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FtGtuary 1999 Yaart Mine * 13-02 Drainage oxd Sedimext CanttnlPlan • 1 <br />• SENECA YOAST MINE <br />DRAINAGE AND SEDIMENT CONTROL PLAN <br />1.0 INTRODUCTION <br />Seneca Coal Company (SCC) proposes to develop the remaining coal reserves south of the existing Yoast Coal Mine <br />operations in the Sage Creek drainage. To control nut-off from the new development area, SCC proposes to build <br />three new sediment ponds (Pond 012, 013, and 014) and associated ditches (ditch 13A and 14A). In addition, to <br />access the new azea, SCC will extend existing haul road B and constmct new haul roads C and D. The design of the <br />haul road ditches aze included in this attachment The location of the proposed ponds, ditches and the haul roads are <br />shown on Exhibit 12-1, Operations Plan. <br />Once mitring is complete, SCC will construct 11 post mining chaturels in conjunction with the reclamation plan. The <br />reclaimed topography and channel locations are shown on Exhibit 20-2, Post Mining Topography (I'ab 20). The <br />design of the post mitring chacurels aze included in this attachment <br />The following is the methodology and assumptions that were used in the designs of the ponds, ditches, haul road <br />ditches and culverts and post mining channels. <br />The hydrologic, hydraulic, and sediment calculations used to model the ponds, ditches, and post mining channels wue <br />performed utilizing the SEDCAD4 computer modeling program developed by Civil Softwaze Design (Civil Software <br />Design, August 1998) and were analyzed according to the following methodologies. <br />2.0 HYDROLOGIC CALCULATIONS <br />• SEDC_t1D4 is the next generation of hydrology and sedimentology modeling software that replaces SEDCAD+. It is <br />a comprehensive, computer-based model used to evaluate storm water, erosion, and sediment control maztagetnent <br />techniques. The SEDCAD4 model calculates run-off and peak flow via a numerical modeling technique based on <br />user inputs of a design storm event (r.e., rainfall volume and duration, selection of rainfall distribution, and convolution <br />increment). Hydrographs aze developed on a subwatershed basis with the input of azea, time of concentration, SCS <br />Curve Number, and the selection of 1 of 3 dimensionless double triangle unit hydrograph shapes. Calculated zunoff is <br />routed to the various structures input into the model Musk+na sn routing parameters aze used. <br />Inputs to the hydrology component of the SEDCAD4 computer model include: <br />• Predpitadon Distribution <br />• Storm Duration <br />• Retum Period/Precipitation <br />• Hydrograph Response Shape <br />• Drainage Basin Area <br />• Time of Concentration <br />• Muskingum Routing Parameters <br />• Curve Number <br />Input values aze explained in following sections. <br />2.9 PRECIPITATION DISTRIBUTION <br />• A precipitation distribution is input to model the mn-o EE hydrograph. SEDCAD4 allows the user to <br />Montgomery Wakan Mrning Group * 165 S. Union Boulevard, Suite 460 * Lokemaad, Colorado 80228 * (303) 763-5140 <br />