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Septembn 1998 Senera l/W SoutS Expansion * Drainage and Sedamrnt Control Plan • 7 <br />SENECA IIW SOUTH EXPANSION AREA <br />M DRAINAGE AND SEDIMENT CONTROL PLAN <br />7.0 INTRODUCTION <br />Seneca Coal Company (SCC) proposes to develop a new coal reserve south of the existing <br />Seneca II-W Coal Mine. To control run-off from the new development area, SCC proposes to <br />build three new sediment ponds (Pond 015, 016, and 017). In addition, to access the new area <br />SCC will construct three new haul roads (Haul Road I, J, and K). The location of the proposed <br />ponds and Roads aze shown on Exhibit 13-2A. <br />Once mining is complete and the area is reclaimed, SCC will construct 4 new post mining <br />channels. The location of the channels are shown on Exhibit 20-2A. <br />The following is the methodology and assumption that went into the designs of the three <br />ponds, haul road ditches and culverts, and post mining channels. <br />The hydrologic, hydraulic, and sediment calculations used to model the Seneca II-W South area <br />were performed utilizing the SEDCAD+ computer modeling program developed by Civil <br />Sofrwaze Design. The results of the SEDCAD+ analyses are presented in the attached <br />calculation exhibit and are summarized in Table 5 of this report. Sedimentation Ponds 015, 016, <br />and 077 were analyzed according to the following methodologies. <br />2.0 HYDROLOGIC CALCULATIONS <br />SEDCAD+ is a comprehensive, computer based model used to evaluate storm water, erosion, <br />and sediment control management techniques. The SEDCAD+ model calculates run-off and <br />peak flow via a numerical modeling technique based on user inputs of a design storm event (i.e., <br />rainfall amount and duration, selection of rainfall distribution, and convolution increment). <br />Hydrographs are developed on a subwatershed basis with the input of area, time of <br />concentration, SCS Curve Number, and the selection of 1 of 3 dimensionless double triangle <br />unit hydrograph shapes. Routing of hydrographs is accomplished by Muskingum's Method. <br />Inputs to the hydrology component of the SEDCAD+ computer model include: <br />• Precipitation Distribution <br />• Storm Duration <br />• Return Period/Precipitation <br />• Hydrograph Response Shape <br />• Drainage Basin Area <br />• Time of Concentration <br />• Muskingum Routing Parameters <br />• Curve Number <br />Input values are shown on the SEDCAD+ printouts and explained in the following sections. <br />2.1 Precipitation Distribution <br />• A precipitation distribution is input to model the run-off hydrograph. SEDCAD+ allows the <br />user to choose between the SCS type I, type IA, type II, type III, and user defined storm umt <br />mass curves. The type II distribution was input as an estimation of the area storms. <br />Montgomery Watson Mining Group * 165 5. Union Boulevard, Surte 460 * Lakewood, Colorado 80228 ' (970f 7635140 <br />