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• POWDERHORN COAL COMPANY <br />Pond 6 Run-off Volume <br />The following calculations are prepared to determine the ramifications of the 25-year, <br />24 hour storm event in the Pond 6 watershed coinciding with the 10-year, 25-year and <br />100-year 24-hour peak flows in the Coal Creek Channel. <br />STORM EVENTS <br />10 YEAR 24 HOUR EVENT 1.4 INCHES <br />25 YEAR 24 HOUR EVENT 1.8 INCHES <br />100 YEAR 24 HOUR EVENT 2.2 INCHES <br />Reclaimed CRDA-1 <br />Curve Number Determination <br />1999 Savage Report <br />Vegetative Cover % 34.4 <br />Litter % 25.8 <br />Ground Cover % 60.2 <br />Table 2-2d-Runoff curve numbers for arid and semiarid rangelands <br />• Cover Type -Sagebrush with grass understory <br />Hydrologic Condition Fair <br />Soil Type -Cameo B <br />Curve Number 51 <br />Coal Refuse <br />Hydrologic Condition Poor <br />Soil Type -Not Class'rfied D <br />Curve Number 88 <br />Consistent with Appendix 13-2 <br />Upland <br />Table 2-2d-Runoff curve numbers for arid and semiarid rangelands <br />Cover Type -Desert Shrub <br />Hydrologic Condition - 30 to 70% Ground Cover Fair <br />Soil Type -Cameo B <br />Curve Number 72 <br />Pond 6 <br />Pond 6 was designed to contain 1.49 acre-feet of water which was the runoff from <br />the 10-year 24-hour storm event. This run-off calculation was based on the assumption <br />the entire Pond 6 watershed would be covered with coal refuse and a curve number of <br />88 was assigned to the entire watershed Since a portion of the refuse pile has been <br />• reclaimed the runoff characteristics of the Pond 6 watershed have changed. <br />CRDA 1 ~ L 6/01 <br />