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Section 3 <br />West Sunday Mine <br />where: <br />Q = runoff (in) <br />P = rainfall (in) <br />S = potential maximum retention after runoff begins (in) <br />S is related to the soil and cover conditions of the watershed through the Curve <br />Number (CN). CN has a range of 0 to 100, and S is related to CN by: <br />S _ 1000 _ 10 <br />CN <br />According to the U.S. Department of Agriculture (USDA) TR 55 report (USDA 1986), <br />the CN for bare soil of hydrologic soil group of "D" is 94. For P equal to 3.0 inches and <br />CN equal to 94, S equals 0.64 and Q equals 2.35 inches. Therefore, CDM used 2.35 <br />inches as the design runoff for the berm design calculation. <br />The total runoff volume was divided by available catchment area within the surface <br />facilities area to calculate the minimum required berm elevation of Basin 3. The <br />calculated minimum berm elevation is 1.5 feet, which includes 1-foot of freeboard. An <br />existing berm encloses Basin 3. However, CDM recommends increasing the height of <br />the existing berm, where it is less than 1.5-feet high, to the calculated minimum <br />elevation. CDM also recommends connecting any breached areas along the berm. <br />Details for this calculation are included in Appendix B. <br />The total runoff volume of Basin 4 was calculated to evaluate the sizing for the <br />proposed catchment basin. Assuming that the proposed catchment basin is 40 feet <br />wide and 170 feet long with 1.5:1 (H:V) bank slope, the minimum required depth of <br />the catchment basin is 4.0 feet. Therefore, by adding 1-foot freeboard, the design <br />depth of 5 feet is recommended for the 40 feet by 170 feet catchment basin to contain <br />the 100-year, 24-hour storm event. Details for this calculation are included in <br />Appendix B. In addition, the existing western berm should be extended to contain the <br />100-year, 24-hour event within the basin boundary (refer to Exhibits B-1 and B-2). <br />Finally, the existing constructed channel along the eastern basin boundary should be <br />extended to the proposed catchment basin (refer to Exhibits B-1). <br />3.5 Conclusions and Recommendations <br />CDM proposed additional control structures to route offsite surface flow around the <br />surface mine area and contain the 100-year, 24-hr storm event within the onsite berms <br />and catchment. CDM recommends the following for West Sunday Mine storm water <br />drainage control: <br />1. A grass-lined channel 2.5-feet deep for proposed Channels 1 and 2. <br />2. Increasing the height of the existing Basin 3 berm, where it is less than 1.5-feet <br />high, to 1.5 feet and connecting any breached areas along the berm. <br />F? <br />3-4 <br />O:t84988 - Denison\Task Order 1 - Sunday Mines Drainage Study\RaporftHnal Reportfinal Sunday Mines Drainage Report _052208.dx