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METHODOLOGIES AND ASSUMPTIONS FOR <br />SEDIMENTATION POND EVALUATIONS <br />• The hydrologic and hydraulic calculations used to model all 10 of the Colowyo mine site <br />sediment impoundments, were performed utilizing the SEDCAD+ computer modeling <br />program developed by Civil Software Design. The results of the SEDCAD+ analyses are <br />presented in Appendices A-J and are summarized in Table 5 of this reporz. <br />1.0 HYDROLOGIC AND HYDRAULIC 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 sub-watershed 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 hvdrographs 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 />• Base Flaw <br />Input values are shown on the SEDCAD+ printouts attached co this report and explained in <br />the following sections. <br />1.1 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 unit <br />mass curves. The type II distribution was input as a estimation of the area storms. <br />1.2 Storm Duration <br />A storm duration of 24 hours was used for the design and evaluation of the Colowyo <br />sedimentation ponds. <br />1.3 Return Period/Precipitation <br />A precipitation amount is required for the appropriate return period. The following return <br />periods and precipitation events were used for the Colowyo mine site: <br />10-year, 24-hour storm event 1.8 in <br />25-yeaz, 24-hour storm event 2.3 in <br />-~v~~,,.,nm.sa <br />:l/I!/M I:H PM <br />REVISED 11/9s <br />