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• Storm Duration <br />A storm duration of 24 hours was used to model the watershed for the Seneca II Mine area. <br />Retum Period/Precipitation <br />A precipitation amount is required for the appropriate return period. The following precipitation <br />amounts were used for Seneca II Mine Area: <br />10-year, 24-hour event 1.7 inches (recent calculations) <br />1.6 inches (original calculations) <br />]00-year, 24-hour event 2.5 inches (recent calculations) <br />2.4 inches (original calculations) <br />The precipitation amounts were obtained from the Seneca II Mine Permit application. <br />Hvdrogranh Response Sha rye <br />A unit hydrograph is chosen for each drainage area or sub-area model to predict the run-off <br />• response. The h}'drograph responses available in the SEDCAD+ model are slow, medium, and fast. <br />A slow response corresponds to a forested area or an area with a number of obstructions. A fast <br />response corresponds to an unvegetated or poorly protected area. Fast and medium hydrograph <br />responses were chosen for disturbed and undisturbed areas, respectively. <br />The internal convolution increment is 0.05 hours and values are saved at the user specified interval of <br />0.1 hours or greater. A convolution increment of 0.] was specified for the Seneca II Mine Area. <br />Drainage Basin Area <br />The drainage areas were determined by direct measurement from a ]' = 500' map, with a contour <br />interval of 10 feet The watershed areas are shown on Exhibit 7-79, Postmining Drainage <br />Control. <br />Time of Concentration. T~ <br />The time of concentration was calculated using the SCS upland method (a utility of SEDCAD+). All <br />h}'draulic lengths, drainage heights and slopes were measured direct]}' from a 1" = 500' map, with a <br />contour interval of 10 feet. The calculated values for each structure are shown on the SEDCAD+ <br />printouts. <br />TR-36 7-12-3 Revised 11/04 <br />