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Unit Hydrograph and Surface Condition <br /> The SEDIMOT II computer program provides three unit hydrograph shapes to <br /> choose from in determining the runoff characteristics of a watershed. <br /> The disturbed hydrograph (No. 1) was used for disturbed and reclaimed <br /> areas. <br /> Time of Concentration <br /> The time of concentration, tc, is defined as the time required for water <br /> to flow from the hydraulically most remote point in the watershed to the <br /> watershed outlet. The runoff time of concentration was calculated using <br /> the following equation (USBR, 1977): <br /> tc = [11.9 L 3]0.385 <br /> H <br /> where: <br /> L = hydraulic length = 0.54 miles <br /> H = watershed height = 133 ft. <br /> tc = 0.194 hr. <br /> SEDIMENTOLOGY <br /> The SEDIMOT II computer program was also utilized to calculate the <br /> sedimentology of each watershed using the modified universal soil loss <br /> equation (MUSLE) . Using a density of 87 lbs/cubic foot, 260.77 tons or <br /> 0.138 ac-ft. of sediment will be deposited in this pond as the result of <br /> a 10-year, 24-hour storm. A total of 0.419 ac-ft. or 3.0 MUSLE 10-year, <br /> 24-hour storm event equivalent storage is provided for sediment storage. <br /> Peabody will restore adequate sediment storage to the pond when the <br /> available sediment storage becomes 0.138 ac-ft. or less. <br /> Particle Size Distributions <br /> The particle size distribution for this sediment pond was taken from <br /> Simons, Li and Associates (Design of Sediment Control Measures for Small <br /> Areas in Surface Coal Mining). This distribution is representative of a <br /> sandy loam soil similar to those described in Tab 9 of the current Nucla <br /> Mine Plan. The inputted soil particle distribution is as follows: <br /> 7 <br />