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• 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 />where: <br />L = hydraulic length = 0.188 miles <br />H = watershed height = 59 ft. <br />tc = [11.9(0.188)3]0.385 = 0.078 hr. <br />59 <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, 46.41 tons or <br />0.024 ac-ft. of sediment will be deposited in this pond as the result of <br />a 10-year, 24-hour storm. A total of 0.068 ac-f t. or 2.8 MUSLE 10-year, <br />24-hour storm event equivalent storage is provided for sediment yield. <br />Peabody will restore adequate sediment storage to the pond when the <br />available sediment storage becomes 0.024 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 />C <br />7 <br />