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1.3 RETURN PERIOD/PRECIPITATION <br />A precipitation amount is required for the appropriate return period. The following return periods and <br />precipitation events were used for the Colowyo Mine: <br />10-year, 24-hour storm event 1.8 in <br />25-year, 24-hour storm event 2.3 in <br />100-year, 24-hour storm event 2.7 in <br />1.4 HYDROGRAPH RESPONSE SHAPE <br />A unit hydrograph is chosen for each drainage area or sub-area modeled to predict the run-off response. <br />The hydrograph responses available in the SEDCAD4 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 />un-vegetated or poorly protected area. Medium and fast hydrograph responses were identified as <br />representative for mine areas. <br />The internal convolution increment is 0.05 hours, and values are saved at the user-specified interval of 0.1 <br />hours or greater. A convolution increment of 0.1 was specified. 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 <br />assumed. As explained in the SEDCAD4 User's Manual (Schwab and Warner, 1998), the time of <br />concentration restriction of 0.125 hours is dictated by a combination of array size restrictions, minimum <br />internal convolution interval, and the user-specified time increment for saving convolution values. <br />1.5 DRAINAGE BASIN AREA <br />Drainage basin areas were determined for each pond from scaled drawings. The final post-mining <br />topography was used as the worst-case representation for all ponds. Watershed (or sub-watershed) areas <br />were delineated consistent with four general categories representing various stages of reclamation and <br />disturbance. The watershed or sub-watershed delineations reflect anticipated disturbance/reclamation <br />status based on Colowyo's current mining and reclamation plans. <br />1.6 TIME OF CONCENTRATION, Tc <br />The time of concentration was calculated using the SCS upland method (Soil Conservation Service, 1986). <br />The hydraulic lengths, drainage heights and associated slopes were determined from scaled drawings. The <br />values calculated for each structure are shown on a sub-watershed basis in the SEDCAD4 printouts. <br />1.7 MUSKINGUM ROUTING PARAMETERS, K, X <br />The Muskingum Routing Parameters were also calculated using the SCS upland method. The hydraulic <br />lengths, drainage heights and associated slopes were determined from scaled drawings. The values <br />calculated between each structure and/or sub-watershed are shown in the SEDCAD4 printouts. <br />1.8 CURVE NUMBER, CN <br />The runoff curve number is a factor relating the amount of rainfall to the amount of runoff for a given area. <br />Curve numbers were jointly developed and agreed upon by the Division of Minerals and Geology and <br />Colowyo. The curve number for the topsoiled and seeded areas was based on a hydrologic soil group B <br />with no vegetation. Table 1 provides a summary of the curve numbers used for the pond design <br />evaluations. <br />The Table I curve numbers provided in bold italics represent values used historically for design purposes <br />and have been previously agreed upon between DRMS and Colowyo staff. In February, 2009, the <br />Colowyo Mine proposed a series of revised curve numbers to provide more typical values relative to <br />previously published sources. This change also provides the design engineer more judgment to adjust for <br />Exhibit 7 Page 2 Revision Date: 6/19/09 <br />Revision No.: TR-73