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** Sedimentation Pond No. 4 contains a small undisturbed area surrounding <br />the pond and the pond's water surface area in addition to the disturbed <br />and undisturbed watersheds contributing to the refuse collection ditches. <br />A curve number was determined for each. <br />*** Magpie Creek Diversion receives runoff from two (2) large undisturbed <br />subwatersheds. Due to their size, curve numbers were calculated <br />separately for each sub-basin based on soil and vegetation types. <br />As previously mentioned, computer (SEDCAD+) printouts for all mine area <br />structures are contained in the respective design calculations sections of <br />this exhibit. These printouts contain all hydrologic input values and <br />calculated peak flow values on a subwatershed basis for each structure. <br />Once peak flow values are calculated by SEDCAD+, Mannings Equation for <br />open channel flow was utilized to size the diversions and ditches. <br />Q = 1.486 ARzi3Svz <br />n <br />Where Q = peak discharge for desired storm event (cfs) <br />n = Mannings roughness coefficient <br />A = cross sectional area of channel flow (ftz) <br />R = hydraulic radius (ft) <br />S = channel slope (ft/ft) <br />A Mannings roughness coefficient of 0.04 was used corresponding to an <br />earthen channel with some weeds and stones (Barfield et. al., 1981, page <br />159). <br />For channel sections with flow velocities exceeding 5.0 feet per second <br />rock riprap is, or will be, installed. The following equation is used to <br />determine riprap size. <br />•~- <br />_-~ ~I <br />r <br />9 R Revised - December, 1991 <br />