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Section 2 <br />Drainage Control Design <br />• CDM used the UDFCD Drainage Criteria (UDFCD 2005) to calculate flow velocity to <br />determine the appropriate channel lining for the diversion channel. UDFCD (2005) <br />recommends flow velocity be less than 5 ft/sec for grass-lined channels and less than <br />12 ft/sec for riprap channels. Based on these recommendations, a Manning's n value <br />of 0.035 was selected for design calculations to check the capacity of grass-lined <br />channels. <br />The calculated channel depth and velocity of the diversion channel are shown in <br />Table 2-3. CDM recommends adding 2 feet of freeboard to the calculated flow depth <br />to account for potential channel overtopping. Therefore, a design depth of 3.5 feet is <br />recommended for the diversion channel. Based on UDFCD criteria, CDM <br />recommends grass-lined channel for the diversion channel since the flow velocity is <br />less than 5 ft/ sec. <br />Table 2.3 Calculation Summa for Diversion Channel <br />h Freeboard Design Depth <br />(ft) I V <br />ft/sec <br />1.45 2 3.45 3.04 <br />2.4.2 Berm Design <br />CDM designed earthen berms to capture and retain surface water runoff from Basins <br />2 and 3. Runoff in Basin 2 will be captured by a berm along the crest of the DRA. <br />i• Runoff in Basin 3 will be captured by a berm along the toe of the DRA (refer to <br />Exhibits A and B). The berms were designed to contain runoff from the 100-year, <br />24-hour storm event. The point precipitation frequency estimate for the 100-year, <br />24-hour storm, obtained from NOAA Atlas 2, Volume 3 for Colorado (NOAA 1973) is <br />3.0 inches (refer to Appendix B). CDM used the SCS runoff equation to estimate <br />runoff from the 3.0 inches of rainfall. The SCS runoff equation is: <br />Q - (P - 0.2S)2 <br />(P+0.as) <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 /> <br /> <br />2-3 <br />TA64986-Denison MineslTask Order I - Sunday Mines Drainage StudAVan 4 Mine Drainage Study\Report%V-4 Mine Drainage Report.dm