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SOMERSET MINING COMPANY PROTECTION OF THE HYDROLOGIC BALANCE • STORM EVENTS 10 YEAR 24 HOUR EVENT 2.0 INCHES 25 YEAR 24 HOUR EVENT 2.4 INCHES 100 YEAR 24 HOUR EVENT 2.8 INCHES The run-off calculations presented below are calculated based on the manual - Procedures for Determining Peak Flows in Colorado - published by the Soil Conservation Service March 1984. The Sanborn Creek Mine area has been broken down into the following drainage areas. The drainage areas are shown on Drawing Nos. 5-018 and 5-021. Curve Numbers are taken from the Mined Land Reclamation Division's adequacy responses dated February 28, 1991. Drainage area descriptions, curve numbers and areas are as follows: Drainage Area ~1, undisturbed area west of Sanborn Creek and east of the drainage area collected by the existing east yard sediment pond. Curve Number 63; 10.2 acres. Drainage Area #2 undisturbed area east of Sanborn Creek west of the portal area. Curve Number 80; 22.1 acres. Drainage Area #3 undisturbed area east of Sanborn Creek °'° directly above the portal area. Curve Number 80; 2.5 acres. Drainage Area #4, undisturbed area east of drainage area #3 directly above the substation area. Curve Number 80; 0.4 acres. Drainage Area #5 undisturbed east of area #4 that provides the flow in the borrow ditch along the old state highway. Curve Number 80; 51.6 acres. Drainage Area #6 undisturbed area which is the Sanborn Creek drainage area. Curve Number 57; 1050 acres. Drainage Area A, disturbed area around portal and substation. Curve Number 89; 0.6 acres. Drainage Area B; haul road, cut slopes and pond area east of Sanborn Creek. Curve Number 92; 1.3 acres. Drainage Area C; disturbed area conveyor corridor Station 0+00 to Station 13+00. Curve Number 89; 0.6 acres. Drainage Area D; haul road and cut slopes west of Sanborn Creek to the east yard. Curve Number 92; 0.8 acres. Drainage Area E; disturbed area conveyor corridor station 13+00 to 21+00. Curve Number 89; 0.2 acres. . B-88