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September 2009 Foidel Creek Mine * Refuse Disposal Area Expansion — Surface Water Control Plan * Page 1 <br />1.0 Introduction <br />This report presents the Surface Water Control Plan (Plan) for the 20 million cubic yard expanion of <br />the Refuse Disposal Area for Twentymile Coal Company's (TCC) Foidel Creek Mine. The purpose <br />of the Plan is to provide minimum design recommendations for the surface water control structures <br />in accordance with the regulations of the Colorado Mined Land Reclamation Board for Coal Mining. <br />The Plan evaluated two phases for the Refuse Disposal Area. The initial phase of the plan covers the <br />initial construction sequence and placement of approximately 3.0 million cubic yards of coal refuse <br />materials. The postmine design addresses surface water control after the placement of approximately <br />20 million cubic yards of material and completion of final grading in anticipation of facility closure <br />and reclamation. <br />The following is the methodology and assumptions used in the design of temporary diversion <br />ditches, culverts and postmine channels. The locations of the hydrologic control structures for the <br />postmine Plan configuration are shown on Drawing C -101, Final Grading Plan; the initial sequence <br />Plan configuration is shown on Drawing C -102, Initial Sequence Grading Plan. The design <br />demonstrations for the surface water control ditches and postmine channels are presented in <br />Attachment A, Channel Design SEDCAD Results. Attachment B, Culvert Designs, presents the <br />culvert designs. <br />• 2.0 Surface Water Hydrologic Calculations <br />The hydrologic calculations were performed using SEDCAD4 (Schwab, 1998). Hydrology inputs <br />are consistent with those utilized for previously submitted and approved Foidel Creek Mine drainage <br />designs. <br />Hydrologic Inputs <br />• Rainfall Distribution: a NRCS Type II storm was used for the rainfall distribution. <br />• Precipitation: the 10 -year and 100 -year, 24 -hour precipitation events were used to design the <br />temporary ditches and postmine channels, respectively. The following precipitation amounts <br />were used: <br />0 10 -year, 24 -hour storm event —1.8 inches <br />0 100 -year, 24 -hour storm event — 2.6 inches <br />• Drainage Area: subwatershed areas were determined by digitizing directly from permit <br />drawings. <br />• Time of Concentration and Muskingum Routing Parameters were calculated using the <br />Natural Resource Conservation Service upland method (SCS, 1986). Measurements were <br />taken from permit drawings. <br />• Curve Number: the curve number (CN) values utilized were taken from previous Foidel <br />Creek Mine refuse disposal area drainage designs. <br />o refuse disposal area — disturbed condition, coarse material; CN value = 86 <br />Hydro - Environmental Solutions, Inc. <br />57 10` Street, Suite B <br />P.O. Box 772996 <br />Steamboat Springs, CO 80477 <br />Revised 09/09 <br />