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September 2009 Foidel Creek Mine * Refuse Disposal Area Expansion — Surface Water Control Plan * Page 7 <br />of the fill). The possibility of infiltrating water moving downward into the underlying spoil or <br />natural soils was ignored even though this could reduce future peak flows through the underdrain. <br />The 10 -year, 24 -hour storm event had the following characteristics: <br />• Precipitation: 1.8 inches in 24 hours <br />• Maximum infiltration: 1.4 inches per hour <br />• Phase I peak surface flow: 15.2 cfs <br />• Closure footprint peak surface flow: 74.5 cfs <br />The volume of water entering the pile was calculated and travel times to the drain were calculated. <br />Travel times through the fill were normalized to provide similar total volume of flow over a longer <br />time period. Assuming very conservative travel time values (all infiltration passes through the pile to <br />the drain in less than 15 days) a peak flow of 1.2 cfs was calculated. Assuming a conservative factor <br />of safety, a drain capacity of 12 cfs was chosen for the refuse disposal area underdrain. <br />The capacity of the underdrain was determined according to a method presented by Office of Surface <br />Mining and Reclamation (1982) which considers the porosity, gradient, D50 of drain rock and an <br />empirical constant which varies according to the D 50 of the rock. Using this method a drain <br />consisting of minimum 9 -inch rock with a minimum cross section of 50 square feet was designed to <br />carry approximately 12 cfs. Filter criteria were developed based upon Bureau of Reclamation (1987a <br />and 1987b) criteria. <br />The recommended location of the drain is shown of Drawing C -101, Final Grading Plan and sections <br />and material requirements are presented on Drawing C -- Sections. All drain and filter aggregate <br />should consist of hard, durable rock. <br />6.0 References <br />Bureau of Reclamation, 1987a. Design of Small Dams, Third Edition. U.S. Department of the <br />Interior. <br />Bureau of Reclamation, 1987b. Embankment Dams, Design Standards No. 13, Chapter 5, Protective <br />Filters. U.S. Department of the Interior, Washington, D.C. <br />Office of Surface Mining and Reclamation, 1982. Engineering and Design Manual for Disposal of <br />Excess Spoil. Office of Surface Mining, U.S. Department of the Interior, Washington, D.C. <br />Schwab, P.J. SEDCAD4 for Windows. Civil Software Design, Ames, IA. <br />Soil Conservation Service (SCS), 1986. Urban Hydrology for Small Watersheds, TR -55. Soil <br />Conservation Service, U.S. Department of Agriculture, Washington, D.C. <br />Stover, M. 1990. Erosion and Sediment Pollution Control Program Method. Pennsylvania <br />Department of Environmental Resources, Bureau of Soil and Water Conservation, Division of <br />Soil Resources and Erosion Control, Harrisburg, PA. <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 />