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Methodologies and Assumptions <br />1.0 INTRODUCTION <br />This attachment provides the methodology and assumptions used to determine runoff from the <br />reclaimed mine site. Calculations for the Hayden Gulch Mine runoff analysis were performed utilizing <br />the SEDCAD'3 (SEDCAD) Computer Model developed by Civil Softwaze Design (Schwab, 1992). <br />Results of the SEDCAD modeling are included in Attachment 2. <br />Section 2 presents the SEDCAD hydrology and sedimentology model methodology and assumptions <br />which were used to calculate runoff volumes, peak flows, and sediment delivery. Section 3 provides <br />the references cited in the methodology and assumptions attachment. <br />2.0 SEDCAD HYDROLOGIC COMPUTER MODEL <br />Calculations for the watershed analysis utilized the SEDCAD computer model (Schwab, 1992). <br />SEDCAD is a hydrologic, hydraulic, and sediment calculation model designed for use on computer <br />systems. The SEDCAD hydrologic model calculates runoffvolume and peak Flow via a numerical <br />modeling technique based on user inputs of a design storm event (i.e., precipitation frequency data, <br />selection of rainfall distribution, and convolution increment). Hydrographs are developed on a <br />subwatershed basis with the input of area, time of concentration, SCS Curve Number, and the <br />selection of a hydrograph shape. Routing of hydrographs is accomplished by the Muskingum <br />Method. <br />2.1 SEDCAD Hydrology <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 used in this model, are shown on the SEDCAD printouts and are explained in the <br />following text of this exhibit. <br />Precipitation Distribution <br />A precipitation distribution is input to model the runoff fiydrograph. SEDCAD allows the user to <br />choose between the SCS Type i and Type II and 6-hour storms. Type II storm distribution is <br />designed for use in all areas of the western continental U.S. except for the coastal side of the Sierra <br />c:~87Thg8~prinuryMethod.how 7/16/96 <br />