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II West Coal Refuse Facility Hydrologic and Hydraulic Design <br />Sedimentation Pond E Retaining Structure <br />5.0 HYDROLOGIC AND HYDRAULIC DESIGN <br />A hydrologic analysis to size the sediment pond for the II West Coal Refuse Facility, <br />including developing the design height of the concrete retaining structure, is detailed in <br />Exhibit 2.05-E3, Drainage and Sediment Control Plan as revised and submitted to the <br />Colorado Division of Minerals and Geology in July 2003. The retaining structure, with a <br />crest elevation of 6,031', was designed to safely pass the 25-yeaz design storm, with an <br />additional 1.5-feet of freeboard between the maximum design water level and the top of <br />the concrete retaining structure, and to store the estimated peak 4-year sediment load. <br />A hydraulic analysis to size the principal and emergency spillways was also completed, as <br />detailed in the permit revision package submitted to the Colorado Division of Minerals <br />and Geology in July 2003. The analysis resulted in an 8-inch diameter ungated pipe with <br />an invert elevation at 6,027.5' serving as the principal spillway for the sedimentation <br />pond. An emergency spillway will also be constructed in the retaining wall. The <br />emergency spillway will consist of a 15-foot wide broad-crested weir located in the right <br />• section of the structure. ff dewatering of the pond is necessary to allow sediment clean- <br />out, aportable pump will be used. <br />• The principal and emergency spillways will be field located at the time of construction, <br />by the owner, based on field conditions (i.e. foundation conditions and materials, <br />construction operations [location relative to concrete pours to optimize strength], and the <br />location of the existing or new drainage structures downstream of the wall. <br />The design of the outlet piping and downstream drainage structures beyond the wall and <br />beneath the railroad tracks were not completed as part of this work. Impacts to the <br />railroad tracks, due to the operation of the spillways or outlet were not considered in the <br />structural design of the structure, but are addressed by a separate railroad crossing permit <br />application, submitted, reviewed and approved by the UPRR. <br />• <br />5 <br />