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Section 3 <br />West Sunday Mine <br />3.1 Existing Drainage Conditions <br />The West Sunday Mine has a drainage area of approximately 9.2 acres. Surface <br />drainage flows north and northwest to an earthen berm located below the WRA, <br />which works as a temporary sediment basin for the site. The drainage from the berm <br />discharges to the north into an unnamed tributary of Big Gypsum Creek, located <br />approximately 3/4 mile north of the mine workings area. <br />3.2 Drainage Plan <br />CDM delineated four basins based on the existing mine surface as illustrated in <br />Exhibit B-1: <br />1. Basin 1- Surface water run-on from offsite southwest of the surface mine <br />facilities. <br />2. Basin 2 - Surface water run-on from offsite southeast of the surface mine facilities. <br />3. Basin 3 - Surface facilities area. <br />4. Basin 4 - WRA slope. <br />3.3 Peak Discharge Estimate <br />CDM estimated the 100-year peak discharge for each drainage area using the <br />following USGS regionalized flood-frequency equations for Colorado (USGS 2000): <br />=I 18.4(Ar.71s <br />Q?oo <br />where: <br />Qioo =100-year peak flow in cfs <br />A = drainage area in square miles <br />As discussed previously, CDM increased the calculated discharge from the flood- <br />frequency equation by the standard error or 78 percent. Table 3-1 presents CDM's <br />estimated 100-year peak discharges for each basin of the West Sunday Mine. CDM <br />used the 100-year peak discharges to design the surface water drainage structures. <br />Table 3-1 100-vear Peak Discharges for the West Siiindeav Mina aae:inc <br /> 100-year peak discha e <br /> Area . Calculated Increased by SE <br />Basin NO s -mi cfs cfs <br />1 1.26 0.00196 1.37 2.44 <br />2 2.04 0.00318 1.94 3.45 <br />3 3.33 0.00520 2.76 4.91 <br />4 2.62 0.00409 2.32 4.13 <br />I? <br />3-1 <br />0.184988- Denison\Tesk Order l- Sunday Mines Drsinege StudVAeport%Final ReponlFiul Sunday Mines Dreiropa Repor 052299.doc