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<br />1.3 RETURN PERIOD/PRECIPTTATION <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 />100-year, 24-hour storm event 2.7 in <br />To meet the requirements of the Colorado State Engineer's Office (SEO), the emergency spillways were <br />designed to pass 50 percent of the probable maximum precipitation value, which is provided below: <br />50% of PMP 4.2 in <br />1.4 IIYDROGRAPH 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 <br />response corresponds to a forested area or an azea with a number of obstructions. A fast response <br />corresponds to an un-vegetated or poorly protected area. Medium and fast hydrograph responses were <br />identified as 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 Users 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 two categories: disturbed/newly reclaimed and undisturbed. The <br />watershed orsub-watershed delineations reflect the worst-case scenario. <br />1.6 TIME OF CONCENTRATION, Tc <br />The time of concentration was calculated using the SCS upland method (Soil Conservation Service, <br />1986). The hydraulic lengths, drainage heights and associated slopes were determined from scaled <br />drawings. The values calculated for each stmcture are shown on asub-watershed basis in the SEDCAD4 <br />printouts. <br />• <br />Exh. 7-20-2 Revision Date: 3/29/07 <br />Revision No.: PR-02 <br />