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INTRODUCTION This technical revision to the permit to conduct coal mining operations at [he Trapper Mine involves changes in the drainage and sediment control plan for the areas drained by Johnson Gulch and Pyeatt Gulch. The revision to the Johnson Gulch plan is necessitated by the impending removal of an existing sedimentation pond due to mining operations. This pond will be replaced by another pond in the revised plan. Detailed plans for the Pyeatt Gulch drain- age and sediment control plan were not included in the original permit appli- cation due to the uncertainty of [he actual mining plan and schedule for this area. Areas in the Pyeatt Gulch drainage will be disturbed by mining opera- tions in 1987 so construction of sediment control structures needs to be com- pleted in 1986. The 1986 and 1987 disturbed areas were submitted as a techni- cal revision in February, 1986. JOHNSON GULCH DRAINAGE AND SEDIMENT CONTROL PLAN A revision to the Johnson Gulch drainage and sediment control plan is neces- sary due to the removal of Johnson Pond lil. Johnson Pond lil will be removed in 1987 to accommodate regrading operations. Johnson Pond lil will be replaced by Johnson Pond lib during 1986. Johnson Pond lib will include enough capacity to compensate for removal of Johnson Dam lil plus the increase in sediment load from the disturbed area in the Johnson watershed over the life of the mine. Johnson Pond lib was previously abandoned because regrading plans called for destruction of this pond. A revised mining and reclamation plan has allowed Johnson lib to remain in place; therefore it is proposed to have Johnson Pond lib replace Johnson Pond lil. The location of Johnson Pond lib is shown on the ~ Pyeatt/Johnson Drainage and Sediment Control Plan Map, Map TR-486-1. The proposed Johnson Gulch sedimentation system consists of six sedimentation ponds. The capacity of each pond is shown in Table 1, Johnson and Pyeatt Sedimentation Pond Summary Information. The overall effectiveness of the ser- ies of sedimentation ponds was determined by modeling the surface water hydro- logy of the watershed. Results of the model indicate that during a 10-year, 24-hour precipitation event, the peak settleable concentration of sediment in the outflow of the final pond in the series, Johnson Dam /i10, will be 0.0011 ml/1. Input parameters for the hydrologic model, along with complete results, are included in Appendix A, Johnson Gulch Hydrologic Model, Appendix D, Curve Number Determination, and Appendix E, Sedimentology Parameters. Trapper has spent considerable time during the summer of 1986 in discussions with the Division in order to determine the applicable curve numbers and sedi- mentology parameters for design of drainage and sediment control systems. The agreed-upon parameters are shown in Appendix D and Appendix E. Trapper's pre- viously-approved method for designing sediment control structures entailed the use of a single curve number of 70. The new hydrology parameters, while not appreciably altering the field configuration of the Johnson sediment control system, have effected the prediction of peak flow. This increased peak flow is safely passed through the principal and emergency spillways in each pond. The capacities of each spillway are listed in Appendix F, Spillway Discharge 1 Revised 8/86