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• l Unit Hydrograph and Surface Condition <br />r 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 (USER, 1977): <br />tc = [11.9 L x]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 Nuc1a <br />Mtne Plan. The inputted soil particle distribution is as follows: <br />7 <br />