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• The input consists of precipitation amount (inches), watershed area <br />(acres), time of concentration (hours), runoff curve number (CN), <br />representative particle-size distributions of the soil types in the <br />watershed, sedimentology information, spillway information, and etc. <br />The precipitation amounts for the Seneca II Mine for the design storm <br />durations were obtained from the NOAA Precipitation Frequency Atlas <br />Precipitation - Frequency Atlas of the Western States Volume III - <br />Colorado, 1973 and are listed below: <br />10-year, 24-hour - 1.60 inches <br />25-year, 24-hour - 2.09 inches <br />The runoff curve number (CN) is a factor relating the amount of rainfall <br />to the amount of runoff for a given area. The soil erodibility factor <br />("k" factor) is a factor of a soils susceptibility to erosion and <br />infiltration capacity. These factors are dependent upon the <br />• infiltration characteristics of the soil types and the type and amount <br />of vegetation. Tables 7-6-1 and 7-6-2 list the values of CN and "k" <br />for different soils types at the Seneca II Mine. <br />The curve number (CN) and "k" factor for each of the ponds was <br />determined as a weighted average. The undisturbed area curve number and <br />"k" factor information is based on the soils and vegetation information <br />found within Tab 9 and 10, Exhibits 9-1 and 10-1, respectively of the <br />Seneca II Mine Permit. <br />• <br />7-6-3 Revised 8/27/86 <br />