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1.4 Hydrograph Response Shape <br />• A unit hydrograph is chosen for each drainage area or sub-area modeled to predict the run-off <br />response. The hydrograph responses available in the SEDCAD+ model are slow, medium, and <br />fast. A slow response corresponds to a forested azea or an azea with a number of obstructions. <br />A fast response corresponds to an unvegetated or poorly protected azea. Medium and fast <br />hydrograph responses were chosen for the subject site. <br />The internal convolution increment is 0.05 hours, and values are saved at the user specified <br />interval of 0.1 hours or greater. A convolution increment of 0.1 was specified for the subject <br />site. It should be noted that a time of concentration Less than 0.125 hours bypasses the unit <br />hydrograph technique and instantaneous run-off is assumed. As explained in the SEDCAD+ <br />User's Manual (Schwab and Warner, 1987), the time of concentration restriction of 0.125 hours <br />is somewhat mandated due to a combination of array size restrictions, minimum internal <br />convolution interval, and the user specified time increment Eor saving convoluted values. <br />1.5 Drainage Basin Area <br />The drainage basin azeas were determined by digitizing directly from scaled drawings. The <br />final post-mining topography was used as the worst case representation of the Streeter, Gulch <br />A, Prospect, Section 16, East Taylor, and West Pit Ponds. The existing condition topography <br />was used as the worst case representation for the Work Area (Warehouse), Gossard, Gossard <br />Loop, and Stoker Siding ponds. The watershed was broken into four sepazate areas <br />representing various stages of reclamation and disrurbance. The areas represent various stages <br />of reclamation according to the most current mine plan developed by Colowyo Personnel. <br />• 1.6 Time of Concentration, T~ <br />The time of concentration was calculated using the SCS upland method (Soil Conservation <br />Service, 1986). The hydraulic lengths, drainage heighu and associated slopes were determined <br />from scaled drawings. The values calculated for each structure are shown on asub-watershed <br />basis in the SEDCAD+ printouts and attached watershed summary sheets. <br />1.7 Muskingum Routing Parameters, K, X <br />The Muskingum Routing Parameters were also calculated using the SCS upland method. The <br />hydraulic lengths, drainage heights and associated slopes were determined from scaled drawings. <br />The values calculated between each structure and/or sub-watershed are shown in the <br />SEDCAD+ printouts. <br />1.8 Curve Number, CN <br />The run-off curve number is a Factor relating the amount of rainfall to the amount of run-off <br />for a given azea. Curve numbers were jointly developed and agreed upon by both the Division <br />of Minerals and Geology and Colowyo. The curve number for the topsoiled and seeded areas <br />was based on a hydrologic soil group B with no vegetation. A summary of the curve numbers <br />used for this evaluation is summarized in Table 1. The curve number for the topsoiled and <br />seeded azeas was added to the existing Colowyo curve number list. <br /> <br />2 <br />i.191MMw]x <br />~~~~~~°,i~ru REVISED 11/98 <br />