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