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Return Period/Precpitation <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 />100-year, 24-hour event 2.5 inches (recent calculations) <br />2.4 inches (original calculations) <br />The precipitation amounts were obtained from meteorological data in the Seneca II Mine permit <br />application package. <br />Hydrograph Response Shape <br />A unit hydrograph is chosen for each drainage azea or sub-area model to predict the run-off <br />response. The hydrograph responses available in the SEDCAD+ model are slow, medium and fast. <br />A slow response corresponds to a forested area or an azea with a number of obstructions. A fast <br />response corresponds to an unvegetated or poorly protected azea. 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 were saved at the user specified interval <br />of 0. ] hours or greater. A convolution increment of 0.1 was specified for the Seneca R Mine area. <br />Watershed Area <br />The watershed areas were determined by direct measurement from Exhibit 7-19, Postmining <br />Drainage Control and Exhibit 7-22, Pond 004 and 008 Watershed Boundary. <br />Time of Concentration, T~ <br />The time of concentration was calculated using the SCS upland method (a sub-routine built into <br />the SEDCAD+ model). The hydraulic lengths, drainage heights and slopes were measured directly <br />from the mine permit watershed maps. The calculated T~ value for each structure is found in the <br />SEDCAD+ demonstrations. <br />Muskingum Routin¢ Parameters, K, X <br />The Muskingum Routing parameters were also calculated using the SCS upland method. The <br />hydraulic lengths, drainage heights and slopes were measured directly from the mine permit <br />watershed maps. The calculated routing values aze also found in the SEDCAD+ <br />demonstrations. <br />TR-41 7-12-3 Revised 04/07 <br />