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METHODOLOGIES AND ASSUMPTIONS FOR <br />SEDIMENTATION POND DESIGN EVALUATIONS <br />The hydrologic and hydraulic calculations used to model all 10 of the Colowyo Mine sedimentation ponds <br />and the Taylor Pump Holding Pond, were performed using the SEDCAD4 computer modeling program <br />developed by Civil Software Design. The results of the SEDCAD4 analyses are presented as attachments <br />in each of the individual pond design sections and are summarized in Table 6, Sedimentation Pond Design <br />Summary. <br />The general procedures described herein will be used for all future hydrologic designs for ponds, ditches, <br />and other facilities to be constructed at the Colowyo Mine. Also refer to a similar discussion presented in <br />Volume 13, Exhibit 7, Item 20 for the structures associated with the South Taylor area. <br />1.0 HYDROLOGIC AND HYDRAULIC CALCULATIONS <br />SEDCAD4 is a comprehensive, computer-based model used to evaluate storm water, erosion, and sediment <br />control management techniques. The SEDCAD4 model calculates run-off and peak flow via a numerical <br />modeling technique based on user inputs of a design storm event (i.e., rainfall amount and duration, <br />selection of rainfall distribution, and convolution increment). Hydrographs are developed on a sub- <br />watershed basis with the input of area, time of concentration, SCS Curve Number, and the selection of 1 of <br />3 dimensionless double triangle unit hydrograph shapes. Routing of hydrographs is accomplished by <br />Muskingum's Method. <br />Inputs to the hydrology component of the SEDCAD4 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 />• Base Flow <br />Input values are shown on the SEDCAD4 printouts included as attachments in each of the individual pond <br />design sections, and explained in the following sections. <br />1.1 PRECIPITATION DISTRIBUTION <br />A precipitation distribution is input to model the run-off hydrograph. SEDCAD4 allows the user to choose <br />between the SCS Type I, Type IA, Type H, Type III, and user-defined storm unit mass curves. The Type H <br />distribution was input as the most accurate reflection of area storm patterns. <br />1.2 STORM DURATION <br />A storm duration of 24 hours was used for the design and evaluation of the Colowyo ponds. <br />Exhibit 7 Page 1 Revision Date: 6/19/09 <br />Revision No.: TR-73