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iiiiiiiiiiiiiiuhi <br />• <br />~/ METHODOLOGY AND ASSUMPTIONS <br />Doc Date:1 2/1 112 0 01 <br />Two (2) methods were used to determine runoff volume and peak discharge <br />values for the drainage and sediment control structures located within the <br />Southfield Mine permit area. In the original permit submitted by <br />Dorchester Coal Company, the "THYDI" computer model was used to determine <br />hydrologic values for all structures. In 1989, all the drainage and <br />sediment control structures utilized at the coal refuse storage area were <br />redesigned to address the necessary modifications resulting from expanding <br />the refuse pile. Also, the refuse collection ditches and Permanent <br />Diversion No. 1 were upgraded to safely pass the 100-year, 24-hour storm <br />event. The refuse area structures were redesigned utilizing the "SEDCAD+" <br />computer model. The redesigns for the refuse ditches and permanent <br />diversion have been approved. Magpie Creek Diversion was also redesigned <br />using SEDCAD+. This Methodology and Assumptions section discusses both <br />methods. <br />Sedimentology calculations for each sedimentation pond were performed by <br />hand and the methodology and assumptions are included in each pond's <br />design calculations section. <br />THYDI COMPUTER MODEL (STRUCTURES MAINTAINING ORIGINAL DESIGN) <br />THYDI is a hydrologic computer model used to calculate the runoff volume <br />and peak discharge for the drainage and sediment control structures at the <br />Southfield mine and loadout areas, with the exception of Permanent <br />Diversion No. 1, the refuse collection ditches, Magpie Creek Diversion and <br />Sedimentation Pond No. 4. THYDI is a computerized version of the SCS <br />Hydrograph Method. Documentation for this computer model was not <br />submitted in the original permit, therefore, none is contained in this <br />exhibit. The computer printouts are contained in the design calculations <br />section for each structure where the model was used. <br />SEDCAD+ COMPUTER MODEL (STRUC7URES REVISED IN 1989? <br />• The required hydrology calculations for all of the mine area structures, <br />_~Z ~ I <br />4 R Revised - December, 1991 <br />