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• PRECIPITATION DISTRIBUTION <br />A precipitation distribution is input to model the runoff hydrograph. <br />SEDCAD+ allows the user to choose between the SCS Type I and Type II <br />storms. For the Southfield Mine area the SCS Type II storm was input. <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 N1 and all refuse collection ditches were designed <br />using the 100-year, 24-hour storm as follows: <br />• Return Period Precipitation (inches) <br />100-year, 24-hour 4.4 <br />Precipitation values were obtained from the NOAA Atlas 2, <br />Precipitation-Frequency Atlas of the Western United States, Volume 3, <br />Colorado. <br />HYOROGRAPH 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 <br />to a forested area or an area with a number of obstructions, where as a <br />fast response corresponds to an unvegetated or poorly protected area. <br />Fast and medium hydrograph responses were chosen for disturbed and <br />undisturbed areas, respectively. <br />u <br />-5- <br />