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Sun Coal Company conducted decommissioned haul road design studies assuming <br />• the rainfall from a 10-year, 24-hour precipitation event is 1.6 inches, with a <br />25-year and 100-year storm event having 2.0 and 2.5 inches, respectively. <br />Culverts and ditches installed to convey water from the road during mining were <br />modified to comply with CMLRD's request to permit the structures to convey the <br />peak flow from the 100-year, 24-hour storm event. Eighteen-inch culverts were <br />used along the haul road (Map 8) and are designed to convey the peak flow <br />without head from the 100-year, 24-hour storm event (refer to Appendix 2.05.3-5A <br />for design calculations). <br />All culverts are 1.5 to 2.0 feet below the road surface at the inlet, and <br />culvert slopes are less than 5 percent. Many approaches to culverts have been <br />riprapped where erosion or entrance velocities have been a problem in the past. <br />Where possible, all culvert outlets discharge below the toe of the fill, and <br />• most have been riprapped to dissipate flow velocity energy. At the outlets of <br />Culverts 2, 3, and 4, a sediment trap with the configuration similar to Figure <br />2.05.3-3 has been constructed; and a trap configuration similar to Figure <br />2.05.3-4 has been installed at outlets from Culverts 7, 8, 9, and 10. <br />The traps were designed to reduce flow velocities and to trap eroded sedi- <br />ment from the drainage above the road. The sediment traps are approximately <br />20 feet long, 15 feet wide, and 5 feet deep. Each trap has a parabolic outlet <br />approximately 5 feet wide. All traps are riprapped and routinely maintained as <br />outlined in Section 2.05.6. <br />L_ J <br />- 174 - (Rev. 5/86) <br />