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g~. <br />October 17, 1996 <br />Ms. E. Cosby <br />Page 2 <br />Input values aze shown on the SEDCAD+ printouts in Attachment B and explained in the following sections. <br />1.1 Precipitation Distribution <br />A precipitation distribution is input to model the run-off hydrograph. SEDCAD+ allows the user to choose <br />between the SCS type I, type IA, type II, type III, and user defined storm unit mass curves. The type II <br />distribution was input as an 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 sedimentation ponds. <br />1.3 Return Period/Precipitation <br />A precipitation amount is required for the appropriate return period. The following retort[ periods and <br />precipitation events were used for the Colowyo mine site: <br />10-year, 24-hour storm event 1.8 N <br />25-year, 24-hour storm event 2.3 in <br />1.4 Hydrograph Response Shape <br />A unit hydrogaph is chosen for each drainage area or sub-area modeled to predict the run-off response. The <br />hydrograph responses available in the SEDCAD+ model are slow, medium, and fast. A slow response <br />corresponds to a forested area or an area with a number of obstructions. A fast response corresponds to an <br />unvegetated or poorly protected area. Medium and fast hydrograph responses were chosen for the subject site. <br />The internal wnvolution increment is 0.05 hours, and values are saved at the user specified interval of 0. L hours <br />or greater. A convolution increment of 0.1 was specified for the subject site. It should be noted that a time of <br />concentration less than 0.125 hours bypasses the unit hydrograph technique and instantaneous run-off is assumed. <br />As explained in the SEDCAD+ Users Manual (Schwab and Warner, 1987), the time of concentration restriction <br />of 0.125 hours is somewhat mandated due to a combination of array size restrictions, ntinimtun internal <br />convolution interval, and the user specified time increment for saving convoluted values. <br />1.5 Drainage Basin Area <br />The drainage areas were determined by digitizing directly from scaled drawings on asub-watershed basis. The <br />sub-water sheds are delineated for each condition on Figures 1 and 2 in Attachment A. <br />1.6 Time of Concentration, T~ <br />The time of concentration was calculated using the SCS upland method (Soil Conservation Service, 1986). The <br />hydraulic lengths, drainage heights, and associated slopes were determined from scaled drawings. The values <br />calculated for each structure are shown on asub-watershed basis in the SEDCAD+ printouts. <br />