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Diversion Ditches <br />~I Several diversion ditches will be used during mining Co control runoff. Some of <br />these ditches will be used only during the mining process (temporary), while <br />others will be more permanent. The major, more permanent diversions are long <br />term in nature, may remain in place until reclamation is complete, and generally <br />drain an area in excess of ten acres. They either route water from disturbed <br />areas into sedimentation ponds or route runoff from undisturbed areas away from <br />sedimentation ponds. The locations of all [he se major diversions at Trapper are <br />shown on Map M51, Drainage and Sediment Control Plan. The designs are shown on <br />Table 4.8-3, Diversion Ditch Sizing. However, the minor or temporary diversions <br />are either constructed on reclaimed lands for erosion control, or are of small <br />size, draining an area of ten acres or less in size. The Locations of minor <br />temporary diversions are not shown on Map M51 due to their temporary nature <br />and/or in significance in terms of flows. <br />A drainage reconstruction plan for Johnson Gulch describing all temporary diver- <br />sions and the postmining drainage characteristics has already been approved by <br />~ the Office of Surface Mining (OSM) and the Colorado Mined Land Reclamation Divi- <br />• Sion (CMLRD). This plan is included as Appendix P (Johnson Gulch Drainage Plan) <br />in this application. The plan was presented on May 20, 1980, and approved May <br />30, 1980, by letter from Mr. Sohn Hardaway, Office of Surface Mining, subsequent <br />to a joint OSM and CMLRD review. Conceptually, this plan requires a series of <br />temporary diversions to accommodate pit advancement through the gulch. Over the <br />last few years of activity in Johnson Gulch, Trapper has modified the diversion <br />ditch section of [his plan with CMLRD approval. These modifications have been <br />incorporated into this revised section. <br />Peak flow rates for ditches routing runoff from disturbed areas were determined <br />by the use of hydrologic modeling techniques. A runoff model was created for <br />each watershed under the 10-year, 24-hour precipitation event. The results of <br />each hydrologic model are presented in Appendix Q, Drainage and Sediment Control <br />Plan. <br /> <br />4-176 <br />~i9ED FEB 13 '87 <br />