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<br />r 1 <br />~J <br />Storm Duration <br />A storm duration of 24 hours was used for design of structures at the <br />Southfield Mine. <br />Return Period/Precipitation <br />A precipitation amount is required for the appropriate return period. <br />Permanent Diversion No. 1 and all refuse collection ditches were designed <br />using the 100-year, 24-hour storm, Sedimentation Pond No. 4 was designed <br />using the 10-year and 25-year, 24-hour storms and Magpie Creek Diversion <br />was designed using the 10-year and 100-year, 24-hour storms as follows: <br />Return Period Precipitation (inches) <br />10-year, 24-hour 2.8 <br />25-year, 24-hour 3.4 <br />100-year, 24-hour 4.4 <br />Precipitation values were obtained from the NOAH Atlas 2, Precipitation- <br />Frequency Atlas of the Western United States, Volume 3, Colorado; and <br />Procedures for Determining Peak Flows in Colorado, SCS 1980. <br />Hvdrograoh Response Shape <br />A unit hydrograph is chosen for each drainage area or sub-area model to <br />predict the runoff response. The hydrograph responses available in the <br />SEDCAD+ model are slow, medium, and fast. A slow response corresponds to <br />a forested area or an area with a number of obstructions, whereas a fast <br />response corresponds to an unvegetated or poorly protected area. Fast and <br />medium hydrograph responses were chosen for disturbed and undisturbed <br />areas, respectively. <br />The internal convolution increment is 0.05 hours and values are saved at <br />the user specified interval of 0.1 hours or greater. (A convolution <br />,,, increment of 0.1 was specified for all structures.) It should be noted <br />that a time of concentration less than 0.125 hours bypasses the unit <br />hydrograph technique and instantaneous runoff is assumed. As explained <br />6 <br />