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JD -6 Mine — Drainage Design Plan 40 <br />Table 23. Calculated Runoff Reporting to the SWRP from the 100 -year 24 -h our Storm <br />Hydr. Area Curve Rainfall Retention Runoff Runoff <br />Area Soil Type Group (acres) Cover Description Condition ) Number P q Q <br />CN (inches) S (inches) (ft) <br />N -F <br />Bodot / Ustic Torriorthents <br />C <br />Sagebrush w /grass understory <br />Fair <br />1 90 <br />75 <br />3.00 <br />3.33 <br />0.96 <br />6,620 <br />N -H <br />Bodot / Ustic Torriorthents <br />C <br />Sagebrush w /grass understory <br />Fair <br />0.35 <br />76 <br />3.00 <br />3 16 <br />102 <br />1,271 <br />N -I <br />Bodot / Ustic Torriorthents <br />C <br />Sagebrush w /grass understory <br />Fair <br />0.68 <br />74 <br />3.00 <br />3.51 <br />0.91 <br />2,225 <br />N -J <br />Bodot / Ustic Torriorthents <br />C <br />Sagebrush w /grass & roadway <br />Fair <br />081 <br />81 <br />3.00 <br />2.35 <br />1.31 <br />3,862 <br />MFA <br />Bodot / Ustic Torriorthents <br />C <br />Disturbed ground <br />Fair <br />3.01 <br />85 <br />3,00 <br />1 76 <br />1 59 <br />17,336 <br />Total (ft) <br />31,314 <br />Total (acre -ft) <br />0 719 <br />Peak flow into and out of the SWRP was calculated using HydroCAD®, which models stage- storage <br />relationships for in -line detention ponds. The hydrographs indicate that the arrival time for the 10 -year <br />storm occurs at approximately 10 hours after rainfall begins, while the 100 -year storm arrives at the SWRP <br />approximate 8 hours after rainfall begins (Figure 19). For the 10 -yr 24 -hr storm, peak inflow of 5.68 cfs <br />occurs at 12.05 hours and no outflow occurs. For the 100 -yr 24 -hr storm, peak inflow of 12.59 cfs occurs <br />at 12.03 hours and peak outflow of 0.19 cfs occurs at 21.99 hours. <br />The spillway was modeled as a broad - crested rectangular weir, which is an appropriate model for the rock - <br />lined excavated earth spillway. During the 100 -yr storm event, free discharge over the spillway occurs <br />with a maximum water surface elevation of H = 5873.53 ft. This stage height corresponds to a depth of <br />water above the spillway crest of 1.03 feet (shown as Hp in Figure 20) and a maximum velocity of 0.42 <br />ft/sec (Table 24). <br />The existing stormwater pond is constructed without a spillway. A 10 -ft wide rock -lined spillway is <br />designed to pass the peak flows associated with the 100 -year 24 -hour storm and allow low intensity <br />vehicular traffic on the existing dirt road. Because the stormwater retention pond exists within the Mineral <br />Joe Mine Permit area, Cotter will install the spillway after the Mineral Joe Mine Permit Amendment is <br />approved. Spillway dimensions are provided in Table 24. <br />4148A.131028 Whetstone Associates <br />