 39 Total: 7,887 ft Runoff from the western areas of the mine is currently routed across the waste rock dump and into the ephemeral drainage near the soils stockpile. Water pools temporarily between the toe of the waste rock pile and the soil stockpile (Sub-Basin 3 in Figure 1) before evaporating or infiltrating. Much of the water evaporates because infiltration capacity of site soils is low (NRCS, 2011) and potential evapotranspiration rates exceed precipitation rates in the region. The existing temporary pool was not engineered to contain the 10-year 24-hour storm or to convey the 100-year 24-hour storm. The existing pool will be expanded to a footprint of 4,583 ft2 with a planned maximum depth of 2.6 feet and side slopes of 5H:1V. This pond (Retention Pond 1)will then hold 3,100 ft3 of water at 1.6 ft. deep (Table 7). Freeboard will be reduced by 0.4 feet during the 100-year 24-hour event at the spillway design capacity. If the excavation encounters bedrock and the pond cannot be constructed as planned,the pond will be excavated to the depth practicable with a larger surface area to meet the specified volume and freeboard requirements. 010 3 Site soils consist of Bodot and Ustic Torriorthents, which is listed as hydrologic soil group C in the NRCS Web Soil Survey (NRCS,2011). 4122A.120702 Whetstone Associates