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~ ~~U ,~ VU ht ~~ ~{~ S Vlict~~L~c Mountain Coal Company West Elk Mine Exhibit 51 Lower Refuse Pile TABLE 13. Clear Water Ditch Design', 100-Year Storm 100-year, 24-hour Normal Riprap Phase Location Peak Discharge Velocity Depth Size= (cfs) (fps) (h) (inches) I East 53 4.4 .63 unlined South 4.6 5.3 S4 unlined II East 5.3 4.4 .63 unlined South 4.6 5.3 .54 unlined III South 4.3 4.0 .60 unlined Southwest 5.1 5 .55 unlined IV South :96 ~ 9 1.7 .43 unlined Upper SVV~ W IOWU( 4.33- ~ q 33 27 ' ~27 ~ 1.2 ~ 1 2 unlined li d Lower S E 4. . > un ne New Access Road 1.64 4.7 .34 unlined Subsoil Pile (Phase I through IV) West 4. 2.5 .74 unlined 'Ditch triangular aMped, 2 [«t deep, 3H:1 V slopps~ (Dt~W-2) 2 Riprap s've represenu 50 percent aize 6.5 'T`reatment oC Water in Sediment Pond Mountain Coal treat the water collected in the sediment pond to meet all applicable efflue t requirements. 6.6 100-Ye~'ir Flood Events in Sylvester Gulch 100-Year Flood Event Determination Illustrated on the various phase diagrams, the realignment of Sylvester ulch Road located the road within 50 feet of Sylvester Gulch. To insure that f e topsoil stockpile and the lower refuse pile would not be flooded, a 100- year flood study was performed to determine the maximum elevation of the 100-year flood. Tah~~ 4`f ~aw5 V~y'~~ The 100-year floodplain in the vicinity of the proposed topsoil stockpile for Sylvester Gulch is shown on Map LWP-002. The floodplain was determined based upon the peak discharge for the 100-year, 24-hour storm. The peak discharge was computed by using a Type II distribution storm. The precipita- tion from a 100-year, 24-hour storm event was determined to be 2.6 inches, based upon Figure S-7 of the manual Procedures for Determining Peak Flows in Colorado Soil Conservation Service, U.S. Department of Agriculture, March 1980. The peak flow for the 100-year, 24-hour precipitation event for ~4