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METHODOLOGY AND ASSUMPTIONS Two (2) methods were used to determine runoff volume and peak discharge values for the drainage and sediment control structures located within the Southfield Mine permit area. In the original permit submitted by Dorchester Coal Company, the "THYD1" computer model was used to determine hydrologic values for all structures. In 1989, all the drainage and sediment control structures utilized at the coal refuse storage area were redesigned to address the necessary modifications resulting from expanding the refuse pile. Also, the refuse collection ditches and Permanent Diversion No. 1 were upgraded to safely pass the 100-year, 24-hour storm event. The refuse area structures were redesigned utilizing the "SEDCAD+" computer model. The redesigns for the refuse ditches and permanent diversion have been approved. Magpie Creek Diversion was also redesigned using SEDCAD+. This Methodology and Assumptions section discusses both methods. Sedimentology calculations for each sedimentation pond were performed by hand and the methodology and assumptions are included in each pond's design calculations section. THYD1 COMPUTER MODEL (STRUCTURES MAINTAINING ORIGINAL DESIGN) THYDI is a hydrologic computer model used to calculate the runoff volume and peak discharge for the drainage and sediment control structures at the Southfield mine and loadout areas, with the exception of Permanent Diversion No. 1, the refuse collection ditches, Magpie Creek Diversion and Sedimentation Pond No. 4. THYD1 is a computerized version of the SCS Hydrograph Method. Documentation for this computer model was not submitted in the original permit, therefore, none is contained in this exhibit. The computer printouts are contained in the design calculations section for each structure where the model was used. SEDCAD+ COMPUTER MODEL (STRUCTURES REVISED IN 1989) The required hydrology calculations for all of the mine area structures, 4 R Revised - December, 1991