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• METHODOLOGIES AND ASSUMPTIONS FOR <br />SEDIMENTATION POND DESIGN EVALUATIONS <br />The hydrologic and hydraulic calculations used to model the three proposed Colowyo Mine sedimentafion <br />ponds were performed using the SEDCAD4 computer modeling program developed by Civil Software <br />Design. The results of the SEDCAD4 analyses are presented as attachments in each of the individual <br />pond design sections and are summarized in Table 4, Sedimentation Pond Design Summary. <br />l.0 HYDROLOGIC AND HYDRAULIC CALCULATIONS <br />SEDCAD4 is a comprehensive, computer-based model used to evaluate storm water, erosion, and <br />sediment control management techniques. The SEDCAD4 model calculates run-off and peak flow via a <br />numerical modeling technique based on user inputs of a design storm event (i.e., rainfall amount and <br />duration, selection of rainfall distribution, and convolution increment). Hydrographs are developed on a <br />sub-watershed basis with the input of area, time of concentration, SCS Curve Number, and the selection <br />of 1 of 3 dimensionless double triangle unit hydrograph shapes. Routing of hydrographs is accomplished <br />by 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 <br />pond design sections, and explained in the following sections. <br />1.1 PRECIPITATION <br />A precipitation distribution is input to model the mn-off hydrograph. SEDCAD4 allows the user to <br />choose between the SCS Type I, Type IA, Type II, Type III, and user-defined storm unit mass curves. <br />The Type II distribution was input as the most accurate reflection of area storm patterns. <br />1.2 STORM DURATION <br />A storm duration of 6 hours was used for the probable maximum precipitation (PMP) and a duration of 24 <br />hours was used for the 10-year and 100-year storm event recurrence intervals in the design and evaluation <br />of the Colowyo ponds. <br />C_~ <br />Exh. 7-20-1 Revision Date: 3/29/07 <br />Revision No.: PR-02 <br />