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4.1 West Hillside Area <br />The west hillside drainage basin consists of the small natural area on the west side and <br />above the mine yard. Runoff from this area is routed through the West Pond and thus is not <br />discussed in the previous sections. The drainage area lies just west of the mine shaft entrance, <br />the scale house and the daysleeper. The area is 6.27 acres with a length to the divide of 970 ft <br />and an average slope of 24.7%. The curve number is 72. <br />4.2 Peak Dischazges and Volumes <br />The eastside and westside diversion systems consists of buried pipe and open channels as <br />previously described. These system will provide conveyance for all runoff resulting from the <br />100yr-24hr event intercepted above the waste pile on the East Basin and at the canyon mouth <br />diversion on the West Basin. The peak discharge is approximately 5.9 cfs for the East Basin and <br />4.6 cfs for the West Basin. <br />The portion of the total watershed which will be collected and routed into sedimentation <br />ponds consists of 8.8 acres of disturbed yard area and 14.5 acres of adjacent undisturbed area. <br />The detailed breakdown is given in the basin characteristic table presented above. The <br />undisturbed azeas were routed into the sedimentation ponds because of the difficulty in diverting <br />them around the yard. The l0yr-24hr volume for the West Pond is 0.98 AF and for the East <br />Pond 0.19 AF. The 25yr-24hr peak discharges for the West and East Ponds are 2.4 cfs and 0 <br />~ cfs respectively. Peak inflows are 2.5 cfs and 0.4 cfs for the West and East Ponds. These values <br />are summarized in the following table. <br />5. Drainage Structures <br />5.1 Pipes and Channels <br />The drainage system consists of 7 sections of open channel (reaches 1, 5, 6, 8, 10, 12 and <br />13) and 6 sections of pipe (reaches 2, 3, 4, 7, 9 and 11). All pipes were sized using a normal <br />depth, flowing full hydtaulic analysis. The pipe roughness used in the analysis is at the high end <br />for standard corrugated metal and plastic pipe. A backwater analysis and normal depth method <br />were employed in sizing drainage channels. The U.S. Army Corps of Engineers' HEC-2, Water <br />Surface Profiles computer model (HEC, 1982) was used for this task. Either as-built or more <br />restrictive dimensions were used in describing the geometry of all drainage structures. <br />5 <br />June 1995 <br />