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,: - <br />--.=._, Cotter CoY•~~o>i•a~ioil <br />'„F <br />-~ ~": Genes al office <br />August 26, 1983 <br />Mr. George Van Slyke <br />Division of Water Resources <br />Colorado Department of <br />Natural Resources <br />1313 Sherman Street <br />Denver, CO 80203 <br />~ECEI~fE® <br />RE: Ore Sorter Flood Protection/Scour Analysis <br />Dear Mr. Van Slyke: <br />n <br />~~ <br />sss -° <br />D I A L ] 0 3~ 2] 2~ o <br />5305 WEST ALAMEDA PARKWAY SUITE 201 <br />LAKEWOO D. COLORADO 80:]n <br />AUG 20198; <br />PAINED LAND RECLAh1ATIDN Di VISION <br />Colo. Dept. of Natural Resources <br />In response to your August 15, 1983 telephone call, Cotter has set <br />forth below the scour analysis pertaining to the footing of the pro- <br />posed flood protection wall described in Cotter's August 5, 1983 <br />letter to the Department of Health. The enclosed scour analysis <br />was prepared by McDermid Engineering Associates, Inc. <br />In accordance with discussions between representatives of Cotter <br />and McDermid Engineering Associates (MEA), MEA prepared this <br />description of the scour mechanism and potential scour depths near <br />the ore sorter site at Cotter's Schwartzwalder Mine. The predicted <br />scour depths may be used to design the depth to which the I-beam and <br />lagging retaining wall should be constructed, as shown on the attached <br />sketch. <br />Ralston Creek Hydrology <br />The 100-year flood design discharge in Ralston Creek at the Schwartz- <br />walder Mine Site has been computed at 5,810 cfs, as described in <br />previous reports. This Tiow rate Was caicuiatea Ter file crairaye area <br />at the mine site based on work by others at Ralston Reservoir down- <br />stream. The earlier reports have been reviewed and accepted by the <br />Colorado Water Conservation Board; they also have approved this <br />design discharge at this site. <br />During this high recurrence interval flood event, the bankfull capacity <br />of Ralston Creek will be exceeded and flood waters will encroach on <br />the ore sorter site. The hydraulics of this condition are described <br />below. <br />Channel Hvdraulics <br />Because of the steep stream bed gradient, velocities in the main channel <br />will be high, decreasing in the overbank area in which the area sorter <br />