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Section 2 <br />Topaz Mine <br />criteria, CDM recommends grass-lined channels for Channels 1 and 2 since the flow <br />velocities are less than 5 ft/sec. <br />2.4.2 Berm Design <br /> <br />Tetra Tech designed earth berms to capture surface water runoff from Basins 3 and 4.'; , <br />Runoff in Basin 3 will be captured by a berm along the crest of the WRA. Runoff in <br />Basin 4 will be captured and routed to a catchment basin. The berms and catchment <br />basin were designed to contain runoff from the 100-year, 24-hour storm event. The <br />point precipitation frequency estimate for the 100-year, 24-hour storm, obtained from <br />NOAA Atlas 14, Volume 1 for Colorado is 3.0 inches (NOAA 1973). <br />Tetra Tech used the SCS runoff equation to estimate runoff from the 3.0 inches of <br />rainfall. CDM performed the same SCS runoff calculations to verify the berm and <br />catchment design. The SCS runoff equation is: <br />_ (P -0.2SY <br />Q (P + 0.8S) <br />where: <br />Q = runoff (inch) <br />P = rainfall (inch) <br />S = potential maximum retention after runoff begins (inch) <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. The total runoff volume <br />was divided by available catchment area within the surface facilities area to calculate <br />the minimum required berm elevation of Basin 3. The calculated minimum berm <br />elevation is 0.38 feet, while the proposed berm elevation by Tetra Tech is 4 to 5 feet. <br />Therefore, Tetra TecNs proposed berm elevation is sufficient to contain the 100-year, <br />24-hour storm event. Details for this calculation are included in Appendix B. <br /> <br />The total runoff volume of Basin 4 was compared with that of proposed catchment <br />basin (60 feet wide, 190 feet long, and 4 feet deep) to calculate the minimum required <br />catchment basin depth. CDM calculated the minimum required depth of the <br />catchment basin as 2.3 feet. Therefore, Tetra Tech's proposed catchment basin of Basin <br />4 is sufficient to contain the 100-year, 24-hour storm event. Details for this calculation <br />are included in Appendix B. <br /> <br />2-4 <br />0:164986 - DentsonWask Order 1 -Sunday Minas Drainage StudykReporhFinal ReporhFinel Sunday Mines Drainage Report 057108.doc