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Sedimentation ponds are proposed for eight separate drainage sub- <br /> basins in the Preparation Plant Area, as shown on Plate II, to miti- <br /> gate the effects of erosion from runoff caused by heavy rains and <br /> snowmelt. The drainage areas, channel characteristics and concentra- <br /> tion times for the eight Preparation Plant drainage sub-basins are <br /> given in Table 3. It is expected that the discharges from these <br /> ponds will meet all federe.l, state and local requirements. <br /> Flood and Runoff Analysis. The Rational Method was adopted for use <br /> with the small drainages in the Preparation Plant Area. The drainage <br /> areas were measured from a Company topographic map having a scale of <br /> 1" = 200' and five-foot contour intervals. The differences in eleva- <br /> tion and average drainage channel slopes were estimated from this <br /> map. Time of concentration values were limited to not less than <br /> fifteen minutes for the reasons discussed in the previous section <br /> dealing with the Road Systems Area. <br /> The rainfall intensities were also estimated in the same manner as <br /> described for the_ Road Systems Area. Rainfall depths used were for <br /> a 10-year/24-hour precipitation event obtained from the NOAA Atlas 2 <br /> 10 <br />