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JD -6 Mine — Drainage Design Plan 39 <br />UDS_mter <br />62.40 25 1.4 868 10.85 104 <br />0.8 <br />083 <br />0.035 <br />0 036 <br />6.75 <br />5858 <br />175 <br />Runoff <br />q <br />(inches) <br />1 5 9.75 11 49 11 <br />0.85 <br />0.89 <br />0 035 <br />0 035 <br />7 1 <br />69.24 <br />1 85 <br />UDS_drop <br />62.40 4064 1.4 868 1085 104 <br />08 <br />0.83 <br />0.25 <br />0.095 <br />6.79 <br />5889 <br />12.47 <br />035 <br />1 5 9.75 1149 11 <br />0.85 <br />0.89 <br />025 <br />0.089 <br />755 <br />73.56 <br />1324 <br />Notes: <br />D84 = 84th - percentile particle size <br />0.68 <br />74 <br />200 <br />3 51 <br />035 <br />857 <br />N -J <br />Bodot / Ustic Torriorthents <br />R = Reynolds' number <br />Sagebrush w /grass & roadway <br />Fair <br />081 <br />81 <br />200 <br />235 <br />060 <br />1,778 <br />n = Manning's n calculated using resistance equations <br />Bodot / Ustic Torriorthents <br />C <br />Disturbed ground <br />Fair <br />301 <br />85 <br />2.00 <br />1 76 <br />Vavg = average velocity <br />8,679 <br />Total (11) <br />Q = flow rate <br />14,457 <br />Total (acre -ft) <br />_ <br />Output reported for Stage -Q values nearest the Q100 value. <br />0 332 <br />5.4 SV;" Capacity <br />As discussed in Section 4.5.1, the existing storage capacity of the SWRP is 12,512 ft3 (0.287 acre -ft). <br />Using methods described in Section 2.2.1 and disregarding stormwater detention by upstream storage <br />(USPs), the volume of stormwater runoff reporting to the SWRP for the 10 -year 24 -hour storm is 14,457 ft3 <br />(0.332 acre -ft) as shown in Table 22. These calculations indicate that the current SWRP capacity is capable <br />of containing 87% of the runoff from the 10 -year 24 -hour storm. Currently, upstream storage at USP -2 and <br />in drainage ditches on site is expected to detain a significant fraction of this runoff. <br />The storage capacity of the planned SWRP is 30,300 ft3 (0.70 acre -ft). Based on a runoff volume of 14,457 <br />ft3 (0.332 acre -ft) for the 10 -year 24 -hour storm, the planned SWRP will be capable of containing the 20- <br />year runoff with 109% excess capacity. The runoff volume for the 100 -year 24 -hour storm is 31,314 ft3 <br />(0.719 acre -ft) (Table 23), and the planned SWRP will be capable of containing 97% of this runoff. The <br />excess runoff from the 100 -yr 24 -hr storm will be released over the engineered spillway into the natural <br />drainage. <br />Storage capacity of the SWRP is maintained by routinely excavating sediment from the floor of the pond. <br />Fine sediment is excavated using a backhoe or other equipment and placed on the topsoil stock pile. The <br />nature of this maintenance precludes the use of a liner in the stormwater and sediment retention pond. It is <br />recommended that the existing and expanded pond not be lined, to facilitate continued maintenance and <br />allow for adequate storage capacity within the pond. Maintenance and inspection are performed according <br />to the checklist and schedule provided in the Stormwater Management Plan. <br />Table 22. Required SWRP Capacity to Contain the 10 -year 24 -h our Storm <br />Area <br />Soil Type <br />Hydr. <br />Group <br />Cover 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 />N -F <br />Bodot / Ustic Torriorthents <br />C <br />Sagebrush w /grass understory <br />Fair <br />1.90 <br />75 <br />2.00 <br />3.33 <br />0.38 <br />2,625 <br />N -H <br />Bodot / Ustic Torriorthents <br />C <br />Sagebrush w /grass understory <br />Fair <br />035 <br />76 <br />200 <br />3 16 <br />041 <br />518 <br />N -I <br />Bodot / Ustic Torriorthents <br />C <br />Sagebrush w /grass understory <br />Fair <br />0.68 <br />74 <br />200 <br />3 51 <br />035 <br />857 <br />N -J <br />Bodot / Ustic Torriorthents <br />C <br />Sagebrush w /grass & roadway <br />Fair <br />081 <br />81 <br />200 <br />235 <br />060 <br />1,778 <br />MFA <br />Bodot / Ustic Torriorthents <br />C <br />Disturbed ground <br />Fair <br />301 <br />85 <br />2.00 <br />1 76 <br />080 <br />8,679 <br />Total (11) <br />14,457 <br />Total (acre -ft) <br />_ <br />_ <br />0 332 <br />4148A.131028 Whetstone Associates <br />