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• METHODOLOGY AND ASSUMPTIONS <br />The required hydrology, sedimentology, and hydraulic calculations for the Grassy <br />Creek Mine permanent drainage structures were performed utilizing a combination <br />of hand calculations and the SEDCAD+ computer model developed by Civil <br />Software Design. SEDCAD+ is a third generation hydrologic and hydraulic <br />calculation model designed for use on IBM compatible computers. <br />Measurements for premine hydrology and sedimentology parameters for Pits 5 <br />and 6 were taken from the Reclamation Map in the Rockcastle Permit. This map <br />was developed prior to mining related disturbance and best represents the <br />premine configuration. Premine measurements for Pits 3 and 4 were assumed to <br />be similar to Postmining conditions. Postmine measurements for Pits 2, 3, 5 and <br />6 were taken from the Postmining Drainage and Sediment Control Map (Map R-1). <br />The Postmining Drainage and Sediment Control Map was developed from the <br />• Reclaimed Topography (1990) & Bond Release Documentation Map, which reflects <br />aerial photography from a flight in October of 1990. Postmine topography for Pits <br />5 and 6, shown on the Drainage and Sediment Control Map was developed by <br />hand. <br />HYDROLOGY <br />Hydrology calculations were performed utilizing the SEDCAD+ computer model. <br />Computer printouts including hydrology inputs and results are contained in Exhibit <br />1, SEDCAD+ Computer Printouts. <br />SEDCAD+ Hydrology Calculations <br />The SEDCAD+ model calculates runoff and peak flow via a numerical modeling <br />technique based on user inputs of runoff basin information and design storm. <br />Design storm inputs include rainfall amount and duration, selection of a rainfall <br />• distribution, and convolution increment. Hydrographs are developed on a <br />subwatershed basis with the input of area, time of concentration, SCS curve <br />1 <br />