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• -2- • stockpile bench floors, which will be graded to act as retention areas; these retention areas will be constructed so as to hold in excess of the 10-year 24-hour discharge of this drainage without overflowing. In addition, these areas will be constructed so as to drain into the east perimeter ditch system along the base of the highwall or rock stockpiles so that overflow in case of a greater than 10- year 24-hour event will not run over the outslopes of the benches. The revised permit map (map C-la) accompanying this report shows the proposed diversion components in detail. AREA I Area I is an ephemeral drainage that originates approximately one mile north of the quarry. It has a drainage area of 137.04 acres as measured by planimeter. Between the quarry and the headwater divide between it and Oasis Creek this drainage has an average gradient of approximately 30%, with elevations varying from 6,700 feet at the quarry to 8,520 feet at its highest point. Area I consists almost entirely of dip slopes developed on the southerly-dipping Leadville Limestone (Pennsylvanian). In lower areas near the quarry these slopes average approximately 300, moderating somewhat near the top of the drainages. Cliffs and ledges up to vertical occur in the area, as well as significant outcrops of Leadville Limestone, The area is sparsely to moderately vegetated with the pinyon-juniper and oakbrush assemblages, and generally thin soils are moderately to well developed except on outcrops. Where exposed, the limestone is moderately to highly fractured, and precipitation infiltration is thought to approach that of the soils. Calculations using the U.S. Department of the Interior Office of Surface Mining's Storm program (ver. 6.21) indicate a peak discharge for Area I of 8.38 cfs, with a total 10-year 24-hour storm drainage volume of 1.94 acre feet. Further calculations indicate that a trapezoidal ditch with a bottom width of 8 feet and sides sloped at a ratio of 3:1 will be adequate to divert this discharge from the quarry. D5O = 6" riprap would be required in any sections exceeding a grade of 11%.